Since 1987, Van Doorne's Transmissie (VDT) BV has produced a steel belt for continuously variable transmission (CVT) applications. To date, over 750,000 vehicles have been equipped with the system, and belt reliability has proved to be very high. During the early stages of transmission production, an intermittent lubricant-related noise problem was encountered. This paper describes the steps taken to isolate the noise source and eliminate it.
The noise, commonly called the “scratch phenomenon,” manifests itself as a transfer-gear rattle under slow-speed, light-load conditions. Initial investigation revealed that the scratch phenomenon could be avoided by using an alternative transmission fluid (1). Experimental work conducted by Lubrizol and VDT to understand the mechanism behind the problem:
-
Established a relationship between the occurrence of the scratch phenomenon and the stick-slip properties of the belt-material/lubricant combination.
-
Led to the development of a transmission test rig to replicate the conditions that produced the scratch phenomenon in the field.
Simple screening tests then were developed to determine the potential of a fluid to produce the scratch phenomenon by evaluating its stick-slip characteristics. During this work, it was found that existing automatic transmission fluid (ATF) specifications did not necessarily contain performance requirements that describe a particular fluid's suitability for CVT applications. Additional work resulted in the development of a fluid designed specifically for CVTs, which eliminated the scratch phenomenon under all driving conditions and also provided superior wear protection under severe conditions (2).