Preventing Premature Cam Bearing Failure

Prevent premature cam bearing failure

For many years, nearly all camshaft bearings were manufactured with a lining
of babbitt. Babbitt is a soft slippery material made up primarily of lead and tin
and is quite similar to solder. As a bearing surface layer, babbitt possesses
the desirable properties necessary to survive under adverse conditions such
as foreign particle contamination, misalignment and marginal lubrication on
start up.

The trend in modern engines has been toward higher operating temperatures
and higher valvetrain loads. Babbitt is limited in its ability to survive under these
conditions due to its relatively low strength. When babbitt cam bearings are
installed under these demanding conditions, the lining may extrude or fatigue.
Fatigue can be identified by craters in the bearing surface where sections of
lining material have flaked out.

To meet the demands of higher loads and operating temperatures in modern
engines as well as the requirements imposed by high performance, babbitt has
been replaced by an alloy of aluminum. This aluminum alloy is much stronger
than babbitt and will withstand several times the load. However, this added
strength is obtained at the expense of some of the more forgiving properties of
babbitt. The aluminum alloy is harder, making it somewhat less compatible with
dirt, misalignment and marginal lubrication. This is typical of the compromises
or trade offs that are frequently necessary when selecting a bearing material to
suit the requirements of a specific application and in this case, higher loading.
Typically, whenever a higher level of loading is encountered, greater precision
is required to maintain reliability. Conditions such as cleanliness, alignment,
clearances, journal surface finishes and lubrication must all be controlled more
closely. The following are some recommendations to help optimize performance
when using aluminum alloy camshaft bearings.

Sufficient clearance is necessary in the initial installation. These stronger
bearings will not wear in rapidly to make their own clearance like softer babbitt
materials. Minimum clearance should be .002” for stock engines and .003” for
high performance. Optimum clearance range for high performance applications
is .003” to .004”. Because of the stack up of tolerances on the block, shaft and
bearing it is impossible to control clearance to this range in the manufacture of
the bearing alone. Clearances must be measured at installation.

Honing the IDs of cam bearings to increase clearance is not recommended
because hone grit may become embedded in bearing surfaces that will cause
shaft wear. Bearing IDs may be reamed, but the most practical means is to
adjust camshaft journal diameters by grinding the journal. Even if not ground
to provide additional clearance, camshaft bearing journals should be polished
to the proper surface finish with the camshaft rotating in the same direction it
will rotate in the engine.

Like clearance, alignment is also extremely important especially for high
performance applications. Any block that has needed to have its main bearing
bore alignment corrected due to distortion is likely to have experienced cam
bearing bore distortion as well. Adequate clearance can help compensate for
minor misalignment of less than .001”.

Installation of bearings into the block must be done with care to avoid shaving
metal off the backs of the bearings. This galling action may cause a build-up
of metal between the bearing OD and the housing bore which will result in a
reduction in clearance. To prevent galling, check housing bores for a proper 25
to 30 degree lead-in chamfer before installing cam bearings. On blocks without
grooves behind the cam bearings, care must be taken to insure that oil holes
line up between the bearings and block. Where the block has a groove behind
the bearing, the bearing should be installed with the oil hole at the 2 o’clock
position when viewed from the front for normal clockwise camshaft rotation.
This will introduce oil into the clearance space outside of the loaded area and
allow shaft rotation to build an oil film ahead of the load.
Source: Mahle

  • Posted on   10/07/15 at 11:30:31 AM   by Sara  | 
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