Lubricant Filtration for Journal Bearings1

8/10/2019 Lubricant Filtration for Journal Bearings1

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Examination of journal bearing filtration requirementsDuchowski, John K

The complex interaction between a hydrodynamic film and contaminant particles ofvarious size and hardness and the effect of these interactions on the operation and

performance of journal bearings are examined. It is shown that for the journal bearings

ranging in size from 5.1 cm (two inches) to 20.3 cm (eight inches) in diameter, a 1-18

(mu)m thick hydrodynamic film forms under standard operating conditions. Data from

several empirical studies suggest that there exists a direct relationship between the film

thickness and the most damaging particle size. That is, particles on the order of the film

thickness and larger in size are responsible for causing the most damage to journal

bearings under normal operating conditions. This relationship indicates that a re-

examination of the optimum means for contamination control for journal bearings is

necessary. It is proposed that in order to eliminate contaminant particle damage in the

5.1-20.3 cm (2-8 inch) journal bearings, filter elements rated at 6 (mu)m (beta^sub 6^ >=200) or finer should be employed. The resultant fluid contamination level should be

maintained at 16/14/12 or lower on the ISO 4406 scale.

KEY WORDS

Journal Bearings; Hydrodynamic Lubrication; Particulate Contamination; Abrasives;

Filtration

INTRODUCTION

Modern bearings can be arbitrarily divided into two classes based on the elements of their

construction. One class is comprised of either ball or roller bearings and the other is

comprised of sleeve or journal bearings. Neither type of bearing is completely

frictionless; however, both are highly efficient in reducing friction. Although sleeve or

journal-type bearings are simpler than rolling element bearings in construction, they can


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be equally complex in theory and operation. Consequently, a considerable amount of

attention and research has been devoted to understanding the critical operating

parameters of journal bearings (1)-(7). Nevertheless, there is only a limited understanding

of the complex relationship between the hydrodynamic film thickness, contaminant

particle size and hardness and their effects on bearing operation. The lack of a clear

understanding of the relationship between these quantities has made it difficult toformulate clearly defined fluid cleanliness requirements. Therefore, even though the

literature often provides extensive details in regards to film thickness, minimum

clearance and other critical operating parameters, it often leaves the question of suitable

filtration open to the interpretation of the designer or operator (1)-(7). The drawback of

this approach is that the choice of a suitable filtration system is often guided by the initial

capital costs rather than by technical requirements and long-term economic

considerations. In particular, it is not certain that the costs associated with the repair,

replacement or maintenance of the failed equipment will outweigh the costs of the

installation of suitable contamination control equipment. This is due to the fact that the

costs associated with the interruption of production, increased inventory, customer

confidence, etc., are often not considered. Therefore, the intention of this paper is to re-evaluate the issues of fluid cleanliness and filtration requirements for journal bearings

and to place them on a firm technical footing. This work presents results of theoretical

calculations of hydrodynamic film thicknesses which form in journal bearings under

normal operating conditions and compares them to film thicknesses achieved in several

experimental studies. The paper also reviews typical film thicknesses expected to form in

journal bearings based on "rule of thumb" calculations presented in original equipment

manufacturer (OEM) literature. Upon examination of theoretical and experimental data

presented in this paper as well as in the bearing OEM literature, it is proposed that

surface damage in journal bearings ranging in size from 5.1 to 20.3 cm (2 to 8 in) caused

by contaminant particles can be reduced most effectively by the employment of filter

elements rated at 6 (mu)m or finer (a filtration ratio beta^sub 6^ >= 200; filter rating

based on filtration ratio is explained later in this paper). Such filters should be capable of

achieving a fluid cleanliness level of 16/14/12 or better on the ISO 4406 scale (8).

FILM THICKNESS CONSIDERATIONS

Filtration requirements for rolling-element bearings have been based on the size of the

minimum thickness of the lubricant film between the operating components. Typically,

these types of bearings exhibit a minimum film thickness on the order of 0.1 to 1(mu)m;

hence, as expected, a substantial reduction in bearing wear and fatigue and, consequently,

a significant gain in bearing service life have been observed when clearance protectionfiltration of 1 to 3 (mu)m (filtration ratio of beta^sub ^1 to beta^sub 3^ >= 200,

respectively) has been applied. In sleeve or journal bearings, the shaft supported by the

bearing is called the journal and the outer portion the sleeve.


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If the journal and sleeve were both made of steel, the bearing surfaces, even if well

lubricated, abrade small pieces of metal from each other. The sleeves of most bearings

are therefore lined with brass, bronze, or babbitt metal. The softer metal in the liner is

more malleable and functions as a cushion against the harder shaft material. Sleeve

hearings are generally pressure-lubricated through a hole in the journal or from the

housing that contains the bearing. The sleeve is often grooved to distribute the oil evenlyover the bearing surface.

In fixed-pad journal bearings, typical machined clearances between the diameters of the

journal and sleeve are on the order of 25 to 50 (mu)m (0.001 to 0.002 in) for every 2.54

cm (1 in) of journal diameter (1), (2), (6). Under certain operating conditions, the

clearance can decrease to as little as 2.5 (mu)m (0.0001 in) on the sleeve side exposed to

the greatest load. In pivoted-pad journal bearings, these clearances may even be smaller

for bearings of otherwise same dimensions. During normal operation the journal is

supported on an extremely thin film of oil, and the two parts should have no actual

contact. As the rotational speed increases, other variables remaining constant, the

lubricant is drawn into the bearing by the rotating action of the journal. Consequently, theoil film becomes thicker and the increase in friction is therefore less than directly

proportional to the speed. Conversely, at lower speeds, the oil film is thinner if other

factors are unchanged. At extremely low speeds the film can rupture and the two parts of

the bearing come into contact. Therefore, friction and wear are highest during startup, and

there is a likelihood that the bearing may fail or be permanently damaged if subjected to

too large of a stress at that time. Equipment such as compressors, which normally operate

with a large number of intermittent duty cycles is therefore exposed to higher levels of

stress than equipment which normally operates under steady-state conditions (large

turbines). A diagram of a journal bearing operating under hydrodynamic conditions is

shown in Fig. 1 (3).

The same procedure can be carried out for a similar 20.3 cm (8 inch) journal bearing

employed in an actual induced draft fan application (9). The bearing in question is

subjected to a load of 17,800 N (4000 lbf) load, operates at 1,800 rpm and is lubricated

with an ISO VG68 oil at 38 deg C (100 deg F) which, at this temperature, would be

expected to exhibit an absolute viscosity (mu) of 7.6 Pa-sec and a kinematic viscosity v

of 8.5 m^sup 2^/sec. Under these conditions, the above bearing exhibits a Sommerfeld

number of 0.05 and a Reynolds number of 32.3. For these S and Re values, the charts

quoted in the literature (1), (2) give a film value of about 24 (mu)m whereas the

aforementioned computer calculation (7) yields a minimum film thickness value of 13

(mu)m.

The above examples show that, even though the dimensional clearance between the pads

and the journal may range from 51 to 203 (mu)m (0.002 to 0.008 inch) for the bearings in

question, the actual film thickness which separates the journal from the pads under fairly

typical operating conditions is on the order of about 10 (mu)m. In addition, the film


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thickness is also dependent on fluid viscosity which, for liquids, varies inversely with

temperature. Under normal operating conditions, the temperature of the bearing and the

lubricant may rise by as much as 10 deg to 15 deg C which can lead to as much as a 20%

reduction in the film thickness for some lubricants (4). Given the above considerations, it

could be expected that the particles which are of roughly the same size as or larger than

the thickness of the film can bridge the gap between the operating components of thebearing, and through this contact, cause abrasive wear of the opposing surfaces.

Moreover, it could be expected that smaller-sized particles will play a more significant

role in this process as the oil film thickness decreases due to changes in viscosity

associated with the rise in temperature.

MOST DAMAGING PARTICLE SIZE

The process of abrasive wear, which occurs when lubricating fluid containing hard

particles comes in contact with operating components, is essentially the same for a

variety of material and equipment designs. The particles first enter the component

through a clearance space or through lubricant supply holes or grooves and are then

carried into the contact area where the film thickness is at a minimum. The particles are

then pressed into the surface material, where they operate as miniature cutting tools,

machining sharpedged grooves into the surface. Subsequently, the sharp-edged grooves

in the first surface machine the opposing surface to complete the cycle (5). An excellent

illustration of this process is the use of sand paper, where the grit between the gap

separating the paper backing from the workpiece is able to cut (abrade) away material

from the work piece surface, because it is hard and abuts against the backing. One way to

interrupt the wear cycle is to make one of the surfaces out of a softer material to allow the

particles to be buried in the softer surface by the pressure exerted on them by the harder

surface. This approach is limited by the fact that only a certain number of solid particlescan be so embedded before the bearing layer is completely covered (5).

The fact that particles on the order of the typical film thickness, or somewhat larger in

size, were responsible for causing the most harm in the operation of roller or ball element

bearings has been well established in the literature (1)-(6) and observed in practice.

Consequently, filtration criteria for these types of bearings are based on the minimum

film thickness which forms under standard operating conditions. In contrast, the filtration

requirements for journal bearings have received a somewhat less extensive scientific

treatment. Nevertheless, a review of technical literature which addresses the topic of the

most damaging particle size in journal bearing operation (10)-(16) reveals that attempts

were made to arrive at a more detailed understanding of this issue as far back as 1927.The main features of some of the more notable efforts in this area are listed in Table 1.

For a more detailed description of these experiments, the reader is referred to the

referenced articles themselves. The main drawback of some of the earlier studies was that

the contaminant particles were either too large to enter into the critical zone of the

bearing, much smaller than the minimum film thickness, or that the particle size


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distribution was too wide to provide direct evidence as to which particles were

responsible for causing the most bearing wear (10)-(12). Nevertheless, the investigators

did agree on a general point that if the particles were small enough to enter the bearing

contact zone and were just slightly greater than the minimum film thickness present in

that zone, they were responsible for wear of the journal and that the rate of wear

increased linearly with the number of particles.

The investigation of the effect of hardness ratios between the journal, sleeve and the

contaminant particles on hearing wear rates followed as a natural extension or the earlier

studies described above (13)-(IS). Results of these investigations revealed that, in

contrast to dry abrasion, both the particle size distribution and the film thickness are

important when analyzing wear processes under fluid lubrication conditions. Among the

observed effects were embedding of abrasives in softer sleeve surface, cutting wear,

formation of ridges and deformation of the sleeve surface. It was also found that abrasion

initiated additional wear processes such as adhesion which, in some cases, resulted in

catastrophic bearing failure. Overall, the types of wear processes occurring within the

bearing were found to be strongly dependent on the bearing materials. In particular, theharder materials, i.e., steel, were more susceptible to pitting which subsequently led to the

production of steel flakes, whereas the softer materials were found to undergo larger

plastic deformations as described above. In this regard, it is worth noting that some

bearing manufacturers require replacement of the bearing surface when these

deformations result in a deviation equal to, or greater than, 130 pm (0.zn (Q.QOS in)

from the original profile of the surface (6).

A more direct relationship between the hydrodynamic film thickness, particle size and

bearing wear was also delineated by several research groups (14)-(16). The results of

these studies showed that the rate of bearing wear increases rapidly as the size of the

particle begins to exceed the thickness of the hydrodynamic film. In particular, the wear

rates observed with contaminated oil were about a factor of 20 higher than those observed

with clean oil. For example, Broeder and Heijnekamp (14) as well as Ronen et al. (15)

concluded that wear in hydrodynamic bearings results from the abrasive action of

particles on the surfaces of both the journal and sleeve, and that this process is most

pronounced when the particles are larger than the protective layer of hydro dynamic film.

The dramatic increase in wear rate for the bearing liner as the particle size begins to

exceed the film thickness as measured by Broeder and Heijnekamp (14) is shown in Fig.

3.

Wear rate measurements which compare bearing wear with clean and contaminated oilobtained by Ronen et al. (14) are also shown in Fig. 4. These tests were conducted over a

20-hour period with interruptions for wear measurements at five-hour intervals.

A detailed study carried out by Watanabe, et al. provides the most illustrative evidence of

a direct relationship between the film thickness, particle size and the bearing wear rate


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(16). In particular, these authors found that the bearing wear rate depended heavily on the

ratio of the minimum film thickness and the particle size. The maximum wear rate was

observed when the ratio of the minimum film thickness and the particle size approached

unity and decreased rapidly as the ratio diverged from unity. The data obtained by

Watanabe et al. (16) is displayed in Fig. 5.

A summary of experimental conditions and results described in all of the above studies

(10)-(16) is provided in Table 1. Overall, the data obtained from these experiments

provide conclusive evidence for a clear and direct relationship between the film

thickness, particle size and bearing wear rate.

MANUFACTURERS' RECOMMENDATIONS

However, even though the bearing manufacturers' literature devotes a considerable

amount of attention to the film thickness, minimum clearance and other critical operatingparameters, the issues of the most damaging particle size and/or suitable fluid cleanliness

level and, consequently, filtration requirements are often not addressed at all or are given

only a cursory treatment at best. This is a somewhat surprising state of affairs, especially

in view of the ever-increasing precision of the operating components and of increasingly

tighter limits of operation. Consequently, some manufacturers have begun to provide at

least some guidelines for filtration in order to address contamination control in their

equipment under typical operating conditions (9). These recommendations are derived

from a thorough technical knowledge of the operating components and consequently

represent a more realistic reflection of contamination control requirements under typical

operating conditions for the equipment in question. In this regard, there seems to be some

agreement among certain manufacturers that a level of filtration suitable to prevent the

entryof particles greater in size than the minimum film thickness should be employed to

provide optimal protection to the operating components (9), (16), (17). However, even

when this criterion is met, erosion of the bearing surface, which may impede the

operation of the bearing, can occur. For example, Ronen et. al (IS) showed that wear can

occur even with clean oil supplied to the bearing, though this process is operative only

under extreme conditions when the layer of hydrodynamic film is too thin to cover the

irregularities of the bearing surface. Under those conditions, the contact between the

opposing surfaces results in adhesive wear which may remove some material from either

one of the interacting surfaces. Ronen et al. (IS) also showed that this process is

accelerated when foreign abrasive material is introduced into the bearing with thelubricating oil It should be emphasized that removal or displacement of material from the

bearing surfaces can significantly reduce the bearing service life because, as was pointed

out earlier, some manufacturers recommend bearing replacement or resurfacing when the

surface deformations exceed a limit of 130 Pm (0.005 in) from the original profile (6).


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USER CONSIDERATIONS

The lack of strict bearing manufacturer guidelines with respect to filtration requirements

presents the equipment designer of operator with a difficult decision when engineeringfiltration requirements to provide a means of contamination control commensurate with

reliable operation of the application. More often than not, the principal guiding factors

are the initial capital costs of installing a high-efficiency filtration system rather than the

actual technical requirements and long-term economic considerations. However, it is

precisely those economic considerations which continually place the hardware

components under increasingly demanding operating conditions. Consequently, in the

short term, the cheapest approach may not always be the most cost effective or

technically best-engineered solution. In practice, field experience has often proven that

the initial cost judgment based on the cheapest purchase price can translate into higher

costs associated with repairs, maintenance and replacement of failed equipment in

addition to the loss of productivity. Results of both theoretical and experimental studies

indicate that the most critical portion of the duty cycle of journal bearings occurs when

the bearings operate under low rpm because that is when the hydrodynamic layer of

lubricating oil is at its thinnest. Consequently, equipment which operates in this regime

for a significant portion of its service life would he at the highest risk to suffer

contamination-related damage. It is therefore strongly suggested that the issues of film

thickness, most damaging particle size and contamination control be given serious

consideration, especially for equipment operated intermittently because it is required to

operate under these most critical conditions which occur during the numerous

startup/shutdown cycles.

FILTRATION REQUIREMENTS

The theoretical and experimental considerations presented in this paper indicate that in

journal bearings with diameters ranging from 5.1 to 20.3 cm (2 to 8 inch), a 1-18 (mu)m

thick hydrodynamic film forms under standard operating conditions. In addition,

significantly thinner films could be expected to form under startup conditions, i.e., when

the journal rotates at low rpm. The experimental evidence presented in this paper showsthat particles which are on the order of the hydrodynamic film in size, or larger, are

responsible for causing the most damage to the operating components of the bearing.

Consequently, suitable contamination control methods which prevent the entry of these

particles into the bearing should be employed. Given the film thicknesses which exist

under standard (steady-state) operating conditions, the filter element appropriate for the


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application should exhibit a high removal efficiency for particles on the order of about 10

pin in size. In addition, the selected filter element should also exhibit a significant

removal efficiency for particles down to about 3 (mu)m in size to allow for thinner film

which exists under startup conditions. A document which provides a standard reference

for rating the efficiency of filter elements for various particle sizes has been published by

the International Standards Organization as ISO Standard 4572 (20). This multipassfiltration performance test describes the performance of filter elements in terms of a so-

called beta ratio, which is the ratio obtained by dividing the number of particles larger

than a given size found in the fluid upstream and downstream of the filter element,

respectively. It is customary to report the beta ratings in the following manner: beta^sub

chi^ >=Y where x represents the particle size and Y represents the ratio of particle counts

upstream and downstream of the filter element for that particle size. For a beta rating of

200 at 6 pm particle size (beta^sub 6^ >= 200), for every 200 particles 6(mu)in size and

larger upstream of the filter, only one particle in that size range would be found

downstream of the filter. This concept is illustrated in Fig. 6 for a filter element rated at

beta^sub 3^ >= 200. Consequently, the higher the beta ratio, the more efficient the filter,

the higher the rate of reduction of particulate contamination and the cleaner the fluiddownstream of the filter.

The relationship between the beta ratio and the rate of reduction of particulate

contamination is shown in Fig. 7.

In addition, the ISO 4572 document provides methodology for rating filter elements for a

range of particle sizes. Thus, to fully describe the effectiveness of a given filter element

in controlling particulate contamination, beta ratios at several particle size ranges should

be provided. Figure 8 presents several examples of beta ratio versus particle size curves

plotted for a number of filter media grades of a given manufacturer.

These curves can he used to derive beta ratios and filtration efficiencies at several particle

sizes. For example, the 6 (mu)m medium rated at beta^sub 6^ >= 200 exhibits beta ratios

of 10, 200 and 1000 for particle sizes of 3, 6 and 9 pun, respectively. Given the film

thicknesses under consideration in journal bearing applications described herein, it is

recommended that the filter element be rated at no less than beta^sub 6^ >= 200, which

translates into a particle removal efficiency of 99.5% for particles 6(mu)m in size and

larger. This recommendation is based on the fact that the above filter element exhibits a

relatively high removal efficiency for particles down to about 3 Fun in size (beta^sub 3^

>= 10 or 90%) and provides protection against smaller particles which become of

significant concern at lower film thicknesses.


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CONCLUSION

The results of the present examination show that, under typical operating conditions, the

5.1 to 20.3 cm (2 to 8 in) journal bearings considered in this article exhibit filmthicknesses on the order of about 1 to 18 (mu)m or less. In addition, a much smaller film

thickness e.g., 2 gum or less, would be expected under startup conditions or at a lower

shaft rotating speed. A IQ-15% decrease in film thickness would also be expected as a

result of the bearing temperature rise in the course of operation. Therefore, it is realistic

to allow for an average film thickness of about 10 (mu)n for this class of bearings.

The results of the experimental studies described in the present paper indicate that all

hard particles which are either on the order of or larger than the thickness of the hydro

dynamic film and which can enter the clearance between the journal and the sleeve can

cause wear damage to the surface of the bearing. In addition, results of some studies

indicate that, although other aspects such as differences in material hardness between thebearing, the shaft and the contaminant also play important roles, the interaction between

these three bodies reaches a maximum when the particle is on the order of the film

thickness in size. Consequently, the maximum damage to the hearing occurs when the

ratio of the particle diameter to the film thickness approaches unity.

Therefore, in view of the above considerations, it is strongly recommended that

contamination control measures to prevent the entry of hard particles, which are on the

order of or larger than the thickness of the hydrodynamic film, be employed in systems

equipped with journal bearings similar to those considered in the present article.

Specifically, it is recommended that, on equipment which employs this type of journal

bearing, filters rated at 6 (mu)m (beta^sub 6^ >= 200 or finer) be used to control

particulate contamination. The use of such filters should result in maintaining fluid

cleanliness levels at 16/14/12 or better on the ISO 4406 scale. It is also recommended that

the system be flushed to a cleaner standard prior to being put into operation under normal

load conditions to offer maximum protection to the system operating components.

Lubricant Filtration for Journal Bearings1

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