Page 83 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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68 Automobile mechanical and electrical systems

Figure 1.95 This bearing on a gearbox shaft is held in place by compression

and ultrasonic welding, which creates a bond through the application of high-
frequency vibration while the parts to be joined are held under pressure.

1.6.13 Shrinking

When parts are to be ﬁ tted by shrinking they ﬁ rst have to be heated so they
expand, or cooled so they contract. In both cases the component to be ﬁ tted
must be made to an exact size. If parts ﬁ tted in this way are to be removed, it is
usual to destroy them in the process. For example, a ﬂ ywheel ring gear has to be
cut through with a hacksaw to remove it.
For a hot shrink ﬁ tting, the part will have a smaller internal diameter than the
one on which it is to be ﬁ tted. It is important not to overheat the components or

damage will occur. An oven is best, but a welding torch may be used with great
care. When the component has been heated and therefore expanded, it is placed
in position at once. It will then cool and make a good tight joint.

Cold shrinking is very similar except the component to be ﬁ tted is made very
slightly larger than the hole in which it is to be ﬁ tted. A cylinder head valve insert
is one example. The process is the opposite of hot shrinking. The component is
cooled so it contracts, after which it is placed in position where it warms back up
and expands, making a secure joint. Cold shrinking is normally a specialist job,
but it is possible to buy aerosols of carbon dioxide under pressure which can be
used to make a component very cold (dry ice).

1.6.14 Compression ﬁ tting

Many parts are ﬁ tted by compression or pressure. Bearings are the most
common example ( Fig. 1.95 ). The key to compression ﬁ tting is an interference
ﬁ t. This means that the component, say a bearing, is very slightly larger than the
hole in which it is to be ﬁ tted. Pressure is therefore used to force the bearing into
place. Suspension bushes are often also ﬁ tted in this way.

The secret is to apply the force in a way that does not make the components go
together at an incorrect angle. They must be ﬁ tted true to each other.

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