How to extend the service life of bearing units

Bearing units (bearings pre-mounted in housings) are being used more commonly in industry because they can be fitted easily and will function efficiently on various machinery parts. However, attention must be paid to the following points when mounting bearing units to ensure extended service life.

Some users have a wrong opinion that mounted units with spherical outside diameter bearings can provide a self-alignment function during operation. In fact, they only provide self-alignment when mounting. Therefore, you must make sure that the bearing and the housing are not misaligned during installation.

Another incorrect opinion is that eccentric collar bearing units can be used in situations where a setscrew system is used. For eccentric collar systems, the shaft and inner ring are connected together by fastening the eccentric collar in the direction of the rotation of the shaft. Although deformation of the inner ring seldom occurs, this system is not suitable for applications where the direction of rotation is sometimes reversed. The tightening direction of the collar when mounting the units should be the same as the rotating direction of the shaft, should the shaft rotate.

The followings are some points for mounting bearing units:

* Check critical dimensions of the mounted units. For example, it must be verified that shaft diameter, shaft height and the distance between the bolt-hole centres for pillow blocks are right for your applications. The shaft size, shaft height and the distance between the bolt-hole centre dimensions are listed in engineering details for the units.

* Check the quality of the frame in which the housing is to be mounted. Make sure the frame is clean and flat and that the rigidity is suitable to the application.

* Check the dimensions of the shaft. Make sure the fit between the shaft and the bearing is appropriate for the application. Some users may neglect this point as they may think the locking system will fix the units. Setscrews or eccentric collars are unable to completely fix the bearing units in the case of improper fit. Also, check for and remove any burrs the shaft may have.

* Make sure that the end of the setscrew is not protruding from the interior surface of the inner ring or the collar.

* Mount the housing of the unit securely on to the frame, or fix the bearing on the shaft first by tightening the setscrews. In addition, it must be determined the unit will not be subjected to any thrust, as this would cause premature failure of the bearings.

* Fasten the setscrews of the inner ring or the eccentric collar on to the shaft according to recommended tightening torques.

Bearing units, in most cases, do not require relubrication under common operating conditions because of their sealing performance. However, in some severe operating conditions, where they are exposed to contamination and high temperature, relubrication of the units is required at intervals through grease fittings in order to secure full performance and longer service life.

James Nie, M. Eng., MBA, is a project engineer with KML Bearing Canada Inc., Mississauga, Ont. For more information, use the reply number below.