WEAR OF COATED AND UNCOATED PRIMARY CLUTCH SHEAVES IN A CVT

In research conducted at Brigham Young University, four coatings were tested for their ability to increase the wear life of primary clutch sheaves made of A390 die-cast aluminum used in continuously variable transmissions (CVT). The coatings tested were: hard chrome, electroless nickel metal, hard coat anodizing and composite ceramic coating.

A wear test stand was developed to duplicate wear found on CVTs currently in use. The wear was evaluated using four methods. The test first method characterized the shift delay properties of the worn CVTs while running on the wear test stand. As the CVT was run on the wear test stand a delay in shifting would develop during the unloading cycles. The second method used an ATV and chassis dynamometer to evaluate the change in performance with wear. The third method used a profilometer to evaluate the amount of material lost, through wear. Finally a scanning electron microscope was used to identify wear scar and wear morphology to determine the dominate mechanism of wear in the sheave material.

The hard chrome coated primary clutches had a 22% lower shift delay than the uncoated clutches in the shift delay test. Statistical analysis was done on the dynamometer test results. This analysis showed that the coatings were a significant factor in the performance of the ATV over time. However due to high variance the analysis was not able to differentiate between coatings. Profilometer test showed that the hard chrome coated clutches had an average of 47% less wear than the uncoated clutches. These tests also showed that the electroless nickel coated clutches had more wear than the uncoated clutches. From the SEM scans the dominate wear mechanism in the hard chrome and the uncoated clutches was adhesive wear and surface fatigue wear in the electroless nickel coating. The hard coat anodized and ceramic composite coatings were eliminated early in testing because of poor performance.