Search Results

Roller tappets are commonly used in automotive valve trains to transform the rotational motion of the cam shaft into axial motion of the valves. Proper design of the cam and roller tappets requires an understanding of the factors that influence the integrity of the contact surfaces. Ideally the roller tappet to cam lobe contact should exhibit pure rolling (equal surface velocities). However, relative sliding (unequal surface velocities) occurs under most conditions. Relative sliding of contacting surfaces operating under marginal lubrication can contribute to deterioration of the surfaces. The relative sliding of tappet roller to cam has been experimentally determined by means of a simulator rig. Sophisticated instrumentation has been used to measure the velocities and synchronize them with the lift event. The experimental results showed high sliding during contact with the cam base circle (up to 20%).

In an effort to reduce the design cycle time and to meet increasingly demanding applications, an improved procedure for bearing retainer design has been introduced. This paper discusses a methodology which allows the designer to predict the life and failure modes of a retainer under application conditions. Specific attention is given to the case of fatigue of the retainer due to the dynamic interactions between the retainer, rolling elements and races. The methodology which has been developed for the life prediction of retainers is based on the dynamic loads and retainer structural integrity. Central to this technique is the ability to predict the loads imposed on the retainer as a function of design and application conditions. The bearing analysis code ADORE has been used for this purpose. The technique will be discussed by means of an example.

A small sample lot of Conrad type ball bearings fabricated of silicon nitride have been fatigue tested under full film liquid lubrication conditions. Analysis of the failed bearings points to a failure mechanism which is different than that of steel bearing fatigue. A failure model has been developed based on the static strength of the material, manufacturing process effects and basic contact mechanics. The failure mode found identifies one limitation of the use of ceramic bearings. The limitation is dictated by the tensile stresses produced in operation and the strength of the finished component. It was demonstrated that silicon nitride fails by mechanical fatigue when subject to high Hertzian contact stress. It must be clearly understood, however, that the high Hertzian contact stresses (above 350 k.s.i.) used in this test program are greater than those of recommended design practice. The bearing manufacturing, testing and failure analysis are discussed.