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Bearings, Types Applications and Maintenance
Author: Seamus Dolly
Since Murphy or whoever it was, took credit for the round wheel,
rolling technology has changed somewhat.
Today, friction whether static or dynamic, is reliant on
bearings to reduce motor or engine load. What is sometimes
overlooked is the inherent features and design of a bearings to
reduce temperature where it is not necessary. The basic
requirement is to reduce contacting surface areas. More extreme
requirements are to reduce vibration, reduce allotted space
required to house such a bearing, and extend their life.
Needle roller bearings are used where the load is spread length
ways, and a typical or common application would be the “big end”
bearings of motorcycles. Plain bearings are still used in the
big ends, of cars and most other crankshaft scenarios. The
principle behind a plain bearing is that the thinner a material
is, the harder it is to compress or displace it. Plain bearings,
often called bushings, can be “steel backed” with a relatively
thin coating of white metal applied to the bearing contact
surface. This would be the common type in the automotive big end
applications, and should a piece of offensive hard material
contaminate the oil, it can be safely embedded within the white
metal. Should it be small enough then it is not an issue.
More plain bearings can be of a bronze base with impurities cast
or sintered into them. The purpose is to absorb oil or lubricant
before application, and become “self-lubricating, for a limited
time anyway.
Many years before the industrial revolution, bearings were made
from anything available that was considered appropriate. Timber,
sometimes from apple trees but not in any way exclusive to them,
have been used in windmills and water mills.
Indeed, stone bushings were not unusual. You’ve heard of the
more expensive watches been said to contain “X number of
Jewels”? The purpose of such jewels was to reduce friction and
associated loads. This would be a loose example of metal to
stone contact.
Thrust bearings are designed to work through a different axis,
and may be ball-type, cylindrical roller type or a combination
of both. Taper roller bearings are an example.
High temperature bearings have in-built allowance for expansion,
or to put it another way; they have more clearance. High-speed
bearings often have “cages” made from non-metallic materials.
Typically, this material is tufnol or plastic.
Incidentally, in low-speed, combustion-risk situations, the
cages can be made from brass as well. This is to help reduce any
incidence of “sparking” between the balls/rollers and the cage
itself. The reason for a cage in the first place, is to maintain
distance between rollers/balls, because it is only in the cases
of high low to space ratios, where no space is needed that cages
are unnecessary and these are low speed applications.
Other low speed bearing/plain bushings are also “steel backed”
with a coating of P.T.F.E. or other plastic on them. Again, this
is to give some self lubrication properties, as well as to
reduce friction. P.T.F.E. happens to be excellent, as it has a
low coefficient of friction.
Bearing failure is primarily due to misalignment and
contamination. Of course, there are many variables, such as
instances where undue load/other mechanical failure happens to
be the case. A common cause of failure in the case of
automotives, is lack of oil or insufficient oil pressure.
Absence of proper cooling, generally, will bring excessive loads.
Bearing failure is all too often due to improper installation or
fitting practices. Indeed, bearings can be “on the way out”
almost immediately. Induction coil heaters are readily available
from manufacturers to ease the fitting operation, and spare the
bearings undue trauma. Mechanical shock from abuse with a hammer
has destroyed many new bearings and they are destined to fail,
as little actual work will verify. Indeed, I’ve seen a case
where a six-year old machine was overdue some maintenance, and
got new bearing fitted six times in six months. It would have
been best to leave it overdue; such was the fitting skill of the
men involved. Even without the hammer, contamination while
fitting, poses massive problems. Allowing foreign bodies to
migrate into the bearing will have a detrimental effect.
Once fitted even, contaminants from its oil/grease/air supply
can make running conditions unbearable. Clean lubricants and
environments are the answer, and in the specific case of oil;
clean filters, lines and the oil itself, will provide an up-time
saving.
About the author:
Seamus Dolly is the webmaster at
www.CountControl and his
background is in engineering.
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