Testing Lubricants Using Reciprocating Systems

Testing of lubricants and surfaces on reciprocating systems, like linear compressors and engines, traditionally involves the use of large-scale tribometers before final component testing. This article explains a tribometry setup that allows time-effective screening of materials and lubricants at the benchtop scale using a UMT TriboLab™ (Bruker, San Jose, CA).

The setup allows the samples to be tested under simulated conditions to grade the performance of surfaces and lubricants while monitoring slight variations in friction with a piezoelectric-based force sensor. This article discusses the method and examines its effectiveness in simulating standard protocols, for example, the ASTM D6245-17 standard.

Furthermore, of significant importance is the fact that the system provides extraordinary flexibility for a broad variety of parameters and conditions, including temperature variation, stroke length, and speed. Finally, the article demonstrates the key significance of the calculation technique used to analyze the data acquired by high-frequency reciprocating tests.

Evaluating the Behavior of Lubricants and Materials on Reciprocating Systems

Continuous development of surfaces and lubricants for reciprocating engine and compressors is required to enhance automobile fuel economy, fulfill varying environmental regulations, and prolong component durability. The accurate understanding of component wear and friction in mechanical systems is important to measure energy losses and to estimate the durability of such surfaces. Therefore, the measurement of lubricant performance remains a vital factor in the estimation of accurate fuel economy.

To successfully perform this, one should consider a range of problems in accurately simulating the particular conditions and factors that might play a key role on the tribological performance of those systems. Luckily, these measurements can now be made with small-scale tribometers.

Tribological assessment of reciprocating systems can be effectively carried out at the laboratory scale only through proper implementation of a motion and velocity profile that precisely simulates the actual motion of the real-world tribosystems. To correctly simulate lubricants or materials in systems that reciprocate, it is important to replicate the sinusoidal motion — normally generated by a slider-crank mechanism in which the lubrication regime could be changing along the stroke — and to monitor the slight variations in friction. This needs high-speed, precise displacement sensors and force transducers.

The automotive and lubricants industry uses various standards. The ASTM D6425-17 (Standard Test Method for Measuring Friction and Wear Properties of Extreme Pressure (EP) Lubricating Oils Using SRV Test Machine)¹ is of particular interest in this case. This particular protocol ranks lubricants using extreme conditions of speed, temperature, and contact pressure.

Performing Reciprocating Tribotests

To simulate the behavior of a reciprocating system, the components that play key roles in the tribological system must be chosen. Such components include:

• Stroke length and reciprocating frequency that will direct the velocity and motion profiles
• A controlled temperature that will trigger tribo-chemical events on the tested surfaces
• Material and geometry of the tested samples
• Contact pressure between surfaces that is dependent on the geometry, load, and material of the contact surfaces

Important resulting parameters that could be measured during the tests include:

• Temperature variations
• Wear measured after the test
• Friction variations along the stroke, because the friction is not stable in reciprocating systems

For this study, a TriboLab system equipped with the latest High-Frequency Reciprocating Rig (HFRR) was used. One important benefit of the use of a small-scale tribometer is that the surfaces and lubricant can be easily characterized after the test with other metrology tools, such as chemical analyzers/spectrometers and profilometers. Figure 1 illustrates the TriboLab setup for high-frequency reciprocating tests. The arrangement comes with a fast reciprocating stage, a 400 °C heater, a normal-force sensor, and a piezo-based sensor. The sensor setup has been developed with the pivoting capability to enable users to rapidly replace lower samples and apply lubricant.

Figure 1. UMT TriboLab with HFRR setup. Left: Setup includes the reciprocating drive, HFRR sensor, 400 °C heater, and load sensor. Right: Pivoting allows easy setup of the samples.

As an example, Figure 2 illustrates a standard result of 5W-30 engine oil tested using the ASTM D6425-17 conditions and a reciprocating ball-on-flat (steel-on-steel) configuration with the HFRR arrangement. Here, friction variations were noted and plotted as a function of the time, and data was gathered and recorded in high resolution to monitor slight changes of friction along the stroke. That change of friction along the stroke is normally the result of a combination of variations on the surface, transition on the lubricating regime, and vibration produced from the mechanical motion. From the high-resolution data, variations in direction can be easily visualized. This enables accurate selection of the peak-to-peak friction (as recommended by the ASTM standard) or calculation of the friction in any way preferred by the user.

Figure 2. Friction and wear results of engine oil tested using ball-on-flat testing with the TriboLab HFRR setup under ASTM D6425-17 conditions (350 N, 50 Hz, 1 mm, 120 °C, 2 hours). Left: Calculated COF using 10% of the top points on each stroke, and inset showing the high-resolution data of the friction force at ~3600 seconds. The table shows the COF at 15, 30, 90, and 120 min, and the minimum and maximum COFs, as well as the wear diameter on the ball. Right: Wear scar after the test.

Analyzing the Results

Obtaining good quality data by using a highly precise system is a vital part of the experiment; however, the way the data is analyzed is equally important. Figure 3 represents the coefficient of friction (COF) as a function of the position, as the velocity is sinusoidal and not linear; this depiction is more realistic of behavior along the stroke when compared to the data collected only as a function of time.

The HFRR system enabled precise recording of position because of the use of a linear variable displacement transformer (LVDT) integrated into the fast reciprocating stage. The graph vividly illustrates how the friction increases in each extreme of the stroke and how the force reduced and dampened along the stroke until the next extreme in a shift of direction.

Figure 3. Friction force as a function of the displacement for data collected between 3600 and 3601 seconds.

Bruker’s UMT TriboLab software offers users complete flexibility in how to analyze the data collected from a fast reciprocating test. To estimate the COF, the user can select between various techniques; a simple technique using a certain percentage of the top values of friction in each stroke (see Figure 4, left), or an advanced technique that chooses a percentage of points in the middle of the stroke (see Figure 4, right). This potential enables users to tailor the data collection and analysis, offering friction values that can improve their understanding of slight variations between lubricants.

Figure 4. Simple and advanced methods to make calculations using the change between positive and negative friction in each stroke.

To gain better insights into the implications of the calculation method, Figure 5 illustrates the differences between the overall COF calculation when using various techniques for the same engine oil.

Figure 5. Results of engine oil tested under ASTM D6425 conditions. Calculations could change the reported value of the COF.

When computing the COF by various techniques, it is possible to observe how the value significantly varies, from overall values of ~0.14 when using the simple technique with 1% of the points, down to ~0.11 when using 30% of the top points, and lower (~ 0.10), when using the advanced technique with 50% of the points in the middle of the stroke. The advanced technique is likely to be more consistent, with 50% and 80% of the points overlapping.

It is apparent that the uncomplicated technique is less consistent than the advanced technique; however, it exists as it is essential to determine the peak-to-peak (a small percentage of the top points) friction demanded by the ASTM standard.

The technique that is chosen to compute the COF is highly significant when comparing lubricants that are very close in performance. With reference to the friction of reciprocating systems, it is not just a simple absolute value but rather highly dependent on the analysis technique. To better describe the effect on lubricant ranking/comparison, Figure 6 illustrates the outcomes of the test carried out using ASTM D6425-17 in two lubricants (oil A: 5W-30 and oil B: 0W-20) that are very close in performance, and how the data varies between the techniques.

Figure 6. Comparison of oils A and B. The COF changes depending upon the method of calculation.

The difference between oils A and B when using the simple technique with 1% of the top points is significantly more obvious than when using the advanced technique with 50% of the points in which the lubricants overlap their behavior. These variations are not just numerical differences but could indicate how different the lubricants behave along the stroke, which means they behave in a different manner mechanically based on the regime or lubricant properties.

Studying Lubricants and Surfaces with Superior Flexibility

The displayed setup/rig has been developed with unmatched flexibility, and not solely for protocols. It enables scientists to conduct measurements at different conditions, which is necessary to understand the performance differences of lubricants at different regimes. The reciprocating stage of the TriboLab can move both at speeds as high as 60 Hz and at speeds as low as 0.01 Hz. At high speeds, the piezoelectric-based force sensor does not drain the current, and the sensor can accurately record quasi-static or extremely slow-motion systems.

The normal force that can be applied is similarly flexible because of TriboLab’s range of 11 distinct sensors covering ranges between a few millinewtons to kilonewtons. In addition, the variable stroke is capable from 10 seconds of microns to 25 mm. Figure 7 shows the quality of the data captured by the piezoelectric-based sensor for tests carried out at various speeds, demonstrating how the test can be performed to understand tribochemical and mechanical events taking place in extremely different regimes.

Figure 7. Oil B tested at 10 Hz, 5 Hz, 1 Hz, and 0.5 Hz.

Conclusions

The UMT TriboLab’s high-frequency reciprocating rig has been established as a reliable method for screening lubricants and materials used in reciprocating applications, such as those found in compressors and engines. The system’s flexibility enables the assessment of lubricants at various regimes, along with the benefit of having complete control of the data analysis. The UMT TriboLab has the potential to execute similar protocols to the ASTM 6425 to assess friction of lubricants, and can help make significant distinctions in a broad array of tribosystem functions.

References

1. ASTM D6425-17, Standard Test Method for Measuring Friction and Wear Properties of Extreme Pressure (EP) Lubricating Oils Using SRV Test Machine, ASTM International, West Conshohocken, PA, 2017, www.astm.org

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.