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This information reprinted courtesy of the Rolling Bearing Institute
and SKF, Inc. It is representative of the type of information
you'll find on the Rolling
Bearings Troubleshooter's Guide on CD-ROM. |
| Part
1: Mounting Methods |
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In order for a bearing to function properly it is important that the correct
mounting method be used. The type of bearing used for a given application
and the method of mounting and dismounting is determined initially at the
design stage. Mounting should, wherever possible, be carried out in a clean
and dust-free room and not where there are dust-producing machines. |
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Dirt and debris can affect the internal clearance of the bearing and the
fit of the bearing on the shaft or in the housing. A small speck of dirt
can pinch the outer ring outside diameter. Also it can keep a split housing
from being tight. This can cause an out-of-round housing bore in which
the bearing's outer ring may turn. Dirt on a shaft at the seal contact
area can cause seal wear which results in lubricant leakage. Dust and dirt
mixes with the lubricant in the bearing and forms a lapping compound that
causes wear in the bearing. |
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Do not remove a new bearing from its original package until immediately
before it is mounted. New bearings are thoroughly coated with slushing
compound to keep out air, moisture and rust. The slushing compound used
by most bearing manufacturers is not normally removed since it is compatible
with nearly all petroleum lubricants. When synthetic oils and greases with
synthetic oils are used, the slushing compound must be removed. However,
with synthetic hydrocarbon oils and greases, the slushing compound does
not have to be removed. Bearings are wrapped in heavy duty, waterproof,
polylaminate paper. Care should be taken not to drop bearings or handle
them roughly. They should not be exposed to large temperature changes which
might cause condensation to form. Do not handle the bearing any more than
necessary. Fingerprints can become a starting point for rust. |
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Three basic methods to mount bearings are cold mounting, temperature mounting,
and hydraulic mounting. |
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Bearings up to a 4"(10.16cm) outside diameter can be cold mounted using
a sleeve and a hammer or a press (see figure 1). An ordinary hammer
should be used. Hammers with soft metal heads are unsuitable as fragments
of the metal may break off and enter the bearing. The end faces of the
sleeve should be flat, parallel and burr free. It should abut the ring
with the press fit.
When a shaft is put in a vise in any assembly or disassembly operation,
it is important to protect the shaft from the jaws with sheets of copper
or brass. A replacement bearing must be an exact duplicate of the failed
bearing. Bearings and shafts are designed for each other and you cannot
make any changes unless a redesign of the machine is made.
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If the bearing fits too loosely on a shaft, it can creep or slip. This
causes the bearing to overheat and also results in abrasive wear to the
bore of the bearing and the surface of the shaft. If the press fit is too
tight, the inner ring of the bearing will be stretched so much that there
will be no room for the balls or rollers to revolve freely.
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An arbor press can be used for mounting small bearings. Place a sleeve
between the bearing and the press. The end faces of the sleeve should be
flat, parallel and burr free. It should be so designed that it abuts the
ring which is to be mounted with an interference fit, otherwise the rolling
elements and raceways can be damaged leading to a premature failure. To
facilitate mounting and also to reduce the risk of damage, the bearing
seating on the shaft and in the housing should be lightly smeared with
thin oil.
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At times it is necessary to have a press fit on the inner ring since it
rotates and also on the outer ring if there is some unbalance load that
could cause the outer ring to creep. The pressing force then must go through
both the inner ring and the outer ring at the same time, otherwise the
bearing will be damaged (see Figure 2).
If a spherical roller bearing was used for inner ring rotation where there
was an unbalance load, a pin could be put through one oil hole in the outer
ring to prevent outer ring creep. With this arrangement a press fit of
the outer ring would not be necessary.
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Temperature mounting is the method of obtaining an interference fit by
first introducing a temperature differential between the parts to be fitted,
thereby making the assembly easier. The required temperature differential
can be obtained as follows:
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a) Treating one part (this
is, generally speaking, the most common method).
b) Cooling one part.
c) Simultaneously heating
one part and cooling the other part.
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The temperature differential method is suitable for any bearing size, both
straight-bore and tapered bore. Because of the equipment required, the
cold mounting method is used wherever possible for bearings under a 4"(10.16cm)
outside diameter.
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The most usual bearing mounting is that in which the inner ring is mounted
with an interference fit on the shaft, and the outer ring is mounted with
a line-to-fine to loose fit in the housing. For non-separable bearings
over a 4"(10.16cm) outside diameter, it is necessary to heat the entire
bearing or just the inner ring, depending on the method of heating, so
that the inner ring easily goes over the shaft. In the case of a separable
bearing, it is only necessary to heat the inner ring. The bearing should
be uniformly heated within a maximum temperature of 250°F(121°C).
Methods for heating a bearing are: hot oil bath, hot plate, induction heater,
and oven. A sealed bearing can never be put in a hot oil bath.
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The hot oil bath is probably the most common method used. Both the oil
and the container should be clean. Quenching oil having a minimum flash
point of 300°F(149°C) should be used. The quantity of oil used
in a bath should be large in relation to the volume of the bearing. An
insufficient quantity heats and cools too rapidly, thus introducing the
risk of in adequately or unevenly heating the bearing.
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A wire rack should be installed 3"(7.62cm) from the bottom. This prevents
direct contact of the bearing with the higher temperature at the bottom
and also separates the bearing from any contamination that may have settled
at the bottom. Alternatively, the bearing can be suspended in the bath
from above.
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After a bearing has been heated in oil, its bore should be wiped with a
clean lint-free cloth before mounting.
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Once the bearing is heated it should immediately be placed on the shaft
and locked in place. If a locking device is not used as part of the mounting,
or if it cannot be fitted until later, some mounting tool should be used
to hold the inner ring against the shaft shoulder until the inner ring
has cooled sufficiently to be firm on the shaft. If this is not done, the
inner will walk away from the shaft shoulder.
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This is actually a simplified method for cold mounting a tapered bore bearing.
It is based on forcing oil between the interfering surfaces, thereby greatly
reducing the required axial force. The pressure is generally supplied with
a manually operated pump with a maximum pressure of 10,000 psi.
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The oil used for oil injection mounting should be approximately SAE 20
or 30. It should be absolutely clean, not only to protect the bearing and
seat, but also to avoid clogging the oil lines.
The shaft has to be designed with an oil groove on the outside diameter
and for a feed line from the end of the shaft hydraulic nut can be used
to easily mount and dismount spherical roller bearings with a tapered bore
(see Figure 3).
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| Part
2: Axial Location of Bearings |
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It is usually unnecessary for the shaft to be located axially by more than
one bearing. In fact, this is usually undesirable because temperature variations
may have a different effect on the length of the shaft than on the distance
between the housing, possibly resulting in damage to the bearings. Therefore,
one bearing usually locates the shaft and this bearing is called the fixed
bearing. The other bearing is free to float on either the shaft or housing
seat. The exception to this is when the span is short, in which case variations
in shaft length, due to temperature, are usually negligible; or when a
pair of angular contact ball bearings or tapered roller bearings are mounted
opposed, in which case proper adjustment of the bearings is necessary. |
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When mounting a free bearing, it is important to ensure that there is sufficient
axial clearance at the side where the bearing would be expected to move
once the application is in operation. It is usually safe to center the
bearing on the seating. |
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If an induction heater is used to heat the inner ring, care should be taken
that no residual magnetism remains in the inner ring. A gauss meter can
be used to measure to see if there is any residual magnetism. |
| Caution:
Never use a torch to heat a bearing. Hot spots can develop in the inner
ring causing a softening of the raceway, which leads to an early bearing
failure. |
| Part
3: Axial Adjustment |
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It is important to ensure full and proper seating of the rolling elements
during adjustment so as not to introduce an error in the final adjustment.
This is quite important for tapered roller bearings, in which it must be
insured that the rollers are not skewed and for thrust ball bearings, whose
washers usually have a radial clearance in the bearing housing. Proper
seating of the rolling elements is insured by rotating the shaft during
adjustment simultaneously pushing and pulling it. |
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The amount of axial looseness which is to be maintained in an adjustable
mounting is determined on the basis of experience. Machine or vehicle manufacturers
usually supply instructions based on their experience. |
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The proper axial adjustment is that which provides the necessary running
accuracy without impairing the design life of the bearings. In most cases,
a small residual axial clearance must be left to allow for shaft expansion
during operation. |
| IMPORTANT:
When tightening all sizes of locknuts, never use a chisel or drift and
a hammer. A spanner wrench should be used as shown in Figure
4, or an impact spanner wrench as shown in Figure
5. |
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If a chisel or drift and a hammer is used a small chip from the locknut
could get into the bearing and ultimately be the reason for a premature
bearing failure. |
| CAUTION:
Whenever mounting or dismounting bearing always use safety glasses. |
| Return
to Bearings Reference Articles Index |
