Camshaft Failure: Camshaft Bearings

A. Camshaft bearing journal wear caused by uneven bearing support

Bearings require the proper support, alignment and fit to function properly. The O.D. of the bearing below shows very little fitment marks to the block bearing bore (yellow arrow). The bearing was measured and found to be in specification on the O.D. and I.D. indicating the camshaft-bearing bore in the block was most likely oversize. The wear marks on the inside of the cam bearing further shows uneven wear most likely from distortion. Excessive clearances allow the camshaft to flex and do not allow for proper lubrication and support.


B. Camshaft failure caused by improperly installed bushings

The picture shows a failed bearing due lack of proper lubrication. The bearing in the second picture has some premature wear, but note the arrow points to a partially covered oil hole.



The next picture shows all the oil holes are partially blocked.



The cause of the failure is improper installation of the cam bushings. The partially blocked oil holes do not allow for proper lubrication.

C. Camshaft failure caused by lack of lubrication

The first picture shows the wear pattern on tappet to camshaft lobe contact area (blue arrow). The yellow arrow points to the only tappet that has been rotating, note circular pattern.



The next two pictures show the wear on one of the lobes of the camshaft. 



The cause of failure is lack of proper lubrication and tappets not rotating.
On a flat tappet engine, the camshaft must be pre-lubed before starting. Other possible causes of failure could be worn or damaged tappet bores.

D. Camshaft failure due to the lack of proper lubrication

The first picture shows a bearing journal blackened from lack of proper lubrication, also note discoloration extending to surrounding shaft (blue arrow).



The next picture shows galling on valve lobe (yellow arrow).



The final picture shows a lobe that is severely gouged from a follower that has turned sideways
(red arrow).



There are two areas where the correct amount of lubrication is required to allow the camshaft and the cam follower rollers to wear normally. The first area is between the pin and roller; the second is between the camshaft lobe and roller. The friction between the cam lobe and the follower comes from the normal loading due to the combined cylinder pressure and the valve spring force. A rolling contact is necessary to minimize friction. If there is a lack of lubrication galling will start to occur.

Galling is defined as material transfer as a result of two surfaces coming into direct contact after penetrating through the lubricating film. Typically, the roller material transfers to the cam surface. Once the surfaces encounter galling, the load carrying capacity of the system is greatly reduced. The breakdown progresses to spalling caused by high contact fatigue stress. Continued running eventually causes the lobe to disintegrate as seen in the above pictures.


E. Camshaft failure from misalignment

The following picture shows the bearing was subjected to severe over heat, note the discoloration of the bearing material.



This type of failure is caused by misalignment, contamination or lack of proper lubrication.


F. Engine over speed damage to camshaft and followers

The following pictures show different degrees of damage to the leading and descending sides of the camshaft lobes. This is caused by the downward impact of the lifter roller after it was instantly separated from the top of the cam lobe. This type of damage is caused by engine over speed.









G. Rocker arm failure caused by mechanical fatigue

The picture below shows the wear on the pin from the adjacent rocker lever roller. Note: the one-sided wear indicating the roller was not loaded evenly. The rollers and camshaft lobes were not running parallel to each other, indicating a distortion in the camshaft or rocker box.



The next picture shows uneven wear patterns on the camshaft reflecting the wear pattern in the previous picture of the pin.



The next three pictures show the fractured surface area of the failed rocker lever. Note beach mark lines in surface pointed out by the red arrow.



The beach mark lines in the picture are on the mating part of the failed rocker lever.



The final picture at 500X magnification shows the stress riser which started the fracture in the rocker lever.





The fracture initiated at one location and propagated across the rocker lever until final failure occurred. The rocker lever failed due to mechanical fatigue. The presence of visual beach marks and microscopic striations characterize the progressions of fatigue cracks. These characteristics are present on the fracture surfaces of the rocker lever as shown in the above picture. The most likely root cause of failure is a stress riser in the rocker lever causing the failure of the rocker lever.



  • Posted on   11/17/15 at 02:24:34 PM   by Sara  | 
  • RSS
  • |
  • More

Reader Comments (0)

Comment Post Comment