305  

Figure 3. Schematic of Mini Traction Machine, image courtesy of PCS Instruments  

The experimental method using the MTM utilized a ball -on-disc configuration. In the MTM, the ball and disc  are driven independently which allows any combination of rolling and sliding. The measurement of  friction  force is done through a load cell that is attached to the bearing housing of the ball motor shaft. In previous  work by the author and when considering other scuffing tests, it was noted that most simulations use a load  stage progression [5,6,13] . This can be seen in the FZG  (Forschungsstelle fur Zahnrader und Getriebebau) ,  4-Ball EP  (Extreme Pressure) , SRV, OK Load Test, and Timken tests . The testing in this work is  done using  progressive speed as opposed to progressive load stages . On benefit of this approach is that  the higher  sliding speeds will allow for a more aggressive wear rate to help differentiate the efficacy of the additives .  However, the primary reason for using the progressive speed approach is the fact that in a test w here the  contact stress increases at every stage, the size of contact patch will also increase at every stage. This  leads to fresh nascent metal being exposed at every stage. This new area of contact has not yet developed  a tribofilm  when it comes into contact , so it is more likely to have aggressive wear. Therefore,  tests run in  a progressive load methodology will typically have failures at the step increase and have lower repeatability.  

The experimental testing methodology is as follows  [1,7]: The testing apparatus is assembled and filled with  oil for the experiment. The temperature of the oil is then heated to 150oC while the ball and disc are rotated  at a slow speed while not in contact. This continues for 30 minutes to allow for any chemical absorption of  the additives on the surfaces. After this, there is a 10- minute run- in period with a Hertzian contact stress of  1.25 GPa and a  Sliding to Rolling Ratio ( SRR ) of 1. Once the run- in has completed, a progressive speed  test starts with running stages for 1 min ute and rest stages for 30 seconds. The SRR is varied at each stage  in order to maintain the entrainment speed for each stage but increase the sliding speed at the contact. The  test continues until either all stages (51) are completed (maximum speed for MT M) or scuffing wear and  seizure occurs.  

Samples   The test plates for the SOT were machined from 440C stainless steel with a surface roughness of  <0.05 microns. Balls used were 12.7- mm or 7.14- mm 440C stainless steel depending on the contact stress  requirements. For the vacuum bearing tests , SKF 7204 Angular contact ball bearings made from 52100  steel were used. In the SRV testing, AISI 52100 steel balls and discs were used with a Young’s Modulus  of 210 GPa and a Poisson ratio of 0.30. The balls had a 10- mm diameter with a roughness (Ra) of 25 nm  and a Rockwell hardness of 62 HRC. The 24mm discs are vacuum arc re- melted and had a hardness of  58 HRC with a lapped surface that has a roughness (Rz) of 500 nm. All the tests were run in duplicate and  with a maximum Hertzian contact stress of 2.12 GPa and a sliding speed of 300 mm/s.  The experimental  MTM test used AISI 52100 steel balls and discs were with a Young’s Modulus of 210 GPa and a Poisson  ratio of 0.30. The balls had a 19.05- mm diameter with a roughness (Rq) of 10 nm and a Rockwell hardness