Investigation of the Failure Mechanism by Bending of a Rack Steering Gear

Abstract

The wheel-drive system of a car has a major influence on the safety of car's occupants. By turning the steering wheel, centrifugal forces are developed and these forces cause a bending moment on the rack steering gear. To avoid plastic deformation of the rack steering gear, sufficient strength and spring/stiffness properties are necessary for the material. In the case under investigation, the vehicle was traveling with 60 km/h on a slippery road. While negotiating a turn along the road, the driver was not able to bring the car back on the road. The car stopped without any crash, and the technical inspection which followed showed that the rack steering gear was deformed plastically at the right arm. The occupants were in great danger during this incident. Metallurgical failure analysis was conducted, revealing that plastic deformation by bending occurred because the mechanical properties of the steel were not adequate because of insufficient deoxidation practices and a defective hard coating. The lack of deoxidation and defects in the hard coating should not be allowed occur, and to avoid such failures, the quality control measures for acceptance of such parts should be modified. The new practices should include better control over the non-metallic inclusions, and measurement of the hard coating's thickness over the whole length of the bar.