**1. Introduction**

Several standards have been developed to establish grease performance and load carrying capability [1–4]. Both good [5] and poor correlation [6] has been reported within the different methods using similar lubricants. These have been explained by either similarities or differences between the contact geometries, configurations, and scuffing detection criteria. One such method, the four-ball tester [1,7] having high precision, has been widely used to manage lubricants' batch production quality control. It has helped scientists to select additives, both conventional as well an environment friendly nanoparticle, for extreme pressure, wear prevention and antifriction grease behavior [5,8,9]. Often, short duration four ball tests of 10 s or 60 s is found to be effective in determining the competing effect of additive molecules in surface deposition, tribofilm formation and protection against friction, seizure, and wear. These short duration test methods are standardized and frequently validated by D02 committee in ASTM—American Standards for Testing Materials. ASTM D2596-15 and ASTM D2783-15 are such test methods that are widely practiced by lubricant manufacturers to determine the extreme pressure (EP) behavior of greases. These standards offer vital information about seizure prevention by EP additives at a given load that is known as "weld load" [1]. Almost every grease specification sheet carries the four ball weld load data, as it is intended to help the consumers to choose the best grease to prevent seizure of critical components under starved lubrication conditions. Therefore, weld load data is important for both the grease consumers and manufacturers, with a

**Citation:** Joysula, S.K.; Dube, A.; Patro, D.; Veeregowda, D.H. On the Fictitious Grease Lubrication Performance in a Four-Ball Tester. *Lubricants* **2021**, *9*, 33. https:// doi.org/10.3390/lubricants9030033

Received: 15 January 2021 Accepted: 9 March 2021 Published: 12 March 2021

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higher weld load indicating better capability against failure. However, this critical data could be "manipulated" within the scope of ASTM D2596-15 or D2783-15, and it can "trick" the consumers to use the lubricant that can be detrimental to critical components.

In ASTM standards like D2596 or D2783 the lubricant is compressed and sheared between the four balls (top ball and three bottom balls) for 10 s (see Figure 1). The mean speed of the top ball is fixed at 1770 rpm. This test is repeated at every load stage from 6 kg to 800 kg or until the full seizure.

**Figure 1.** Four ball tester and four ball test configurations in the ball pot.

Seizure is represented by the sudden jump in the motor torque due to melting and fusion of steel balls followed by welding of four balls. To achieve higher weld load the lubricants are formulated with high performing EP additives [5]. In contrary, the desired weld load could also be achieved by tuning the speed "ramp up time" in the four-ball tester. Speed ramp up time is the delay in time taken to reach the mean speed of 1770 rpm. Although the mean speed is described in the ASTM standards, the ramp up time is not mentioned, that could result in fictitious grease lubrication performance. There have been four ball test reports that showed effect of speed on grease wear [10] and effect of delay in applied load on lubricant scuffing loads [11]. However, there are no reports on the effect of ramp up time on grease seizure load or weld load.

In this study, we have developed a four-ball test method to control and measure the speed ramp up time or delay in motor speed in the four-ball tester, whose effect on weld load, friction and wear is investigated for two types of greases. And we propose mechanisms that can explain the changes in weld load and friction due to delay in reaching the mean speed.

### **2. Materials and Methods**
