Application of the Flash Temperature Concept to Cam and Tappet Wear Problems

Abstract

In the lubrication of a modern automotive engine the cams and tappets are among the most sensitive parts and are expected to become more difficult to lubricate as engine speeds increase. This paper deals with the two types of cam and tappet distress that occur mainly at high engine speeds; scuffing and polishing (or burnishing). The occurrence of these phenomena is believed to be controlled by the temperatures in the contact zone, and in order to treat these in a quantitative manner the flash temperature theory of Blok has been applied to cam and tappet systems. For this purpose, some preliminary measurements have been made of the coefficient of friction between cam and tappet. The contact temperature reaches a maximum at three points in the valve operating cycle; at the cam nose and at the two points of zero tappet acceleration. The relative values of these maxima depend on the cam contour, spring forces, materials of construction, etc. If the temperatures at these points are excessive, either scuffing or polishing is liable to occur. A case of polishing wear is examined, and it is shown that most of the observed details may be explained on the flash temperature theory. It seems necessary to postulate that a steel surface with a low velocity relative to a rubbing contact will scuff if the contact temperatures are high enough, but that the surface will become more liable to fail by polishing wear as this relative velocity increases. This is related to some phenomena observed with steel spur gears. The theory may also be used to evaluate the relative scuffing properties of different cam profiles, with certain limitations which are explained in the paper, the metallurgy being constant. Some indications as to the design of a cam profile to avoid scuffing are also given, but there is no experimental backing for these.

Tribological Study for a Low-Friction Cam/Tappet System Including Tappet Spin

“…and Therefore, from Eqs. [5], [50] a n d [51], modified sliding speeds are: a n d These expressions are comparable to the ones derived in (5).…”

Development of valve train rig for assessment of cam/follower tribochemistry

“…This is shown at regions 5 & 6. The presence of CaCO 3 and phosphate has been supported by Raman analyses which are discussed in the next section. At higher temperature (Figure 12b), significant abrasive marks were observed on the worn surface with patchy phosphate films which appear to have been slightly removed due to rubbing action from the camlobe.…”

“…Therefore, for improved engine efficiency, research into this tribopair is necessary. Accordingly, numerous authors have designed and used valve train rigs to study friction [3][4][5][6][7], wear [8][9][10], film thickness [11][12][13][14] and tribochemistry [15,16] at the cam tappet contact. Historically speaking, Dyson and Naylor [3] were perhaps the first to investigate friction at the cam follower interface using a push rod assembly where the tappets were held in place by a pair of flat springs, and the corresponding stresses measured by means of piezo electric gauges.…”

Measuring the tribological performance of all the tappets in a production engine using magnetometer sensors and the effect of lubricant rheology

“…In the past, researchers used different experimental techniques to understand the mystery of tappet rotation. [1][2][3][4][5][6][7] Most of the techniques developed were either adopted on specially designed rigs or on extensively modified engine heads. One of the techniques that required relatively less modifications and can be used in production engines was developed by the author 8 in the year 2007.…”