2。8(a), the axially ground roller has a roughness of Rq                         m, the highly   polished

roller in Figure 2。8(b) has Rq                         m and the chemically polished roller trace in

Figure 2。7 (a) Ground, (b) highly polished, (c) chemically polished, and (d) NiB coated test specimens used in this study。

Figure 2。8。 Example measured roughness profiles for (a) ground, (b) chemically polished, (c) highly polished, and (d) NiB coated rollers。

Figure 2。8(c) has Rq                        m。  Meanwhile, although it was applied to the chemically

polished (relatively smooth) specimens, nickel-boron coated specimens were found to be rather rough。  The measured trace in Figure 2。8(d) for a coated roller has Rq                   8 

m。

Lubrication types were selected to properly coincide with the steel gear applications。 The AISI 9310 specimens were tested using MIL-PRF-23699 lubricant to represent aerospace application。 For this study, this MIL lubricant was selected as the commercial blend Aeroshell555。 The AISI 5120 specimens were tested with SAE 80W90 oil that is commonly used in commercial truck differentials as well as various military ground vehicles such as armored tanks。  The viscosity of the   MIL-PRF-23699

at  100˚C  is  typically  5。25  mm2/s,  where  the  viscosity  of  the  80W90  at  the  same

condition is 14。2 mm2/s。

2。3 Test Procedures

Each of the three tests that comprised this study, traction, wear, and scuffing, all were setup in the same manner。 First, the disk was heated using an induction heater to approximately 150˚C to increase its bore diameter as to fit onto the disk shaft。 The specimens were designed to have a slight interference to prevent any relative motion between the disk and shaft。 A locknut was then tightened to secure the disk axially。 Next, the roller was placed between the forks of the loading fixture and the two precision ball bearings were placed into the applicable diameters within the lower  fork。

The roller shaft was then manually pushed through the upper fork of the loading fixture,

until interference with the inner diameter of the roller occurred。 A small arbor press was used to apply axial force onto the roller shaft to insert it through the remaining thickness of the roller and then through the ball bearings。 A lock washer and nut were threaded onto the end of the shaft to secure the inner race of the bearings。 Finally, on each side of the forks, there was a retaining component that secured the positions of the outer races of the bearings axially。

This roller assembly was then moved to the testing platform and the load cell was inserted into the end of the loading fixture。 A cylindrical pin, below the contacting surfaces of the disk and roller, at the base of the loading assembly, was inserted to form the pivot point。 The roller shaft was then connected to the drive shaft through a flexible coupling。 By tightening this coupling down, no further axial motion was allowable。 Great care was taken to ensure the same contact region throughout each test。

2。3。1 Traction Tests

The traction tests were performed first in the study。 The goal of the study was to characterize traction (friction coefficient) values for each surface finish and oil variation。 The rotational speeds of the roller ( ) and disk ( ) were both linearly varied at the same time interval to achieve a constant rolling (entraining) speed value