Viscosity is defined as a physical characteristic of a lubricant, while friction is said to be a system property. Predicting friction by simply knowing the viscosity of the lubricant is rather difficult. Therefore, tribometers with friction measuring system are generally used to examine the relationship between friction and viscosity for a specified system operating in mixed, starved, boundary, or elastohydrodynamic lubrication condition (see Figure 1).
Figure 1. Friction behavior of lubricants as a function of operating parameters, from various Ducom tribometers.
The following sections provide some examples of various friction measurement techniques used for connecting viscosity and friction in a wide range of system conditions.
Friction Response from a Four Ball Tester in Boundary/Mixed Lubrication
Ducom Four Ball Tester, or FBT-3 (see Figure 2), is an automated tribometer integrated with high and low friction measuring system. With the help of an in-house designed flexure and load cell, the friction of the lubricant can be determined with a precision of 1% at a load of 10,000 N, 120 °C, and 3000 rpm.
Figure 2. Images of Ducom FBT-3, friction measurement during the four ball test, the measured friction profiles of lubricants and its relationship with the viscosity of the lubricants.
The friction graphs shown in Figure 2 were recorded for base oils with varied viscosity grades, at a contact pressure of 1.7 GPa and at a speed and lubricant temperature of 1200 rpm and 75 °C, respectively. The results of the FBT-3 denote that the quantified friction depends on the viscosity of the lubricant. Base oil with the highest viscosity grade was found to have the lowest friction.
Friction Response from a Microforce in Starved Lubrication
Fitted with a patented friction measuring system, the Ducom MicroForce (see Figure 3) provides a high friction resolution — that is, 30 µN at a load of 2500 mN.
Figure 3. Images of Ducom MicroForce, friction measurement system under microscale sliding, and the friction graph showing the transition from slip to stick phase.
Using parallel plate elastic sheets and capacitive sensors, the normal force and the friction force are determined independent of each other. Stroke length ranges between 0.02 and 50 mm, while the range of operating load varies from 10 mN to 10 N.
The friction graph shown in Figure 3 indicates the lubricants’ “slip-stick” behavior in the starved condition. Friction is obtained as a function of an increase in lubricant starvation (decrease contact diameter/stroke length). The conversion from the slip phase to the stick phase signifies total starvation at the microscale. These results demonstrate that viscosity does not influence friction.
Friction Response from a KRL Shear Stability Tester in Elastohydrodynamic Lubrication (EHL)
Ducom KRL Shear Stability Tester (see Figure 4) utilizes a dedicated flexure to determine the friction response of lubricants sheared in a tapered rolling element bearing.
Figure 4. Images of Ducom KRL shearing stability tester, friction measurement system under rolling-sliding condition, and the friction behavior with respect to viscosity.
Friction measurement system precisely records the behavior of the lubricants under the rolling-sliding friction (rolling element bearing), for a testing period of more than 100 hours. The accuracy is 1% and the least count is 100 mN.
Figure 4 shows the friction response of lubricants with four varied viscosity grades. The test load was 5000 N, lubricant temperature was 60 °C, and speed was 1450 rpm. As demonstrated, the friction is mostly affected by the viscosity, and as the viscosity of the lubricants increases, the friction decreases.
Friction Response from an Extreme Load Tribometer in Elastohydrodynamic Lubrication (EHL)
The automated tribometer — Ducom extreme load tribometer (see Figure 5) — allows lubricant testing at contact pressure less than 300 MPa at 10,000 N. The friction measurement at that extreme load condition should be stable and also independent of the loading unit. An exclusive flexure system with self-aligning potential can precisely transfer friction to the load cell, which reports the friction force.
Figure 5 shows the friction response from three pins on a plate configuration, demonstrating that lubricant with a high viscosity index (Oil B4) exhibits lower friction. In this test, the shearing condition is extremely aggressive, and it produces immense friction heating that can have an impact on the viscosity of lubricants. Conversely, this effect can be reduced and low friction can be maintained by a high viscosity index, as demonstrated in the results.
Figure 5. Images of Ducom extreme load tribometer, friction measurement system for three pin on plate configuration, and the friction behavior with respect to viscosity index.
In conclusion, Ducom tribometers are extremely useful in linking the viscosity of the lubricants with their friction behavior in a system.
This information has been sourced, reviewed and adapted from materials provided by Ducom.
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