proper lubrication is essential to successful performance o

8/10/2019 Proper Lubrication is Essential to Successful Performance o

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inimum film thickness (based on "owson quation) :

hmin - " (o)0.1a0.%23&0.'450.24

where :hmin - minimum lubricant film thickness" - constant containing moduli of elasticity

o - lubricant *iscosity at atmospheric pressure - relati*e surface *elocity

a - lubricant pressure *iscosity coefficient3 - load per unit length5 - equi*alent radius

+*erage film thickness (based on 6rubin quation) :

h - 0.047 (a)0.189 (P/ l)&0.07' ( '/ 5)0.4;2

where :h - lubricant film thickness (mm) - *iscosity of lubricant - surface *elocitya - lubricant pressure *iscosity coefficientP - load between inner race and rollersl - effecti*e length contact between rollers and inner race '/ 5 - sum of in*erses of contact radii

The ma#or factors influencing the lubricant film thickness are *iscosity and speed whereas load has less importance. These thin!" films are often not much larger than the surface roughness height.

The fatigue life of a bearing is related in a comple< way to speed, load, lubrication, temperature, setting and alignment. Thelubricant=s role in this interaction is determined primarily by speed, *iscosity and temperature> and the effects of these factors onbearing fatigue life can be dramatic. $or e<ample, in a test program, table %&+, two bearing test groups were sub#ected toconditions of constant speed and load. "iffering film thicknesses were achie*ed by *arying operating temperature and oil grade,and thereby, oil operating *iscosity. ife was dramatically reduced at higher temperatures, with lower *iscosity, and thinnerresultant films.

?n another in*estigation, table %&@, *iscosity and load were held constant, but speed was *aried producing results similar to thosein table %&+ & higher speeds produce thicker films and longer li*es.

Test6roup Temperature isc. A Test Temp.mm8 / sec (ct) !" $ilm (hmin) ife B

CD C$ m in

+ & ' '4% 81% 8.0 0.049 '.% '4 & '7

+ & 8 ;; '%0 '7.2 0.8;2 '0.2 '00

Table %&+5elati*e bearing fatigue life *s !" film thickness(*ariable speed & constant temperature)

Test6roup

peedre*/min

!" $ilm (hmin) ife B

m in

@ & ' 4;00 0.'08 2.0 '00

@ & 8 ;00 0.089 '.' 20

Table %&@5elati*e bearing fatigue life *s !" film thickness(ariable speed & constant temperature)

Film thickness at rib/roller end contact

The contact between the large end of the roller and the inner race rib is described as an elastohydrodynamic contact or ahydrodynamic contact (elastic deformations are negligible). +s the roller/rib loads are much lower than the roller/race loads, thefilm thickness at the rib/roller end contact is usually larger than the film thickness on the roller/race contact (appro<imately 8



times more). Ee*ertheless, in se*ere conditions, scoring and/or welding of the rib/roller asperities can occur. This may be relatedto speed, oil *iscosity, load or inadequate oil supply to the rib/roller end contact. ?n these conditions, the use of <treme Pressure(P) lubricant additi*es may help pre*ent scoring damage.

proper lubrication is essential to successful performance o

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