HPM High load Lubricant Specifications -
How Reliable is Your Four Ball Weld Data ?

The NLGI high-performance multiuse (HPM) grease updated specification delivers performance improvements over the existing GC-LB specifications for newer and emerging applications. Developed in 1989, GC-LB is recognized as a mark of quality for grease users. Improvements in materials, technologies and applications have led to a new set of high-performance multiuse (HPM) grease specifications. The new High-Performance Multiuse Grease (HPM) certification offers a core specification and four performance tags which are
- Water resistance (HPM + WR)
- Corrosion resistance (HPM + CR)
- High load carrying capacity (HPM + HL)
- Low temperature performance (HPM + LT)
For greases in HPM + HL category, the most significant change has been an increase in weld load requirement from 250 kg to minimum of 400 kg in the ASTM D2596 extreme pressure test.
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 which run for an hour. There are several standards regulating EP tests. A summary is given in Table 1.
The shorter test duration makes the acceleration rate quite important for EP tests. 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 unreliable and “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

Greases
A commercial high weld load grease that can offer protection against wear, scuffing and pitting in gear drives was chosen for this study. It was NLGI grade 00 having a density of 0.92 g/cc at 20 °C, kinematic viscosity of 500 cSt at 100 °C, flash point greater than 200 °C and FZG scuffing load stage was equal to or better than 12.
Four Ball Tester with Speed Ramp Control
Computer controlled and automated four ball tester (Model - FBT3) from Ducom Instruments, USA was used in this study (Figure 1). A variable speed direct drive motor without any belt or pulley arrangements was used to control the speed between 100 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 working in closed loop with the variable frequency drive system that controlled the flow of current to the motor, and the motor speed. In this study we chose speed ramp up time 0.15 s, 0.25 s and 0.95 secs which is the time delay for motor to reach a preset mean speed of 1770 rpm (Figure 2). The above time intervals were chosen considering the motor capabilities and technology used in commercial four ball testers.
Pass Load and Weld Load
According to ASTM D2596, the grease is packed into the ball pot with three stationary steel balls with diameter of 12.7 mm, Grade 25 extra polish, hardness 64 to 66 HRc at a temperature of 27 ± 8 °C. 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 fuse and get welded resulting in a high friction torque that 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.
Load Wear Index
This parameter is crucial for evaluating a lubricant’s ability to prevent wear under high loads, helping engineers select the right lubricant for specific applications and extend equipment lifespan. 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 lubricant at a speed ramp up time of 0.15 s, 0.25 s, and 0.95 s.
Impact on Weld Load, Pass Load and Load Wear Index
Speed ramp up time had a significant influence on the pass load, corrected load and weld load of grease (Figure 3). 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
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).
Load-Wear Index (LWI) was calculated from MWSD, Mean Hertz diameter and corrected load. LWI was lower (133 kg) for a faster acceleration 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 (Figure 5). This shift in LWI, can be rationalized given the higher weld loads required for slower acceleration rates.
Discussion
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.

Conclusion & Recommendation

ASTM D2596 was developed with an understanding that seizure prevention by EP lubricants is largely affected by factors like actual load, ball diameter, sliding speed and lubricant temperature. However, this study shows that weld load can significantly vary for the same grease by using a test parameter called the speed ramp up time. 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., 600 kg weld load (t < 0.15 s or t > 0.15 s). As an instrument manufacturer, we are working with ASTM D02 committee to set up an inter-lab study to investigate the role of “speed ramp up time”.
This article summarizes the study described in a paper titled “On the Fictitious Grease Lubrication Performance in a Four-Ball Tester” published in the peer-reviewed journal Lubricants.