FLUID CONTAMINATION SURVEY OF 143

NAVAL AIRCRAFT HYDRAULIC SYSTEMS

13 March 1963

Prepared under Navy, Bureau of Weapons

Contract NOw 62-0297-tS'.-'D Task Order No. 62-2

. •.Supplemental Agreement NSE-312

Final Report

Qualfie :--~C'ter,3 inariobtain copicc o" this -

report, froLi AS:TA. (

ISUPA U BU'R•U ".o,..-.A.'. "'.

LB-31228

FLUID CONTAMINATION SURVEY OF 143

NAVAL AIRCRAFT HYDRAULIC SYSTEMS

13 Maroh 1963

Prepared under Navy, Bureau of Weapons

Contraot NOw 62-0297tTask Order 62-2

Supplemental Agreement NSE-312

Prepared by Approved byPrepred ¥ . roinsen K.W. Dubois

li~dro-Meg h. SectionCheHydro-Keoh. Seation

Approved by¥

Hydro-Mech. SeotionTest Group Engineer

DOUGLAS AIRCRAFT COMPANY, INC.LONG BEACH, CALIFORNIA

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DOUGLAS AIRCRAFT COMPANY, INC.

L.0 A•STRAVT

A. A survey of hydraulic fluid contamination was madein which 79 airplanes, plus several test stands,were sampled. The results are presented in tabularand graphical form.

B. An analysis of the data of the survey leads to theconclusion that substantially cleaner systems canbe maintained with no great Increase In cost.

0, Recommendations are made concerning the maximumpermissible level of contamination, and procurementof components which will operate reliably at thefluid contamination levels to which they will besubjected.

II _ _ _ _ _ _ _ _____

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2.0 TABLI OF CONTENTS

1.0 SUMMARY

2.0 TABLE OF CONTENTS

3.0 INTRODUCTION

4.0 FIELD SURVEY OF HYDRAULIC FLUID CONTANDITION

4.1 Data Obtained and Procedures Used.

4.2 Systems Tested.

4.3 Tabulation and Analysis of. Results.

4.3.1 Tabulation

4.3.2 Analysis

5.0 ANALYSIS OF CONTAMINATION SURVEY TO DETERMINE MINIMUMPRACTICABLE CONTAMINATION LEVELS

5.1 General

5.2 Prevention of Contamination

5.2.1 Sources of Contamination

5.2.2 Built-In Contamination

5.2.3 Contamination in New Oil

5.2.4 Airborne Dust Infiltrating the System

5.2.5 Contaminants Generated Within the System

5.2.6 Contamination Introduced by Interchange ofFluid With Ground Test Equipment.

5.3 Improved Filtration in Systems.

5.3.1 Filters Available

5.3.2 Filter Applications

5.4 Other Factors Atfecting Contamination Levels

5.5 Estimate of Practical Minimum Contamination Levels

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2.0 TALE OF CONTENTS (CONTINUED)

6.0 PROCUREMENT OF CONTAMINATION RESISTANT HYDRAULIC SYSTEMCOMPONENTS.

6.1 Types of Failures

6.1.1 Single Particle Failures

6.1.2 Wear Failures

6.1.3 Frictional Failures

6.1.4 Outlook for Contaminant Resistant Compenents

6.2 Specifications and Recommended Changes

7.0 DISCUSSION

7.1 Maintenance Practices

7.2 System Flushing

7.3 Filter Servicing Policy

7.T Filter Performance in a System

7.5 Contamination Teats.

8.0 CONCLUSIONS

9.0 REFERENCES

,I__________________________________________________

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FIGURE NO. LIST OF ILLUSTRATIONS

1. Sketch - Particle Size Comparison

2. Table - Contamination Levels - Tentative Standards

3. Table - Aircraft Hydraulic Systems - Sampling Dataand Design Information. (3 sheets)

4. Table - Composite Data - Hydraulic System Tests(6 sheets)

5. Graph - Total Flight Time vs Contaminatien Class

6. Graph - Fluid Viscosity vs Contamination Class

7. Graph - Cumulative Percent of Hydraulic SystemsDirtier Than a Given Contamination Level(All Models)

8. Graph - Cumulative Percent of Hydraulic Systems DirtierThan a Given Contamination Level (By Model)(2 sheets)

9. Graph - Total Systems and Types of Filtration in EachContamination Level

10. Graph - Average Contamination Level vs Number ofActuators in System

11. Graph - Contamination Classes and MIL-V-27162 MaximumLimit for Servo-Valve Tests.

APPENDIX

Sampling Prooedure and Zquipment

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3.0 INTRODUCTION

This test program was undertaken by Douglas AircraftCompany under Navy contract NOw 62-0297t, Task Order 62-2.Stated objectives of the contract are as follows:

1. Investigate actual contamination levels in serviceaircraft.

2. Determine minimum contamination level which can beconsistently maintained in service.

3. Establish methods and procedures which can be used toinsure that system components will be sufficientlycontaminant resistant to operate in contaminated fluidwithout performance degradation or loss of reliability.

The handbook of maintenance instructions for each aircraftcontains instructions for maintaining the hydraulic systemin a clean condition. Most maintenance personnel have noclear conception of the nature of the contaminants mostprevalent in hydraulic system, and are satisfied if thecomponents, the filter elements, and the oil, look clean.Figure 1 shows some typical particle sizes related tofilter ratings, valve clearances, etc.

While hydraulic fluid contamination has long been known toadversely affect some components, notably hydraulic pumps,failures usually occurred gradually, and could be detectedbefore they became catastrophic.

The advent of hydraulically powered control systems, andmore recently, the use of electro-hydraulic servo-controlvalves with these control systems, has made hydraulic fluidcleanliness vital to the satisfactory function of theaircraft and to the performance of its mission. Muchattention has been focused on this area in recent yearssso much so, that malfunctions traceable to other causesare sometimes blamed upon fluid contamination.

This contamination survey and analysis are intended,therefore, to answer the questions, (1) how clean are thehydraulic systems now? (2) how clean can we make them,economically? and (3) how can system reliability beassured at the contamination levels which it is practicalto maintain?

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4.o FIELD SURVEY OF HYDRAULIC FLUID CONTAMINATION

4.1 Data Obtained and Procedures Used

Particle count of solid contaminants: 100cc ofhydraulic fluid was filtered through a 0.8 micronfilter membrane and the particles counted undermicroscopic examination per SAE Aircraft RecommendedPractice 598. An average tare count was then sub-tracted. The hydraulic systems were sampled, andthe fluid filtered, by methods and portable fieldequipment described in Appendix A.

Viscosity: Viscosity in centistokes at 100F wasdetermuine by ASTM Method D445-53.

Neutralization Number: The acidity of the hydraulicfluid, an index to" its corrosiveness, was determinedby ASTM Method D974-58T. This is done by titrationto an end point, using potassium hydroxide as thebase and para-naphthol-benzein as an indicator. Theresults are expressed in milligrams KOH per gram ofsample.. The accepted limit is 0.5.

4.2 Systems Tested

One hundred forty four samples were obtained from79 airplanes of 17 models plus several ground servicetest and fill stands. The table, Figure 3, liststhe models and certain significant characteristicsof their hydraulic systems. Figure 4 lists for eachsample the airplane, system, squadron, and airbase.A total of 13 bases was vilited.

4.3 Tabulation and Analysis of Results

4.3.1 Tabulation

Figure 4 is a composite table showing all lab-oratory analysis data for each sample, inaddition to the identifying information suchas airplane model and BuNo, total flight hours,squadron, base, etc.

The particle count is a rathez') unwieldy des-cription of fluid contaminatioh. A tentativeset of standards has been adopted Jointly bythe SAE, ASTM, and AIA, defining contaminationclasses, according to the table, Figure 2.This is a convenient but imperfect index, asthe particles are seldom present in the correctproportions to fit this table. To improvethis correspondence for purposes of this report,use is made of "class plus" designations. Forexample, if the particle count in four of the

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five size ranges is within the limits ofClass 3, but the fifth size range belongs inClass4, the class is given in the compositedata table as 34.

4.3.2 Analysis of Results

Cleanliness level distribution, all aircraft.

The curve, Figure 7 shows the contaminationclass (per Figure vs the percentage of allsystems which exceed that class. This curveindicates that while the mean contaminationlevel for all systems is about Class 4, 23%are dirtier than Class 5, and 11% are dirtierthan Class 6. Figure 9 shows this informa-tion in bar-graph form.

Cleanliness level by model.

Figure 8 shows the cleanliness distributionby aircraft model, provided there are suffi-'cient data to generate a curve. On severaltypes of aircraft, three or less airplaneswere tested. Figures 8E through 8H comparecontamination levels of utility and flightcontrol systems for four models. These curvesshow the flight control systems to be cleanerin each case, although the margins vary.

Contamination vs Flight time

The graph, Figure 5, shows contaminationclass vs total flight time on the airplane.This graph shows no trend, but has a randomdistribution, demonstrating that flight timeis no index to contamination.

Contamination vs Viscosity

Figure 6 shows the contamination class vsviscosity of the fluid. There appears to besome relationship in thise case, but not suf-ficiently well defined to use viscosity as anindex to contamination.

Contamination vs Geographical Location

While the data is not conclusive, there appearsto be little correlation between hydraulicfluid contamination and geographical location,The P-2 aircraft, which was the only modelchecked in several locations, was quite con-sistent in contamination level. The threeP2 airplanes in the Pacific Northwest

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(Whidbey Island), which probably has a lowairborne dust level, were slightly cleanerthan average.

Identification of Contaminants

Microscopic examination, which is not con-sidered a dependable method, indicates that alarge part of the contamination in virtuallyall samples is rubberlike particles. This isindicated by color (black), texture and form(similar to eraser dust).

Some of the more heavily contaminated mem-branes were analyzed on a Jarrel-Ash 3.4meter emission spectrograph.

This machine shows the presence and relativequantity of metallic elements, and except forrubber and plastic, most of the contaminantsare metals or metallic compounds. The cleanermembranes do not contain enough material forreliable analysis.

The results of the spectrograph analysis areshown on the composite data table, Figure 4.The probable sources of the elements shown are:

Silicon (Si) Airborm dust; grindingcompounds.

Chromium lCrt stools

Copper (Cu) Pump wear AluminumAluminum (l " bronze

Titanium (Ti) Paint pigmentCadmium ( Plating

Tin (Sn) ) Solder - brazingsilver

stt

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5.0 ANALYSIS OF CONTAMINATION SURVEY TO DETERMINE MINIMUM

PRACTICABLE CONTAMINATION LEVEIL

5.1 General

Cleaner hydraulic systems might be obtained bypreventive measures, that is, by reducing or eliminat-ing contamination at its source, by improving system 4filtration, or a combination of both. The data of thecontamination survey, Paragraph 4.0, provides abasis for analyzing these approaches.

5.2 Prevention of Contamination

5.2.1 The sources of contamination are:

(a) "Built-in" contamination. Dirt in hydrau-lic components, lines, etc., as fabricatedand not removed by system flushing beforedelivery.

(b) Contamination in new oil added to thesystem.

(c) Airborne dust infiltrating the system.

(d) Contaminants generated within the systemby pump wear, packing wear, etc.

(e) Contamination from hands, tools, eto.,introduced when a system is opened formaintenance reasons (includes overhaulingof components).

(f) Contamination introduced by interchange

of fluid with ground test equipment.

These will be discussed in order.

5.2.2 "Built-In" Contamination

All components of hydraulic systems containsome contamination, such as metal from machin-ing and grinding, abrasive compounds fromhoning and lapping, dirt from hands and tools,etc. More is introduced during assembly ofthe system. Flushing of the system may removea portion of this contamination before theairplane is delivered. The test data does notindicate that"new" airplanes have significantlymore or loss, contamination than older ones.It may be of a different type however. Forexample, two low-time A-5 aircraft show thepresence of silver, probably from solderingor brazing operations during manufacture.

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This element is not significant in older air-craft. The aircraft manufacturers expend con-siderable effort to control contamination, buthave generally rejected, as too expensive,clean-room assembly of components and othertechniques applied to missile systems. Thissurvey indicated that, at present at least,such efforts would largely be wasted as thesystems would soon become contaminated fromother sources.

5.2.3 Contamination in New Oil

New MIL-O-5606 hydraulic oil as purchased isusually moderately clean to dirty, (Class 4 to6), although a cleaner grade (MIL-O-5606B) isavailable at a premium price.

Most of the aircraft currently in the Navy'sarsenel require pressure-filling of the hydrau-lic systems, rather than pour-in filling.Most of these are filled with a hand operatedfill stand such as the Alemite Model 7181 or alocally manufactured equivalent. This standconsists of a reservoir, hand pump, filter,and delivery hose.

The filter utilizes a paper AN 6235-3A element.In moat cases, the oil is purchased in onegallon cans (sometimes one quart) and when acan is opened, the entire contents are usedat once, (that is poured into the fill stand).The cans are opened with beer-can openers orservice station type pouring spouts.

Three such fill stands were checked during thesurvey, and the oil they discharged variedfrom exceptionally clean to unacceptably dirty.(Class 0 to 7).

It appears that new oil may represent a signi-ficant source of contamination. It can begreatly reduced quite inexpensively. Throw-away filter elements per SpecificationMIL-F-27656 (5 micron absolute) can be obtainedto fit in present filter housings, and will,in a single pass, remove virtually all par-ticles. (These elements are net interchange-able with presently installed airplane filtersbecause of their reduced flow and temperaturecapability). The delivery hose should bechanged if it shows any evidence of deteriora-tion, or be replaced with plastic-lined hose.

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5.2.4 Airborne Dust Infiltrating the System

The majority of systems are sealed againstinfiltration of dusts many employing airlessreservoirs, others use filtered air to pres-surize the reservoir. The wide-spreadincidence of silicon as a contaminant indicatesthat airborne dust does, nevertheless, find itsway into the system. There appears to be nospecific geographical area where this is mostprevalent. The Naval air stations coveredduring this survey were mostly in coastalareas, so may not represent the worst dustenvironment.

Dirt and dust probably enter the hydraulicsystem by any of 4 routes (1) by infiltratingthe filling stands and entering the hydraulicsystem with new oil (required to make up fornormal attrition,) (2) with air vented to thehydraulic reservoir, or used to pressurize thereservoir, (3) by clinging to surfaces(piston rods) which are alternately exposed toatmosphere and to hydraulic fluid, and (4) fromhands and tools during maintenance in which thesystem is opened.

5.2.5 Contaminants Generated Within the System

Any moving parts within the hydraulic systemmust be expected to generate wear particleswhich will contaminate the hydraulic fluid.By far the most important such source inmost systems is the hydraulic pump. Particlesof steel and bronze resulting from pump wearoccur to some degree in most of the systemstested in this survey. Another significantsource is the seals in the actuators, valves,etc., in the system. Particles from thissource may be less damaging to hydraulic pumpsthan metallic particles, but may neverthelesscause valve sticking, silting,etc. Theseparticles contribute greatly to "backgroundcolor" in test filter membranes, because oftheir carbon black content. The most effec-tive way to combat the generation of con-taminants within the system is to maintaina clean system.

5.2.6 Contamination Introduced by Interchange of OilWith Ground Test Equipment

Most aircraft hydraulic systems are periodicallconnected to hydraulic ground test stands forfunctional or leakage tests, and this inevit-

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ably involves some interchange of fluid betweenairplane and test stand. Although the aircrafthydraulic system reservoir serves during teatstand operation, the volume of oil in the teatstand pump, filters, valves, and hoses maystill be a substantial percentage of the air-plane system volume. For example, one modelof test stand uses two filters containingAN-6236-3 elements, each filter housingholding approximately one gallon of hydraulicoil. This stand is used with F-8 airplanes,whose power control systems have a capacity of2.6 gallons. Test stand operation of such asystem thus implies a 50% chanSe of hydraulicoil. This fact emphasizes the importance ofmaintaining test stands in good condition, andfurther, indicates that by incorporating finerfilters in the test stands, airplane systemswould be cleaned whenever connected to a teststand. From the test results, it appears thatthe cleanliness of hydraulic oil in teststands was in most cases comparable to thatin the airplanes in the same activity, whichwould be expected with the large exchange ofoil with the aircraft systems. However,there were cases where the test stand fluidwas cleaner, and dirtier, than the fluid inthe airplanes.

Test stand maintenance was generally entrustedto a base maintenance activity, rather than tothe aircraft squadron, and varied widely inquality. In some cases, it appeared question-able as to whether the prescribed servicingintervals were being observed. One squadronsupplied photographs of ruptured filterelements found in their test stands.

5.3 Improved Filtration in Hydraulic System

5.3.1 Filters Available

(a) The Specification MIL-F-5504B (pleatedpaper element) filter is the most widelyused on Naval aircraft. The elementsconform to drawing AN 6235 or AN 6236.The plastic impregnated paper media, witha random distribution of pore sizes,traps a percentage of all sizes of par-Sticles. It has no "absolute" rating, buthas a nominal rating of 10 microns.II _________________________________________________

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(b) High performance aircraft of recent designgenerally use a stainless steel wovenwire mesh filter element with a nominalrating of 10 microns and an absoluterating of 25 microns. These filterswere usually bought to an airframemanufacturer's specification, but may nowbe purchased to SpecificationMIL-F-25862 (USAF). The uniform poresstop all large particles, but pass mostparticles smaller than the nominal ratedsize.

(e) Specification MIL-F-8815 describes afilter rated at 15 microns absolute, andapproximately 2 microns nominal. Thoughno qualified product list had been issuedas of this writing, qualification testinghad been completed and submitted by atleast one vendor. Filters conforming tothis specification are used on A-6 air-craft (none of which were included inthis survey) and have been flight testedon A4C aircraft (Ref. C).

(d) Filters made of sintered metal particles(bronze) are available in a variety ofratings, conforming to airframe manu-facturer's or vendor's specifications.

(e) Specification MIL-F-27656 describes afilter with an absolute rating of 5microns, and a nominal rating less thanone micron. At least one qualifiedproduct is available, employing anepoxy-impregnated fiber media.

5.3.2 Filter Applications and Performance

When used within their limitations, theMIL-F-5504 paper filters do an excellent Job.Of all airplanes checked in the contaminationsurvey, those with the cleanest hydraulicsystems were equipped with these elements.However, many of the dirtiest systems alsoused MIL-F-5504 filters. The reason forthese dirty systems may be failed filter media,open by pass valves due to. clogged elements,or damaged filter element seals. (The airbubble test of Specification MIL-F-5504B shouldpreclude faulty elements being purchased).The handbooks of maintenance instructions forseveral models permit these elements to beoleaned with solvent and a soft brush and re-used. This practice should be stopped it itactually exiats,.

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Visible particles on the surface may indicatetrouble elsewhere, but will not clog thefilter. The fine particles which clog thefilter are deep in the pores and cannot besatisfactorily removed. Attempts to cleana paper filter can do little good, but maydamage it.

The stainless steel wire filter elements arewidely used because they will withstandhigher temperatures and higher pressures, arefree of media migration and are recleanable.Cleaning as practiced at squadron level,however, is ineffective. These filterseffectively remove large particles, as shownin the particle counts, Figure 4, but do notstop many particles below their nominal rating.References A and B indicate that it is thesesmaller particles that affect servo-controlvalve performance. A comparison of systemscleanliness (Figure 8A) indicates that thewire mesh filters are not as effective asMIL-F-5504 paper filters used within theirlimitations.

Sintered bronze filter elements were employedin only one aircraft included in this survey;the F-8. In this aircraft, its performancevaried widely, much like the paper elements,with systems both cleaner and dirtier than anysystem with wire mesh filter elements (Ref.Figure 9). The overall performance of thesintered bronze filters appears somewhat betterthan the wire mesh filters, or roughly equalto the paper filters (Figure 8A).

The MIL-F-8815 filter is a relatively newdevelopment, and combines the best featuresof the MIL-F-5504 filter (fine particleremoval) with the advantages of the wire meshfilters. The media used in the A-6 aircraftapplication, and tested in A-4C aircraft,consists of a stainless steel wire mesh backingand an overlay of stainless steel particles,all sintered together.

In the A-4C flight test program, conducted by.NATC (Ref C) the use of the MIL-F-8815 filterelement resulted in a gradual but significantimprovement in the performance of the auto-matic flight control system.

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Mil-F-8815 filtration appears to be the bestchoice for new aircraft designs, in spite ofhigher initial cost. The use of a pressuredifferential indicator permits maximum usageto be obtained from each element, and permitselimination of frequent inspections (whichoften result in contaminating the system).

The MIL-F-27656 filter, which employs epoxyimpregnated fiber media, is reported to beperforming well in commercial airline tests,and may be adopted by at least one airline asstandard equipment (Ref. H). At least onequalified product is available. Because ofits temperature limitation (approx. 2 500 F)and the fact that a larger unit is requiredfor equivalent flow, fighter and attack air-craft may find it unsuitable. For other air-craft, and for ground support equipment, itshould be seriously considered.

5.J Other Factors Affecting Contamination Levels

A study of the test data shows that in general,the systems with the most contaminant generatingcomponents (actuators, etc.) are the dirtiest.

Figure 10 is a graph showing average contaminationclass vs. number of actuators in the system. Forsystems with paper (MIL-F-5504 )filters, the distri-bution appears random, but for systems with metalfilters, the correlation is surprisingly good. Theimplication is clear that complex systems requiremore filtration than simple ones.

It does not necessarily follow that the actuatorsproduce all, or even most of the contamination.More flow demand requires the hydraulic pump tooperate under load more often, thus increasing itswear.

5.5 Estimate of Practicable Minimum Contamination Level.

Figure 7 shows that in current Naval aircraft, morethan half the hydraulic systems meet a contaminationlevel of Class 4 or better. If all systems were thatclean, there would be little trouble caused by con-tamination. However, about a quarter are dirtierthan Class 5, the recommended limit, and more than10 percent are dirtier than Class 6, which is 4 timesan dirty as Class 4.

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A number of positive steps toward cleaner systemsis listed below:

(1) Use MIL-F-8815 or better filtration on airbornesystems.

(2) Use MIL-F-27656 or equivalent filtration on allsystem filling and ground test equipment.

(3) Discontinue cleaning and re-use of paper elements(as now permitted by several HMI's).

(4) Use redundant filtration (two or more filtersin series).

(5) Use filter media in proportion to the number ofcomponents in the system, rather than the maxi-mum pump flow.

(6) Purchase only hydraulic pumps which have satis-factorily completed a run-in test.

On new designs, where most or all of the above stepscan be taken, class 4 cleanliness level or bettershould be consistently attainable.

On existing alrcraft, where only a few of theforegoing suggestions are practical, Class 5 or bettershould be a practical goal.

a

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6.0 PROCUREMENT OF CONTAMINANT-RESISTANT HYDRAULIC SYSTEM

COMPONENTS

6.1 Types of Failures

Contamination induced failures of hydraulic systemcomponents may be grouped into three categories,as follows:

A. Malfunctions due to single large particles (ora few large particles) interfering with themotion of moving parts (reseating of valves,etc.).

B. Loss of performance due to wear and eventualfailure (hydraulic pumps and motors, etc.).

C. Lose of performance when frictional forcesbecome significant with respect to drivingforces, or when contamination affects thedriving forces (servo control valves).

A secondary type of failure has also been reported.Worn hydraulic pumps produce greater pulsationfluctuations in the delivery pressure, thus inducingfatigue failures in lines and other components.Data to substantiate this seems to be lacking.

6.1.1 Single Particle Failures

A particle large enough to plug an orifice, preventa valve from seating, etc. is large enough to beeasily seen, and would be stopped by the coarsestfilter. Its presence is indicative of carelessnesson the part of manufacturing, overhaul or mainten-ance personnel. Failure to deburr a drilled passageat the time of manufacture, for example, mayeventually result in the burr being dislodged intothe system. Good maintenance, and the applicationof screens (coarse filters) at critical points willprevent this type of failure.

6.1.2 Wear Failures

All moving parts are subject to wear at points ofcontact, but few hydraulic system components aresubject to continuous movement. The principleexceptions are hydraulic pumps, and continuous dutyhydraulic motors (such as alternator drives).Infiltration of the precision fits by fluid-borneparticle results in wear, scoring, and Increasedclearances. This sometimes causes sudden failure,but more often results in a gradual loss of*fficiency.

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It has been shown (Reference D) that by monitoringthe contamination produced during a running-inperiod, potentially troublesome pumps can bedetected by the manufacturer. This procedurehas been incorporated in specificationMIL-P-19692A (WEPS) (Reference E) but is notgenerally applied because of the costs involved.This run-in test, in abbreviated form, is used inthe procurement of some hydraulic pumps (A-4 air-craft) and has reduced pump rejections. ReferenceD claims a 50% increase in pump life was achievedon F-8 aircraft. If this is true the cost of therun-in procedure should be justified. ReferenceD also describes typical design and manufacturingchanges which may increase pump durability. Toour knowledge, there is no data which relates pumpendurance to fluid contamination levels, so nospecific goal can be defined for a contaminationlevel which will result in improved pump life.

Wear in other components is usually not seriouswithin the life expectancy of military airplanes.Testing of actuators to specification KIL-C-5503,for example, serves to reveal potentially trouble-some wear points, which can usually be eliminatedthrough redesign.

6.1.3 Frictional Failures

Control valves, in particular electrohydraulicservo valves, may have limited force availableto drive the valve spool. Frictional forces maybecome great enough to cause degradation ofperformance, although little or no wear hasoccurred.

Servo valves, also called transfer valves orhydraulic amplifiers, are usually designed toproduce hydraulic flow output proportional toan electric current input, and are used in mostautopilots, missile guidance systems, etc. Itis the performance of these units, more than anyother factor, which has focussed attention onhydraulic fluid contamination in the last fewyears.

Tests per References A and B were conducted bythis contractor in an attempt to find thecontamination level which will yield satisfactoryservo valve performance. The results may besummarized as follows:

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Class 6 - Hydraulic Fluid - Performance is degraded

Class 5 - Hydraulic Fluid - Performance is stable

Class 4 - Hydraulic Fluid - Performance improves

These results were obtained with three differentmodels or servo valve, so are regarded as beinggenerally applicable. Flight test corroborationwas obtained (Reference C) although the fluidcontamination was not monitored. Three A-4Cairplanes were equipped with filters manufactured

.to meet specification MIL-F-8815 (identical withthe elements used in the laboratory tests perReferences A and B.). Figure 4 of this reportindicates the initial contamination level in A-4Cutility systems would have been Class 5 to Class 6.Reference C states "Prior to the installation ofthe 2-15 micron APM filter element in the modelA4D-2N airplane, the performance of the AFCS wasslowly deteriorating to an unacceptable level.This deterioration was evidenced by increasingrandom control movements and feedback near thetrimmed position when in the Control Stick Steering(CSS) mode. After filter installation theperformance appeared to level off for a period ofapproximately 8-10 hours, then improve slowly.Random control movements and stick feedback werenot eliminated completely, but did decrease to anacceptable level." This test program involved127 sorties flown by 23 pilots.

It was found that the electrical circuit could beadjusted to minimize the effect of servo valvedegradation. Nevertheless, it seems clear thata Class 5 contamination level is marginal forthis type of equipmen,., with Class 4 or betterto be desired.

6.1.4 Outlook for Contaminant Resistant Components

A review of the nature of contaminant-causedfailures and the possible courses of action toobtain contaminant resistant components leads tothe conclusion that little progress can beexpected in this area.

In hydraulic pumps, for example, the combinationsof metals and hardnesses used are normally thosefound by experience to have the best wearresistance. The clearances are dictated by

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volumetrio efficiency and mechanical considerations.Future developments in metallurgy may bringimprovements, but the most positive action atpresent appears to be use of a run-in procedure(Per MIL-P-19692A, or some variation thereof).

The other major contaminant-sensitive components,servo-control valves or other special closetolerance valves, have close tolerance slidesand small orifices dictated by performance andleakage requirements. In specific applications,greater leakage might be tolerable, but servo-control valves are seldom designed for a specificapplication. Increased slide-driving force canbe attained by using larger diameter slides,but this affects frequency response. Since manypresently available servo-control valves workwell with Class 4 contamination levels and fairlywell with Class 5 contamination (at least for air-plane applications) it appears the most feasibleapproach is to provide hydraulic systems whichmeet these levels.

6.2 Specification Changes

The only military specifications covering contamin-ation resistance of hydraulic system componentsare:

A. MIL-H-8775B (Reference 0), which requires a25 micron absolute filter to be used forqualification or preproduction tests ofcomponents. Finer filtration is not to beused unless called for in the detailspecification.

B. MIL-P-19692A (WEPS) (Reference E) specifiesMIL-F-8815 filtration for the "normal"endurance test of a hydraulic pump, but nofiltration for the "overload" endurancetest. This specification also specifies abreak-in run for every pump, the effluentfluid to be checledby a filter patch test.

C. MIL-V-27162 (USAF) (Reference F) for electro-hydraulic servo-control valves limits thecontamination level in the test equipment byby particle count. (Figure 11)

No change is recommended for MIL-H-8775B, asa the filter specified should result in a

__________________________________________________

LB-31228Page 21

DOUGLAS MAICRAFT COMPANY. 04C.

contamination level comparable to that of theairplane system. No change is recommended forMIL-P-19692A (WEPS), as the test withoutfiltration is sufficiently severe.

Specification MIL-V-27162 (USAF) should berevised to specify a minimum, rather than amaximum, contamination level for the preproductionlife test. This minimum level should be aboutthe same as that now specified as maximum, whichis about Class 5 (Figure 11).

A,1p.

LB -31228Page 22

DOULU AIRCRAFT COMPANY. INC.

7.0 DISCUSSION

7.1 Maintenance Practices

The majority of maintenance personnel contactedduring the contamination survey had some awarenessof the importance of hydraulic system cleanlinessand made an attempt to keep their systems clean.They have little idea of the degree of cleanlinessrequired, as evidenced by the fact that one squadron,,informed in advance of the contamination survey,had obtained some fruit jars into which to drawoil samples. The jars were carefully wiped outwith a clean rag.

Most maintenance officers stated that they hadlittle or no hydraulic contamination trouble.Several were concerned that the hydraulic fluidin shock struts became dark and apparently moreviscous.

It appears that system cleanliness must be inherentin the system design, the design of the groundsupport equipment and the prescribed servicingpractices. Although there should be a continuouseffort to acquaint maintenance personnel with theimportance of hydraulic system cleanliness,"better maintenance" alone will do little toachieve it.

7.2 System Flushing

Apparently flushing of hydraulic systems is rarelypracticed. None of the squadrons visited everflushed a hydraulic system unless a completefailure of a hydraulic pump occurred. One 0 & Rfacility changes oil in airplanes undergoingrepair, provided an oil sample is shown to beexcessively contaminated by the SAE-ARP 598procedure. One airplane was checked during itsfirst engine run after completing this procedure(Figure 4, Sample No. 51) and found to be littlecleaner than other airplanes of this model. Thesmall particle count (5 to 15 microns) wassubstantially lower however. This system hadbeen pronounced satisfactory by the 0 & Rlaboratory. Conclusions to be drawn from thisexample are:

A. A hydraulic system should be drained immediatelyafter an engine run, so the maximum amount ofdirt-will be in suspension in the fluid.

L

LB-31228Page 23

DOUGLAS NARCRAT COMPANY. INC.

B. Very fine filtration (MIL-F-27656 for example)should be used on the filling equipment.

C. Evaluation of contamination by samples drawnfrom a static system is of questionable validity.

When it was suggested that a particularly dirtysystem be flushed, a squadron maintenance officerreplied that he did not have suitable equipmentfor the Job. Apparently that judgment was correct;however, an ordinary test stand with a finelyfiltered discharge flow is the principal requirement.(At the time of the survey, two of the threehydraulic stands in that squadron were inoperative.)

Considerable publicity has been given to flushingstands which 'recondition" hydraulic fluid inaddition to filtering it. The composite datatable, Figure 4, shows such fluid properties asviscosity and acidity remain within acceptablelimits, which indicates that such equipment isunnecessary. A separate study (Reference H)indicates that the use of such equipment isprohibitive (at least for commercial operations)in terms of airplane down-time and man-hoursexpended.

Specification MIL-H-5606 fluid is inexpensiveenough to permit it to be discarded and replacedperiodically. Such replacement should be madea part of every PAR procedure. Even if notrequired for other reasons, it would tend toreduce the accumulation of wear particles toosmall to be removed by filtration.

7.3 Filter Servicing Policy

The Handbook of Maintenance Instructions for mostmodels of aircraft calls for periodic removal andvisual inspection of filter elements at intervalsof 30 or 60 hours. The element is to be cleanedor replaced "if contaminated".

Visual inspection of filter elements is noteffective. Large, visible particles do not clogthe filter though they are indicative of troubleelsewhere le.g a failing pump). Particles whichclog a filter are not visible, and even undermagnification can hardly be detected. Also, thecleaning accomplished at squadron levels is oflittle value.I

LB-31228Page 24

DOUGLAS ARCIAFT COMPANY. INC.

A far better approach is the use of a differentialpressure indicator to show when the element isclogged. This method is incorporated in specifi-cation MIL-F-8815. Not only does this give apositive indication of a loaded filter, but alsoassures that the full capacity of the elementwill be used.

In addition, unnecessary opening of the system,with the attendant danger of introducing contamin-ation, is eliminated. As currently praeticed, itis possible that a filter element which is fullyloaded but looks clean will remain in service, andall flow will go through the by-pass relief valvein the filter housing, so that effectively thereis no filtration.

It is recommended that no attempt be made to re-clean hydraulic filter elements at squadron level.Paper elements should be discarded upon removal.Metal elements should be forwarded to a facilityequipped for ultrasonic and/or chemical cleaning,flow checking, bubble point checking, etc.

7.4 Filter Performance in a System

A filter will remove various percentages ofdifferent particles, and these percentages varywith concentration of particles, rates of flow,and as the "filter cake" acts as an additionalfilter. The contaminants generated within thesystem vary with system activity and withcontamination level (that is, a pump wears outfaster in a dirty system.). All these variablesmake it virtually impossible to design a filterwhich exactly meets the system requirements.Choosing the filters strictly on the basis ofrated flow appears to be inadequate.

It has been reported that a system which has been"supercleaned" by intensive filtration tends tostay clean. This is undoubtedly because pumpwear particles are generated at a reduced rate,which the normal filter can cope with. Indirty systems, wear particles are generated at arate faster than the filter can remove them.This suggests a balance point of contaminationmay exist, where dirtier systems tend to getdirtier, and cleaner systems tend to get cleaner.This would only occur, however, with filterelements which remove a substantial percentageof all sizes of particles. Thus, several systems

I

LB-31228Page 25

DOUGLAS AIRCAFT COPANY, INC.

with paper filters were found in the "superclean"category-class 0 or 1, whereas no metal-filteredsystem was that clean. Some paper-filtered systemswere also among the dirtiest tested. The metalfilters give more consistent results, even thoughthe average cleanliness level is not as good aswith paper.

7.5 Contamination Tests

Contamination test procedures such as SAE-ARP 598are beyond the capability of squadron maintenance,though they are employed to some extent at the0 & R base level.

It is suggested that a sampling system such asdescribed in Appendix A could be used at eithermaintenance level. Instead of microscopicexamination of the filter membrane, it can bevisually compared with standard "go-no-go"membranes. This method has been used successfullywith a missile system, where requirements were muchmore stringent.

i

LB-31228Page 26

DOIULAS AICRAFT COMPANY. INC.

8.0 CONCLUSIONS

1. Approximately one-fourth of the hydraulic systemsin Naval aircraft are contaminated beyond themaximum level recommended for reliable, trouble-free operation. (Class 5)

2. Filter elements of 25 micron absolute rating wovenwire mesh, per specification MIL-F-25682 orequivalent, are only marginally acceptable orare unsatisfactory in many applications.

3. Existing systems can be maintained in a cleanercondition by employing better test standfiltration, better filling stand filtration andperiodic system flushing. In some aircraftsystems, the use of additional filters or adifferent filter media may be justified.

'4. Cleaning and reuse of MIL-F-5504 paper filtersshould be discontinued, except as an emergencymeasure. Cleaning of metal filters at squadronlevel should be discontinued.

5. The number and sizes of filters in new systemsshould be in proportion to system complexityrather than maximum pump flow.

6. Special flushing and fluid reconditioning standsbeing contemplated will be unnecessary if useis made of the improved filter media which hasbecome available (MIL-F-8815 and MIL-P-27656).

7. No great improvement in contaminant-resistanceof hydraulic system components appears likely.No procurement specification changes arerecommended, except a minimum contaminationlevel for life-testing servo-control valves.

iI _________________________________________________

LB-31228Page 27

DOUGLAS AIRCRAFT COMPANY. INC.

9.0 REFEENCES

(A) Report ES 29862 - Evaluation of ContaminationTolerance of Servo Valves and ContaminationLevels in Hydraulic Systems, Aircraft DivisionDouglas Aircraft Company.

(B) Report ES 40348 - Servo Valves, Comparative Testof the Effect of Contamination on Performance,Aircraft Division, Douglas Aircraft Company

(C) Report P49AE-32-1 - Evaluation of 2-15 MicronFilter Element in the A4D-2N Utility HydraulicSystem, Service Test Division, Naval Air TestCenter - 6-13-61

(D) Contamination Control for Cleaner, More ReliablePumps - by J. H. Ballantoni and A. B. Billet,Aviation Age (Periodical) January 1958, Page 138

(E) Specification MIL-P-19692A (WEPS) - Pumps,Hydraulic, Variable Delivery, General Specifi-cation for - 12-1-60

(F) Specification MIL-V-27162 (USAF) - Valves ServoControl, Electro-Hydraulio, General Specificationfor - 10-6-59

(0) Specification MIL-H-8775B - Hydraulic SystemComponents, Aircraft and Missiles, GeneralSpecification for - 11-8-61

(H) Society of Automotive Engineers Technical Paper575B - Commercial Jets - Hydraulic SystemContamination versus Airline Maintenance - byR. H. Lesser - October 1962

i

LB-31228Page 28

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LB-31228Page 29

DOUGLAS AIRCRAFT COMPANY. INC.

CONTAMINATION LEVELS

SAE, ASTM, AND AIA TENTATIVE STANDARDFOR HYDRAULIC FLUIDS

Size Contamination Class

Range 02 31 4 -5 6* 7-10,(Micron4 • 3 •5•7I

2.5-5 Pending

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25-50 93 210 380 780 1,510 3,130 6,50 D1N

50-100 16 28 56 110 225 430 1,0 a

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FIGURE 2

Notesa (1) The particle size ranges used in this programwere 5-15 microns and 15-25 microns, insteadof 5-10 and 10-25. The table was used with-out change.

(2) The use of a plus sign with a class designationin this report means that four of the fivesize ranges were within the limits for thatclass, but one size range had a count whiohexceed the limit for that class, but not ofthe next higher class.

/

LB -3122 8Page 30

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LB-31228Page 47

DOUGLAS AInCRAFT COMPANY. INC.

APPENDIX A

SAMPLING PROCEDURE AND EQUIPMENT FORHYDRAULIC SYSTEM CONTAMINATION SURVEY

Particulate contamination in the hydraulic systems testedwas determined by preparing a filter membrane on-site,filtering 100 cc of hydraulic fluid. The samples wereobtained from pressurized, circulating systems, utilizingthe Dou$las 5819486 hydraulic fluid contamination tester(Figures A-1 and A-3).

The sampler was connected to the system under test, usinga parallel, series, or open circuit hookup as required(Figure A-2). With the aircraft engine(s) running andvarious portions of the system actuated (control surfaces,speed brakes, etc.) fluid,was circulated through thesampling chamber. After at least three minutes ofoperation, the system was shut down and the samplerdisconnected. The trapped 100 cc sample was then passedthrough a .08' micron filter membrane of approximately9 sq. cm. effective area. The membrane was rinsed withfiltered solvent to remove all hydraulic oil, then driedwith filtered nitrogen. The membrane, in a plasticcapsule, was then sealed, identified, and stored forlater evaluation. For about half the systems, a liquidsample was also taken in a clean can for evaluation ofviscosity, acidity, etc.

In general, airplanes belonging to regular operatingsquadrons were tested. In some cases, squadrons had beensuddenly deployed, leaving only a few airplanes assignedto the air base, or undergoing progressive aircraftrepair (PAR) etc.

Testing was carried out on the flight lines, in such amanner as to avoid interfering with regular flightoperations. This frequency involved waiting until air-planes and/or flight-line personnel were available. Wherepossible, airplanes which required an engine run formaintenance purposes were sampled, and no more than threeairplanes in a squadron were tested to avoid excessivedemands on a squadon's manpower.

The squadon maintenance personnel were cooperative andhelpful. The Douglas Customer Service Organizationrendered invaluable assistance in making arrangements withvarious squadons, shipping samples, etc. Equally helpfulwere individual representatives of the Lockheed, Grumman,Chance-Vought, and North American Companies, and localBureau of Weapons Representatives.

LB- 31228Page ~48

DOUGLAS MO~W COMPAW. INC. It UOUNDO DOMIIO it SOUNCO. CA"MO

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LB-31228Page 49

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