Application of disc machines for testing the shock loading capability of hypoid and industrial gear oils

V. K. Jain, V.P. Sharma* and A. Sethuramiah+

Hypoid rear axle oils are normally evaluated for performance on full-scale test benches under shock loading and high torque low-speed conditions. Shock load tests have rather poor repeatability and are expensive and time consuming. The present work describes a method to rate the relative shock load capabilities of reference oils and commercial additive systems on the Amsler disc machine. The technique effectively distinguishes the relative shock loading tendency as a function of additive concentration. The same technique is also useful in predicting the present day requirement of industrial gear oils with regard to their shock loading capability.

Keywords: oil performance testing, shock loading capability

Automotive gear lubricants which meet the specification requirements of MIL-L-2105B are expected to continue for some time at the present level of performance. The specification prescribes L-42 high-speed shock load rear axle’ and L-37 high torque low-speed rear axle tests2. Experience with the L-42 test3 has indicated the need for improvements in test repeatability and differentiation. In order to eliminate the effect of some variables, a simpler method4 known as the variable severity scoring test (VSST) was developed. Besides this, various other laboratories576 are also working on different axles. The major difficulties with these tests are the procurement of required axles and quality control of materials as large variations have been observed between batches of test gears. Secondly, these tests are not capable of differentiating the entire range of RGO reference oils.

Recently, Seitzinger’,’ reported successful differentiation of RGO oils by a modified FZG procedure, but again this test method is not capable of differentiating the entire range of RGO oils. It is, therefore, desirable to develop a simple test method in order to eliminate the many variables and overcome the difficulties. Recently, the authors’ laboratory ‘-12 had made a successful attempt to differen- tiate the entire range of RGO reference oils as a function of scuffing load under intermittent loading conditions with the Amsler disc machine. However, this method failed to match the performance of the current additives used by various oil companies at their recommended concentrations with that of RGO-110 oil. On the other hand, these additives attained the level of RGO-110 at higher concen- trations. The additives recommended to meet the RGO-105 level performance at the recommended percentage, however, matched well. Furthermore, it was suggested that the scuff load level of 40 kg”>‘* for RGO-105 oil is to be improved. In order to meet these requirements, modification of the simpler test method was undertaken. This paper describes the complete study undertaken on the modified test condition with RGO oils and commercial additives.

*Indian Institute of Petroleum, Dehradun 248005, India.

findustrial Tribology Machine Dynamics and Maintenance Engineering Centre (ITMEC), Indian Institute of Technology, New Delhi-l 10016, India

Experimental details

Equipment

An Amsler type A 135 disc described elsewhere’?” was used The lower EN 3 1 steel disc was 40 mm in diameter and the upper disc was 35 mm in diameter. Both the discs were 10 mm wide and form the test pair. The tests were conduc- ted under opposite peripheral velocities at a relative sliding velocity of 74.9 cm/s and oil temperature of 135 f 5°C. Normal load was applied by tension from a spring and intermittent loading between test discs was achieved by an eccentric wheel that rotates by means of a worm gear provided in the machine. The displacement between loading and unloading of testpieces was adjusted in such a way that the discs were loaded for one-third of a revolution of the eccentric wheel in each revolution. The arrangement results in a loading cycle of 4 revolutions per 12 revolutions of the lower disc.

Surface preparation

The test discs, hardness 62 f 1 Rc, were ground on the machine itself by emery paper and kerosene in order to achieve proper conformity at a load of 10 kg and rotational speed of 60 r/min. This procedure ensures proper surface roughness and adjustment (0.6 f 0.05 pm CLA). Each pair of discs was numbered and thoroughly cleaned in benzene.

Materials used

Table 1 shows reference oils and additives at the recommen- ded concentration to meet MIL-L-2105 and MIL-L- 2105B specifications. For making blends with commercial additives, RGO-100 was used as the base oil.

Test procedure

The discs are mounted in such a way that they can be offset between 1-3 mm as required. The lower disc was partially dipped in the lubricant bath (capacity 100 ml) maintained at a temperature of 135 f 5°C and carries the lubricant into the contact zone. The discs were first run-in at a load of 10 kg and sliding speed of 74.9 cm/s. A series of tests were conducted at different run-in times, ranging from 15 to 60 min. Also different offsets were given for obtaining

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Jain et al - shock loading capability of hypoid and industrial gear oils

Table 1 Samples used in testing

Sample Sample Properties number

1 RGO-100 oil 2 RGO-100 oil

3 Additive A

4 Additive B

5 Additive C

6 Additive D

7 Additive E

8 RGO-105 oil

9 RGO-110 oil

SAE-90 grade base oil SAE-90 base oil containing 10% additive by weight Commercial additive recom- mended at 6.5% weight to meet MI L -L -2105B, at 3.5% weight to meet M I L - L-2105 Commercial additive meeting 2105B specification at 6.5% weight, at 3.5% weight to meet 2105 specification Commercial additive meeting 2105B specification at 7% weight, at 3.5% to meet 2105 specification Commercial additive meeting 2105B specification at 7.5% weight, at 3.5% to meet 2105 specification Commercial additive meeting 2105 specification at 5% weight Reference oil meeting M I L - L - 2 1 0 5 specification Reference oil meeting M I L - L - 2 1 0 5 B specification

variation in pressure. It has been observed that the RGO- 110 oil did not fail within the load capacity of the machine with zero offset position of the disc, while the large offset resulted in lower scuffing values and less differentiation. For good differentiation between RGO-100 to RGO-110 level oils, various combinations of offsets and run-in times were tried and it was observed that a condition of 2.25 mm offset of discs and 30 rain run-in time at 10 kg load offer the best solution for the test requirements. During the course of these trials, it was observed that both high run-in load and time improved the scuffing values significantly.

At given conditions, a load of 20 kg was applied under a loading-unloading cycle. If no visible scoring of the discs was observed, a new pair of run-in discs was subjected to a 40 kg load in a similar manner. Subsequent tests were con- ducted in steps of 20 kg until failure occurred. For more accurate observation of scuffing, a 10 kg step can be used. The test duration of each loading was 1 rain. The load at which the scuffing occurs was referred to as scuffing load for characterization of lubricants.

Descr ipt ion o f f ina l test me thod

The steps in the final test method adopted are:

(1) Grind the discs properly on the machine at 60 r/rain, 10 kg load and number them.

(2) Clean the discs with benzene and allow them to air dry.

(3) Mount the discs in their respective shafts in such a way that they are offset by 2.25 mm.

(4) Heat 100 ml test oil to a temperature of 135°C and maintain within + 5°C.

(5) Run the lower disc at 200 r/min under double sliding conditions in order to obtain a sliding speed of 74.9 cm/s.

(6) Run-in the discs at 10 kg load with test oil for 30 min. (7) Adjust the machine for loading-unloading cycle as

previously described. (8) Adjust load to 20 kg. (9) Start the machine and observe whether scuffing

occurs within one minute. (10) Subsequently increase the load in steps of 20 kg/10 kg

as per requirement by employing a new pair of run-in discs during each load stage until scuffing is observed.

(11) Duplicate test results should not differ by more than a value of + 10 kg. If more, a third test is required.

Test results and discussion

Evaluat ion o f commerc ia l addit ives and reference oi ls

Reference off RGO-105 was prepared by blending RGO- 103 and RGO- I l0 oils in suitable proportion by weight. Reference oils RGO-105 and RGO-110 are expected to meet MIL-L-2105 and MIL-L-2105B specifications respectively and have shown a scuffing load of 60 and 180 kg by the proposed method. Hence, there is a clear differentiation between two specifications with very good repeatability. Reference oils above the RGO-110 level could not be evaluated within the toad capacity of the machine.

The utility of the technique was judged further by differen- tiating RGO-108 and RGO-110 oils, which is one of the general requirements for differentiation. These oils show a scuffing load of 130 and 180 kg, respectively, which can be considered a good differentiation.

Commercial additives A, B, C and D were blended into RGO-100 base oil at the recommended percentage to meet MIL-L-2105B specification (Table 1). As shown in Fig 1, additives C and D fail at 120 kg, whereas additives A and B fail at loads of 150 and 140 kg respectively, at a concentra- tion of 6.5 %. None of them, however, match the perform- ance level of RGO-110. It is interesting to observe that additives C and D tend to match the RGO-108 level oil (130 kg), while A at 150 kg and B at 140 kg proved to be higher than the RGO-108 level oils. However, the manu- facturer of these additives, observes that additives C and D

2 0 0

1 8 0

1 6 0

140 o

1 2 0

~= 1 0 0

~, 80

60

40

20

M I L - L - 2 1 0 5 level M I L - L - 2 1 0 5 B level

~ 03 O a uJ ~ (~ m t j 13

o ' ' ' o ' '

. - < < < < < t r < < <

Fig l Commercial additives and reference oils for MIL L 2105 and 2105B specification

TR I BO LOGY international 93

Jain et al - shock loading capability of hypoid and industrial gear oils

O

E

180

160

140

DBDS

x J x

120 ,-

100 -

8 0 - X / / ' 2 / / / /

6 0 - / ,"

/ /

/ /

/ /

• / /

/ /

/ / / mC~/J

Add - A v'

/

i /

f / Z

P / /

/ " 7 ' / t / /

f i / /

f

Add - B

Add - C

Add - D

Add - E

40

0 1 2 3

. . . . -Zk TCP

ZnDP I 1 I I I 4 5 6 7 8

Concentration, %

Fig 2 Scuffing level as function of additive concentration are inferior to additive B with regard to shock loading tendency so that differentiation within the 2105B level is not altogether satisfactory. On the other hand, additives A, B, C, D and E, which meet the M1L L-2105 level oils at the recommended percentage, fail at around a load of 60 kg and match the performance level of RGO-105 oil. There- fore, there is no doubt that the technique is capable of differentiating between M I L - L - 2 1 0 5 and M I L L 2105B level oils very well.

Ef fec t o f addi t ive concent ra t ion

The impor tance o f the technique was fur ther ver i f ied by studying the scuff load level at various additive concentra- tions. The re'sults are plotted in Fig 2 and it is interesting to observe that these additives differ markedly from each other with respect to their action as a function of concentration. For example, additive E matches the M1L-L-2105 level oils at 5% and its concentration effect beyond 5% is less than the other oils. At 1% concentration, the scuff load value is 20 kg, the minimum load level of this method. It is, therefore, felt that additive E has the capability to improve shock loading tendency but only up to the level of R G O - 105 oil. With increasing concentration the scuffing load of additive C appears to be unchanged beyond a level of 7.5%, at which the scuff level is 140 kg. Similarly, with additive D,

I I 9 10

the effect of additive concentration beyond 6.5% is not pronounced but below this, it is very effective.

Additives A and B, which are current versions being used in automative gear lubricants, have shown better performance than others. Additive A seems to be better than B beyond a level of 6.5% but both additives differ markedly in their action although the scuff load level at the recommended concentration, ie 6.5% is more or less the same. However, the performance of additive A matches that of R G O - 1 1 0 at a concentration of 7.8% instead of the recommended 6.5%, while additive B produces a match at 9.5%.

Study of DBDS and ZnDP

Dibenzyl disulphide (DBDS) which is known to be good as an antiseizure additive and is used in many gear lubricant formulations was evaluated at different concentrations. As expected, the scuff load improves significantly with increas- ing concentration, while ZnDP has shown little or no effect on scuffing load. Similarly, other phosphate compounds, such as TCP, show the same effect. It can be said that phos- phorous compounds do not perform well as antiscuff additives, while sulphur compounds have good antiseizure properties.

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Jain et al - shock loading capability o f hypoid and industrial gear oils

Discussion

The problem with existing rear axle tests is the differentia- tion ability of the entire range of RGO oils from R G O - 1 0 0 to R G O - 1 1 0 with a single set of test conditions. Different rear axles are required to study the range of RGO oils but, besides this, axle tests are expensive. As a result, it is difficult to know the actual level of additive performance relative to others. The main object of this work was to modify the test conditions of an earlier method H for better differentiation, while maintaining a single set of conditions. This would enable grading of the additive system on individual merit and would facilitate the selection of a proper additive system not only for automotive gear oils but for industrial gear oils also.

In the earlier method the scuff level of 40 kg with R G O - 105 was too small for differentiation amongst oils having a performance level below that of R G O - 1 0 5 . This range could be useful for determining the shock loading tendency of industrial types of gear lubricants, an important require- ment for current lubricants in many applications where shock load operations are involved. Hence, there is a need to rationalize the test method in order to grade the additive system with respect to shock loading tendency.

The proposed modified test method for evaluating hypoid gear oils under loading-unloading conditions at a sliding speed of 74.9 cm/s and temperature of 135 + 5°C differen- tiates the RGO oils from RGO-101 to R G O - 1 1 0 on the basis of scuffing load which varied from 20 to 180 kg. The R G O - 1 0 5 oil scuffed at a load of 60 kg. However, the range below this level is sufficient to determine the relative behaviour required for industrial gear lubricants. As a result, even RGO-101 which scuffed at a load of 20 kg, could be evaluated. With various commercial additives the scuffing load differs significantly with concentration. Interesting trends were observed with the plot of scuff load level against concentration. The scuffing values are not improved notice- ably beyond a particular concentration with additives C, D and E, although the scuffing values are different. Additive B, very widely used today, seems to be more active beyond a level of 3.5% and its effectiveness decreases in this test above 6.5% concentration. The scuffing load of additive A increases linearly from 1 to 7.5% concentration. Beyond this concentrat ion no other studies were carried out. Sulphur compounds like DBDS seem to be effective in improving antiscuffing properties while phosphorous com- pounds like ZnDP have little or no influence on it.

The interesting feature of this technique is its ability to differentiate the entire range of RGO oil series and com- mercial additives with one set of condit ions not available with other test benches. The analysis of results in terms of application of the test method shows that the method is useful in evaluating the hypoid gear oils matching R G O - 1 0 5 or M I L - L - 2 1 0 5 performance levels. Regarding the use of this method as a substitute for shock loading tendency test as per L ~ 2 test specified in the M I L - L - 2 1 0 5 B specifica- tion, a compromise approach is needed. This is due to the fact that the current additives A and B did not match exactly the performance level with R G O - 1 1 0 at the recom- mended dosage of 6.5%. However, they meet at concentra- tions of 7 .5 -7 .8 and 9.5% respectively. The efficiency of additives A and B at 6.5% is little higher than the R G O - 108 oil and well below the level of R G O - 1 1 0 . However, the difference between R G O - 1 0 8 and R G O - 1 1 0 is not large in the rear axle test. As such, it can be assumed that one

may get a reasonable idea about the performance of oils in the absence of the rear axle test. Based on current additive performance at 6.5% by this procedure, a scuff load of 140 kg can be considered safe, to avoid the danger of premature failure. The technique can be used as an effective simple screening technique in additive develop- ment programmes. AlsQ in the absence of rear axle tests the prescribed seizure load provides a good alternative to specify performance. Further, the technique differentiates well between RG O -101 and R G O - 1 0 5 oils which can be considered a requirement for shock loading tendency of industrial type gear lubricants.

In spite of reasonable correlation and the util i ty of the method, no clear scientific reasoning for correlation could be advanced, as the condit ions between the two methods vary so widely. Basically, the method tests the ability of the ep lubricant to react fast enough at the new fresh metal contacts being generated when a sudden load is applied, thus protecting the surface from scuffing. This is directionally at least similar to what happens in a shock load test.

Conclusions

(1) A laboratory test method has been developed on a disc machine to evaluate the shock loading tendency of gear oils.

(2) The technique is useful for evaluating hypoid gear oils and can be used as a substitute for the L - 1 9 test specified in the M I L - L - 2 1 0 5 specification. A load of 60 kg can be considered for this level. In the absence of L - 4 2 test facilities specified in the M I L - L - 2 1 0 5 B specification, the technique is useful for predicting the performance level of oils equivalent to RGO 110 oil and a minimum load of 140 kg may be considered for this level.

(4) The technique is also useful for predicting the shock loading behaviour of oils required for industrial gear oils. This may be included in the specification of industrial gear oils as a means of evaluating the shock loading tendency of industrial gear oil, which is becoming important for today 's lubricants.

(3)

Acknowledgements

The authors are grateful to Dr I.B. Gulati, Director, Indian Institute of Petroleum, for permission to publish this work.

References 1 CRCRep. No. 330, April 1958

2 CRCRep. No. 308, February 1957

3 White R.B. Automotive gear lubricant test method develop- ment, Proc. Symp., 1P, Switzerland, April 1973

Schenk A.E., Powell D.L. and Barton H.R. Development of variable-severity scoring test for hypoid gear lubricants, SAE pap. 720152, January 1972, 10-74

Morris P.R. A progress report on the development of a high torque test procedure using a modern axle system, Proc. Syrup., IP, Switzerland, April 1973

Morris P.R. Development of a high speed shock test rig for the qualification of gear oils to ministry of defence specification of CS 3000 B, Proc. Symp., 1P, Switzerland, April 1973

Seitzinger K. Limitations and possibihties of the FZG gear testing of hypoid gear oils, Schmiertechnik und Tribologie, 19(1) February 1972, 22-29

Seitzinger K. Limitation and possibilities of application of the FZG gear test rig, particularly for the testing of hypoid gear oils, Schmiertechnik und Tribologie, October 1972, 19(5), 145-148

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9 Sethuramiah A. and Jain V.K. Evaluation of rear axle EP lubricant by a disc machine, Wear, 52, 1979, 49-56

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