BEARING CLEAR
GLYCO explains the vital role of bearing clearances in modern engines.
As engine builders continue to extract more power from less fuel,
modern engines are subjected to greater temperatures and loads. With
bearings taking the strain, bearing clearances are microscopic spaces
with huge impact.
BEING CLEAR ABOUT CLEARANCES
A bearing clearance is the difference between the external diameter
of the crankshaft and internal diameter of the installed bearing.
Bearing clearances can be less than the thickness of human hair and it
depends on:
• The component’s dimensional tolerance
• Wear to the bearing during operation
• Material properties of the component
absorption of shock loads,
and less overall wear.
Less bearing clearance
also helps to reduce
engine noise. However, it
also reduces the flow of oil
through the lubrication
gap. Bearing cooling is less efficient as a result. Whilst cooling can
be improved with larger clearances, this can significantly shorten the
lifetime of the bearings. Larger clearances reduces oil pressure, so a
high volume oil pump may be needed. A rule of thumb is a minimum oil
pressure of 10lbs per 1,000rpm.
OIL TEMPERATURE & CLEARANCE
The materials of the crankshaft, bearing and housing have different
thermal expansion coefficients which significantly influence the bearing
clearance at different temperatures. The pressfit of a bearing also has an
effect that usually leads to a minor widening of the housing bore.
Oil temperature can affect the expansion and contraction of the bearing
case and shaft material. This increases or reduces the bearing clearance.
If the case and shaft are made from the same material, expansion and
contraction will take place at the same rate. If made from different
materials (e.g. aluminium case and steel shaft), the change in oil
temperature will affect each component differently.
Calculating bearing tolerances
Aluminium expands faster than steel as temperature rises. As a result,
the clearance will increase and the pressfit will decrease. Conversely, if
oil temperature drops dramatically, the clearance can reduce to zero,
making it impossible to start the engine. This can be overcome by
increasing the crush height.
NARROW MEASURES
Bearing clearance tolerance is
calculated by adding the tolerances
of the individual parts together. For
example, typical engine will have a
tolerance of 19µm for the case boring,
19µm for the crankshaft diameter and
10µm for the bearing wall thickness.
20µm clearance for the connecting
rod bearing and 25µm for the main
bearing also have to be allowed for.
The result is clearances from 20-
78µm for the connecting rod bearing
and 25-83µm for the main bearing.
CLEAN & CLEAR
It is essential to maintain impeccable cleanliness
when building and assembling an engine as the
smallest speck of dirt can affect the clearance.
By maintaining a spotless assembly, the bearing and
engine can perform to specification, offering greater
fuel efficiency, lower emissions and longer bearing
lifespan.
Mind the gap! Bearing tolerances can be less
than the width of a human hair
SMALL SPACE FOR HUGE IMPACT
If bearing clearance is minimal, there will be good conformability
between the bearing and the crankshaft journal. This will be due to
material in some parts of the bearing being worn by a few microns
(µms). The result is less local stress to the sliding layer, better
6 ENGINE BEARINGS
The Glyco range now comprises more than 2,500
bearings and supports over 10,000 engine
applications. Glyco bearings are
catalogued on MAM Autocat.
For more information,
please speak with your local
FPS representative.
VITAL NEWS ISSUE 48