journal bearings and their lubrication

7/23/2019 Journal Bearings and Their Lubrication

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Journal Bearings and Their Lubrication

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Robert Scott Tags: bearing lubrication, viscosity

Journal or plain bearings consist of a shaft or journal which rotates freely in a supporting

metal sleeve or shell. There are no rolling elements in these bearings. Their design and

construction may be relatively simple, but the theory and operation of these bearings can be

complex. This article concentrates on oil- and grease-lubricated full fluid film journal

bearings but first a brief discussion of pins and bushings, dry and semilubricated journal

bearings, and tilting-pad bearings.

!ow-speed pins and bushings are a form of journal bearing in which the shaft or shell

generally does not ma"e a full rotation. The partial rotation at low speed, before typically

reversing direction, does not allow for the formation of a full fluid film and thus metal-to-

metal contact does occur within the bearing. #ins and bushings continually operate in the

boundary lubrication regime. These types of bearings are typically lubricated with an

extreme pressure $%#& grease to aid in supporting the load. Solid molybdenum disulfide

$moly& is included in the grease to enhance the load-carrying capability of the lubricant.

'any outdoor construction and mining e(uipment applications incorporate pins and

bushings. )onse(uently, shoc" loading and water and dirt contamination are often major

factors in their lubrication.

*ry journal bearings consist of a shaft rotating in a dry sleeve, usually a polymer, which

may be blended with solids such as molybdenum, graphite, #T+% or nylon. These bearings

are limited to low-load and low-surface speed applications. Semilubricated journal bearings

consist of a shaft rotating in a porous metal sleeve of sintered brone or aluminum in which

lubricating oil is contained within the pores of the porous metal. These bearings are

restricted to low loads, low-to-medium velocity and temperatures up to /) $0/+&.

Tilting-pad or pivoting-shoe bearings consist of a shaft rotating within a shell made up of

curved pads. %ach pad is able to pivot independently and align with the curvature of the

shaft. 1 diagram of a tilt-pad bearing is presented in +igure . The advantage of this designis the more accurate alignment of the supporting shell to the rotating shaft and the increase

in shaft stability which is obtained. 1n article on tilting-pad bearings appeared in the

'arch21pril 03 issue of Machinery Lubrication magaine.

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Figure 1. Kingsbury Radial

and Thrust Pad Bearing

Journal bearings are meant to include sleeve, plain, shell and babbitt bearings. The term

babbitt actually refers to the layers of softer metals $lead, tin and copper& which form the

metal contact surface of the bearing shell. These softer metals overlay a stronger steel

support shell and are needed to cushion the shell from the harder rotating shaft.

Simple shell-type journal bearings accept only radial loading, perpendicular to the shaft,

generally due to the downward weight or load of the shaft. Thrust or axial loads, along the

axis of the shaft, can also be accommodated by journal bearings designed for this purpose.

+igure shows a tilt-pad bearing capable of accepting both radial and thrust loads.

Figure 2. Layers of Journal Bearing Structure

Journal bearings operate in the boundary regime $metal-to-metal contact& only during the

startup and shutdown of the e(uipment when the rotational speed of the shaft $journal& is

insufficient to create an oil film. 4t is during startup and shutdown when almost all of the

damage to the bearing occurs.0 4nformation on plain bearing failures was discussed in an

article in the July - 1ugust 03 issue of ML magaine. 5ydrostatic lift, created by an



external pressuried oil feed, may be employed to float large, heavy journals prior to

startup $shaft rotation& to prevent this type of damage. *uring normal operation, the shaft

rotates at sufficient speed to force oil between the conforming curved surfaces of the shaft

and shell, thus creating an oil wedge and a hydrodynamic oil film. This full hydrodynamic

fluid film allows these bearings to support extremely heavy loads and operate at high

rotational speeds. Surface speeds of 67 to 07 meters8second $9, to 7,feet8minute& are common. Temperatures are often limited by the lubricant used, as the lead

and tin babbitt is capable of temperatures reaching 7/) $9/+&.

4t is important to understand that the rotating shaft is not centered in the bearing shell

during normal operation. This offset distance is referred to as the eccentricity of the bearing

and creates a uni(ue location for the minimum oil film thic"ness, as illustrated in +igure 9.



Figure 3. Shaft otion !uring Startu"

:ormally, the minimum oil film thic"ness is also the dynamic operating clearance of the

bearing. ;nowledge of the oil film thic"ness or dynamic clearances is also useful in

determining filtration and metal surface finish re(uirements. Typically, minimum oil film

thic"nesses in the load one during operation ranges from . to 9 microns, but values of7 to 67 microns are more common in midsied industrial e(uipment. The film thic"ness will

be greater in e(uipment which has a larger diameter shaft. #ersons re(uiring a more exact

value should see" information on the Sommerfeld :umber and the Reynolds :umber.

*iscussion of these calculations in greater detail is beyond the scope of this article. :ote

that these values are significantly larger than the one-micron values encountered in rolling

element bearings.



The pressures encountered in the contact area of journal bearings are significantly less than

those generated in rolling bearings. This is due to the larger contact area created by the

conforming $similar curvature& surfaces of the journal and the shell. The mean pressure in

the load one of a journal bearing is determined by the force per unit area or in this case,

the weight or load supported by the bearing divided by the approximate load area of the

bearing $the bearing diameter times the length of the bearing&. 4n most industrialapplications, these values range from <= to 0,6 "#a $ to 9 psi&. 1t these low

pressures, there is virtually no increase in the oil viscosity in the bearing contact area due to

pressure. 1utomotive reciprocating engine bearings and some severely loaded industrial

applications may have mean pressures of 0.6 to 97 '#a $9, to 7, psi&. 1t these

pressure levels, the viscosity may slightly increase. The maximum pressure encountered by

the bearing is typically about twice the mean value, to a maximum of about 6 '#a $,

psi&.

>il whirl is a phenomenon that can occur in high-speed journal bearings when the shaft

position within the shell becomes unstable and the shaft continues to change its position

during normal operation, due to the fluid forces created within the bearing. >il whirl may be

reduced by increasing the load or changing the viscosity, temperature or oil pressure in the

bearing. 1 permanent solution may involve a new bearing with different clearances or

design. >il whip occurs when the oil whirl fre(uency coincides with the system?s natural

fre(uency. The result can be a catastrophic failure.9

#il Lubrication

>ils are used in journal bearings when cooling is re(uired or contaminants or debris need to

be flushed away from the bearing. 5igh-speed journal bearings are always lubricated with

oil rather than a grease. >il is supplied to the bearing by either a pressuried oil pump

system, an oil ring or collar or a wic". @rooves in the bearing shell are used to distribute the

oil throughout the bearings? surfaces.

The viscosity grade re(uired is dependent upon bearing R#', oil temperature and load. The

bearing speed is often measured strictly by the revolutions per minute of the shaft, with no

consideration of the surface speed of the shaft, as per the AndmB values calculated for rolling

bearings. Table provides a general guideline to selecting the correct 4S> viscosity grade.

The 4S> grade number indicated is the preferred grade for speed and temperature range.

4S> <C- and -grade oils are commonly used in indoor, heated applications, with 90-

grade oils being used for high-speed $, R#'& units and some outdoor low-temperature

applications. :ote in the table that the higher the bearing speed, the lower the oil viscosityre(uired and that the higher the operating temperature of the unit, the higher the oil

viscosity that is re(uired. 4f vibration or minor shoc" loading is possible, a higher grade of

oil than the one indicated in Table should be considered.

Bearing S"eed Bearing $ #il Te%"erature &'()

&r"%) to 7 < 67 =



9 to ,7 - <C to 7 -

D,C 90 90 to 3< <C to

D9,< 90 90 3< to <C <C to

D, 90 90 90 90 to 3<

Table 1. Journal Bearing *S# +iscosity ,rade Selection

1nother method of determining the proper viscosity grade is by applying minimum and

optimum viscosity criteria to a viscosity-temperature plot. 1 generally accepted minimum

viscosity of the oil at the operating temperature for journal bearings is 9 cSt, although

some designs allow for an oil as thin as 6 or C cSt at the operating temperature. The

optimum viscosity at operating temperature is 00 to 97 cSt, for moderate-speed bearings if

no shoc" loading occurs. The optimum viscosity may be as high as =7 cSt for low-speed,

heavily loaded or shoc"-loaded journal bearings.

Esing this method re(uires some "nowledge of the oil temperature within the bearing under

operating conditions, which can be difficult to determine. +ortunately, an accurate oil

temperature is not needed for most viscosity determinations. 4t is common to determine the

temperature of the outer surface of the pipes carrying oil to and away from the bearing. The

temperature of the oil inside of the pipes will generally be higher $7 to /), to C/+&

than the outer metal surface of the pipe. The oil temperature within the bearing can be

ta"en as the average of the oil entering versus the temperature exiting the bearing.3

1 third and more complex method is to calculate the oil viscosity needed to obtain a

satisfactory oil film thic"ness. #ersons wishing to learn more about this method should see"information regarding the Sommerfeld e(uation and either eccentricity ratios or Reynolds

:umbers.3

4f the oil selected is too low in viscosity, heat will generate due to an insufficient film

thic"ness and some metal-to-metal contact will occur. 4f the oil is too high in viscosity, heat

will again be generated, but due to the internal fluid friction created within the oil. Selecting

an oil which is too high in viscosity can also increase the li"elihood of cavitation. The high-

and low-pressure ones, which are created within the oil on each side of the area of

minimum film thic"ness, can cause oil cavitation in these bearings. )avitation is a result of

expansion of dissolved air or a vapor $water or fuel& in the low-pressure one of the bearing.

The resulting bubble implodes, causing damage, as it passes through the high-pressure

portion of the bearing. 4f the implosion or collapse of the vapor bubble occurs next to the

metal surface, this can cause cavitation pitting damage to the metal. 4f the implosion of the

bubble occurs within the oil, a micro hot spot or micro-dieseling can occur, which may lead

to varnishing within the system.

Typically, a rust and oxidation $RF>& inhibited additive system is used in the oils employed

in these applications. 1ntifoam and pour point depressant additives may also be present.



1ntiwear $1G& hydraulic oils may also be used as long as the high-temperature limit of the

inc 1G component is not exceeded and excessive water is not present. RF> oils tend to

have better water separation characteristics, which is beneficial, and the 1G properties of a

hydraulic oil would be beneficial only during startup and shutdown, assuming a properly

operating bearing.

,rease Lubrication

@rease is used to lubricate journal bearings when cooling of the bearing is not a factor,

typically if the bearing operates at relatively low speeds. @rease is also beneficial if shoc"

loading occurs or if the bearing fre(uently starts and stops or reverses direction. @rease is

almost always used to lubricate pins and bushings because it provides a thic"er lubricant

than oil to support static loads and to protect against vibration and shoc"-loading that are

common in many of these applications.

!ithium soap or lithium complex thic"eners are the most common thic"eners used in

greases and are excellent for most journal bearing applications. The grade of grease used is

typically an :!@4 grade H0 with a base oil viscosity of approximately 7 to 00 cSt at

3/). @reases for low-speed, high-load, high temperatures and for pins and bushings may

use a higher viscosity base oil and be formulated with %# and solid additives. @reases for

improved water resistance may be formulated with heavier base oils, different thic"eners

and special additive formulations. @reases for better low-temperature dispensing may

incorporate a lower viscosity base oil manufactured to an :!@4 H specification. Iearings

lubricated by a centralied grease dispensing systems typically use a H, or grade of

grease.

The apparent viscosity of grease changes with shear $pressure, load and speed& that is,

greases are non-:ewtonian or thixotropic. Githin a rotating journal bearing, as the bearing

rotates faster $shear rate increases&, the apparent viscosity of the grease decreases and

approaches the viscosity of the base oil used in grease. 1t both ends of the bearing shell,

the pressure is lower and therefore the apparent viscosity remains higher. The resulting

thic"er grease at the bearing ends acts as a built-in seal to reduce the ingression of

contaminants.

,reasing Procedures

The greasing procedures for journal bearings and pins and bushings are not as well-defined

or as critical as for rolling bearings because the grease is not subjected to the churning

action created by the rolling elements. The volume of grease to inject and the fre(uency of

application are dictated more by trial and error. @enerally, most journal bearings cannot beovergreased. )aution must be ta"en when pumping grease into a bearing that is fitted with

seals, so they are not damaged or displaced by the force and volume of the incoming

grease. The harshness of the environment, shoc" loading and especially the operating

temperature will be major factors in determining the fre(uency of relubrication.

Journal bearings are generally a simpler design and not as difficult to lubricate as rolling

element bearings. The proper viscosity matched to the operating conditions and a clean and



dry lubricant will usually suffice to form a full fluid lubricating film and provide excellent

bearing life.

+or the latest information about bearing lubrication, begin a free subscription to 'achinery

!ubrication magaine.

References

. Strec"er, Gilliam. ATroubleshooting Tilting #ad Thrust Iearings.B Machinery

Lubricationmagaine, 'arch-1pril 03.

0. Strec"er, Gilliam. A+ailure 1nalysis for #lain Iearings.B Machinery

Lubricationmagaine, July-1ugust 03.

9. Ierry, James. A>il Ghirl and Ghip 4nstabilities within Journal Iearings.B Machinery

Lubrication magaine, 'ay-June 07.

3. Tribology Data Handbook . )hapter <, Journal Iearing *esign and 1nalysis.

;honsari, '. )R) #ress, ==6.

-ditors /ote0

#ortions of this article have been previously published in the Society of Tribologists and

!ubrication %ngineers $ST!%& 1lberta Section, Basic Handbook of Lubrication, Second

%dition, 09.

Tilting Pad Thrust Bearings

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5ein #. Iloch Tags: bearing lubrication

Tilting pad thrust bearings are designed to transfer high axial loads from rotating shafts

with minimum power loss, while simplifying installation and maintenance. The shaft

diameters for which the bearings are designed range from 0 mm to more than , mm.The maximum loads for the various bearing types range from .7 to 7 tons. Iearings of

larger sie and load capacity are considered nonstandard but can be made to special order.

%ach bearing consists of a series of pads supported in a carrier ring each pad is free to tilt

so that it creates a self-sustaining hydrodynamic film. The carrier ring may be in one piece

or in halves with various location arrangements.

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ulti"le #"tions

Two options exist for lubrication. The first is to fully flood the bearing housing. The second,

which is more suitable for higher speed applications, directs oil to the thrust face. This oil is

then allowed to drain freely from the bearing housing.

Similarly, two geometric options exist. The first option does not use e(ualiing or levelinglin"s $+igure &. This option is used in many gear units and other shaft systems where

perpendicularity between shaft centerline and bearing faces is assured.

Figure 1. Flooded Lubrication0

Ty"ical !oublethrust rrange%ent

Iearings for both flooded and directed lubrication are intended for machines where an

e(ualied thrust bearing is specified by 1#4 re(uirements, or where the bearing may be

re(uired for other reasons.

Flooded s. !irected Lubrication

The conventional method of lubricating tilting pad thrust bearings is to flood the housing

with oil, using an orifice on the outlet to regulate the flow and maintain pressure. 1 housing

pressure of .6 to . bar $. to 3.7 #S4& is typical, and to minimie lea"age, seal rings

are re(uired where the shaft passes through the housing.



1lthough flooded lubrication is simple, it results in high parasitic power loss due to

turbulence at high speed. Ghere mean sliding speeds in excess of 7 meters per second

$m8s& are expected, these losses may be largely eliminated by employing the system of

directed lubrication. 1s well as reducing power loss by typically 7 percent, directed

lubrication reduces the bearing temperature, and in most cases, oil flow.

Some typical double-thrust bearing arrangements using directed lubrication are shown in

+igure 0.

Figure 2. !irected Lubrication0 Ty"ical !oublethrust

rrange%ents !esigned to Preent Bul4

#il fro% (ontacting the (ollar

4t should be noted that

• *irected and flooded bearings have the same basic sies and use identical thrust

pads.

• #referred oil supply pressure for directed lubrication is .3 bar $0.9 #S4&.

• >il velocity in the supply passages should not exceed three meters per second $m8s&

to ensure full pressure at the bearing.

• The bearing housing must be "ept free of bul" oil through an ample drain area

around the collar periphery.



• :o seal rings are re(uired on the shaft.

'anufacturers offer a variety of pad materials. Some polymeric materials are capable of

operating at temperatures up to 0/) $03C/+& higher than conventional white metal or

babbitt. 1lso, pad pivot position can have an effect on thrust pad temperature.

1ll pads can be supplied with offset pivots, but center-pivoted pads are preferred for

bidirectional running, foolproof assembly and minimum stoc"s. 1t moderate speeds, the

pivot position does not affect load capacity however, where mean sliding speeds exceed 6

m8s, offset pivots can reduce bearing surface temperatures and thus increase load capacity

under running conditions.

Thrust bearings can be fitted with temperature sensors, proximity probes and load cells.

4n hydraulic thrust metering systems, a hydraulic piston is located behind each thrust pad

and is connected to a high-pressure oil supply. The pressure in the system then gives a

measure of the applied thrust load. +igure 9 shows a typical installation of this systemcomplete with control panel, which incorporates the high-pressure oil pump and system

pressure gauge calibrated to read thrust load.

Figure 3. 5ydraulic Thrust etering

rrange%ent

+or systems incorporating load cells or hydraulic pistons, it is typically necessary to increase

the overall axial thic"ness of the thrust ring.

+inally, thrust bearings incorporate hydraulic jac"ing provisions. These provisions ensurethat an appropriate oil film exists between thrust runner and bearing pads while operating

at low speeds.

1t startup, the load-carrying capacity of tilting pad thrust bearings is restricted to

approximately < percent of the maximum permissible operating load. 4f the startup load on

a bearing exceeds this figure and a larger bearing is not an option, the manufacturer can

supply thrust bearings fitted with a hydrostatic jac"ing system to allow the bearing to

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operate with heavy loads at low speeds. This system introduces oil at high pressure

$typically to 7 bar $,37 to 0,67 #S4& between the bearing surfaces to form a

hydrostatic oil film.

4t should be noted that a similar approach is ta"en when ma"ing hydraulic jac"ing

provisions for radial bearings. 1 hybrid thrust bearing is offered by ;ingsbury and )olherne)ompany $based in the Enited ;ingdom& under the name ;ing)ole.

The bearing housing re(uirements for the ;ing)ole !%@ bearing are similar to those of

standard thrust bearings. >il seals at the bac" of the carrier rings are not re(uired because

the inlet oil is confined to passages within the base ring assembly. +resh oil enters the

bearing through an annulus located at the bottom of the base ring. The discharge space

should be large enough to minimie contact between the discharged oil and the rotating

collar. The discharge oil outlet should be sied so that oil can flow freely from the bearing

cavity.

The manufacturer recommends a tangential discharge opening, e(ual in diameter to C

percent of the recommended collar thic"ness. 4f possible, the discharge outlet should be

located in the bottom of the bearing housing. 1lternatively, it should be located tangential to

the collar rotation. The bearing pads and carrier ring are constructed so that cool undiluted

inlet oil flows from the leading edge groove in the bearing pad directly into the oil film. The

cool oil in the oil film wedge insulates the white metal face from the hot oil carryover that

adheres to the rotating collar.

4n contrast to the !%@ bearing, the oil for spray-fed bearings is injected between the bearing

surfaces, not directly on them. This can result in uneven bearing lubrication and the need to

supply nonpractical high pressure to achieve true effective scouring of the hot oil carryover

adhering to the thrust collar. There is also a possibility for the small jet holes to clog with

foreign material.

+riction power loss is claimed to be lower than both flooded and spray-fed bearings due to

the reduced oil flow. The flow of cool oil over the leading edge lowers pad surface

temperatures and increases the ;ing)ole?s capacity.

The resulting performance improvements are shown in +igure 3.



Figure 6. L-, Bearings s. Standard FloodedBearings and S"rayfed Bearings

1ssuming an oil inlet temperature of 7/) $00.3/+&, it is possible to estimate the white

metal temperature of ;ing)ole leading edge bearings from +igure 7. These temperatures

are a function of surface speed and contact pressure.

Figure 7. L-, 8hite etal Te%"eratures at97$97 Position &: and ;"ad series< steel

"ads)

Bearing Selection

Thrust load, shaft R#', oil viscosity and shaft diameter through the bearing determine the

bearing sie to be selected.



!eading edge bearings are sied for normal load and speed when transient load and speed

are within 0 percent of normal conditions.

1ll curves are based on an oil viscosity of 4S> K@90, with an inlet oil temperature of 7/)

$00.3/+&. The manufacturer recommends 4S> K@90 oil viscosity for moderate- through

high-speed applications.

Table 1.

Thrust Bearing !esignation

/u%bers and Bearing rea

&King(ole ;"ad thrust bearings)

Tilting Pad Radial Bearings

The basic principles of tilting pad journal bearing operation are explained in the selection

guides and related literature of many competent manufacturers. >ne of these is Gau"esha

Iearings of Gau"esha, Gisconsin.

Sources

The @lacier 'etal )ompany in !ondon, %ngland and 'ystic, )onnecticut ;ingsbury 4nc. in

#hiladelphia, #ennsylvania and Gau"esha Iearings in Gau"esha, Gisconsin.

-ditors /ote0

This article was published in 5ein Iloch?s boo", Practical Lubrication for Industrial Facilities.

This and other lubrication-related boo"s are available through Machinery

Lubrication’sIoo"store. +or a complete listing, chec" out www.machinerylubrication.com.

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