Page 211 - The Tribology Handbook

Cams and followers B10

COMMON MODES OF FAILURE Heavily loaded surfaces will continue to pit with
increasing severity with time. Figure 10.1 shows some
Three main forms of cam and tappet failures occur. These pitted cam followers.
are pitting, polish wear and scuffing. Failure may occur on
either the cam or tappet, often in differing degree on both.
Polish wear
Pitting This is the genera! attrition of the contacting surfaces.
When conditions are right this will be small, but occa-
This is the failure of a surface, manifested initially by the sionally very rapid wear can occur, particularly with
breaking-out of small roughly triangular portions of the chilled and hardened cast iron flat-faced tappets. Often a
material surface. This failure is primarily due to high casual look will suggest that the surfaces are brightly
stresses causing fatigue failure to start at a point below the polished and in good condition but dimensional checks
surface where the highest combined stresses occur. After reveal that considerable wear has occurred. Polish wear
initiation a crack propagates to the surface and it may be appears to be an intermediate case between pitting and
that the subsequent failure mechanism is that the crack scuffing assisted by some form of chemical action involving
then becomes filled with lubricant, which helps to lever the oil - certainly surfaces which develop a bloom after
out a triangular portion of material. running do not normally give ‘polish wear;

1

4 5 6

7 8 9

Figure 10.1 Examples of varying degrees of pitting
failure severity for flat automobile cam followers.
Numbers indicate awards for ratings for lack of
10 damage during oil standardisation rests (courtesy
Orobis Ltd.)

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