*2.1. Greases*

Two commercial high weld load greases that can offer protection against wear, scuffing and pitting in gear drives were used in this study. These greases are labelled as grease X and grease Y. Both the greases were NLGI grade 00, they had same density of 0.92 g/cc at 20 ◦C, kinematic viscosity of 500 cSt at 100 ◦C, flash point greater than 200 ◦C, thermal stability of tribofilms was equal to 120 ◦C and FZG scuffing load stage was equal to or better than 12. There was no information about composition of extreme pressure additives used in these greases. And it is not critical for this study because we are focused on consequences of the test method on grease lubrication behavior. Moreover, we are not investigating lubrication mechanism based on the composition of greases.

#### *2.2. Controlling the Speed Ramp Up Time and Load in a Four-Ball Tester*

Computer controlled and automated four ball tester (Model–FBT3) from Ducom InstrumentsTM (Groningen, The Netherlands) was used in this study (see Figure 1). A variable speed direct drive motor without any belt or pulley arrangements was used to control the speed between 300 rpm to 3000 rpm. Speed ramp up time, that is time delay in motor speed to reach 1770 rpm starting from 0 rpm was controlled using the position encoders. Position encoders can precisely identify the angular position of the spindle in the motor. And they were in closed loop with the variable frequency drive system that controlled the flow of current to the motor, and the motor speed. Variable frequency drive ensures that the speed ramp up time is not affected by starting motor torque that is crucial for ASTM D2596. The direct drive motor without any gear box was compatible with peak load of 10,000 N, to sustain maximum torque at zero speed. Safety controls were used to prevent the overflow of current to the motor at the peak torque operating conditions. The labview based WinDucom software was used to set the desired speed ramp up time for each test. In this study we chose speed ramp up time 0.15 s, 0.25 s and 0.95 s, that is the time delay for motor to reach a preset mean speed of 1770 rpm (see Figure 2A). The above time intervals were chosen considering the motor capabilities and technology used in commercial four ball testers.

The data acquisition and display system in WinDucom software allowed the user to view and store the real time changes in speed profiles.

Ducom four ball tester is equipped with an automated pneumatic loading system that can control the actual load between 100 N to 10,000 N. The standard error at 10,000 N was ±20 N or 0.2%. The test balls were preloaded to a desired load at zero rpm and the load was maintained stable during the spindle rotation for the entire test duration of 10 s and at all the different speed ramp up time (see Figure 2B). The data acquisition and display system in WinDucom software allowed the user to view and store the real time changes in load profiles.

#### *2.3. Pass Load and Weld Load*

According to ASTM D2596, the grease is packed into the ball pot with three stationary steel balls (supplied by SKF, E-52100, with diameter of 12.7 mm, Grade 25 extra polish, hardness 65 to 66 HRC) at a temperature of 27 ± 8 ◦C, the top steel ball connected to the motor is brought in contact with the bottom three steel balls at a fixed load. The top steel ball rotates at a mean speed of 1770 ± 60 rpm for a test duration of 10 s. If there was no welding of the test balls, the load is increased to the next load step, using a look up chart for load steps given in the ASTM D2596.

The weld load is the load step at which the test balls local temperature reached the melting point of steel, that fused the four balls. At this point the friction torque sensor in the four-ball tester exceeds the safety value and shuts down the motor. This represents the failure by grease lubricants to prevent seizure. The load step prior to the weld load is the pass load. The pass load represents the state of the grease lubricant after incipient seizure and before the full seizure. The pass load and weld load for grease X and Y was measured at a speed ramp up time of 0.15 s, 0.25 s, and 0.95 s. There were new steel balls used for each test.

The cleaning procedures in this study followed the ASTM D2596.

**Figure 2.** Measurement and controls for speed ramp up time and normal load in a Ducom four-ball tester. Real time changes in the speed profiles (**A**) and load profiles (**B**) at ramp up time of 0.15 s, 0.25 s and 0.95 s. Note: Ramp up time is the time taken to reach the average speed of 1770 rpm (average speed follows ASTM D2596-15, see the figure inset) and the motor was designed for full torque or load at zero speed.

#### *2.4. Ball Mean Wear Scar Diameter and Corrected Load*

At every pass load there is severe wear on the three test balls in the ball pot. The mean value of the wear scar diameter on these three test balls can be measured using a microscope to determined ball mean wear scar diameter. The corrected load is a pass load that is compensated with the wear. It is calculated by multiplying the pass load with the ratio of Hertzian contact diameter to ball mean wear scar diameter. The corrected load was determined for grease X and Y at a speed ramp up time of 0.15 s, 0.25 s, and 0.95 s.

## *2.5. Friction Coefficient*

The friction measuring system in the four-ball tester has been extensively described in the US patent US 2017/0176319 A1. Friction torque was measured using a load cell, that was in contact with the moment arm fixed to the ball pot in the four-ball tester. Coefficient of friction was calculated from the measured friction torque and applied load as per ASTM D5183 [12] The data acquisition and display system in WinDucom software allowed the user to view and store the real time changes in friction coefficient profiles. The average friction coefficient was calculated by determining the mean of all the friction coefficient values acquired during a pass load test for grease X or grease Y.
