Friction Measurement of Grease in a Wheel-Rail Flange type Contact

Introduction
Greases are used on railways to reduce friction between the wheel and the rail flange in corners. This reduces wear of the components and noise. This study was designed to emulate the conditions of the wheel-rail flange contact, to quickly screen the performance of different greases.

Method
A Mini Traction Machine (MTM2) was used to allow emulation of the conditions within the wheel-rail flange contact. The MTM2 uses two independently driven specimens, a ball and a disc, to produce a contact with varying slide/roll ratios. The ball and disc are loaded together at 50 N to produce a point of contact with a pressure of 1.1 GPa. Two types of tests were used:

  • Resilience test – to screen the ability of the grease to maintain low friction in the contact over a large sliding distance, which corresponds to many wheel passes over a point on a rail.
  • High speed test – to study the variation of friction with slide/roll ratio at high rolling speeds.

 

Grease scoop Insitu (Small)

Grease Scoop in Situ, MTM2

Grease Resilience Test
A grease resilience test was designed to study the ability of the grease to maintain a low friction coefficient during long sliding distances, which corresponds to many wheel passes of the rail flange. The rolling speed was set at 0.2 m/s, with a slide/roll ratio of 2 %. The speed was set purposely low to minimise any flash temperature build up on the small specimens, which would be unrealistic compared to the real conditions. Over time the grease will be slowly squeezed from the contact. This will be indicated on the MTM2 with a rise in friction. Failure will occur once all the lubricant has been squeezed from the contact and a large jump in friction is seen which indicates high wear.

Upper Specimen                                                                           AISI 52100 super finished steel ball
Lower Specimen                                                                           AISI 52100 polished steel disc (Ra ~ 10 nm)
Rolling Speed                                                                                0.2 m/s
Slide/Roll Ratio                                                                            2 %
Temperature                                                                                 Ambient
Load                                                                                                50 N (1.1 GPa)

High Speed test

A high speed test was designed to investigate the dependence of the friction on the slide roll ratio. This test uses the MTM2, with a rolling speed of 3.8 m/s and a varying slide/roll ratio of between 1 and 10 %.

Upper Specimen                                                                            AISI 52100 super finished steel ball
Lower Specimen                                                                            AISI 52100 polished steel disc (Ra ~ 10 nm)
Rolling Speed                                                                                 3.8 m/s
Slide/Roll Ratio                                                                              1 – 10 %
Temperature                                                                                   Ambient
Load                                                                                                 50 N (1.1 GPa)

Greases

Grease Thickener Base Oil Additives/Comments
A Ca/Li stearate Ester & Veg Oil
B Ca stearate 100cSt mineral oil
C Li stearate Mineral Oil Polybutene, MoS2
D Ca/Li stearate Ester & Veg Oil
E Ca/Li stearate Veg oil
F Ca Sulphonate Ester
G Li Stearate Mineral Oil

Results
Figure 1 shows the results from the grease resilience test. Greases A, D and E, show an initial low coefficient of friction of below 0.05. This then rises sharply during the test to the maximum value of 0.21, indicating loss of the lubricant from the contact and the occurrence of wear. Greases B, C, F and G show a gradual increase of the friction coefficient during the test, but do not show a characteristic failure.

Railway grease figure 1

Figure 2 shows the variation of friction with slide/roll ratio for one grease at a rolling speed of 3.8 m/s. At SRR’s above 4 %, the friction coefficient remains constant; below this the friction coefficient decreases with decreasing SRR.

Railway grease figure 2

Discussion
The resilience test, the results of which are shown in Figure 1, shows that the greases that fail during the test contain vegetable base oils. The addition of a synthetic ester delays this failure somewhat. Greases A and D, which have a similar composition, fail at the same sliding distance.
Greases B, C, F and G maintain a low coefficient of friction throughout the test. These greases use mineral base oils and synthetic esters. An extended sliding distance test was performed for grease G. This eventually showed failure after 50 m of sliding.
The addition of solid lubricants, which are present in greases C and G, have no noticeable effect on friction in this test. Although this may help to increase the sliding distance before failure, the solid lubricants are likely to help maintain a low friction coefficient even after the base oil and thickener have been squeezed from the contact. This is done by forming low shear strength layers on the metal surfaces.

Elbow Grease: The work behind the Art, Afton Chemical

Reprinted with permission from the November 2012 issue of TLT, the official monthly magazine of the Society of Tribologists and Lubrication Engineers, an international not-for-profit technical society headquartered in Park Ridge, Illinois.