Camshaft Engineering Company

CAMSHAFT AND FOLLOWER FAILURE ANALYSIS

It is important in the event of camshaft lobe and follower failure, or on dismantling an engine for repair or reconditioning, to be aware of factors which can lead to camshaft and/or follower failure, so that these factors can be avoided on subsequent re-assembly of the engine.

As with most engine components, camshafts, particularly the lobes, and camshaft followers will suffer wear over time to a point where valve lift is reduced and engine performance degraded. This wear will generally affect most lobes, although once the lobe hardening has been worn through, further wear will be more rapid.
Followers will generally wear to a concave shape, and will be contacting the cam lobe on their edges, sometimes with material breakup on the lobe flank edges. Camshafts and followers in this condition would be considered to have suffered normal or mature wear.

Where premature lobe or follower failure has occurred there are various pointers to possible causes.

Hard contact or scuffing on the lobe tips only:-

Coil binding of springs, valve guides fitted incorrectly (too high), excessive valve spring pressure, start-up without lubricant,

Scuffing on lobe flanks and tips:-

As above but including more general lubrication failure, incorrect finish on cam followers, incorrect startup and run-in procedures, faulty or poor quality cam followers. Of recent times, we have observed poor lobe finish on both aftermarket and performance camshafts being such as to be the likely reason for lobe failures. Camshafts from the same source have been reground by us to the same profiles and superfinished on the lobes before fitting to the engines. This has overcome prior repeated lobe failures.

Follower contact on one edge of the lobe, lobe tip unworn:-

Worn or incorrectly faced follower fitted, excessive taper ground to the lobe.

Follower contact on both edges of the lobe, lobe tip unworn:-

Worn follower has been refitted.

Major lobe tip wear, major follower wear on isolated lobes only:-

Eventual outcome of any or all of the above causes applying to particular lobe/follower assemblies, possible damage of a lobe surface during storage or fitting. Check for breakup or pitting of the follower surface. If present this can be indicative of follower material structure inadequate for the lobe to follower loads applied. Periodically observed with high rocker ratio valve train designs, or where modified (extra lift) camshafts and/or stronger valve springs have been fitted.
Higher lift camshafts automatically increase the contact forces between lobe and follower due to the extra compression applied to the standard springs. This additional load in itself will offset any reduction of valve float speed the modified profile may introduce, reducing the need for heavier springs. Fitting stronger springs further increases the contact force, the critical measure being the force at full valve lift.This is most damaging at low engine speeds, particularly idle and engine cranking speeds.
Under these conditions, there is little lubrication from oil throw-off from the crank bearings, and the lobe to follower rubbing speeds are low. This minimises the development of any hydrodynamic oil film to separate the surfaces and avoid metal to metal contact. Any shortcomings of lobe finish, lobe material, follower material, finish or profile, will reduce the ability of the lobe and lifter combination to survive wthout scuffing wear, leading to catastrophic failure.
Many of the cam followers supplied these days are made using cost cutting processes where the material structure at the critical centre contact face is only partially chill hardened, and with associated intergranular flake graphite. This is a structure barely adequate for the level of contact forces imposed in engines even at standard specification.
Under repeated high contact forces, cracks form along the graphite flakes and material grain boundaries, such that grains become dislodged, causing pitting of the surface. This further reduces the ability to support the loads, and wear debris which increases the rate of wear to the point of rapid major failure. See the microstructure analysis images below.
Therefore, wherever possible, select cam followers which have a fully hardened 'button' attached to the cam contact face. Where such followers are not available, extreme care must be taken in component selection, particularly valve springs, camshaft lobe surface finishes, lubricant quality, startup and run-in procedures to minimise the potential for failure.

Scored or seized camshaft bearings:-

failure of lubrication to the camshaft, blocked oil galleries or pipes, debris or foreign matter built in or penetrating the lubrication system. For overhead camshaft engines, blockage of oil galleries or drillings, blocked oil flow restrictors or anti flow-back valves, warped cylinder heads.

Unfortunately failure analysis is hindered in the case of hydraulic followers or adjuster elements, as the initial stages of failure are not signaled by the onset of tappet noise, as applies with solid follower designs. Failures are therefor more likely to become catastrophic, destroying evidence of the initial cause before being detected.
Examination of other lobes on the camshaft could show evidence of incipient failure leading to isolation of the cause.

Prevention is the best insurance against failure, by selection of good quality mating components (lifters), and by correct and careful assembly checks, fitting, and first start-up procedures as described in the fitting instruction page.

[FAILED FOLLOWER AND LOBE]
Failed cam follower and lobe

[PITTED FOLLOWER]
Pitted face of follower due to material structure deficiencies

[SATISFACTORY CHILL STRUCTURE]
Satisfactory chilled structure, hard phase white, graphite black

[POOR CHILL STRUCTURE]
Incomplete chill structure showing less hard phase and flake graphite.
Follower face pitting and cracking along graphite flakes visible at top

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