Lubricant ageing effects on the friction characteristics of wet clutches

Abstract

The friction characteristics and performance of wet clutches have been investigated by several authors. Studies have also been made to understand the frictional performance during the service life of the clutch system. However, most lifetime studies have been conducted for systems with paper-based friction material so that systems using sintered bronze friction material remain largely unexplored. To study the friction performance of how these systems can vary over time, the friction characteristics for a clutch system using lubricants aged in three different ways were compared. The effects on friction characteristics resulting from oxidation of the lubricant, reduced additive concentration, and ageing under real operating conditions in a wet clutch test rig were studied.The oxidation effects on friction characteristics were examined using a modified waterless turbine oil oxidation stability test on a fully formulated lubricant. Five oxidation time periods from 48 to 408 h were investigated. For each period of oxidation, a friction performance test was run using a pin-on-disc machine.The ageing carried out in a wet clutch test rig is a standard test of a wet clutch systems manufacturer which is used in order to verify that an oil-friction disc combination will last the full service life of the specific application. This test gives a realistic ageing process similar to that in a wet clutch in a field test.Under boundary-lubricated conditions, additives are vital to the performance of wet clutches. Therefore, the effect of reducing the additive concentration in the oil was also studied, in the range of 10 to 100 per cent of the original additive package used in the fully formulated wet clutch lubricant.Results showed that a general friction increase can be observed for oxidation, additive reduction, and test rig ageing. It was also concluded that different methods of simulating the wet clutch ageing process differ and cannot be directly correlated with each other.

Evaluating lifetime performance of limited slip differentials

“…The remaining concentrations of antioxidants are measured periodically during ageing using RULER™ (remaining useful life evaluation routine), a quantitative linear voltammetry method . Friction has earlier been shown to increase with decreasing levels of antioxidants when ageing is performed according to a modified dry‐TOST (Waterless Turbine Oil Oxidation Stability Test) ASTM D943 …”

“…After the initial friction coefficient decrease, the friction coefficient increases as ageing continues. Increased friction coefficient levels in limited slip differential systems have also been observed previously …”

“…The analysis of the μ − v curve yields information on the condition of the system. Increased friction levels and tendencies towards a negative slope of the μ − v curve have been shown previously to result from wet clutch ageing . All μ − v curves in this work are produced from weighted least‐squares smoothed data of the second part of the engagement, maximum sliding speed to standstill.…”

“…developed similar models to predict lubricant degradation of automatic transmission fluids using standard oxidation procedures. The severe degradation of friction characteristics due to oxidation of a limited slip differential lubricant was shown in the study of Berglund et al …”

“…Much of the previous research performed on wet clutch degradation emphasise the significance of temperature on wet clutch degradation. The reason for this is that temperature plays such an important role in many of the wet clutch lubricant degradation mechanisms, e.g.…”