Field Experience with Selected Lubricants for Commercial Vehicle Manual Transmissions

Laboratory testing is an essential part of product development. However, it usually only reflects a small portion of the experience that a lubricant may see in actual service conditions. Many laboratory tests are designed to only address one or two facets of what is deemed to be critical performance areas. Since it is difficult to cover all of the critical performance conditions problems sometimes arise in service that were not anticipated by the laboratory test. Or, conversely, some above average performance evolves during service that was not observed in a specific laboratory test.

This paper highlights the overall performance of four manual transmission fluids approved or accepted by the manufacturer for this application. The evaluations were conducted in a city bus fleet with the test buses assigned to the same route for approximately 300,000 km over 30 months. The route chosen for this operation was the most severe for the fleet and included several long, steep grades on the outbound and inbound legs from the garage. While all of the fluids met the laboratory acceptance criteria for this equipment the severe nature of this service showed notable differences among the fluids at the end of the test period. Interim inspections were conducted and samples taken at regular intervals in an effort to track progress. The gearboxes were assessed for synchronizing capability, gear wear, and overall cleanliness at the conclusion of the testing.