When the bearings are handled properly, they will respond reliably to a wide range of operating conditions. Considering that a bearing is a precision component of a machine, it can be damaged due to improper handling, even before it is put into operation.
The following general principles must be kept in mind:
• Keep the bearing and its surrounding area clean – wait till the last moment before removing it from its packaging.
• Bearings which are heat treated to reach a certain level of hardness must be considered vulnerable to impacts and excessive forces during careless assembly or disassembly.
• Do not heat the bearings to temperatures above 120ºC as they may lose their hardness, with a consequent reduction in their service life.
• Use appropriate tools for each size of bearing.
The following basic instructions must be kept in mind:
• Choose a clean work area.
• Check the shaft, housing and radiuses for their dimensions, surfaces and geometric shapes.
• Clean the shaft, housing and radiuses.
• Take care when touching the finished surfaces of the bearing to avoid possible traces of rust.
• When mounting the inner and outer rings separately, apply force to each one separately, avoiding mounting them. For example: the outer ring by hitting the mounted inner ring.
Now that the good assembly practices have been explained, we will concentrate specifically on tapered roller bearings.
Tapered roller bearings are mounted by placing them against another bearing, generally of the same type. This adjustment is carried out with lock nuts or compensation discs, amongst other methods.
These adjustments subject the bearings to a preload, which must consider the load to be supported, once the desired operating temperature has been reached.
It is important to keep this operating temperature factor in mind, seeing as when the mechanism heats up, the heat dissipation is different in the shaft, the hubs and the bearing components, which means that initial clearance may be much reduced and may even lead to locking the system.
When mounting the bearings, the machined state of the bearing housings is very important. A possible error of perpendicularity with respect to the shaft of the bearing or an error of parallelism causes the bearings to operate misaligned.
Misalignments are also caused when the bearings are mounted in housings with burrs, deformations, etc.
These misalignments will cause a reduction in the service life of the bearing as they cause additional loads in certain areas of the raceways.
On other occasions they can cause fractures and deformations in the cage.
Shaft and housing design
Due to the importance of how the areas where the bearings are to be mounted (shafts and housings) are machined, a series of general instructions are given which the manufacturer must keep in mind when designing these parts of the machine.
Design the shafts as rigid as possible, with maximum diameters and short lengths.
Maintain a good geometry between the shaft and the side seating, as well as a good surface finish.
The fillet radius "R" will always be smaller than the radius or chamfer of the bearing to avoid the existence of interference.
The height of the projection (db) will be less than the diameter of the inner ring, to facilitate future disassembly, but will also be as large as possible to make the shaft and seating as rigid as possible.
Shafts and housings
A polished finish is recommendable to minimize vibrations and noise as far as possible.
Therefore, on the bearing seatings on the shaft, a finish of at most 1.5 μm of Ra must be obtained. In the housings, a finish of at most 3 μm of Ra must be obtained.
If the above mentioned steps have been followed, the tapered roller bearings will now be mounted. In order to verify the correct operation of the equipment, the following steps must be kept in mind:
• Small machines will first be tested by turning the rotating parts by hand, and will thereafter be connected to the motor.
• For machines where this is not possible, they should be turned at low speed, and the speed must gradually be increased.
• The temperature increase is gradual, until it reaches a maximum after a certain time (1–2 hours according to application). Once this maximum has been reached and with an adequate lubrication system, the temperature will drop and stabilize.
• During the first rotations, check for possible friction which prevents the moving parts from rotating freely. The adjustments made may be very large.
• If slight bumps are noticed when rotating, there may be dirt present or, in the worst case, a damaged raceway.
If any anomaly arises, it is advisable to stop the operational test and fix the problem (cleaning, lubricating, replacing the bearing, adjusting the bearing, etc.).
Two main reasons exist for disassembling bearings. Either they are disassembled to carry out preventive maintenance and are mounted again afterwards, or they are disassembled because their service life has expired and they must be replaced.
In the first case, the care taken with the bearing will be the same as if they were new, and so the recommendations mentioned in this Chapter will be followed.
In the second case, the procedures may vary considerably and may even be severe, seeing as the bearings are to be disposed of. Nevertheless, care must be taken so as not to damage the seatings and shafts, as new bearings are to be mounted on these.
Either way, in both cases, it is recommendable to use the appropriate tools for these operations.