A four ball tester is typically used to run either Wear Preventive (WP) or Extreme Pressure (EP) tests. EP tests are run over a test time of 10 to 60 s, a much shorter test duration compared to WP test protocols. The shorter test duration makes the acceleration rate quite important for EP tests. There are several standards regulating EP tests. A summary is given in Table 1.
Table 1. Summary of the test parameters from the EP test protocols.
A weld load > 400 kg is an important requirement in the HPM-HL grease classification for High Load characteristics. Standard EP test protocols only prescribe an average speed for the rotating upper ball; however, the top ball acceleration, or the ramp-up time (the delay in time taken to reach the mean speed), is not specified. Can this lead to fictitious weld loads for the same HPM-HL grease ? This article describes the effect that the top ball delay in reaching the mean speed has on the Extreme Pressure behavior of greases.
Materials and Methods
A commercial high weld load grease was selected for this work. Its main characteristics are summarized in Table 2. The chemical composition of the grease was not known, as the scope of this work was to evaluate the EP response to different ramp-up times, irrespective of the composition or additive package.
Table 2. Characteristics of the grease used in this study.
Speed and Ramp-Up Time Control
The tests were conducted with a tabletop Four Ball Tester (FBT-3), the latest evolution of the traditional lubricant tester. Ducom FBT-3 features a variable speed direct drive motor with high precision position encoders to monitor the angular position of the motor’s spindle. The position encoders are in a closed loop with the variable frequency drives that ensures accurate control of the speed ramp-up time and average speed, regardless of the applied normal load or the starting motor torque. This system is so robust that the belt-less motor can deliver its maximum torque (corresponding to a maximum load of 10.000 N) even at zero speed.
Figure 1. Ducom Four Ball Tester (FBT-3) with Image Acquisition System.
Three values of ramp-up time were selected : 0.15, 0.25 and 0.95 seconds delay in reaching the target speed of 1770 rpm (Figure 2). The three values were chosen considering the ramp up times and technology available in most commercial four ball testers.
Changing the speed ramp-up time from 0.15 to 0.25 sec, an increase in weld load from 800 kg to 1000 kg was observed. With a ramp-up time of 0.95 sec, the grease did not weld even at a load of 10.000 N (Figure 3 ).
Figure 3. Weld loads of the same grease at different ramp-up times.
The evolution of wear with increasing loads in a load-wear index sequence was analyzed. Wear on the test balls appear to nearly unaffected for loads up to 315 kg. Above 400 kg, there was significant influence with a lower wear scar for a slower ramp up-time of 0.95 secs compared to 0.15 secs (Figure 4).
Figure 4. Evolution of mean wear scar diameter (MWSD) with test load.
Load-Wear Index (LWI) was calculated from MWSD, Mean Hertz diameter and corrected load. LWI was lower (133 kg) for a faster aceleration of 0.15 sec compared to a slower acceleration of 0.95 secs which gave a LWI of 201 kg, a fictitious 50% increase in LWI for the same grease. This shift in LWI (Figure 5) can be rationalized given the higher weld loads required for slower acceleration rates.
A possible explanation for this phenomenon could be related to the flash temperature at the contact. Welding occurs when the flash temperature reaches the melting point of steel (around 1400 °C) and therefore the difference in the weld loads are the results of different loads being required to reach the melting temperature. We hypothesized that a faster acceleration (0.15 secs) leads to a reduced heat dissipation (Figure 6) away from the contact, resulting in a higher flash temperature and a lower weld load. On the other hand, when slower acceleration (0.95 secs) was imposed, the greater heat dissipation led to a lower flash temperature at the contact, requiring a higher normal load to achieve the welding of the test balls.
Figure 6. Illustration explaining the effect of ramp-up speed in heat dissipation and weld load.
Conclusion & Recommendation
This work demonstrated the importance of speed ramp up in extreme pressure test. A motor ramp up delay of 1/10th of a second can lead to a 25% higher weld load and load-wear index, indicating a fictitious performance for the same grease. This is prominent for HPM-HL greases with a weld load > 400 kg. So, how should manufacturer's report grease data ? We strongly recommend that the weld load and LWI data be accompanied with the “speed ramp up time” in the grease specification sheet. (i.e., 800 Kg weld load (t < 0.15 s or t > 0.15 s). As an instrument manufacturer, Ducom is working with ASTM D02 committee to set up an ILS study to investigate the role of “speed ramp up time”.
Note - This article summarizes the study described in a paper published in the peer-reviewed journal Lubricants. The paper can be downloaded through the link below (a valid email address is required).
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