Improving repeatability of laboratory-based friction and wear with a modified running-in procedure

Abstract

Repeatability of friction and wear testing results is a problem often experienced with laboratory-based tests. To determine repeatability of friction and wear testing results on the SRV test rig (Schwingung, Reibung und Verschleiß), the factors that affect repeatability were investigated. Repeatability was improved with a gradual load increase procedure to the test load from the running-in load.

The test method used throughout was based on the American Society for Testing and Materials (ASTM) standard test method as described in ASTM D 6425, using the standard test specimens. This test utilizes a ball-on-disc configuration with sliding oscillating motion. Different base oils with different viscosities were used. A friction modifier and an anti-wear agent were included as additives to enable the friction and wear test to be completed at a load of 200 N. The additives were used separately in the selected test fluid formulations. The factors that affect repeatability are: • Base oil viscosity. • Base oil composition. This relates to the viscosity index and polarity of the base oil. • Additive. • Step load increase from the running-in load to the operating load. • Wear scar measurement.

The first 3 factors affect the extent to which the metal surfaces can make direct contact. The better the fluids’ ability to prevent contact, the less interactions will occur. This in turn decreases the probability of deviations and so better repeatability will be obtained. Calculation of the Stribeck curves showed that test procedure used, operates from the boundary to the elastohydrodynamic lubricating regime. Since the ball motion is oscillatory, the ball speed reached a maximum in the middle of the stroke, where the highest Stribeck parameter value was obtained.

The step load increase showed that the apparent contact area increased rapidly once the operating load was reached. This new area contains an area with unworn surface where wear occurs rapidly. This resulted in poor repeatability of the friction coefficient.

The wear scar diameter was adjusted with an image measurement correction technique. This improved repeatability of the friction and wear results for most base oils. The only exception was that the polyalphaolefin 6 (PAO)-based oils did not show any improvement. The main contribution of the correction is that it improves the accuracy of the measurement.

When the gradual load increase was implemented, the consistency of the wear profile improved. This in turn resulted in more consistent interactions between the metal surfaces which lead to improved repeatability of the friction coefficient and the extent of wear. The gradual load increase also showed a smooth transition when the load was increased compared to that of the step load increase.

The novelty of the investigation is in identifying the factors affecting repeatability and improving repeatability with a gradual load increase. The gradual load increase improves the consistency of the surface profiles generated. Contributions were also made regarding adjustment of the wear scar diameter toward the accuracy of measurements. An improved method to evaluate the repeatability of the friction coefficient was also developed. This method is based on integration of friction coefficient plots for the entire duration of the test. Furthermore, by also analysing the wear rates, better insight into the duration of the running-in process on the SRV test rig was obtained.

In this investigation, the running-in procedure was extended to 5 minutes at an operating load of 50 N. After 5 minutes the load was stepped up to the operating load of 200 N. The gradual load increase procedure had a gradient of 30 N/min, for the duration of the running-in procedure. The atmospheric conditions were controlled throughout the test with a humidity of 45 % relative to 22 °C. The block temperature was set at 50 °C and the stroke was 1 mm with a frequency of 50 Hz. Each test was repeated 5 times.