Crossed Helical Gears for High Speed Automotive Applications: Some Observations on the Problem of Failures, the Effects of the Changes of Design and Manufacturing Processes

Abstract

The paper indicates that the problem has arisen in the last 10 years and is related to the increase in speed and output of automotive petrol and diesel engines. Failure incidence is furthermore associated with the change over from gear type to the cheaper multi-lobe constant displacement type lubricating oil pump. The latter have a very pronounced cycle of high torque pulses. A similar severe cyclic loading of the gear teeth is experienced when driving a distributor type diesel fuel injection pump. Changes of component design leading to reduced gear tooth loading are beneficial but cannot compensate for insufficient quality of surface finish of the tooth flanks. Failures of four distinctly different types of drives are illustrated and cause of failure analysed in detail. The part played by lubrication and oil additives is discussed. The author suggests that no drive will be successful unless the surface finish roughness of the tooth flanks is equal to or less than the oil film thickness generated. This film thickness diminishes rapidly with increase in sliding velocity and/or lubricating oil temperature. A relationship between tooth surface finish roughness and maximum safe tooth sliding velocity is demonstrated, based on the analysis of success and failure of 14 different engine types. The importance of obtaining a consistent quality of tooth flank surface finish is stressed and manufacturing methods of obtaining this are compared. The information on hobbing, shaving, grinding, lapping, buffing, phosphating, cutter blunting, shaving techniques for difficult abrasive and hard materials, detailed floor to floor production times etc. can be safely applied to gears of automotive proportions generally. The author gives a summary of his findings concerning successful and unsuccessful combinations of materials for worm-wormwheel as well as crossed-helical-gear drives. The stage by stage tooth flank surface finish improvement of an extremely successful crossed-helical-gear drive is followed through its different phases of production and demonstrated by photographic records.