In general, tribometers used in the labs have contacts largely scaled down compared to friction contacts in the real field conditions. It has resulted in several key advantages for the users including short test duration, lower cost of testing, improved test repeatability and ease of testing. However, the largely scaled down contact conditions may not simulate the real life shearing state of lubricants. This often translates into reports with unrealistic friction and wear.
Figure 1. Description of Ducom Extreme Load Tribometer (ELT 1.0) and multiple contact conditions.
The manufacturers of tribometers are partly responsible for this, due to limited technological advances in loading capacity Tribometer with a maximum load of 60 N (and 500 rpm) will yield an unrealistic contact pressure of 2 MPa (pin diameter of 6 mm). Neither the shearing condition nor the contact pressures are realistic, for example the engine lubricants often experience contact pressure above 200 MPa. To achieve high contact pressure, the manufacturers instead of increasing the loading capacity they advise the buyers to further scale down the contact conditions, for example the use of point contacts instead of area contact. Such a practice will result in unrealistic friction and wear due to overstressing of lubricants. It will mislead our tribological investigation, as a result we might end up offering a poor quality solutions to our customer.
Figure 2. Three pins on a plate configuration in Extreme Load Tribometer.
Details about the test setup
Ducom Extreme Load Tribometer (ELT 1.0) is equipped with an innovative load control system that can exert a maximum load of 10 kN (see Figure 1). At this maximum load, the three pins on a plate set up, as shown in Figure 2, can yield a contact pressure of 300 MPa (applicable to lubricants research in IC engines). Furthermore, the three pins will triple the shearing cycle per motor spindle revolution. This translates to 9000 rpm (150 Hz) at a motor speed of 3000 rpm. High load, large contact area and high shearing speed can result in a realistic shearing state for automotive lubricants. Here we would like to draw your attention to the Figure 3 that shows an increase in lubricant temperature to 90 °C due to friction heating (area contact), and the friction coefficient was between 0.004 to 0.025. This represents the lubricant shearing conditions in an IC engine.