Hydraulic Fluids

MNL37-EB/Jun. 2003

Hydraulic Fluids W. A. Givens^ and Paul W. Michael^

T H E PRIMARY PURPOSE OF A HYDRAULIC FLUID is to transfer

power. The concept of fluid power is based on a principle articulated by Blaise Pascal, which is usually given as follows: "Pressure applied to an enclosed fluid is transmitted undiminished to every portion of that fluid and the walls of the containing vessel" [1]. Within the context of fluid power, pressure is related to the force acting on a confined fluid as illustrated in Fig. 1 [2]. This principle has given rise to mode m hydraulics, which entails highly engineered systems for efficiently controlling fluid flow to transfer energy and accomplish work.

The heart of any hydraulic system is the pump, which pulls in fluid from a reservoir by creating a vacuum at its inlet and then forces the fluid through its outlet, usually against pressure created by flow controllers and/or actuators downstream of the pump. Pumps, actuators, and other system components have surfaces that move relative to each other, often at high speeds, pressures, and temperatures . These components require cooling and lubrication for efficient performance and durability. Consequently, hydraulic fluids not only must transmit power, they serve critical functions as lubricant and heat transfer medium.

P o w e r Trans fer

To transfer power efficiently, a hydraulic fluid must exhibit minimal compressibility. Low compressibility allows all of the pressure applied to the fluid to be available for direct and effective transmission to system components such as motors, cylinders, or other actuators. The compressibility of a fluid is generally discussed in terms of its "bulk modulus," which describes the change in fluid volume as a result of applied pressure [3]. The bulk modulus of a fluid, which is the reciproccd of compressibility, is described by Eq 1. There are a number of methods available for est imating the isothermal secant bulk modulus of a fluid based upon its viscosity and density characteristics [4,5]. As depicted in Fig. 2, the bulk modulus for oil also varies with temperature [6]. For petroleum oils, compressibility is often assumed to be 0.5% for each 1000 psi pressure increase up to 4000 psi [7].

Where: K = Bulk modulus Vo = Original volume AP = Pressure change Ay = Change in volume

H e a t Trans fer

Heat is generated as a by-product of normal operation of a hydraulic circuit. Friction between the moving parts of a p u m p or hydraulic motor, as well as friction between the fluid and surfaces of valves, pipes, and other circuit devices generates heat. In addition, heat is generated in a hydraulic system as a result of the dissipation of the potential energy of pressurized fluid [8]. As a hydraulic fluid is circulated through a system, heat is transferred from high temperature areas to coolers, reservoirs, and other regions of the circuit where it is dissipated. As can be seen in Table 1, typical specific heat and thermal conductivity values for hydraulic oils are a fraction of that of water [4]. These factors are an important consideration in sizing hydraulic system coolers because the inherent cooling efficiency of petroleum based hydraulic fluid is less than that of water. ASTM D 2717, Test Method for Thermal Conductivity of Liquids and ASTM D 2766, Test Method for Specific Heat of Liquids and Solids are used to determine these properties of fluids.

L u b r i c a t i o n

The durability of hydraulic equipment depends to a large extent upon the lubricating properties of the fluid. As a lubricant, the key function of the hydraulic fluid is to reduce friction between contact surfaces. A reduction in friction lowers contact temperatures and wear. This is accomplished through a combination of hydrodjoiamic and boundary lubrication mechanisms. The hydrodynamic lubricating properties of a fluid are governed by its physical properties while boundciry lubrication is a function of fluid chemistry. A discussion of hydraulic fluid wear testing is presented in the Wear Protection section of this chapter.

Bulk modulus {K) = -Vo (\PI\V) (1) TRENDS

* Exxon Mobil Research & Engineering, Paulsboro Technical Center, 600 Billingsport Rd., Paulsboro, NJ 08066.

^ Benz Oil, 2724 West Hampton Avenue, Milwaukee, WI 53209.

A brief outline of major trends in the motion control industry, particularly with respect to hydraulic equipment design and fluid requirements, is presented as a backdrop for the discussion of hydraulic fluids test methods. As motion con-

353

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354 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK

Force

Area

F = force in pounds p = pressure in pounds / sq. incli (psi) A = sq. in.

FIG. 1—Relationship of force, pressure, and area in fluid power. Any one of the parameters equals the other two in the relationship depicted by the triangle.

40

^ 30 M Q.

z CO

I 3 m

20

10

100 200 300

FIG. 2-fluid.

-Effect of temperature on the bulk modulus of petroleum

trol technology advances, there is a trend towards higher performance and efficiency. For hydraulic equipment, this translates into a concentrat ion of horsepower in smaller components. There are a number of reasons for such a trend. Equipment manufacturers are looking for ways to minimize raw material usage and cost. Users of the equipment demand smaller systems for better space utilization in industrial environments cind compact multifunctional capabilities in mobile equipment. These advancements in mechanical design along with encroachment of environmental , health, and safety regulations fuel the following trends: • Hydraulic equipment builders will continue to push com

ponent manufacturers to design parts to accommodate high pressures and temperatures . For example, hoses, valves, and other fittings will continue to evolve in terms of materials used as well as actual functional design.

TABLE 1—Thermal conductivity and specific heat values for oil and water.

Thermal Conductivity Btu/h/ft^/F/Ft

@ 212°F

Thermal Conductivity

W / m - K @373K

Specific Heat

BTU/lb°F @68°F

Specific Heat

J/kg • K @293K

Oil Water

0.08 0.39

0.14 0.67

0.47 1.0

1966 4184

Smaller components will mean smaller p u m p displacements [cubic inches or cc per pump revolution]. To maintain flow rates at present or higher levels, pump speeds will be increased [cubic inches/minute = displacement X speed (rpm)]. Smaller reservoir sizes will mean shorter fluid residence times and will therefore dictate use of hydraulic fluids with improved air release characteristics. Smaller dimensional clearances will be required. These smaller clearances will dictate more stringent fluid cleanliness requirements to prevent abrasive wear from particulate contaminants and failure of servo or proport ional valves.

Fluid cleanliness will increasingly be emphasized as an effective way of increasing equipment durability and controlling warranty costs. As a result, users will move to finer filtration and specify pre-filtered hydraulic fluids [9]. Consequently, the filterability of the hydraulic fluid will continue to grow in significance. (Filterability is described in section 4.6.) Quieter hydraulic systems will be required in order to meet workplace noise restrictions and compete with electric motors. Reduction of noise levels in hydraulic equipment has been attained by the insulation that absorbs the noise. This insulation results in higher system temperatures, as heat is not as readily dissipated. Components and actuators, such as cylinders, will be designed with tighter seals to increase efficiency and reduce


CHAPTER 13: HYDRAULIC FLUIDS 3 5 5

leakage. The effects of this trend include increased stress on seal materials and cylinder chatter resulting from reduced lubrication between seal and cylinder wall. In addition, certain applications will require fill-for-life systems that translate into lower maintenance and disposal costs. Consequently, fluids will remain in a system for longer periods, since meike-up fluid is not required.

• A growing awareness of the environmental impact of chemicals will lead to further restrictions on performance additives eind base stocks used in lubricants. As a result, lubricant producers are required to address such issues through alternative (usually more costly) chemistry and the development of environmentally friendly (non-toxic/biodegradable) lubricants.

• The hydraulic fluid industry has evolved from the use of plain water in hydraulic systems to the use of advanced fluid technologies that continue to evolve as performance requirements become more stringent and equipment designs become more sophisticated [10]. Due to environmental health and safety issues, hydraulic systems are once again being designed to employ pure water as hydraulic fluid [11].

Solvent refining yields base oils that fall into Group I while hydroisomerization and deep hydrogenation processes yield low sulfur, high paraffin content Group II and Group III base stocks. Because of their lower aromatic and sulfur content, hydraulic fluids formulated from Group II and Group III base stocks typically have superior oxidation stability. However, more highly refined stocks tend to be less effective at dissolving additives. Not only is additive solubility a concern, additive chemistries and their functional mechanisms may be both synergistic and antagonistic. Thus, additive chemistry must be ceirefully balanced to achieve opt imum performance. In the following section, test methods for evaluating key fluid properties such as oxidation stability, wear prevention, and corrosion inhibition are discussed. These methods have been developed to measure characteristics of hydraulic fluids that are thought to correlate to performance in "real-life" applications as well as gage additive response for the fluid formulator. In order to provide a link between fluid tests and additive chemistry, a description of the generally accepted functional mechanisms of additives is also included.

PETROLEUM BASE STOCKS

Most hydraulic fluids consist of a base fluid and additives that are designed to impart chemical characteristics and functionality to the finished product. Operating conditions and equipment builder specifications generally dictate the type of fluid that is needed and thus, the kind of base stocks and additives employed. In petroleum based hydraulic fluids the typical concentration of additives is less than 3.0% by weight. Paraffinic oils are the primary base stock utilized in hydraulic fluids but other materials, from polyglycols to vegetable oil, serve as the basis for formulating hydraulic fluids.

From a historical standpoint, solvent reflned paraffinic oils have been the most widely used base stock for hydraulic applications. In recent years alternative refining processes such as catalytic isomerization and deep hydrogenation have been developed to yield higher purity base oils that are bet ter suited to withstand severe operating conditions [12]. These base stocks are categorized by the American Petroleum Institute (API) according to their composition and viscosity index [13]. Groups I through III consist of crude derived base oils while Group IV is reserved for synthetic polyalphaolefins. Low viscosity index naphthenic oils and other base stocks that do not meet Group I through IV criteria are classified as Group V. The API Base Oil classification is described in Table 2.

TABLE 2—API base oil classifications.

Category

Group I

Group II

Group III

Group rV Group V

Composition

<90% Saturates or >10% aromatics

£ 9 0 % Saturates or <10% aromatics

>90% Saturates or <10% aromatics

All polyalphaolefins (PAO) All others not included in

Groups 1,11, m or IV

Suli^ir

>0.03%

<0.03%

<0.03%

Viscosity Index

80-120

80-120

>120

F L U I D C H A R A C T E R I S T I C S A N D P E R F O R M A N C E

O x i d a t i o n a n d T h e r m a l Stabi l i ty

An important characteristic of a hydraulic fluid is its ability to withstand high temperatures. This is because horsepower losses in hydraulic systems directly result in transfer of heat to the fluid. Resulting high temperatures can cause hydraulic fluids to react with oxygen. The rate of this reaction accelerates exponentially with increasing temperatures and is further catalyzed by metals like copper and iron, especially at temperatures above 200°F [14]. Rate constants for the oxidation of saturated hydrocarbons at 125°C are as much as 40 times higher than rate constants at 60°C [15]. Thus, fluid oxidation is highly dependent upon hydraulic system operating temperatures. Lubricants expected to operate in high temperature environments are tjrpically fortified with additives known as antioxidants, which are discussed in the Antioxidants section. Oxidative stabilization of the fluid translates directly into extended oil service life. Failure to resist oxidation can result in thickening of the oil (viscosity increase), formation of acidic byproducts, and subsequent deposit formation.

Not only can heat cause oxidation, fluids may thermally degrade upon exposure to high temperatures with litde or no oxygen present. The thermal stability of a hydraulic fluid is dependent mainly on the intrinsic ability of the base fluid or its components to resist decomposition at high temperatures. Unlike oxidation, controlled thermal degradation of certain types of additives [such as Zinc Dialkyldithiophosphate (ZDTP)] is desirable, because it is the very mechanism by which they react with the metal surfaces they are designed to protect [16]. Similar to oxidation however, the negative effects of thermal degradation may include increased acidity, thickening of the oil, and deposit formation. Therefore, good control of thermal degradation results in the retention of desired fluid properties.


3 5 6 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK

High Temperature/Oxidation Tests

One of the most commonly sited methods for measuring the abihty of a fluid to resist oxidation is ASTM D 943, Standard Test Method for Oxidation Characteristics of Inhibited Mineral Oils (also known as the Turbine Oil Oxidation Stability Test - TOST). In this test, 300 ml of fluid and 60 ml of distilled water are placed in a large test tube together with coils of copper and iron wire (Fig. 3). The fluid is heated to 95°C (203 °F) and oxygen is bubbled through the fluid at a controlled rate. The test is complete when the Total Acid Number (AN) of the fluid reaches 2.0 mg KOH/g. As can be seen from the reaction scheme in Fig. 5, cJdehydes eire among the chemical by-products of hydrocarbon oxidation. These aldehyde compounds are readily converted to carboxylic acids in the hydraulic system [17,18]. Since carboxylic acids are corrosive to yellow metals and agglomerate to form deposits, they have a detrimental effect upon fluid performance when their concentration becomes excessive. The concentration of acidic oxidation debris present in a fluid can be determined by titration with potassium hydroxide. For the D 943 test, a variation on ASTM D 664 Acid Number of Petroleum Products by Potentiometric Titration is used. This method, ASTM D 3339, Test Method for Acid Number By Semi-Micro Color Indicator Titration is utilized because it permits a 0.2-1.0 g sample size for total acid numbers in the 0.5-3.0 mg KOH/g

OXYGEN DELIVERY

TUBE

CATALYST COILS

FIG. 3—Oxidation cell and sampling tube for ASTM D 943 apparatus.

range. The hours to form 2.0 mg of KOH equivalents of acidic oxidation products per gram of some typical fluids are shown in Table 3. In general, turbine oils provide longer TOST oxidation life than antiwear hydraulic fluids because turbine oils typically do not contain zinc dialkyldithiophosphate (ZDTP). Zinc dialkyldithiophosphate reduces the t ime it takes for a fluid to reach 2.0 mg KOH/g because it is acidic and its mere presence raises the acid number of the fluid. In addition ZDTP is subject to hydrolysis and forms acidic compounds as it degrades. Ester based fluids such as rapeseed oils are also subject to hydrolysis, which accounts for their poor performance in the D 943 test. When the D 943 test is run without water (dry method), the oxidation life of a synthetic ester can be extended by nearly a factor of 100.

The amount of sludge produced in the TOST test may be measured by ASTM D 4310, Test Method for Determination of the Sludging Tendencies of Inhibited Mineral Oils. In this test, the fluid is subjected to D 943 test conditions for 1000 h. At the end of this time, the sludge produced is determined gravimetrically by filtration of the oxidation tube contents through 5-/u,m pore size cellulose acetate filter disks. To a certain extent the D 943 and D 4310 tests evaluate different mechanisms of high temperature degradation. In the D 943 test, acidity is measured and this acidity is predominantly due to formation of carboxylic acids by the conventional liquid phase oxidation mechanism shown in Fig. 4. In essence D 943 measures the stability of the base oils and the effectiveness of oxidation inhibitors. Sludge formation in hydraulic oils is to a greater extent due to theimal degradation of the ZDTP antiwear additive. Consequently, the result of a D 4310 test is an indication of the thermal stability of ZDTP. Figure 5 shows a model for the mechanism of sludge formation by zinc dialkyldithiophosphate [19].

Another method for measuring the sludging tendency of hydraulic fluids is the Cincinnati Machine Heat Test [20]. This test has been adopted as an ASTM procedure and is designated ASTM D 2070, Standard Test Method for Thermal Stability of Hydraulic Oils. In this test, polished pre-weighed copper and steel rods are placed in a beaker containing 200 cc oil and heated to 135°C (275°F) for 168 h. At the end of the test, the copper and steel rods are examined for discoloration due to corrosion caused by carboxylic acids and sulfur compounds formed by thermal degradation. Sludge content and viscosity increase are also measured (Table 4).

Antioxidants

Oxidation inhibitors, commonly referred to as antioxidants, are chemicals that reduce the rate at which oxidative degradation of a lubricant occurs. Degradation begins with the reaction of hydrocarbon molecules at elevated temperatures to form unstable reactive species known as free radicals. These

TABLE 3—D 943 turbine oil oxidation test life of typical hydraulic fluids.

Hours to TAN of Fluid Type 2.0 by D 943 Method

Synthetic ester without antioxidant 65 Mineral oil without additives 300 Antiwear hydraulic oil. Group I base stock 2016 Antiwear hydraulic oil, Group 11 base stock 5040 Synthetic ester with antioxidant, dry method 5500 R & O hydraulic oil >10,000


Initiation RH Temperature _ Light, catalyst

CHAPTER 13: HYDRAULIC FLUIDS 357

Alkyl radical

Propagation R • + O2

ROO• + RH

- • ROO* Peroxy radical

- • ROOH + R* Hydroperoxide

Branching ROOH

RO» + RH

• OH + RH

-*• RO • + • OH Alkoxy radical

-> ROH + R • Alcohol

-> H2O + R • Water

Termination R • + ROO •

RO • + ROO •

ROO • + ROO •

RO • + R •

R« + R«

Alcohols

Aldehydes

Ketones

Acids

Longer chain hydrocarbons

FIG. 4—Reaction scheme for liquid hydrocarbon oxidation.

Hydraulic Oil

RO S S OR

RO S — Z n — S OR

Base Oil (Paraffinic)

and Additives

Reaction withi

Thermal P^°''^^ Deterioration witii Water Degradation

T

Decomposition Oxidation

Reaction with l\̂ etai Ions

ZnSq Polyphosphates

RO 0

\ ^ RO 0 -

0 OR

- Z n — 0 OR

Oxidation Products

and Metal Soaps

T

Machines and Outside Environment

Wear Particles, Dust, Rust, Water

andOtliers

Sludge

FIG. 5—Mechanism of sludge formation by zinc dialkyldithiophosphate.


358 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK

TABLE 4—Cincinnati machine thermal stabihty test performance requirements.

Property Requirement

Condition of steel rod Visual Deposits Corrosion Condition of copper rod Visual Corrosion Condition of fluid Viscosity Sludge Total acid number

No discoloration 3.5 mg maximum 1.0 mg maximum

5 rating maximum 10.0 mg maximum

5% change maximum 25 mg/100 mL max ±50 % maximum

species react with oxygen and non-oxidized oil to form additional free radicals, which propagate the oxidation process. This generally accepted mechanism is described as free radical chain reaction and is illustrated by the steps shown in Fig. 4.

Antioxidants interrupt this chain reaction and thus, reduce the rate of oxidation and the resulting viscosity increase and acid and deposit formation. There are two general mechanisms by which these additives inhibit oxidation. The antioxidants are therefore categorized as primary or secondary, depending upon the mechanism of oxidation inhibition. Primary antioxidants, commonly referred to as "free radical scavengers," react with the peroxy radicals and hydroperoxides to form inactive compounds (Fig. 6) [21]. Examples of primary antioxidants include hindered phenols and aromatic amines. Secondary antioxidants, commonly referred to as "peroxide decomposers," react with hydroperoxides or peroxy radicals to form less reactive compounds. Examples of secondary antioxidants include sulfur and/or phosphorus compounds and metal dithiophosphates (Fig. 7). Antioxidants genereilly function in the bulk lubricant and are consumed as they do their job [22].

Detergents IDispersants

Detergents and dispersants are used to delay formation and subsequent deposit of insoluble oil degradation species. The terms detergent and dispersant are often used interchangeably, but are generally differentiated by their composition and primary functionality. Detergents are metallo-organic compounds that neutralize acidic deposit precursors, while dispersants are predominantly organic chemicals that keep insoluble materials dispersed and suspended in the lubricant. The term "ashless" dispersants, meaning non-metallic, is used to further differentiate dispersants from detergents. Some detergents have the ability to disperse and suspend in-solubles, while some dispersants are capable of neutralizing precursors of deposits. Typical lubricant detergents include barium, calcium, and magnesium phenates, phosphates, salicylates and sulfonates. Ashless dispersants are typically alkyphenol-based or alkyl succinimides.

(R0)3P + R'OOH (R0)3P?-0—O3

H

(R0)3P=0 + HOR'

FIG. 7—Secondary antioxidants such as the phosphite compound depicted above inhibit oxidation by decomposing hydroperoxides. This prevents the oxidation process from progressing beyond the branching stage In the reaction mechanism.

o»

+ R00»

ROO^^R

FIG. 6—Reaction scheme for primary antioxidants. Primary or free-radical trapping antioxidants work by donating a hydrogen radical H* to the peroxy radical formed during mineral oil oxidation. Due to steric hindrance, the antioxidant radical does not attack mineral oil molecules, i.e., R-H bonds. Consequently, the radical chain is terminated.



Wear Protection

Reduction of friction and prevention of wear is the fundamental purpose of a lubricant. Lubricants reduce friction in machine components by producing a physical or chemical barrier between surfaces that slide or roll past each other. Depending on equipment design and function, lubricants function within three commonly recognized regimes: hydrody-namic, mixed-film, and boundary lubrication (Fig. 8) [23].

Hydrodynamic lubrication is often the dominant lubrication regime under conditions of moderate temperatures and loads. According to the ASM Handbook on Friction, Wear and Lubrication Technology, [24] hydrodynamic lubrication is "a system of lubrication in which the shape and relative motion of the sliding surfaces causes the formation of a fluid film that has sufficient pressure to separate the surfaces." In this regime, viscosity is the most important fluid characteristic because it, in combination with sliding speed, contact geometry and load, determines the thickness of the lubricating film, and determines whether or not the surfaces will contact each other.

Fluid viscosity plays an important role in hydraulic applications. A hydraulic fluid that is too low in viscosity will cause low volumetric efficiency, fluid overheating, and increased pump wear. A hydraulic fluid that is too high in viscosity will cause poor mechanical efficiency, difficulty in starting, and wear due to insufficient fluid flow [25]. Since viscosity is a function of fluid temperature, the temperature operating window (TOW) for a particular viscosity grade of hydraulic fluid is a function of temperature. Figure 9 depicts

the TOW for straight grade mineral oil based hydraulic fluids. The viscosity grade indicated in the TOW corresponds to ASTM D 2422, Classification of Industrial Fluid Lubricants by Viscosity System. For example, ISO 32 hydraulic oil generally will provide satisfactory performance in a temperature window of - 8 to 64°C.

There are several methods for measuring the viscosity of hydraulic fluid. The most widely utilized method is the ASTM D 445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids. In this test, the time is measured for a fixed volume of liquid to flow under gravity through the capillary of a calibrated viscometer at a closely controlled temperature. The kinematic viscosity is the product of the measured flow time and the calibration constant of the viscometer. Based upon D 2442 and ISO 3448, the standard temperature for measuring hydraulic fluid viscosity is 40°C [26]. Typically, the viscosity of a hydraulic fluid is 15-68 mm^/s (centistokes) at 40°C. ASTM D 446, Standard Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers, describes more than 15 types of viscometers that may be employed in performing a D 445 viscosity test. With the exception of invert-emulsion type fluids, hydraulic fluids are generally transparent. Consequently, a tube suitable for transparent liquids such as the popular Cannon-Fenske viscometer may be used. For opaque liquids, a reverse-flow tube is required because it is difficult to see the meniscus as the fluid flows by the timing marks on a standard viscometer. Cannon-Fenske tubes for viscosity measurement of transparent and opaque liquids are depicted in Fig. 10.

c o o

c g> o

» ^

O

o

0.1

0.01

0.001

B LI

OUNDARY JBRICATION

1 MIXFn FN M LUBRICA-•|ON

hULL-hlLM LUBRICATION

1 0.001 0.01 0.1 1

Sommerfeld number, (rjA// P) x 10"^

10

FIG. 8—Stribeck Curve of coefficient of friction versus Sommerfeld Number (S), where S = r}N/P. N shaft speed; P, average pressure between shaft and bearing due to applied load; 7), lubricant viscosity.



100

90

80

70 o <D

60

5 50 -I—•

I" 30 j 5 ^ 2 0

10

0

-10

-20

-30

-40

94 —

84

73

64

55

44

32

+10 +4

-2 -8

-15 -23

— 3 3

10 15 22 32 46 68 100

212

194

176

158

140

122

104

86

C O

LL.

CD V—

CO CD O.

E .<!>

50

32

14

-4

-22

-40

ISO Viscosity Grade FIG. 9—^Temperature Operating Window (TOW) for 100 VI mineral oil based hydraulic flu

ids. Based upon survey of viscosity requirements for hydraulic pumps and motors, fluids will generally provide satisfactory performance at the temperature range that corresponds to 13 to 860 cSt.

FIG. 10—Cannon-Fenske standard and reverse flow kinematic viscosity tubes, respectively.

Frequently in high-pressure hydraulic applications, the loading conditions are sufficient to rupture hydrodynamic lubricating films. Consequently, boundary and mixed-film lubrication regimes play an important role in controlling wear in hydraulic applications. In boundary lubrication, friction cind wear between two surfaces in relative motion are de

termined by the properties of the surfaces and by properties of the surfaces and lubricants other than viscosity [27 ]. In hydraulic equipment, these surfaces are typically composed of ferrous or yellow metals. Under magnification, tribologicsJ surfaces in hydraulic components reveal the presence of surface asperities. High load conditions cause these aperities to



make contact, resulting in friction and weeir. In most cases, mixed-film lubrication takes place and some hydrodynamic lubrication occurs, even as "asperity contact" creates boundary conditions. Depending upon the extent of asperity contact, scuffing or adhesive wear may occur. A schematic description of the various wear processes specific to hydraulic pumps is shown in Fig. 11. When cavitation, corrosion, or scuffing wear processes generate particles that are the same approximate size as p u m p clearances, synergistic wear may take place. Sjmergistic wear ultimately leads to failure that may appear to be abrasive in origin [28].

Wear protection under conditions of boundary lubrication may be enhanced through the use of additives that interact with surfaces to form protective chemical films. (See the An-tiwear Performance Testing section for description of the boundary lubrication additives utilized in hydraulic applications.) These chemical films reduce friction by decreasing the shear strength of the surface relative to the underlying material. Thus, surface interaction under boundary lubrication conditions is primarily between the low-shear strength chemical films rather than the metal substrate. Good wear protection and friction reduction result in enhanced equipment durability, reduced heat generation, improved energy conservation, and many other operational advemtages.

Antiwear Performance Testing

The majority of hydraulic fluids are formulated with anti-wear additives because surface loads associated with high-pressure pump operation necessitate the use of fluids with enhanced wear protection. There are a variety of test methods available for assessing the antiwear performance of hydraulic fluids. These tests may either be bench-top or full-scale tests employing high-pressure piston and vane pumps. Bench tests are generally less expensive to perform than pump tests. However, translating bench test data into real-world performance can be problematic because of the complexity involved in simulating all of the materials, velocities.

pressures, and entry angles in a functioning hydraulic system [29,30].

One of the more common bench tests used for screening the antiwear performance of a hydraulic fluid is the Four-Ball Method. There are two versions of the test for liquid lubricants: ASTM D 2783, Standard Test Method for Measurement of Extreme-Pressure Properties of Lubricating Fluids (4-Ball Method) and ASTM D 4172, Standard Test Method for Wear Preventive Characteristics of Lubricating Fluids (4-Ball Method). The former method is generally used for evaluat ing extreme pressure gear lubricants while the latter method is used for evEiluating antiwear hydraulic fluids. In the 4-Ball Wear Test (D 4172), three half-inch diameter steel balls are clamped together and covered with the lubricant to be evaluated. A fourth ball of equal diameter is pressed with a force of 1 5 ^ 0 kg into the cavity formed by the three stationary balls making a three-point contact (Fig. 12). Lubricants are evaluated by rotating the top ball under load at 1200 rpm for 60 min and measuring the average scar diameters worn in the three lower balls.

In cooperative testing of fluids performed by members of ASTM D02.L on Industrial Lubricants, the addition of zinc dithiophosphate to 46 cSt mineral oil decreased the scar di-

(a)

FIG. 12—The four-ball test: (a) perspective view, (b) plan view.

CAVITATION JEl

ASPERITY CONTACT

ELECTROLYTE (WATER)

r FATIGUE WEAR

ADHESIVE WEAR

CORROSIVE WEAR

EXTE PAR"

INGRE

\Air:Ap VVtArl

DEBRIS

WEAR DEBRIS

WEAR DFRRI.q

ERNAL riCLE ESSION

1 > — ^ •

ABRASIVE WEAR

WEAR DEBRI

TOTAL WEAR

S

FIG. 11—Synergistic view of pump wear process. Fatigue, adhesive, and corrosive wear can be triggered Independently. Resulting wear debris generation leads to abrasive wear.


3 6 2 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK

ameter in 4-ball wear tests from 0.72 m m to 0.42 m m at 40kg [31]. These results are tjpical of a mineral oil based antiwear hydraulic fluid where average scar diameters of less than 0.50 m m are the norm (P. W. Michael, unpublished data).

While four-ball tests are effective in screening antiwear additive response, they do not directly correlate with p u m p tests [32]. This is in part due to the fact that loads in the four-ball tests are constant and do not pulsate in the same way that a hydraulic p u m p does as sliding surfaces transit ion from high pressure to low pressure regions of the pump. In an effort to enhance the correlation between the four-ball test and full-scale p u m p eveJuations Penn State University has performed investigations involving sequential four-ball wear tests. In the sequential four-ball test, wear scars are evaluated at 10 and 40 kg and 600 rpm and the diameter of the scar is measured after the fluid has been replaced by white oil in order to measure the durability of the antiwear film [33,34]. This method yields better correlation with vane pump tests.

The FZG Test is another bench test used for screening hydraulic fluids. FZG test equipment consists of two gear sets arranged in a foursquare configuration (Fig. 13). The FZG procedure is described in ASTM D 5182 Standard Test Method for Evaluating the Scuffing (Scoring) Load Capacity of Oils. In this test, pre-examined gears are immersed in 1600

mL of oil that is heated to 90C (194°F). The test gear set is run in the test fluid for 15 min at successively increasing loads until the failure criteria is reached. According to the ASTM procedure, failure criteria are reached when the summed total width of scuffing wear damage from all 16 teeth is estimated to equal or exceed one gear tooth width. In DIN 51524, Part 2, a maximum weight loss of 0.27 mg/kW h for antiwear hydraulic oil is specified as well as a minimum damage stage of 10. While Reichel reported a correlation between FZG Test results and hydraulic fluid performance in vane pumps, correlation with piston pump performance has proven difficult to establish [35].

The most widely referenced vane pump wear test for hydraulic fluids is ASTM D2882, Standard Test Method for Indicating the Wear Characteristics of Petroleum and Non-Petroleum Hydraulic Fluids in a Constant Volume Vane Pump (Vickers 104C). In this test, a hydraulic fluid is circulated through a rotary vane pump for 100 h at a pump speed of 1200 r/min and a p u m p outlet pressure of 2000 psi. The fluid temperature is controlled to 150°F at the pump inlet for most fluids. Petroleum based fluids with a viscosity greater than 46 mm'^/s and some synthetic fluids must be evaluated at 175°F. At the end of the test, the total cam ring and vane weight losses are measured and reported. Based upon ASTM

Drive gear case

Test gears with long addenda

FIG. 13—The Neimann (FZG) Four-Square Gear Test Rig.



D 6158, Standard Specification for Mineral Hydraulic Oils, less than 50 mg of total wear is expected from properly formulated petroleum based antiwear hydraulic oil. For invert-emulsion type fluids, higher wear rates in the 100-200 mg range are common while water glycol fluids routinely generate less than 50 mg wear in the D 2882 test.

While the D2882 test is a popular benchmark for evaluating hydraulic fluids, this method is not without its problems. First of all, Vickers has discontinued product ion of the V104C pump. This will ultimately necessitate the use of substitute hardware or abandonment of the test procedure. Second, rotor and bushing failures are common in the first few hours of the test. This may be due to the fact that the pump was originally designed for a maximum pressure of 1000 psig. Fluid performance in the V104C pump is evaluated at 1000 psi using the ASTM D 2271, Standard Test Method for Preliminary Examination of Hydraulic Fluids (Wear Test). In this procedure, the pump stand is operated for 1000 h, which provides an extended evaluation of pump wear behavior under normal operating conditions. Xie et al. provide a detailed discussion of the D 2882 Test Method in the Handbook of Hydraulic Fluid Technology [36].

For higher pressure and mobile applications Vickers prefers their 35VQ25 vane p u m p for screening hydraulic fluid wear performance (Table 5). In the 35VQ25 test, three 50-hour tests are conducted on the same charge of test oil. For each 50-hour test a new pump cartridge is used. The test rig is operated at 3000 psi and 200°F with a pump speed of 2400 rpm. Vickers limits the amount of wear on each test kit to 90 mg: 75 mg ring, 15 mg vanes. In addition there must be no sign of scuffing on the cam ring.

The Denison T6C vane pump test is a variable pressure vane pump test. In this test, a Denison T6CSH 020 pump cycles between 7 bar (—100 psi) and 250 bar (—3600 psi) at one-second intervals for 300 h [37]. The pump speed is nominally 1700 r/min and fluid tempera ture is maintained at 80°C (176°F) for mineral oil based fluids and 45°C (113°F) for those based on water. The test is run in two 305-hour sequences. Each 305-hour test consists of a 5-hour break-in period followed by 300 h of high pressure cycling. After the first 305-hour test, the p u m p cartridge is removed for inspection and a new cartridge is installed for the second sequence. The second 305-hour sequence is run with 1% distilled water added to the fluid. The first stage of the T6C test serves as an aging mechanism and increases the susceptibility of the fluid to the deleterious effects of water contamination. After the second 305-hour sequence the pump cartridge is again removed for inspection. As with the 35VQ25 test, weight loss of cam ring and vanes, vane tip profile, and visual appearance of all components are all reported. In addition, a wet filter-ability test is performed on the fluid to determine if water contamination will lead to filter blinding. (See the Filterabil-ity section for a discussion of filterability tests.)

Although the V104C and 35VQ25 vane pump tests have served the industry well for many years, these tests are not sufficient to screen hydraulic fluids that will be used in high-pressure piston pumps applications [38]. Thus, piston pump tests have been to qualify the antiwear capabilities of hydraulic fluids. Komatsu, Rexroth, and Sundst rand piston pump tests are described below.

Komatsu developed a piston p u m p test to evaluate

biodegradable vegetable oil based hydraulic fluids [39]. This test is based on a Komatsu HPV35+35 twin-piston pump using cycled pressure test conditions. In this test pump efficiency change, wear and surface roughness, formation of lacquer and varnish, and hydraulic oil deterioration are evaluated.

Rexroth has proposed a three-stage piston pump test based on the Brueninghaus A4VSO piston pump [40]. Stage one is conducted at the maximum operating pressure and temperature and at the minimum viscosity specified for the fluid being tested. The test duration is 250 h at which time the pump is dismantled and inspected. The second stage of the test is pulsed pressure test at the maximum displacement of the pump. This stage is operated for one million cycles. When this stage is complete, the pump is dismantled and inspected. The third stage is a variable displacement stage at maximum pressure, maximum temperature, and minimum fluid viscosity. The test duration is 280 h at which time the pump is dismantled and inspected again. The final pass/fail assessment is made with reference to a standard damage catalog.

The Sundstrand Water Stability Test Procedure test originally employed a Sundstrand Series 22 piston pump at a constant pressure [41]. Currently, this test procedure is conducted using a Sundstrand Series 90 piston pump with a 55-cc displacement. The objective of the test is to determine the effect of water contamination on mineral oil hydraulic performance and yellow metal corrosion. However, other fluids, including water-containing fluids such as HFB and HFC fluids, may also be evaluated using this test. The test duration is 225 h, at which time it is disassembled and inspected for wear, corrosion, and cavitation. If the flow degradation is equal to or greater than 10%, the test is considered to be a "fail."

Antiwear and Extreme Pressure (EP) Additives

Antiwear and EP additives prevent wear of metal surfaces by forming a protective chemical film between moving parts. These additives have traditionally been labeled as antiwear or extreme pressure (EP), depending on the mechanism of protection. Antiwear additives are generally considered to form protective films that adsorb on the metal surface and function effectively under relatively mild conditions of load and temperature . Extreme pressure additives form protective films by reacting with the metal surfaces at localized high temperatures to form low shear strength films that are relatively insoluble in the bulk oil. In either case, tribological contact is between the surface films rather than the metals.

Various types of chemistry are employed in the prevention of wear in hydraulic applications. Typical compounds include zinc dialkyldithiophosphates (ZDTP), tricresylphos-phates (TCP), sulfur compounds, amine phosphates, dithio-carbamates, and other chlorinated, phosphorus/sulfur, and molybdenum compounds.

W a t e r C o n t e n t a n d H y d r o l y t i c Stabi l i ty

In many hydraulic systems, the lubricant is susceptible to contamination with water. Contamination with water can lead to a host of problems including loss of lubricity, corrosion, additive degradation, and filter plugging. Consequently, machine builders and equipment users often attempt to limit the amount of water that enters their hydraulic systems. At the same time, fluid formulators endeavor to manufacture


364 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK

TABLE 5—Machine builder specifications for antiwear hydraulic oil.

Properties Method(s) ISOVG Kinematic Viscosity, cSt D445

0°C max., calc. D 5133 40°Cmax. 40°C min.

100°Cmin. Flash Point °C min. D92 Fire Point, C min. D 92 Pour Point, °C, max D97 Color, max D 1500 ISO Contam. Code, max ISO 4406 Density @15°C D1298 TAN, mg KOH/g, max D664/ D974 Rust Test A D665 A Rust Test B (Salt Water) 0665 B Cu Rating (3 hr, 100°C), max. D130 TOST Oxidation, Hours to 2.0 ,^^, TAN " * ^ Air Release @ SOX, minutes Q » J ~ , (max) Foam tendency/statxiity D892 Seq 1 max Seq II max Seq III max DemulsilJility @ 54°C D1401

FZG Fail Stage D 5182 Change in Hardness

NBR1,168hrs@100°C Change in Volume (%)

NBR1,168hrs@100°C Viscosity Index, min D 2270 Aniline Point C min. D 611 CM Thermal Stabtity D 2070 A Viscosity Change, % max TAN Variation, % max * Comparative IR Scan Sludge, mg/100 ml max Cu metal removed, mg/200 ml, max. Copper rod appearance, rating (max.) Steel deposits, mg/200 ml, max Steel metal removed, mg/200 ml, max Steel rod appearance, rating (max) Oxidation (1000 h) D4310 AN, mgKOH/g max Total sludge, mg max. Copper, mg max Iron, mg max Hydrolytic Stability D 2619 Copper wt loss, mg/cm^ max Water layer TAN. mgKOH max V104 C Pump mg wear, max D 2882 Vickers 35 VQ 25 Pump Test Vane Wear, mg max Rir^ Wear, mg max Denison P-46 (100 h) DenisonTBC, vane wear TP-30283 Cam ling wear Denison Fnterability Test, sec TP 02100

Dry, max Wet, max

Denison HF-0

Requirements

0.84 - 0.90

Pass Pass

-10 - /O -10

90 100

100 10

Report

2 200 50 SO (1) 0.2 4.0

Satisfactory Satisfactoiy

600 2xdry

Vickers

Requirements

I-2S6-S SO

M-2950^ 15 75

Cincinnati iWachine

. P68 P70 P69 32 46 68

35.2 50.6 74.8 28.8 41.4 61.2

188 196 196 215 218 218

2 3 3

<1.S> Pses

<90>

< 5 > <50>

< 25 mg. /100ml > <10> < 5 >

<3.5> < 1 >

<1.5>

< 50 > (2)

6M LS-2

LH-02 LH-03 LH-04 LH-06 22 32 46 68

300 420 780 1400 24.2 35.2 50.6 74.8 19.8 28.8 41.4 61.2 4.1 5 6.1 7.8 175 190 190 195

-21 -18 -15 -12

19/16/13

Pass <1b>

<1500>

5 5 10 10

<:50/0> <SO/0> <50/0>

Timeto40/40«)(O/W/E) <30> <10>

Oto-8 Oto-7 Oto-7 Oto-6

OtolS 0to12 0to12 OtolO <95>

< 5 > <50> Record

< 25 mg. /100ml > <10> < 5 >

<1.5>

<0.2> < 4 >

<10> <50>

no smear, scratch, etc < 0.01 >

No distress

<600> < 2 X dry >

(1) Rqmnt. Sut>ject to Denison discretion (based on other pump/fietd history) (2) D 2882 mn at 79,4C (higher temp.) for ISO 68 and higher grade.



hydraulic fluids that resist chemical degradation or hydrolysis in the presence of water and heat. Several ASTM methods are used to monitor water content of hydraulic fluids as well as their ability to resist hydrolytic degradation.

Distillation, Centrifuge and Karl Fisher Titration Tests

In ASTM D 95, Standard Test Method for Water in Petroleum Products and Bituminous Materials by Distillation, the material to be tested is diluted with a water-immiscible solvent such as toluene and heated under reflux conditions. The resulting distillate is condensed and separated in a trap. The amount of water present in the sample is determined by observing the volume of water settled in the graduated section of the trap.

Centrifuge tests such as ASTM D 96, Standard Test Method for Water and Sediment in Crude Oil by Centrifuge Method, can also be used for un-emulsified or insoluble water contamination in fluids. While distillation and centrifuge methods provide reasonably accurate results for samples that contain free water contamination, these methods are generally not sensitive enough for hydraulic applications. A more accurate method for quantifying water in hydraulic fluid is the Karl Fischer test (ASTM D 1744, Standard Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent) [42]. In this test, the fluid is dispersed in a solvent such as methanol and titrated with standard Karl Fisher reagent to an electrometric endpoint (Fig. 14). The endpoint of the titration, at which free iodine is liberated.

may be registered either potentiometricly or by color indication. Although this method has the capability to be more accurate than distillation or centrifuge techniques, the Karl Fisher Test is susceptible to chemical interference. Calcium sulfonate, magnesium sulfonate, ZDTP and other oil additives react with iodine and have been known to interfere with the titration [43].

Hydrolytic Stability Testing

Hydrolytic stability refers to the lubricant's resistance to chemical interactions with water that result in undesirable changes to fluid properties. Certain chemical components may react with water to decompose or form undesirable byproducts of hydrolysis. Heat and catalysts such as copper can accelerate the process of hydrolysis. Hydrolytically unstable oils form insoluble contaminants and acidic compounds that create hydraulic system malfunctions similar to those produced by oxidation and thermal degradation of fluids. Furthermore, antiwear additives and corrosion preventatives that are susceptible to hydrolysis are likely to lose their ability to perform their critical functions in the presence of heat and water.

ASTM D 2619, Standard Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method) is used to measure this fluid property. In this test, 75 g of fluid and 25 g of water are sealed in a beverage bottle with a copper strip. The test bottie is rotated in an oven for 48 h at 93°C (200°F). At the end of the test, the oil and water layers are separated

FIG. 14—The Karl Fisher apparatus (a) titrant solution, (b) burette, (c) titration cell with electrode, (d) solvent, (e) waste.


3 6 6 MANUAL 3 7 ; FUELS AND LUBRICANTS HANDBOOK

and insolubles are weighed. Viscosity and acid numbers are also determined. Based upon the Denison HF-0 specification (see Table 5 for details of this specification), the weight change of the copper specimen should be less than 0.20 mg/cm^ and the water layer acidity should be less than 4.0 mg KOH. Since exposure to water can be expected throughout the life of a fluid, hydrolytic stability is an important design characteristic of hydraulic fluids.

In genereJ, there are no additives specifically used to improve hydrolytic stability. Instead, hydrolytic stability is achieved by appropriate selection of stable components that maintain effectiveness even in the presence of water. Hydrolytic stability is also a key factor in the wet filterability behavior of hydraulic oils (see the Filterability section) [44].

D e m u l s i b i l i t y

Demulsibilty is the term used to describe a fluid's ability to separate from water. As discussed above in the Water Content and Hydrolytic Stability section, water contamination of the hydraulic oil may lead to various problems that adversely affect both fluid and equipment durability. Thus, it is desirable for hydraulic oil and water to separate as quickly as possible. In many industricJ applications, water is drained from the hydraulic oil reservoirs as it separates and settles on the bottom. For fluids with poor demulsibility, the separation is either very slow or unlikely to occur to any significant degree.

Demulsibility Testing

The speed at which water is separated from oil and the tendency of an oil to form a cuff of emulsified oil at the interface between the oil and water phases may be measured by ASTM D 1401, Standard Test Method for Water Separability of Petroleum Oils and Synthetic Fluids. In this test, a 40 ml sample of oil and 40 ml of distilled water are stirred for 5 min at 54°C (130°F) in a graduated cylinder. The time required for the emulsion to separate into water and oil phases is recorded. An oil with good demulsibility will completely separate in 30 min or less without a "cuff' of emulsified oil between the phases [45].

Demulsifiers

Demulsifiers are chemicals used to alter the surface tension at the oil/water interface to accelerate separation. T3rpical demulsifiers include alkylphenol ethers, low molecular weight synthetic sulfonates, and polyoxyalkylate resins.

A e r a t i o n a n d F o a m

Under normal conditions there is always air present in a hydraulic fluid. By volume, it is present at about 7-9% at room temperature and atmospheric pressure [46]. In this state, it is not visible to the human eye and thus referred to as dissolved air. Higher temperatures and/or lower pressures (such as vacu u m conditions) lead to lower dissolved air levels. (See chapter on compressor lubricants for detailed discussion on gas solubility and methods of measuring gas solubility.)

Fluid circulation through hydraulic systems and reservoirs may cause mecheinical introduction of air into hydraulic fluids, particularly if reservoir size or design does not allow sufficient residence time for air separation to occur. At elevated

levels, entrained air is visible to the human eye as larger bubbles and can cause the oil to become cloudy. Uncontrolled air contaminat ion results in a n u m b e r of undesirable consequences. Entrained air increases the compressibility of the fluid and can adversely affect its response to hydraulic control mechanisms or devices, especially in high-pressure systems. Dissolved or entrained air expands into larger bubbles as its solubility in the fluid decreases as a result of exposure to vacuum conditions at the p u m p inlet. This leads to noise and cavitation, which is the dynamic process of gas cavity growth and collapse in a liquid [47]. Several studies of this phenomenon have suggested theoretical mechanisms and documented experimental evidence of wear and increased oxidation due to cavitation [48].

Foaming is very much rooted in the fundamentcJ problem of air contamination and consequently, results in many of the same negative effects of air entrainment. It is characterized by the formation of a mass of relatively large bubbles on the surface of the fluid and is usually brought about by turbulent return of oil to the reservoir or migration of entrained air to the surface. It is desirable to have fluids with a low tendency to form foam in the first place and have the foam collapse quickly once formed. For effective foam control, the rate of foam collapse must be faster than the rate at which entrained air migrates to the surface to form the foam. Otherwise, the foam layer will continue to increase and air may eventually be re-dispersed in the bulk fluid [49]. In severe cases, oil that produces a significant amount of foam may bubble out of hydraulic reservoir breathers, creating a fluid spill.

Air entrainment has increasingly become a concern due to a trend toward smaller reservoir sizes. Shorter fluid residence times therefore dictate use of hydraulic fluids with improved air release characteristics for the reasons discussed above. Several studies have shown that fluid viscosity is a critical factor influencing air release properties. Within a given class of fluids, higher viscosity and lower oil temperatures translate into slower air release characteristics. While different classes of base fluids have demonstrated unique air release advantages, there has been little success in identifying additives that improve air release properties of a base fluid.

Foam and Aeration Tests

Because of the importance of properly managing air contamination in hydraulic fluids, there are a number of standardized test methods for evaluating this feature of fluid performance. The foaming tendency and stability of oil may be measured by ASTM D 892, Standard Test Method for Foaming Characteristics of Lubricating Oils. In this test, an oil sample is equilibrated at 24°C (75°F). Air is bubbled through oil for 5 min, and then the oil is allowed to settle for 10 minutes. The volume of foam is measured at the end of both periods. The test is repeated at 93.5°C (200°F) and again at 24°C (75°F) after the foam breaks. Various levels of foaming tendency are permitted by industry standards, but stable foam is generally not tolerated [50,51].

Not only must a hydraulic fluid resist the tendency to form stable foam, it also must allow air to rapidly rise and separate from the fluid. The Waring blender test is one test method that may be used to measure the air release properties of fluids [52]. In ASTM D 3519, Standard Test Method for Foam in Aqueous Media (Blender Test), 200 ml of the fluid is stirred



at an agitation rate of 4000 to 13000 rpm for 30 s. The meixi-m u m total height at zero t ime, at 5 min and 10 min is recorded in order to assess the foaming and aeration tendency of a fluid under high shear conditions.

Air release properties of a hydraulic fluid may also be quantified by IP 313, DIN 51381 or ASTM D 3427, Standard Test Method for Air Release Properties of Petroleum Oils. In these tests, the time in minutes for finely dispersed air in oil to decrease to 0.2% under standard test conditions is measured using a density balance. Air release times and specifications typically vary with oil viscosity.

Defoamants

Antifoam additives, generally referred to as defoamers or defoamants, are materials that destabilize the liquid film that surrounds air bubbles. The most commonly used defoamants are silicone polymers (particularly polydimethylsiloxanes), which function as finely dispersed marginally soluble liquid particles. Since silicon defoamants have very low surface tensions, they tend to accumulate at air/oil interfaces. When the larger bubbles rise to the surface and join other bubbles to form foam with only very thin films separating them, silicone defoamants cause these films to rupture, thus accelerating collapse of the foam. While silicone defoamants reduce the foaming tendency of a fluid, they may also tend to increase air entrainment (Fig. 15) [53].

Besides affecting air entrainment in hydraulic fluids, silicone defoamants tend to have poor filterability and storage stability due to their marginal solubility in oil. Non-silicone defoamants are increasingly used to address these disadvantages. Polyalkylacrylate additives are the most common class of non-silicone defoamants recognized in the industry. Although they do not possess the disadvantages of the silicone types, these polyalkylacrylates must be used at higher concentrations to deliver equivalent performance.

Filterability

It is widely recognized that beyond proper fluid selection, good fluid maintenance is the key to reliability and durability of hydraulic equipment. Fluid maintenance is closely linked to fluid cleanliness and filtration. Filtration devices, therefore, are critical tools for maintaining hydraulic fluids and system components . Hydraulic fluid "filterability" is concerned mainly with the appropriate flow characteristics of the fluid through filter media. For proper operation, the fluid should readily flow with minimum pressure drop across the filter and with negligable depletion of additives. The viscosity and chemistry of the lubricant will affect filterability. Therefore, filter size and materials should be compatible with the circulating fluid. The drive to increase hydraulic system reliability through the use of fine filtration magnifies the importance of this performance parameter.

Filterability Tests

Due to the likelihood of water contamination in many hydraulic systems and its potential impact on fluids, most of the filterability tests are designed to run dry and wet (with water added). Hydraulic fluid filterability tests generally consist of filtering a specified quanti ty of fluid through a s tandard medium while monitoring changes in flow rate (Table 6). The results are tj^pically reported in terms of a ratio between flow rates with and without water added to the fluid. This approach attempts to account for changes in filterability behavior independent of viscosity. In Denison TP 02100 the time required for complete flow of a standard volume of fluid through a specified filter is evaluated. In the Pall Filterability Test the differential pressure across a specified filter assembly is monitored over the duration of the test and cin appropriate limit is established to discriminate between fluids with good and poor filterability behavior. While key equipment

O O eg <

VOLUME OF AIR BLOW IN

AIR RELEASED DURING BLOWING PHASE

h- BLOWING OR TURBULENT PHASE"

OIL WITH SILICONES

SETTLING OR "TRANQUIL PHASE" > »

TIME

FIG. 15—Impact of silicone defoamer on foaming tendency and air release. Silicone de-foamer decreases the tendency of the oil to generate foam while increasing the tendency of the fluid to retain air below its surface.


3 6 8 MANUAL 37: FUELS AND LUBRICANT HANDBOOK

TABLE 6

Method

Medium pore size Percent water added Aging time Temperature

1—Filterability

AFNOR

0.8jU.M 0.2 70 h 70°C

tests.

Pall"

3/LiM 1.0 24 h 70°C

Denison

1.2 ^M 2.0 None 25°C

"Parkhurst, H., Pall Filterability Index Test for Paper Machine Oils, SLS Report No. 5669, April 1995.

builders and industrial manufacturers may require fluids to meet certain filterability criteria as measured by these tests, global hydraulic oil specifications (i.e. ASTM D 6158, ISO 11158, DIN 51524) have not yet incorporated these procedures.

Filterablility Additives

From a formulation standpoint, identifying and replacing additives with potential filterability problems (i.e., filter material incompatibility, gel-forming tendency, hydrolytic instability, etc.) has been the primary method of improving fluid filterability. Recently, dispersants have been identified that enhance filterability by preventing agglomeration of insoluble species present in the fluid. These dispersants are typically alkyphenol-based or alkyl succinimide polymers of varying molecular weights.

C o r r o s i o n P r o t e c t i o n

Chemical contaminants and corrosive by-products of fluid degradation can cause surface attack of metallic hydraulic system components. Ferrous metal corrosion in a hydraulic system is most often caused by water contamination, while copper and its alloys are susceptible to attack by the products of high temperature fluid degradation. Rusting of ferrous metal is an electrochemical reaction that occurs between the parent metal and the thin oxide layer on the metal surface formed as a result of exposure to the atmosphere [20]. Rust, which is hydrated iron oxide, compromises the integrity of the metaJ surface and adversely affects other important fluid propert ies when it contaminates the bulk fluid. Ferrous metal corrosion protection in hydraulic systems is usually accomplished by incorporating surface-active additives such as rust inhibitors. There are several ASTM methods for evaluating the corrosion inhibition properties of hydraulic fluids.

Corrosion and Rust Testing

The ability of fluids to prevent rusting of ferrous parts due to water contaminat ion may be measured by ASTM D 665, Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water. In Part A of this test, 10% distilled water is added to oil that has been heated to 60°C (140°F). Round steel rods are polished to remove their oxide coating and immersed in the oil. The oil-water mixture is continuously stirred to avoid separation while the temperature is maintained at 60°C. At the end of 24 h the specimens are inspected for rust (Fig. 16).

In Part B of the method, the same procedure is used, except synthetic seawater is substituted for distilled water. As described in Part B, synthetic seawater is made by the addition

of sodium chloride, magnesium chloride, calcium chloride, and several other ionic compounds to distilled water. Part B is particularly pertinent to marit ime hydraulic fluid applications where seawater, rather than fresh water or condensation, is a likely source of contamination.

The standard test method for measuring vapor phase corrosion inhibition of hydraulic fluids is ASTM D 5534, Test Method for Vapor-Phase Rust-Preventing Characteristics of Hydraulic Fluids. In this test, a steel specimen is attached to the cover of an ASTM D 3603 test apparatus that contains hydraulic fluid maintained at a temperature of 60°C (140°F). ASTM D 3603 is the Horizontal Disk Method for Rust-Preventing Characteristics of Steam Turbine Oils in the Presence of Water. The specimen is then exposed to water and hydraulic fluid vapors for a period of 6 h. At the end of this time, the specimen is inspected for evidence of corrosion and results are reported on a pass-fail basis. The ASTM D 5534 test is particularly relevant for water-glycol and invert-emulsion hydraulic fluids because corrosion of the underside of reservoir covers has been observed in systems that use these fluids.

Accelerated corrosion can also occur when dissimilar metals are in electrical contact in the presence of an electrolyte (i.e., conductive solution). This corrosion mechanism, known as galvanic corrosion, has been found to be particularly relevant for certain biodegradable oils [54], The ability of a fluid to prevent galvanic corrosion may be measured by ASTM D 6547, Test Method for Corrosiveness of a Lubricating Fluid to a Bi-Metallic Couple. In this test, a brass clip is fitted to the oil coated surface of a steel disk. The bi-metallic (brass/steel) couple is then stored in 50% relative humidity for ten days. At the end of the ten-day period, the surfaces are inspected for evidence of staining like that depicted in Fig. 17. The steel disks are rated on a pass-fail basis.

Sulfur containing additives such as zinc dithiophosphate, sulfurized olefins, organic polysulfides, and carbamates may be used as antiwear and extreme pressure additives in hydraulic fluids [55]. Depending upon the chemical activity of these sulfur compounds, hydraulic fluids exhibit varying degrees of corrosiveness to copper when activated by high temperatures. ASTM D 2070, Standard Test Method for Thermal Stability of Hydraulic Oils is one of the most effective methods for predicting the corrosiveness of a hydraulic fluid to copper and its alloys. The ASTM D-2070 test measures the aggressiveness of chemical constituents in the fluid toward yellow metals when aged under high temperature conditions. (See the section on High Temperature Oxidation Tests)

In some cases, such as when a hydraulic fluid is contaminated with sulfur containing metalworking fluid, the fluid may exhibit corrosivity to copper without requiring thermal degradation. The standard test method for measuring the copper corrosion properties of oil is ASTM D 130, Standard Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test. In this test, a polished copper strip is immersed in oil and heated for a predefined period of time. At the end of the test, the copper strip's appearance is compared to a standard. The rating system used for the D 130 test appears in Table 7. The rating system is on a scale of one to four. The higher the copper strip rating, the greater the degree of copper corrosion. Color standards are also available from ASTM for rating copper strips [56].



FIG. 16—ASTM D 665 passing vs. failing rod.

FIG. 17—Galvanic corrosion: staining on test specimen by vegetable oil.

Corrosion Inhibitors, Rust Inhibitors, and Metal Passivators Corrosion Inhibitors, Rust Inhibitors, and Metal Passivators are designed to prevent deterioration of metal surfaces that are in contact with the lubricant. Corrosion inhibitors are polar molecules that are surface active. They adsorb on the metal surface and inhibit the electrochemical reaction that produces rust. Some hydraulic fluids, particularly those used in applications that require enhanced fire resistance, are for

mulated with water. Such fluids have entirely different corrosion inhibition requirements. For instance, water glycol hydraulic fluids must prevent corrosion in the vapor phase above the liquid due to evaporation. Thus they are formulated with vapor phase corrosion inhibitors such as morpho-line. Typical classes of rust inhibitors include metallic sulfonates, amine phosphates, simple fatty acids, and succinic acid esters. Triazoles, or derivatives thereof, are commonly used metal passivators.


3 7 0 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK

TABLE 7—Copper strip classifications. Rating

l a

l b 2a 2b 2c

2d 2e 3a 3b

4a

4b 4c

Designation

Slight tarnish

Slight tarnish Moderate tarnish Moderate tarnish Moderate tarnish

Moderate tarnish Moderate tarnish Dark tarnish Dark tarnish

Corrosion

Corrosion Corrosion

Description

Light orange, almost the same as freshly polished strip

Dark orange Claret red Lavender Multicolored with lavender blue or

silver overlaid on claret red Silvery Brassy or gold Magenta overcast on brassy strip Multicolored with red and green

showing (peacock), but no gray Transparent black, dark gray or

brown with a trace of peacock Graphite or lusterless black Glossy or jet black

TABLE 8—Recommended property change limits for determining compatibility of elastomer seals for

industrial hydraulic fluid applications.

S e a l Compat ib i l i t y

Very critical to the successful operation of a hydraulic system is the ability to prevent leakage and accidents that are a result of failed seals. Leaks can lead to contamination, loss of pressure, loss of lubricating fluid, and environmental damage depending on the severity of the spill. In extreme temperature and pressure operations, sudden failure of seeds may have life threatening consequences, considering the potential for explosions, fires, etc. [57]. Hydraulic fluids and elastomeric seals are composed of complex chemical components that can interact as they come into contact. Depending on the chemistries involved, t ime, temperature , and mechanical stresses cause fluid interactions with the seal material, resulting in swelling or shrinkage of the elastomer compound. It is desirable to select seal materials that exhibit minimal change in hardness, volume, tensile strength etc. in service. Slight swelling of seals is preferable to shrinkage as indicated in Table 8. This is because a reduction in seal volume may result in leakage of fluid due to failure of the seal to fill the gland that retains it in place.

Seal Compatibility Testing

In general, industry recognized seed compatibility tests entail exposure of the elastomer material to the test fluid for a specified duration and at a standard temperature under static conditions. Familiar industry seal compatibility tests include ISO 7619, ISO 6072, DIN 53 538, and ASTM D 6546-00, Standard Test Methods for and Suggested Limits for Determining Compatibility of Elastomer Seals for Industrial Hydraulic Fluid Applications. Other major organizations such as ASTM and SAE also have related specifications for sealing devices. Due to variations in elastomer chemistry, it is necessary to perform compatibility tests on the specific materials being used. While most standard tests measure changes in hardness, stress/strain properties, and volume changes after exposure to the test fluid, translation of these results to a practical application may be difficult, since geometry and mechanical conditions of the targeted application profoundly impact the elastomer. It is therefore recommended that seal materials be tested under conditions that closely simulate the actual application [58].

Time in Hours

24 70

100 250 500

1000

Maximum Volume Swell,

% 15 15 15 15 20 20

Maximum Vol.

Shrinkage, % - 3 - 3 - 3 - 4 - 4 - 5

Hardness Change, Shore A Points

±7 ±7 ±8 ±8

±10 ±10

Maximum Tensile

Strength Change, %

- 2 0 - 2 0 - 2 0 - 2 0 - 2 5 - 3 0

Seal Swell Agents

These chemicals react with the elastomeric materials to cause slight swelling or softening to counteract the typical effects of temperature and mechanical stress. Seal swell agents are typically used with base fluids having very low aromatic content. Aromatic derivatives or phosphate esters are typically used to enhance the seal swell characteristics of a fluid.

C o o l a n t S e p a r a b i l i t y

Hydraulic systems used in machine tool operations are susceptible to contamination by aqueous cutting fluids, which contain components with poor oxidation resistance, high deposit forming tendency, and/or high corrosivity. In metcd-working applications, the hydraulic fluid may be considered a contaminant of the cutting fluid that alters its effectiveness in metal removal operations. Regardless of the perspective, a mix of these two categories of fluids is undesirable, especially if they have not been designed to be compatible. In this case, compatibility is defined as the ability of either fluid to complement, enhsince, or at least have no impact on the performance of the other when mixed. The lubricant's ability to readily separate from coolants is highly desirable in most cases. However, the variety and complexity of coolant chemistries makes it difficult to ensure good separability of the hydraulic oil from all metalworking fluids [59].

There are generally no additives specifically designed to improve coolant separability, since coolant chemistries vary so widely. The t3?pical approach is to formulate a lubricant to have good demulsibility (water separability) and then test its compatibility with specific coolants with which it is expected to come into contact.

Coolant Separability Testing

A standard industry test method for assessing lubricant compatibility with coolants has not yet been established. However, some Icirge industrial manufacturers and lubricant suppliers do have in-house test procedures designed to simulate oil contamination by a low percentage of coolant, as well as coolant contamination by a low percentage of oil (typically referred to as t ramp oil). In general, these procedures consist of mixing the lubricant with the coolant at a specified ratio and temperature for a s tandard durat ion. The fluid container, t5^ically a graduated cylinder, is then allowed to sit while the degree of separation between the coolant and the lubricant is observed at specific t ime intervals. Properties such as additive leaching and foam stability may also be ob-



served. Rapid separation, implying absence of a stable emulsion or cuff (the layer between way oil and coolant) at the interface, is very desirable (Fig. 18).

S h e a r Stab i l i ty

Mobile hydraulic equipment such as excavators, farm tractors, cranes, and timber harvesters frequently are required to operate under extreme high and low temperature conditions. To accommodate wide-ranging environmental conditions, hydraulic fluids with enhanced viscosity - temperature properties are often employed. These fluids t3^ically contain viscosity index improving polymers that thicken oil at high temperatures , while having little impact upon their low temperature fluidity. Viscosity index (VI) is a common means for expressing the variation of viscosity with temperature. The viscosity index of an oil is calculated from the measured viscosity of the fluid at 40 and lOOX using ASTM Method D 2270, Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C. A high VI indicates less relative change in viscosity for a given change in temperature. Vl-improved oils are commonly referred to as multi-grade oils, because they meet both the low temperature requirements of low viscosity oils and the high temperature requirements of higher viscosity oils. Conceptually, an SAE

lOW-30 multigrade oil consists of a lOW base oil and sufficient polymer to thicken the oil at 100°C to a viscosity equal to that of an SAE 30 weight oil (Fig. 19).

Viscosity Index Improvers are typically subjected to mechanical degradation due to shearing of the molecules in high stress areas such as between gear teeth in gear pumps and vane-ring interface in vane pumps. High pressures generated in hydraulic systems subject fluids to shear rates up to 10^ s~' [60]. Not only does hydraulic shear cause fluid temperature to rise in a hydraulic system, but shear may bring about permanent viscosity loss in hydraulic fluids [61]. Permanent viscosity loss results from mechanical scission of polymer molecules in multigrade hydraulic fluids and often occurs after a relatively short period of time (<24 hours of operation). The polymer (as opposed to the base oil) is susceptible to mechanical shear because it has a higher molecular weight and therefore a larger molecular volume. As a result, with polymer-containing multigrade hydraulic fluids, the functional viscosity of an oil may differ from that predicted from kinematic viscosity measurements of new oil [62].

Shear Stability Tests

It is desirable to formulate hydraulic fluids with shear-stable VI improvers so that the fluid retains its viscosity properties throughout its service life. Several laboratory test methods are

Good Fair Poor

FIG. 18—Good vs. Bad coolant separability.



Multigrade Oils 100,000 10,000

1,000

2-i : . ^--40 -20 40 100 150

Temperature, ° C FIG. 19—Impact of VI improver on lubricant viscosity.

designed to stress multigrade oils so that they produce a permanent viscosity loss such as would take place in service. The two methods generally used are mechanical shearing with a Bosch diesel fiiel injection pump and sonic shearing with a high frequency sonic oscillator. In ASTM D 6278, Test Method for Shear Stability of Polymer Containing Fluids Using a European Diesel Injector Apparatus, the polymer-containing fluid is passed through a diesel injector nozzle at a shear rate that causes polymer molecules to degrade. Under standard test conditions, the kinematic viscosity of the fluid is measured after 30 to 250 cycles through the injector pump to determine the extent of permanent viscosity loss that has taken place. In ASTM D 5621, Standard Test Method for Sonic Shear Stability of Hydraulic Fluid, the polymer-containing oil is irradiated with a sonic oscillator for 40 min and changes in kinematic viscosity are measured. Based upon data from Kopko and Stambaugh, the Fuel Injector Shear Stability Test lacks the necessary severity to predict permanent viscosity loss produced by hydraulic equipment [62]. However, 40 min of irradiation with a high frequency sonic oscillator produced viscosity changes that closely correlate to that experienced in the ASTM D 2882 Vane Pump Test. Consequently, this test method has become the basis for ASTM D 6080, Practice for Defining the Viscosity Characteristics of Hydraulic Fluids.

Viscosity Index Improvers

Viscosity Index Improvers (also referred to as viscosity modifiers) are high molecular weight poljTuers that reduce the magnitude of viscosity change as a function of temperature. They function by enabling the oil to retain thickness at higher temperatures while having minimal impact on viscosity at lower temperatures. In general. Viscosity Index Improvers are oil-soluble organic polymers with molecular weights ranging from about 10000 to 1 million. The oil temperature controls coiling of the polymer molecules, which in turn controls the degree to which the polymer increases viscosity. The higher the temperature, the less the coiling and the greater the "thickening" effect of the polymer. Therefore, as temperature increases, there is less thinning of the lubricant compared to non-polymer-containing oils.

The performance of VI Improvers is also described in terms

of resistance to mechanical shear, as well as their chemical and thermal stability. For a given polymer type, shear stability decreases with an increase in molecular weight. Shear is indicated by a loss in fluid viscosity (Fig. 20). The "thickening efficiency" of the viscosity modifiers generally increases with an increase in molecular weight for a given polymer type. Selection of the best VI Improver must entail consideration of viscosity requirements, shear stability, and thermal and oxidative stability in actual equipment operation.

Low Temperature Pumpability

Paraffinic mineral oils, which comprise the bulk of hydraulic fluids, contain some amount of wax that forms crystalline structures at low temperatures. As these structures form, the oil becomes more viscous. At very low temperatures the fluid may become gel-like or even solid. For hydraulic systems, poor low temperature flow characteristics can result in catastrophic failures. During start-up at very low temperatures, significant pump cavitation can occur due to inadequate oil flow.

Low Temperature Pumpability Tests

A number of bench tests are commonly used to evaluate low temperature flow characteristics of lubricants. One of the most common tests specified for this purpose is ASTM D 97, Standard Test Method for Pour Point of Petroleum Products, which measures the lowest temperature at which a lubricant will flow. ASTM D 6351, Test Method for Determination of Low Temperature Fluidity and Appearance of Hydraulic Fluids is used for evaluating the pour characteristics of biodegradable oils. While this test gives an indication of low temperature flow characteristics of the fluid, it does not necessarily address fluid performance in many applications subjected to very low temperatures. Pumps, motors, engines, and many types of lubricated machinery require that the lubricant be pumped or circulated effectively at start-up. As a result, several tests have been developed to determine fluid viscosity at low temperature. For hydraulic systems, tests such as Brookfield (ASTM D2 983), Scanning Brookfield SBV (ASTM D 5133), and Mini Rotor Viscometer MRV (ASTM D

Multigrade Oils and Shear Stability Quiescent Polymer Coil in

Oil Solution

Reversible/

Orientation of Coil Under Shear Forces

sNonreversible

Rapture of Coil and Subsequent Orientation Under Shear Forces

Temperature Viscosity Loss

Permanent Viscosity Loss

FIG. 20—Impact of shear stress on VI improver molecule.



TABLE 9—Low temperature viscosity grades for hydraulic fluid classifications.

Viscosity Grade

L5 L7 LIO L15 L22 L32 L46 L68

Min.

-49 -41 -32 -22 -14 - 7 - 1

Max.

-50 -42 -33 -23 -15 - 8 - 2

4

4684)—all of which include specified cooling cycle and low shear rates simulating field conditions—can be used to assess fluid pumpability. Performance specifications that include low temperature pumpability requirements, such as ASTM D 6080, Standard Practice for Defining the Viscosity Characteristics of Hydraulic Fluids, typically specify a temperature range for different viscosity grades. In Table 9 (from Standard D 6080) the temperature range for a given L-grade is approximately equivalent to that for an ISO grade of the same numerical designation and having a viscosity index of 100. For instance, the temperature range for the L32 oil is approximately the same as an ISO VG 32 grade with a Viscosity Index of 100.

Pour Point Depressants

Pour point depressants are additives designed to reduce formation of rigid wax crystals in the lubricant at low temperatures. Conventionally refined mineral oils typically require the use of pour point depressants because they contain wax

that will crystallize at low temperatures and cause the fluid to solidify. These additives do not entirely prevent wax from crystallizing in the oil. Rather, they lower the temperature at which large wax crystal structures are formed. By reducing the size of the crystal matrix, pour point depressants permit lubricants to flow at lower temperatures.

Two widely used types of pour point depressants are alky-laromatic polymers, which adsorb on wax crystals to inhibit growth and adherence of crystals to each other, and poly-methacrylates (PMA), which co-crystallize with wax to minimize growth of crystals. Depending mainly on the type of base fluid, the pour point of oil can be lowered typically by 20-30°F (I1-17°C).

TYPES OF HYDRAULIC FLUIDS

The major compositional categories of hydraulic Quids are Petroleum Based, Synthetic Based, Water Based, Vegetable Oil Based, and Water (Fig. 21). As expected, these different categories have properties that make them especially desirable in particular applications. In this section, the types of hydraulic fluids will be discussed in terms of their defining functionality rather than composition. For example, fire resistant fluids, which are typically water-based or ester-based fluids having high flash points, are used extensively in the basic metals industry where the risk of ignition is high, while "environmentally acceptable" fluids are used in environmentally sensitive areas.

Hydraulic system hardware is usually designed and formally rated to work with mineral oils, since they are the predominant hydraulic fluid in use. Systems may have to

HVDRMjuc mxs

Pelrolaum 8a»«l SynlHtBc Ba»9Cl Witer Baaed V9a* fcb 01 Based

R&OlfWbHed AnCweartAW)

MuBgrade

SynttieBc Hydiocarfcona (SHC)

Polyalphaolat>i8(PAO)

PoW/«'}

wsot 0 ^ SynHiellc SoluUon

Polyaloxarwa EMere EWare HatoaanalKl Cemiiounds

Rapwwed

Sc»Bean

—Phosphale Eaan

SMeones

Silanas

Pol»olEg»re

Polyetherg

Aryieftera

FluowEstere

Water-h-QI (hvBrt) O U n - W W f ;So>i>le OB

Ghlorinatad Hydrocarbons Raflutor Mcra Emulalm

WiiBd Haloflan Coropounds

FIG. 21—Schematic of hydraulic fluid types.



be modified to accominodate alternative types of hydraulic fluids.

M i n e r a l H y d r a u l i c Oi ls

The majority of hydraulic fluids in service are mineral oil based because they generally provide excellent performance at a relatively low cost. Within the mineral hydraulic oil category there is a wide range of viscosity grades and fluid types. The International Organization for Standardization (ISO) established a classification system for hydraulic fluids that is designated ISO 6743-4: 1999, Lubricants, Industrial Oils and Related Products (Class L)-Classification-Part 4: Family H (Hydraulic Systems). Using this classification system as a foundation, ASTM created ASTM D 6158, Standard Specification for Mineral Hydraulic Oils, which defines the physical propert ies and performance requirements of mineral hydraulic fluids. Table 10 provides a list of mineral oil based fluids listed in ASTM D6158.

HH

Type HH fluids are straight base oils without any additives. They may be used in air-over-oil hydraulic systems such as is found in car lifts at automotive service centers. They are also used in manual hydraulic pumps, jacks, and other low-pressure hydraulic systems. While type HH fluids are able to perform the primary function of a hydraulic fluid, which is to transmit power, they are unable to withstand high temperatures and have limited lubricating capabilities. Thus these fluids find limited application in industry.

HL

Type HL fluids are also mineral oil based, but they contain rust and oxidation inhibitors to protect equipment from the detrimental effects of water contamination and chemical deterioration due to heat. These fluids are also known as R & O oils because they contain rust and oxidation inhibitors. Type HL fluids are often recommended for use in machine tool applications where system pressures are limited to 2000 psi or less. They are also recommended for some piston pump applications. For example, type HL fluids are the preferred fluid for Denison piston pumps [63]. This is because some ZDTP containing oils can be aggressive to yellow metal (brass and bronze) and silver alloyed components in piston pumps.

R & O oils often are formulated using a rust inhibitor chemistry that contains succinic acid derivatives [64]. These additives may be incompatible with metallic sulfonate or phenate rust inhibitors or ZDTP antiwear additives used in many antiwear hydraulic fluids, resulting in formation of

TABLE 10—Mineral oil based hydraulic fluid classifications.

Symbol

HH HL

HM

HV

Classification

Non-inhibited refined mineral oils Refined mineral oils with improved

rust protection and oxidation stability

Oils of the HL type with improved anti-wear properties

Oils of the HM type with improved viscosity index properties

Commercial Designation

Straight base oils R&O oils

Antiwear oils

Multigrade oils

precipitates that can cause hydraulic valve sticking and filter plugging [65].

HM

Type HM fluids contain antiwear additives in addition to the rust and oxidation inhibitors found in HL fluids. They are the most widely used mineral oil based hydraulic fluids because antiwear additives provide enhanced performance in high-pressure hydraulic applications. The requirements for HM oils are listed in Table 11. While early versions of HM oils lacked the thermal stability necessary for satisfactory piston pump performance, modern fluids are able to perform quite well in piston pump applications.

Zinc dialkyldithiophosphate is the most widely used anti-wear additive for hydraulic applications. In recent years, concerns about the environmental effects of ZDTP have led to development of zinc-free or ashless antiwear hydraulic fluids. These products utilize sulfur and phosphorus compounds to achieve satisfactory antiwear performance. Thus, a type HM fluid may contain zinc or some other type of antiwear additive chemistry.

HV

Type HV fluids contain the same basic chemistry as HM fluids plus a viscosity index (VI) improver. (See the Shear Stability section.) Viscosity index improvers impart multigrade functionality to type HV fluids. While a wide range of polymers may be used for VI enhancement, these additives all function in the same basic manner. At low temperatures, VI improvers have a minimal effect upon fluid viscosity and at high temperatures they have a thickening effect. This enables the fluid to provide satisfactory performance at a wider operating temperature range [66].

Tractor F lu ids , ATF, a n d E n g i n e Oils

Tractor fluids are unique in that they are formulated to lubricate transmissions, final drives, wet brakes, clutches, and hydraulic systems from a common fluid reservoir on the tractor [67]. Consequently, tractor fluids are often used in agricultural equipment, off-highway machinery, backhoes, and turf applications where a multifunctional hydraulic fluid is required. To lubricate gears, wet brakes, and hydraulic systems, tractor fluids typically utilize phosphate ester based friction modifiers and ZDTP.

Automatic transmission fluids (ATFs) are similar to tractor fluids in that they are designed for multiple functionality, however, they generally utilize ashless antiwear additives ra ther than ZDTP. ATFs tend to be used in applications where superior low temperature performance is desired because they are designed to remain fluid at temperatures as low as —40°C. Years ago it was common to use lOW engine oils in hydraulic applications. Until recently lOW diesel engine oil was the primary hydraulic fluid recommendation for Caterpillar equipment because lOW diesel engine oil contains ZDTP antiwear additives and is compatible with engine oil [68]. The disadvantage of using ATFs and engine oils in hydraulic applications is that they tend to have poor water separation properties, which reduces wet filterability performance due to hydrolysis of the metallic sulfonates and phen-ates. Consequentiy, fluids designed specifically for hydraulic performance are generally more desirable.



Properties ISOVG Kinematic Viscosity, cSt

40'C max. 40°C min.

Flash Point, °C min. Flash Point, °C min. Brookfield Vis < 750 cP, Max Pour Point, °C, max Appearance Water Content, wt% Density @1S°C TAN, ma KOH/g, max Rust Test A Rust Test B (Salt Water) Cu Rating (3 hr, 100°C), max. TOST Oxidation, hours AN after 1000h, max. Air Release @ 50°C, minutes (max) Foam tendency/stability Seql SeqII SeqIII Demulsibility @ 54°C Minutes to 37 mL water . FZG Fail Stage Ctiange in Hardness

NBR1,168 hrs @ 100°C Change in Volume (%)

NBR1,168hrs@100°C Viscosity Index, min CM Thermal Stab. Sludge, mg/100 ml max

TABLE 11—International specifications for antiwear hydraulic oil.

ASTM

D445

D92 093 D2983 D97 Visual D1744 01298 0664 or 0974 D665A 06658 0130 0943

D3427 0892 max max max 01401

0 5182 0 471 ISO 7619

0 2270 02070A

Copper rod appearwice, rating (max.) Oxidation (1000 h) AN, mgKOH/g max Total sludge, mg max. Total Metals in oil/water/sludge Vickers 104C mg. wear max.

0 4310

0 2882

22

24.2 19.8 140 128

-18

2

2

5

30 10

ISO 11158 HM (Antiwear) 32

35.2 28.8 160 148

-15 < Report > <-ao5-> < Report > < Report > <Pass>

2

2

5

<i5ao> <75/Q> <150«)>

30 10

< Report >

< Report >

46

50.6 41.4 180 168

-12

2

2

10

30 10

S8

74.8 61.2 180 168

-12

2

2

13

30 10

< Report >

2 2 < Report >

•: Report >

2 2

DIN 51524 Part 2

22

24.2 19.8 165

-21

2

2

5

40

-Oto-8

0to15

Requirements 32 46

35.2 50.6 28.8 41.4 175 185

-18 -15

< Report > < Report > < Report > <Pass>

-2 2

2 2

5 10

<15Qro> <7SI0> <i5oro>

40 40 10 10

Oto-7 Oto-7

0to12 0to12

68

74.8 61.2 195

-12

2

2

10

60 10

Oto-6

OtolO

< 150 in exterxled test>

ASTM D6158

22

24.2 19.8 165

-15 -21

2 1000

5

30

HM (Antiwear) 32

35.2 28.8 175

-8 -18

< C & B >

< Report > < Report > <Pass> < Pass>

2 1000

5

<15C/0> < 7 » 0 > <150«)>

30

46

50.6 41.4 185

-2 -15

2 1000

10

30

68

74.8 61.2 195

4 -12

2 1000

13

30

r288 hrs. f 000) Oto-8

Oto15

Oto-7

0to12

Oto-7

0to12

Oto-6

OtolO <-90->

< Report > < Report >

200

50

25 5

200 < Report >

50

25 5

200

50

25 5

200

50

Fire R e s i s t a n t F l u i d s

Fire resistant hydraulic fluids are used in the basic metals industry, die casting, military, and foundry applications. They may be found in any application where a ruptured hydraulic line presents a potential fire hazard. Fire resistant hydraulic fluids are formulated with materials that have a lower BTU content than mineral oils, such as polyol esters, phosphate esters, and water-glycol solutions. As a result, they b u m with less heat generation than mineral hydraulic oils. As with mineral hydraulic fluids, the International Organization for Standardization has established a classification system for fire resistant fluids based upon composition. Table 12 provides a list

TABLE 12—ISO designations for fire resistant hydraulic fluids.

Symbol Classification Commercial Descriptions

HFAE Oil-in-water emulsions containing typically >80% water

HFAS Chemical solutions in water containing typically >80% water

HFB Water-in-oil emulsions containing approximately 45% water

HFC Water-polymer solutions containing approximately 45% of water

HFDR Synthetic fluids containing no water and consisting of phosphate esters

HFDU Synthetic fluids containing no water and of other compositions

Soluble oils

High water based fluids

Invert emulsions

Water-glycols

Phosphate esters

Polyol esters

of the ISO designations for fire resistant hydraulic fluids [69]. While power transmission, heat transfer, and lubrication

are essential requirements for all types of hydraulic fluids, it is sometimes necessary to compromise these properties to accommodate a critical fluid characteristic. This is especially true of fire resistant hydraulic fluids. Fire resistant fluids differ from mineral hydraulic fluids in density, compatibility, and lubricating properties. As a result, hydraulic systems are often modified when utilizing a fire resistant fluid. To optimize the performance of fire resistant fluids, the National Fluid Power Association and ISO have published guides for their use [70,71]. These NFFA and ISO documents detail the operational characteristics of fire resistant fluids and provide suggestions for storage, use, and handling of these fluids. Table 13 provides a comparison of the properties of common fire resistant hydraulic fluids.

HFA

HFA fluids contain greater than 80% water. These products are sometimes referred to as 95:5 fluids, because 5% concentrations are commonly employed. The ISO 6743-4 classification divides HFA into two sub-categories: HFAE and HFAS. HFAE fluids are oil-in-water emulsions. HFAS fluids are chemical solutions or blends of selected additives in water. Typically these products are sold as concentrates and diluted prior to use in service. Because of the high vapor pressure of water, the maximum recommended bulk fluid temperature for HFA fluids is 50°C [72]. The antiwear properties of these fluids are inferior to mineral hydraulic fluids because the vis-



TABLE 13—Comparison of common fire resistant fluid properties.

Property Antiwear Hyd.

Oil Invert

Emulsion Water Glycol

Phosphate Ester

Polyol Ester

ISO Designation Heat of Combustion" Autoignition Temp, "F* Maximum" Temperature Vapor Pressure, mbar Specific Gravi ty Viscosity @ 40°C, cSt Water Content Vane pump rating" Compatible Seals

HM 29.1 kJ/g 650 150°F 0.001 @ 50°C 0.85-0.88 32-68 0.05%

100% Buna-N, Viton

HF-B 16.3 kJ/g 830 120°F NA 0.91-0.93 80-100 43%

33% Nitroxyl, Buna-N

HF-C 5.3 kJ/g 830 120°F 80 @ 50°C 1.05-1.10 40 43%

67% Buna-N

HF-DR 19.0 kJ/g 1100-H 150°F < 1 @ 150°C 1.02-1.16 22-100 0.05%

67% Butyl, EPR

HF-DU 21.1 kJ/g 750" 150°F NA 0.91-0.96 46-68 0 .1%

100% Viton, Buna-N

"Roberts and Brooks Flammability Data, NFPA T2.13.8-1997, a calculated estimate was used for HFDU.

cosity of HFA fluids is comparable to water, approximately 1 cSt. Performance is satisfactory with HFA fluids when suitable components are used but is apt to be poor if used in conventional hydraulic systems. Special precautions also are required in the selection of filter construction materials and plumbing of pump inlets. Thus, it is necessary to work closely with fluid and component suppliers when utilizing HFA fluids.

HFB

HFB fluids are water-in-oil emulsions consisting of pet roleum oil, emulsifiers, selected additives, and water. They are commonly referred to as invert emulsions. In an invert emulsion the oil phase, which provides lubricity and rust protection, encapsulates the water phase, which provides fire resistance. The water content of an HFB fluid is normally in the 43-45% range (w/w). When water content of these fluids drops below 38% due to evaporation, the fire resistance of the invert-emulsion deteriorates. Maintenance of invert emulsions is complicated by the fact that when these fluids lose water through evaporation, a high-shear mixing device is normally necessary for proper addition of make-up. The viscosity propert ies of invert emulsions are unusual in that evaporation of water results in a viscosity decrease.

Several ASTM methods have been developed specifically for invert emulsion hydraulic fluids. ASTM D 3709, Standard Test Method for Stability of Water-in-Oil Emulsions Under Low to Ambient Temperature Cycling Conditions, is used to evaluate the freeze-thaw stability of invert emulsions. ASTM D 3707, Standard Test Method for Storage Stability of Water-in-Oil Emulsions by the Oven Test Method is used to determine if the emulsion has a propensity to separate after 48 h at 85°C. As with HFA fluids, special precautions also are required in the selection of filter construction materials and plumbing of pump inlets. Thus, it is necessary to work closely with fluid and component suppliers when utilizing HFB fluids.

HFC

HFC Fluids are solutions of water, glycols, additives, and thickening agents. They are commonly referred to as water-glycol hydraulic fluids. Typically, water-glycol fluids are formulated with diethylene glycol or propylene glycol and a polyalkylene glycol based thickening agent [73]. The low molecular weight glycol reduces the vapor pressure of tlje

fluid (relative to water) while high molecular weight polyalkylene glycol acts as a thickening agent, much like a viscosity index improver. This combination thickeners and glycols enhance the lubricating properties of a water-glycol and reduces the propensity of the fluid toward cavitation erosion. Nonetheless, operating temperatures for water-glycols are limited to a maximum of 50°C because of the effect of temperature on vapor pressure [74].

Water glycol fluids are highly alkaline due to the presence of amine based corrosion inhibitors. As a result, these fluids can attack zinc, cadmium, magnesium, and non-anodized aluminum, forming sticky or gummy residues. Consequently, these metals should be avoided when selecting system components. Special precautions also are required in the selection of filter construction materials and plumbing of pump inlets. Thus, it is necessary to work closely with fluid and component suppliers when utilizing HFC fluids.

HFD

HFD Fluids are non-water containing fire resistant fluids. The first edition of International Standard ISO 6743-4 classification (1982) divided HFD into four sub-categories: HFDR, HFDS, HFDT, and HFDU. In 1999 the standard was revised, deleting the HFDS and HFDT fluids from the classification system. HFDS and HFDT fluids are no longer commercially viable because they were based upon chlorinated materials such as polychlorinated biphenyls (PCBs) or other chlorinated aromatic compounds. Environmental concerns associated with chlorinated hydrocarbons led to withdrawal of these products from the market. On the other hand, HFDR and HFDU fluids continue to be widely used in a variety of commercial and military hydraulic applications.

HFDR fluids are composed of phosphate esters. The majority of phosphate ester type hydraulic fluids used in industrial applications are based upon triaryl phosphate [75]. Trialkyl and mixed alkylaryl phosphate esters are used in aviation because of their lower density [76]. Phosphate esters are difficult to ignite because they are non-volatile and chemically stable. The stability of phosphate esters is demonstrated by the fact that they do not propagate a flame in the Standard Test Method for Linear Flame Propagation Rate of Lubricating Oils and Hydraulic Fluids (ASTM D 5306-92). The principal reason they do not propagate a flame is that the chemical reactions that take place during



combust ion of phosphate esters are endothermic. Thus, phosphate esters generate less heat when burned relative to other HFD fluids. In addition, because their Are resistance is not dependent upon the presence of water or mist suppressing additives, the fire resistance of HFDR fluids does not degrade in service.

HFDR fluids have been used in hydraulic applications for more than forty years and are known for excellent inherent lubricating properties [77]. In fact, aryl phosphate esters serve as antiwear additives in mineral oil based hydraulic fluids [78]. However, phosphate esters have a steep viscosity temperature curve, which makes their temperature operating window rather narrow [79]. Hydrolysis is the most common form of degradation in HFDR fluid, and can occur in the presence of a small amount of water and heat. When hydrolysis takes place, phosphate esters break down into their constituent acids and alcohols. Due to the frequent presence of water in hydraulic applications, the sensitivity of phosphate esters to water has limited their use and significantly reduced their service life.

Phosphate esters are compatible with all common metals except aluminum. Phosphate esters do not "wet" the surface of a luminum and thus aluminum should not be used in tri-bological contacts such as bearings [80]. Phosphate esters should never be added to systems containing mineral oil or water-based fire resistant fluids. Not only are these materials chemically incompatible with each other, in all probability preexisting gaskets, seals, hoses, and coatings are also incompatible. Special precautions also are required in the selection of filter construct ion materials and plumbing of pump inlets. Thus, it is necessary to work closely with fluid and component suppliers when utilizing HFD fluids.

HFDU fluids typically are composed of polyol esters although other materials such as polyalkylene glycols are included in the HFDU category. Trimethylol propane oleate, neopentyl glycol oleate, and pentaerythritol esters are the most common of the synthetic polyol esters. Triglycerides derived from soybeans, sunflower, and rapeseed plants are naturally occurring polyol esters that also are used in HFDU fluids. Polyol esters derive their fire resistance from a combination of factors. First, polyol esters have a relatively high flash, fire, and autoignition point. Second, they b u m with less energy than oil because of the presence of oxygen in the molecule. And finally, polyol ester fire resistant fluids employ antimist additives that enhance their spray-flammabil-ity resistance [81]. Depending upon the shear stability of the polymer, the fire resistance of the fluid may deteriorate in service.

Like phosphate esters, polyol esters have excellent lubricating properties but are prone to hydrolysis in the presence of water [82]. In addition, they are vulnerable to oxidation because of unsaturation irl the fatty acid portion of the ester. These factors tend to limit their service life relative to mineral oils. Most common metals used in hydraulic applications are compatible with polyol ester hydraulic fluids, with the exception of lead, zinc, and cadmium. Unlike other fire resistant fluids, polyol esters performance is satisfactory with common filter construction materials and system designs. Thus it is relatively easy to convert a hydraulic system that operates on mineral oil based hydraulic fluids to HFDU fluids.

E n v i r o n m e n t a l l y A c c e p t a b l e H y d r a u l i c F lu ids

Environmental ly acceptable hydraulic fluids have found their way into hydraulic applications where there is risk of fluid leaks and spills entering the environment (especially waterways) affecting aquatic and terrestrial life. Some examples of these niche markets include forestry, construction, locks and dams, heavy-duty lawn equipment, amusement parks/enter tainment industry, offshore drilling, and maritime. Most environmentally acceptable hydraulic fluids exhibit two key environmental characteristics: virtual non-toxicity to aquatic life and aerobic biodegradability. Organizations such as the Organization for Economic Co-operation and Development (OECD), the Co-ordinating European Council (CEC), and the U.S. Environmental Protection Agency (EPA) have developed standard test methods to determine the toxicity and biodegradabili ty of substances. More recently ASTM has developed a Guide for Assessing Biodegradability of Hydraulic Fluids (ASTM D 6006) and a Classification of Hydraulic Fluids for Environmental Impact (ASTM D 6046) based on the above organizations' methods. Utilizing the methodology from these organizations, standard classifications and performance requirements for environmental fluids have also been established by the International Organization for Standardization (ISO) and regional environmental organizations that award Eco Labels (i.e., German Blue Angel, Nordic Swan, Japanese EcoMark). ISO environmental hydraulic fluid classifications are described in Table 14.

HETG

Type HETG fluids are based on naturally occurring vegetable oils or triglyceride esters. Without the addition of a thickener, vegetable oils are limited to a narrow viscosity range between ISO 32 and 46. While HETG fluids biodegrade rapidly, have excellent natureil lubricity and have a natural VI in excess of 200, they are unsuitable for use at high and low temperature extremes. This is because they tend to gel at low temperatures and oxidize at high temperatures. The practical temperature limits for uses HETG fluids is —25°F to 165°F.

HEES

Type HEES fluids are based on unsaturated to fully saturated synthetic esters. Common ester chemistries utilized for hydraulic fluids consist of TMP oleates, neopentylglycols, pentaerythritol esters, adipate esters, and complex esters. The synthetic esters provide better performance over HETG t5T3e hydraulic fluids with wider operating temperature ranges, broad range of ISO viscosity grades, and better oxidation stability while still maintaining biodegradability.

TABLE 14—ISO environmental hydraulic fluid classifications. Symbol

HETG HEES

HEPG HEPR

Classification

Vegetable oil types Sjmthetic ester types

Polyglycol types Polyalphaolefln types

Commercial Designation

Vegetable oils and natural esters Polyol esters, neopentylglycols,

syntiietic adipate esters Polyglycols Polyalphaolefins (PAO) or

synthetic hydrocarbons (SHC)



HEPG Type HEPG fluids are polyethyleneglycols (PEG), which possess good oxidation stability and low temperature flow characteristics. At molecular weights of up to 600-800, HEPG type fluids are ecotoxicologically harmless and readily biodegradable (>90% in 21 days) [83]. Some disadvantages of this class of fluids include miscibility with water, incompatibility with mineral oils, and aggressiveness toward some common t5^es of elastomer seal materials.

HEPR

HEPR type fluids are polyalphaolefins (PAO) or synthesized hydrocarbon (SHC) base fluids, which have significantly better viscometric properties over a wider range of temperatures than mineral base fluids with the same standard viscosity classification. Some low viscosity PAOs have shown acceptable primary biodegradability, though not as rapid as vegetable or synthetic ester base fluids (Fig. 22). Additional advantages claimed for synthetic lubricants over comparable petroleum-based fluids include improved thermal and oxidative stability, superior volatility characteristics, and preferred frictional properties.

CONCLUSIONS

A well formulated hydraulic oil consists of a properly selected base fluid and the appropriate balance of additives, optimized to provide the best possible overall performance required for the targeted application. The versatility of hydraulics makes fluid power advantageous in a wide variety of industrial and mobile applications. With this versatility comes the challenge of developing fluids that function appropriately in a wide range of conditions, even as environmental health and safety requirements become more and more stringent. New fluid technologies continue to emerge to meet these challenges.

Another challenge that comes with the various hydraulic applications is that of developing test methods that are truly representative of performance in actual systems. Bench-top tests are to be used as logical indicators of a fluid's response to expected conditions of temperature, pressure, contamination, etc. A significantly higher number of variables concurrently influence the fluid more than any single bench test can simulate. Therefore, standards and specifications consist of multiple bench tests as well as more realistic full-scale test stands that use actual pumps in typical hydraulic circuits. Test methods will continue to evolve as more sophisticated techniques are developed to predict field performance of hydraulic fluids.

ASTM STANDARDS

No. Title D 92 Test Method for Flash and Fire Points by Cleveland

Open Cup D 95 Test Method for Water in Petroleum Products and

Bituminous Materials by Distillation D 96 Test Method for Water and Sediment in Crude Oil

by Centrifuge Method D 97 Test Method for Pour Point of Petroleum Products D 130 Test Method for Determination of Copper Corro

sion from Petroleum Products by the Copper Strip Tarnish Test

D 287 Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method)

D 445 Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity)

D 446 Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers

D 471 Test Method for Rubber Property-Effect of Liquids D 664 Test Method for Acid Number of Petroleum Prod

ucts by Potentiometric Titration

Polypropylene glycols

Mineral oils

I I Mininnum • Maximum

Hydro-treated mineral oils

Polyethylene glycols

—1 Vegetable oils

§ Synthetic esters

20 40 60 80 100%

FIG. 22—Chart comparing primary biodegradation of base fluids by CEC method.



D 665 Test Method of Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water

D 892 Test Method for Foaming Characteristics of Lubricating Oils

D 943 Test Method for Oxidation Characteristics of Inhibited Mineral Oils

D 974 Test Method for Acid and Base Number by Color-Indicator Titration

D 1298 Test Method for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum Products by Hydrometer Method

D 1401 Test Method for Water Separability of Petroleum Oils and Synthetic Fluids

D 1744 Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent

D 2070 Test Method for Thermal Stability of Hydraulic Oils D 2270 Practice for Calculating Viscosity Index from Kine

matic Viscosity at 40°C and 100°C D 2271 Test Method for Preliminary Examination of Hy

draulic Fluids (Wear Test) D 2272 Test Method for Oxidation Stability of Steam Tur

bine Oils by Rotating Bomb D 2422 Classification of Industrial Fluid Lubricants by Vis

cosity System D 2619 Test Method for Hydrolytic Stability of Hydraulic

Fluids (Beverage Bottle Method) D 2717 Test Method for Thermal Conductivity of Liquids D 2766 Test Method for Specific Heat of Liquids and Solids D 2783 Test Method for Measurement of Extreme-Pressure

Properties of Lubricating Fluids (Four-Ball Method) D 2882 Test Method for Indicating the Wear Characteris

tics of Petroleum and Non-Petroleum Hydraulic Fluids in a Constant Volume Vane Pump

D 2983 Test Method for Low-Temperature Viscosity of Automotive Fluid Lubricants Measured by Brookfield Viscometer

D 3339 Test Method for Acid Number of Petroleum Products by Semi-Micro Color Indicator Titration

D 3427 Test Method for Air Release Properties of Petroleum Oils

D 3519 Test Method for Foam in Aqueous Media (Blender Test)

D 3603 Test Method for Rust-Preventing Characteristics of Steam Turbine Oils in the Presence of Water (Horizontal Disk Method)

D 3707 Test Method for Storage StabiHty of Water-in-Oil Emulsions by the Oven Test Method

D 3709 Test Method for Stability of Water-in-Oil Emulsions Under Low to Ambient Temperature Cycling Conditions

D 4172 Test Method for Wear Preventive Characteristics of Lubricating Fluid (Four-Ball Method)

D 4310 Test method for Determination of the Sludging and Corrosion Tendencies of Inhibited Mineral Oils

D 4684 Test Method for Determination of Yield Stress and Apparent Viscosity of Engine Oils at Low Temperatures

D 5133 Test Method for Low temperature. Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils Using a Temperature Scanning Technique

D 5182 Test Method for Evaluating the Scuffing Load Ca

pacity of Oils (FZG Visual Method) D 5306 Standard Test Method for Linear Flame Propaga

tion Rate of Lubricating Oils and Hydraulic Fluids D 5534 Test Method for Vapor-Phase Rust-Preventing

Characterisitics of Hydraulic Fluids D 5621 Test method for Sonic Shear Stability of Hydraulic

Fluid D 6006 Guide for Assessing Biodegradability of Hydraulic

Fluids D 6046 Classification of Hydraulic Fluids for Environmen

tal Impact D 6080 Practice for Defining the Viscosity Characteristics

of Hydraulic Fluids D 6158 Specification for Mineral Hydraulic Oils D 6278 Test Methods for Shear Stability of Polymer Con

taining Fluids Using a European Diesel Injector Apparatus

D 6351 Test Method for Determination of Low Temperature Fluidity and Appearance of Hydraulic Fluids

D 6546 Test Methods for and Suggested Limits for Determining Compatibility of Elastomer Seals for Industrial Hydraulic Fluid Applications

D 6547 Test Method for Corrosiveness of a Lubricating Fluid to a Bi-Metallic Couple

OTHER STANDARDS

AFNOR NF E48-690: Hydraulic Fluid Power. Fluids. Measurement of Filtrability of Mineral Oils

AFNOR NF E48-691: Hydraulic Fluid Power. Fluids. Measurement of Filtrability of Minerals Oils in the Presence of Water

ANSI/(NFPA) Standard T2.13.7R1-1996: Hydraulic Fluid Power - Petroleum Fluids - Prediction of Bulk Moduli

ISO 6743/4 Part 4: Family H (Hydraulic Systems), Lubricants, Industrial Oils and Related Products (Class L ) : Classification Part 4: Family H (Hydraulic Systems)

ISO 12922: Lubricants, Industrial Oils, and Related Products (Class L)—Family H (Hydraulic systems)—Specifications for categories HFAE, HFAS, HFB, HFC, HFDR and HFDU

ISO/DIS 15380: Lubricants, Industrial Fluids and Related Procedures (Class L), Family H (Hydraulic Systems)-Spec-ifications for Catagories HETG, HEPG, HEES and HEPR

REFERENCES

[1] Halliday, D. and Resnick, R. Physics, 3rd ed., John Wiley & Sons, NY, 1978, p. 376.

[2] Henke, R., Diesel Progress, Fluid Power Buyer's Guide, Diesel and Gas Turbine Publications, Waukesha, WI, 1994, p. 4.

[3] Vickers Industrial Hydraulics Manual, Ch. 3, Vickers, Inc., Rochester Hills, MI, 1992, p. 3-1.

[4] Booser, E. R., Handbook of Lubrication, Volume II Theory and Design, CRC Press, Boca Raton, FL, 1987, pp. 242-244.

[5] NFPA Standard T2.13.7R1 - 1997: Hydraulic Fluid Power -Petroleum Fluids - Prediction of Bulk Moduli, National Fluid Power Association, Milwaukee, WI, 1997.

[6] Lightening Reference Handbook, 7th ed., Paul-Munroe Hydraulics, Whittier, CA, 1990, p.l22.

[7] Totten, G. E., Webster, G. M., and Yeaple, F. D., "Physical Prop-



erties and Their Determination," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., 2000, Marcel Dekker, NY, p. 253.

[8] Esposito, A., Fluid Power with Applications, Prentice Hall, En-glewood Cliffs, NJ, 1988, p . 478.

[9] Michael, P. W. and Wanke, T. S, "Surgically Clean Hydraulic Fluid - A Case Study," Proceedings of the 47th National Conference on Fluid Power, National Fluid Power Association, Milwaukee, WI, 1996, pp. 129-136.

[10] Esposito, A., Fluid Power with Applications, Prentice Hall, En-glewood Cliffs, NJ, 1988, p.3.

[11] Ruble, L. R., "The Expanded Focus, Use, and Future of Water Powered Rotary Actuators," Proceedings of the 48th National Conference on Fluid Power, National Fluid Power Association, Milwaukee, WI, 2000, pp. 567-574.

[12] Perrier, R., "Lubricant Basestocks," Speech, STLE Chicago Lube School, Downers Grove, IL, 17 Mar. 1999.

[13] Engine Oil Licensing and Certification System, 14th ed.. Publication 1509, American Petroleum Institute, Washington DC, 1996.

[14] Wills, J. G., Lubrication Fundamentals, Mobil Oil Corporation, Fairfax, VA, 1980, p . 30.

[15] Denisov, E., Handbook of Antioxidants, CRC Press, Boca Raton, FL, 1995, p . 19.

[16] Lubricant and Fuel Additives, Advanced Lubrication Education Program, Report 1525, May 1992.

[17] Rasberger, M., "Chemistry and Technology of Lubricants," Oxidative Degradation and Stabilisation (Sic) of Lubricants , Blackie and Academic Professional, London, 1992, pp. 83-123.

[18] Igarashi, J., "Oxidative Degradation of Engine Oils," Japanese Journal of Tribology, Vol. 35, No. 10, 1990. p. 1098.

[19] Saxena, D., Mookken, R. T., Srivastava, S. P., and Bhatnagar A. K., "An Accelerated Aging Test for AW Oils," Lubrication Engineering, Vol. 49, No. 10, 1993, pp. 801-809.

[20] Approved Lubricants Manual, Cincinnati Machine Pub No. lO-SP-95046, Cincinnati Machine, Cincinnati, OH, 1995, pp. 2-40.

[21] Reyes-Gavilan, J. L. and Odorisio, P., "A Review of the Mechanisms of Action of Antioxidants, Metal Deactivators and Corrosion Inhibitors," NLGI Spokesman, Vol. 64, No. 11, Feb. 2001, pp. 22-33.

[22] Gatto, V. J. and Grina, M. A., "Effects of Base Oil Type, Oxidation Test Conditions and Phenolic Structure on the Detection and Magnitude of Hindered Phenol/Diphenylamine Synergism," Lubrication Engineering, Vol. 55, No. 1, Jan. 99, pp. 11-20.

[23] Pike, R. and Conway-Jones, J. M., "Friction and Wear of Sliding Bearings," ASM Handbook, Volume 18, Friction, Lubrication, and Wear Technology, 1992, ASM International, Materials Park, OH, p. 519.

[24] Blau, P. J., "Glossary pf Terms," ASM Handbook, Friction, Lubrication, and Wear Technology, Volume 18, 1992, ASM International, Materials Park, OH, p. 11.

[25] Michael, P. W., Herzog, S. N., and Marougy, T. E., "Fluid Viscosity Selection Criteria for Hydraulic Pumps and Motors" Proceedings of the 48th National Conference on Fluid Power, Chicago, National Fluid Power Association, Milwaukee, WI, 2000, pp. 313-325.

[26] ISO 3448: Industrial Liquid Lubricants-ISO Viscosity Classification, International Organization for Standardization, Geneva, 1992.

[27] Michael, P.W., "Lubrication Basics," Plant Services, Vol. 14, No. 5, May 1993, pp. 19-22.

[28] Silva, G., "Wear Generation in Hydraulic Pumps," presented at the SAE International Off Highway Congress, Milwaukee, WI, 1990, Paper 901679, Society for Automotive Engineers, War-rendale, PA.

[29] Voitik, R. M., "Realizing Bench Test Solutions to Field Tribology Problems Utilizing Tribological Aspect Numbers," Tribol

ogy - Wear Test Selection for Design and Application, ASTM STP 1199, A. W. Ruff and R. G. Bayers, Eds., ASTM International, West Conshohocken, PA, 1993, pp. 45-59.

[30] Hogmak, S. and Jacobson, S., "Hints and Guidelines for Tri-botesting and Evaluation," Lubrication Engineering, Vol. 48, No. 5, May 1992 p. 401.

[31] ASTM Round Robin Test Program RR: D02-1152, ASTM International, West Conshohocken, PA, 1994.

[32] Xie, L., Bishop, Jr., R. J., and Totten, G. E., "Bench and Pump Testing of Hydraulic Fluids," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 2000, p. 526.

[33] Perez, J. M., Klaus, E. E., and Hansen, R. C , "Comparative Evaluation of Several Hydraulic Fluids in Operational Equipment, A Full-scale Pump Stand Test and the Four-Ball Wear Tester. Part II - Phosphate Esters, Glycols and Mineral Oils," Lubrication Engineering, Vol. 46, No. 4, April 1990, p. 249.

[34] Perez, J. M., Weller, D. E., and Duda, J. L., "Sequential Four-Ball Study of Some Lubricating Oils," Lubrication Engineering, Vol. 55, No. 9, 1994, pp. 28-32.

[35] Reichel, J., "Importance of Mechanical Testing of Hydraulic Fluids," Tribology of HydrauUc Pump Testing, ASTM STP 1310, George E. Totten, Gary H. Kling, and Donald J. Smolenski, Eds., ASTM International, West Conshohocken, PA, 1996.

[36] Xie, L., Bishop, Jr., R. J., and Totten, G. E., "Bench and Pump Testing of Hydraulic Fluids," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 2000, p. 526.

[37] Denison Test Equipement and Instruct ions, Consigne TP-30283, Denison Hydraulics, Marysville, OH, 1999.

[38] Kling, G. H., Totten, G. E., and Ashraf, A., "Strategies for Developing Performance Standards for Alternative Hydraulic Fluids," in Lubricants for Off-Highway Applications, SAE SP-1553, Society for Automotive Engineers, Warrendale, PA, 2000, pp. 1-9.

[39] Ohkawa, S., Konishi, A., Hatano, H., Ishihama, K., Tanaka, K., and Iwamura, M. "Oxidation and Corrosion Characteristics of Vegetable-Base Biodegradable Hydraulic Oil," SAE Technical Paper Series, Paper Number 951038, Society for Automotive Engineers, Warrendale, PA, 1995.

[40] Melief, H. M., Totten, G. E., and Bishop, R. J., "Overview of the Proposed Rexroth High-Pressure Piston Pump Testing Procedure for Hydraulic Fluid Qualification," Fluid Power for Off-Highway Applications, SAE SP-1380, Society for Automotive Engineers, Warrendale, PA, 1998.

[41] "Sundstrand Water Stability Test Procedure," Sundstrand Bulletin No. 9658, Sauer-Danfoss Corp., Ames, lA.

[42] Hodges, P. K.B., Hydraulic Fluids, John Wiley, NY, 1996, p. 116. [43] Karl Fischer Applications, Mettler-Toledo AG., CH-8606

Griefensee, Switzerland, 1992. [44] Lloyd, B. J., "Water Water Everywhere," Lubes-N-Greases,

Vol.11, No. 3, 1997, pp. 44-53. [45] Tsong-Dsu, L. and. Mansfield, J., "Effect of Contamination on

the Water Separability of Steam Turbine Oils," Lubrication Engineering, Vol. 51, No. 1, 1995, pp. 81-85.

[46] Barber, A. R. and Perez, R. J., Air Release Properties of Hydraulic Fluids, The Lubrizol Corporation, Wickliffe, OH.

[47] Totten, G. E., Sun, Y. H., Bishop, R. J., and Lin, X., "Hydraulic System Cavitation: A Review," SAE 982036, Society for Automotive Engineers, Warrendale, PA, 1998.

[48] Brew, D., Khonsari, M., and Ball, J. H.,"Current Research in Cavitating Fluid Films," STLE Special Publication SP-28, Society of Tribologists and Lubrication Engineers, Park Ridge, IL, 1990, pp. 63-65.

[49] "Foaming and Air Entrainment in Lubrication and Hydraulic Systems," Mobil Technical Bulletin, Mobil Oil, Fairfax, VA, 1989.

[50] HVLP HydrauUc Oils, Minimum Requirements, DIN 51524 Part II, Deutsche Norm, Beuth Verlag GmbH, BerUn, 1990.

[51] Denison Fluid Standard HF-0: Hydraulic Fluid-For Use in Axial



Piston Pumps and Vane Pumps in Severe Duty Applications, Denison Hydraulics, Marysville, OH, 1995.

[52] Claxon, P. D., "Aeration of Petroleum Based Steam Turbine Oils," Tribology, Februaiy 1972, pp. 8-13.

[53] Hatton, D. R., "Some Practical Aspects of Turbine Lubrication," The Canadian Lubrication Journal, Vol. 4, No. 1, 1984, p . 4.

[54] Rhee, I., Velz, C , and Von Bemewitz, K., Evaluation of Environmentally Acceptable Hydraulic Fluids, Technical Report No. 13640, U.S. Army Tank-Automotive Command, Research Development and Engineering Center, Warren, MI, March 1995.

[55] Rizvi, S. Q. A., "Lubricant Additives and Their Functions," ASM Handbook 10th ed., Vol. 18, Friction, Lubrication, and Wear Technology, ASM International, Materials Park, OH, 1992, pp. 98-112.

[56] Available from ASTM International Headquarters , 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA.

[57] Fainman, M. Z. and Hiltner, L. G., "Compatibility of Elas-tomeric Seals and Fluids in Hydraulic Systems," Lubrication Engineering, Vol. 37, May 1980, pp. 132-137.

[58] Ashby, D. M., "O-ring Specifications - Some Important Considerations," Hydraulics & Pneumatics, August 1999, pp. 53, 54, 90.

[59] Leslie, R. L. Sculthorpe, H. J., "Hydraulic Fluids Compatible with Metalworking Fluids," Lubrication Engineering, Vol. 28, May 1970, pp. 165-167.

[60] Carpsjo, C , Proceedings of International Symposium on Performance Testing of Hydraulic Fluids, Institute of Petroleum, London, 3-6 Oct 1978.

[61] Stambaugh, R. L. andKopko, R. J. "Behavior of Non-Newtonian Lubricants in High Shear Rate Applications," SAE Transactions 82, Paper 730487, Society of Automotive Engineers, Warren-dale, PA, 1973.

[62] Stambaugh, R. L., Kopko, R. J., and Roland, T. F., "Hydraulic Pump Performance - A Basis for Fluid Viscosity Classification," SAE International Off Highway Congress, Milwaukee, WI, Paper 901633, Society of Automotive Engineers, Warrendale, PA, 1990.

[63] Hydraulic Fluids Information, Denison Corp., Marysville, OH, 1995, p. 2.

[64] Rizvi, S. Q. A., "Lubricant Additives and Their Functions," ASM Handbook Volume 18 Friction, Lubrication, and Wear Technology, ASM International, Materials Park, OH, 1992, p . 106.

[65] Sharma, S. K., Snyder Jr., C. E., Gschwender, L. J., Lang J. C , and Schreiber, B. F., "Stuck Servovalves in Aircraft Hydraulic Systems," Lubrication Engineering, Vol. 55, No. 7, July 1999.

[66] Michael, P. W. and Webb, S., "Future Fluids - What's Coming Down the Hydraulic Line," OEM Off-Highway, January 1998, pp. 34-36.

[67] Lubrizol Ready Reference for Lubricant and Fuel Performance, The Lubrizol Corporation, Wickliffe, OH, 1998, p. 111.

[68] Colver, R., Chek-Chart Publications, Simon & Schuster, NY, 1995, p . 45.

[69] ISO 6743-4: Lubricants , Industr ial Oils and Related Products (Class L) - Classification - Part 4: Family H (hydraulic systems). Internat ional Organization for Standardization, Geneva, 1999.

[70] NFPA/T2.13.1: R3-1997, Recommended Practice - Hydraulic Fluid Power - Use of Fire Resistant Fluids In Industrial Systems, NFPA, Milwaukee, WI, 1997.

[71] ISO 7745: Hydraulic Fluid Power - Fire Resistant (FR) Fluids -Guidelines for Use, International Organization for Standardization, Geneva, 1999.

[72] Vickers Guide to Alternative Fluids, Eaton Corp., Southfield, MI, November 1992.

[73] Totten, G. E. and Sun, Y., "Water-Glycol Hydraulic Fluids," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 2000, pp. 917-982.

[74] NFPA/T2.13.1: R3-1997, Recommended Practice - Hydraulic Fluid Power - Use of Fire Resistant Fluids in Industrial Systems, NFPA, Milwaukee, WI, 1997.

[75] Phillips, W. D., "Phosphate Ester Hydraulic Fluids," in Handbook of Hydraulic Fluid Technology G. E. Totten, Ed., Marcel Dekker, NY, 2000, pp. 1025-1027.

[76] Parker O-Ring Handbook Y2000 Edition, Parker Hannifin Corp., Cleveland, OH, 1999, pp. 3-18.

[77] O'Connor, J. and Boyd, J., Standard Handbook of Lubrication Engineering, McGraw-Hill, NY, 1968, pp. 2-16.

[78] MIL-H-5606G Military Specification: Hydraulic Fluid, Petroleum Base, Aircraft, Missile and Ordinance, NATO code number H-515, U.S. Department of Defense, Fort Belvoir, VA, 1994.

[79] Parker O-Ring Handbook Y2000 Edition, Parker Hannifin Corp., Cleveland, OH, 1999, pp. 3-18.

[80] Phillips, W. D., "Phosphate Ester Hydraulic Fluids," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 2000, pp. 1025-1027.

[81] Gere, R. A. and Hazelton, R. A., "Polyol Ester Fluids," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 2000, pp. 983-1022.

[82] Hohn, B. R., Michaelis, K., and Dobereiner, R., "Load Cjirrying Capacity of Fast Biodegradable Gear Lubricants," Lubrication Engineering, 1999, Vol. 55, p. 37.

[83] Bartz, W. J., Environmentally Acceptable Hydraulic Fluids, Technische Akademie Esslingen, Ostfildern, Germany.


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