Reducing Compressor Wear with Low-Viscosity Lubricants: A Four Ball Tester Case Study (ASTM D5183)

Introduction

As energy efficiency becomes an increasingly critical factor in appliance design, refrigerator manufacturers are under pressure to meet stricter energy labelling standards. A key contributor to a refrigerator's overall energy performance is the efficiency of its air compressor, which is heavily influenced by the lubricant used. The current compressor oil formulation - while reliable - has a relatively high viscosity, which can lead to increased friction and wear, ultimately impacting energy consumption.

To address this challenge, the compressor manufacturers are seeking a new lubricant solution that minimizes internal friction and wear, thereby enhancing compressor efficiency and contributing to a better energy label for the refrigerator. In this study, we evaluated two newly developed oil formulations with lower viscosities compared to the current formulation in use (i.e., 5.5 cSt and 7.5 cSt, respectively) to determine their potential as performance-boosting alternatives to the existing oil.

Ducom Four Ball Tester (FBT-3)

Materials & Methods

Two different lubricant formulations with the same low viscosity of 5.5 cSt were evaluated. In porticular, Oil A consisted in a naphthenic base oil with anti-wear additives, while Oil B presented anti-wear additives with PAG in a paraffinic base oil.

Tests were carried out using Ducom Four Ball Tester FBT-3, manufactured by Paltro, Ducom’s Automation and AI division, according to the ASTM D5183 test standard (“Standard Test Method for Determination of the Coefficient of Friction of Lubricants Using the Four-Ball Wear Test Machine”). The test procedure comprises two phases:

1. Wear-in phase – During this preconditioning stage, the test balls were lubricated with white oil (refer to Table 1 for test conditions).
2. Wear test phase – After thoroughly cleaning the ball pot, it was placed under the Ducom Image Acquisition System (IAS) with the bottom balls still in place, eliminating the need to remove them. The mean wear scar was measured at this point. Subsequently, the ball pot was filled with the lubricant under investigation, and the second phase of the test began (refer to Table 1 for test conditions).

Table 1 – Designation and formulation of the tested oils.

The normal load, friction force, temperature, and speed were continuously monitored and acquired throughout the test. The mean wear scar diameter (MWSD) was measured at the end of the "wear-in" phase and again at the conclusion of the wear test, following lubricant failure. The Ducom Image Acquisition System (IAS) was employed to measure the ball wear scar, while Mooha A.I. was used to automatically detect and quantify the mean wear scar diameters.

Results and discussion

Figure 1A presents the real-time data of the friction coefficient during the wear-in phase, with the step load increments highlighted in red. The first observed friction peaks indicate points of lubricant failure, which were used to determine the incipient seizure load. Figure 1B displays the temperature profiles for the two oils throughout the test duration. In each case, the temperature began to rise after approximately 90 minutes. For Oil A, the test was automatically terminated at around 150 minutes due to the safety trip limit of 120 °C set in the FBT-3 system. The tests for the Oil B ran the full duration of 200 minutes, with the temperature reaching the trip value of 120 °C by the end of the test.

Figure 1. Friction coefficient live data (A), temperature live data (B).

Figure 2A shows the incipient seizure load, while Figure 2B depicts the coefficient of friction (COF). Figure 2C shows representative wear scar images of one bottom ball for each oil, captured after both phase 1 (wear-in) and phase 2 (wear test). The percentual increment of the mean wear scar diameter (MWSD), calculated among the MWSD value after phase 1 (wear-in) and phase 2 (wear test), is reported in Figure 2D. Consistently, Oil B guaranteed the best overall performance, with the highest incipient seizure load, the lowest COF value and the smallest percentual increase in MWSD.

Figure 2 - Incipient seizure load (A), % increase in wear scar diameter (B), wear scar images (C) and percentual increment In terms of mean wear scar diameters (MWSD) (D) of the two oils.

Conclusions

Oil B exhibited the best overall performance in terms of friction coefficient, temperature stability, incipient seizure load, and mean wear scar diameter. Based on these results, Oil B is currently undergoing a “field” test in a real air compressor, with additional data to be published soon.

Table 2 – Summary of the results