---
title: Fictitious Grease Weld Load in a Four-Ball Tester
description: The weld load in a Four Ball Tester is strongly influenced by time needed to accelerate to the test speed. Smaller acceleration lead to fictitious results.
---

[Ducom Instruments Blog](https://blog.ducom.com)

# [Fictitious Grease Weld Load in a Four-Ball Tester](https://blog.ducom.com/fictitious-grease-performance)

 Written by [Global Applications Team](https://blog.ducom.com/author/global-applications-team) | March, 2023

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  
*Grease*  
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)](https://lp.ducom.com/en/product-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.

Results  
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.

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.

*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).*

Download the full paper for free through the form below

[View full post](https://blog.ducom.com/fictitious-grease-performance)

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