tech talk
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Part Three: Anti-rotational features
Part two of this tech talk series reviewed how to secure bearing axially to keep them moving from their
housings. Another important aspect of bearing security is the anti-rotational feature and why it is necessary.
How to properly secure bearings
Why would an anti-rotational feature be needed?
In a plain bearing system, the press-fit in the housing hole
keeps the bearing in place. The bearing supports the
moving shaft, and the low coefficient of friction on the inner
diameter of the plain bearing allows the shaft to easily rotate.
Since the plain bearing is usually much less expensive than
the shaft, it is designed to be a wear part. This means the
plain bearing just needs to be replaced every so often, and
not the more expensive shaft.
Many metal plain bearings are lined with a thin PTFE layer
on the inner diameter. The metal outer layer serves as the
backbone of the bearing, while the PTFE layer keeps the
shaft running smoothly. Plastic bearings, however, are more
flexible. They offer sliding capabilities on both inner and
outside diameter. In theory, it does not matter if the shaft
rotates inside or if the bearing itself turns in the housing.
Keep things spinning
It is significant if the plain bearing rotates around the shaft in the housing, or if the bearing remains still and
the shafts turns inside of it. Typically, shafts are optimized for sliding. Housing holes are not. The machining
quality of housing holes is often quite cheap. They are simple holes drilled into a piece of metal, stamped
or cut by lasers. Creating holes with a smooth inner diameter is expensive, and the surrounding material
of the hole is usually a cheap construction steel or sheet metal. All these circumstances are essentially a
collection of nonstarters for wear or sliding components, which is why the bearing should not turn with the
shaft in the housing hole. While the bearing itself will not have a problem running on its outer diameter, the
surface it will run on will.
Both sliding surfaces in a bearing system are of equal importance. If the metal sliding surface is too rough
and abrasive, the bearing will suffer from highly increased wear, resulting in more frequent downtimes
or even damage to the bearing system. This is why a classic press-fit, non-rotating plain bearing with a
rotating shaft inside is ideal. It is also why alternative methods should be prepared, if a press-fit cannot be
maintained.
