The Effects of Lubricant Additives on the Wear of Rolling Bearings Can Be Determined on Test Rigs

Norbert Geheeb and Jörg Franke

Article orginally published “Maschinenmarkt”No. 6; February 6, 2000Vogel-Verlag Würzburg

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The Effects of Lubricant Additives on the Wear of Rolling Bearings Can Be Determined on Test RigsNorbert Geheeb and Jörg Franke

The thickness of lubricant film andlubricant additives are both criticalfactors determining rolling bearing wear. Their effects havebeen demonstrated through testingon a thrust bearing test rig. Measured variables include electrical contact, temperature,slip, speed and frictional torque.Wear is determined gravimetrically.The impact of additives is due tothe effects of oxygen, sulfur andphosphorus on the steel surface.

1 IntroductionTests requisitioned by the Forschungs-vereinigung Antriebstechnik (FVA) using a FZG-test rig and a FE-8 test rig (with thrust bearings series 811 and 812)show that additives have a significanteffect on bearing life in the mixed frictionand boundary friction region.The classic rolling bearing life theory(such as that proposed by Dowson/Higginson) consider only film thicknesswhile effects of additives are disregarded.In the mixed friction region a simplificationis made since pure lubricating oil is assumed in the calculations for the

required film thickness. Figure 1 showsthe interaction of oxygen, sulfur andphosphorus with the steel surface. This element analysis, conducted as partof FVA project no. 126, clearly shows anenrichment of these elements in an approx. 20 nm “reactionary layer”; oftenreferred to as the ‘tribological layer’.

In tests conducted by INA, the bearingmanufacturer based in Herzogenaurach(Germany), it was found that rolling bearings achieve a long life if this kind oftribological layer is formed and brings about quasi elastohydrodynamic frictionconditions (EHD) in the mixed and boundary friction region.

Using these basic insights, INA has developed the lubrication function test rig(LFT) (see Figures 2 and 3).

The test rig parameters are as follows:motor speed 11 to 4,000 rpm, maximumload 100 kN, maximum oil temperature120 °C. Depending on the test condi-tions, either oil bath or recirculating oil lubrication can be used. Measurable variables include frictional torque, contactvoltage, cage speed, lubricating filmthickness, oil temperature and bearingtemperature.

2 Components on the Four Column Frame

The thrust bearing test rig has a four-column frame for the test rig components. The drive unit with the testshaft and the top test-bearing support aswell as the appropriate bearing supportsare located on the test rig motor plate.The bottom test-bearing support with thehydrostatic friction balance is mountedon a height adjustable intermediate platethat is guided by linear bearings on thecolumns of the test rig frame. Load is applied by means of a hydraulic cylinder.

To support the test bearing shaft washer,the support is equipped with a shaft journal. In the case of thrust cylindricalroller bearings, this shaft journal alsoserves to guide the cage. The shaftwasher is press-fit on the shaft journal to prevent it from rotating.

The top test-bearing support is connected to the test shaft by means ofan electrically isolating ceramic washer inorder to allow the measurement ofcontact voltage.

The bottom test-bearing support alsocontains heating elements to heat thetest-bearing and sensors for measuring

Reaction layer (20 nm)

Oil film(300nm)

Plasticallydeformed

layer(103 nm)

Hardenedmaterial(106 nm)

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lubricating film heights, cage speed andtest-bearing temperature. The test-bearing housing washer has aclearance fit in the test head recess designed for this washer. A pin is usedthat locks on the outside diameter of thehousing washer to prevent rotation.Figure 4 shows the wear of rolling elements when several kinds of oil areused.This series of tests was conducted onthrust bearings (type 81112 TN) at a

speed of 11 rpm, a load ratio C/P of 2and a temperature of 80 °C. According toEHD theory the low speed results in mixed friction conditions during the test.The results show that the types of oilused generate significant differences inthe amount of wear after 80 hours oftesting despite the fact that they may beof similar viscosity or specification.

Further INA tests performed on thrustbearings at 800 rpm showed a much lesspronounced effect of oil additives on

wear protection behavior. It may thusconcluded that the increasing height ofthe lubricating film overrides the effects ofadditives.

It is advisable to check lubricant behaviorwhen the contact elements are separatedby an EHD lubrication film since extremely active additives cover the surface even under these conditions. Inextreme case, this can lead to deviationsin the actual bearing fatigue life comparedto the conventional calculation methods.

Figure 3 This test rig can be used to determine the following: frictionaltorque, contact voltage, cage speed, lubricating film thickness,oil temperature and bearing temperature

Figure 2 This test rig allows rolling bearings to be tested at speedsranging from 11 to 4,000 rpm and loads up to 100 kN

Figure 1 The additive components oxygen, sulfur and phosphoruspenetrate the steel surface. A reaction layer forms (FVA oil no. 3 with 4% additive A99, test time: 42 hrs., bearing temperature 120 ° C.)

Bearing unit

Four-column frame

Test shaft

Test bearing support

Guide plate

Hydraulic cylinder

Bearing speed

test bearing

cage speed

Oil temperature

Lubricationgap

hydrostaticfriction scale

Bearing load(frictional torque)

Iron

Oxygen

Reaction layer

Phosphorus

Carbon

Sulfur

a Depth profile (measured using SNMS)

b Cross-section (TEM photo-graphy, magnification 160,000)

peak-to-valley height(± 50 nm)

3 Impact of Additives Subjectto Temperature Effects

It was also observed that the impact of additives or additive packages isinfluenced by temperature. For instance,test results show limited effects of addi-tives at 50 °C, poor effects at 80 °C and good effects at 120 °C. As a resultselected transmission oils must be testedat temperature ranges used in theapplication (Figure 5).The test method described above can be used for industrial transmission oils,manual transmission oils, automatictransmission fluids (ATF) and oils forcontinuously variable automatic trans-missions (CVT oils). Here it is importantthat the lubrication function test rig (in contrast to the conventional FE-8 test rig) also enables the capacitivemeasurement of contact between rollingelements and washers. In addition, theintegrated hydrostatic friction balanceallows the coefficient of friction for theappropriate fluid to be determineddirectly. Figure 6 shows an oil that weregard as acceptable, and Figure 7 anunacceptable oil.

In Figure 6 the rubbing structure is stillrecognizable on the washers after 80 testhours. By contrast, Figure 7 shows heavycontact tracks on the rolling elementsand shaft washer.In the graph in Figure 6,it can be seen that there is no metalliccontact between the rolling elements andwashers after a brief run-in phase. A separating layer (tribological layer) hasbeen formed. In Figure 7 however, metallic contact is present throughout the entire run time. The test rig also allows discontinuousmeasurements of the lubricant gap heightto be made.

4 Lubricant Behavior Can BeAssessed

It has been shown that wear measurements performed for shaft washers and rolling elements, capacitivemeasurements and frictional torque measurements allow fluids to be differentiated. The lubricant behavior ofan oil in terms of wear protection in therolling bearing can be clearly assessed.However, for the overall picture it is absolutely necessary that other

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machine elements also be considered.For instance, gray stain load capacity andthe pitting test could serve as importantcriteria for gears. It can often be determined that lubricating oils displaydiffering tendencies for differing machineelements. For this reason, it is necessaryto test the clutch in an automatic transmission or synchronizer rings in amanual transmission to analyze frictionbehavior for the lubricants selected. The results for part-specific lubricanttests normally reveal that a compromiseis the best solution.

ReferencesNN: Forschungsvereinigung Arbeitstech-nik (FVA) Nr. 126 I + II, RWTH Aachen

About the authorsDr. Norbert Geheeb is manager of theDepartment for Tribology / Chemistry atINA Wälzlager Schaeffler oHG in Herzogenaurach (Germany).Jörg Franke is employed in the same department.

Figure 4 Rolling bearing wear is dependent on the lubricant, as demonstrated by thrust bearing tests (motor speed 11 rpm,run time 80 hrs, bearing temperature 80 °C)

Figure 5 Besides the lubricants (a to e), bearing temperature also hasan effect on wear behavior

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Figure 6 If the proper lubricant is selected, no contact is present between rolling elements and shaft washers after a certain run-in time. As a result, hardly any track patterns are generated

Figure 7 If the incorrect lubricant is used, no separation of rolling elements and shaft washers occurs. This will cause the generation of heavy contact tracks.

Rolling Elements

Shaft washer

Rolling element

Shaft washer

Friction, contact and bearing temperature with respect to test time

Friction, contact and bearing temperature with respect to test time

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91072 Herzogenaurach (Germany)Phone +49/9132/82-0Fax +49/9132/82-49 50info@ina.comwww.ina.com