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Bearing load, life and lubrication

General

The assumption is usually made when constructing the electric motor that the electric motor will be coupled to a machine with a flexible couple or a V-belt transmission. However, electric motors are also often coupled to the machine directly. Therefore, the electric motor bearing is also directly loaded with axial and/or radial forces originating from that machine. Usually such a load is significantly larger than the load of the electric motor itself. The total axial and/or radial ­load in combination with the rotational speed and the applied bearing design must always be taken into account when calculating the bearing service life.

 

The service life of grease-lubricated bearings depends on the following factors as well as others:

 

  • total axial and/or radial load of the bearing - bearing type ‑peripheral velocity of the bearing (in part dependent on the electric motor speed)
  • temperatures of the bearing and the bearing grease - quality and lubricating properties of the bearing grease
  • operational conditions (for example, influence of moisture, pollutants, external vibrations, etc.)

 

Ball bearings or cylindrical bearings?

Electric motors are provided with ball bearings by preference. Only when the radial load is too high (for ball bearings) must cylindrical bearings (NU bearings) be used on the drive end (DE). They can absorb great radial forces but the disadvantage is that they have to be re-lubricated twice as often when compared to ball bearings.

A relatively high peripheral velocity is involved with regard to large bearings in combination with high rotational speeds. This means that the grease is mechanically loaded heavily, which means that the soap structure of the grease will deteriorate must faster due to pulverization. The service life of the grease also depends on the set-up of the electric motor. The theoretical service life is halved when set up vertically compared to it being set up horizontally. External vibrations also have a negative influence on the service life of the grease. “Bleeding” may occur where oil seeps out from the grease.

The bearing temperature is not only determined by the heat development in the bearing with regard to electric motors but by the added heat from the electric motor or heat that is transferred at the electric motor shaft and originates from the machine. It is important to know that 2-pole electric motors (3000 min-1) have a higher rotor temperature when compared to multiple-pole electric motors (1.500 and 1.000 min-1). The same applies to the use of electric motors on a voltage/frequency inverters where usually the loss in the rotor is a lot larger. This will also cause a higher temperature of the rotor shaft and subsequently of the bearings.

 

Open bearings

If, however, in case of open bearings, a favorable operational temperature of the bearing will in part be realized if equipped with the correct quantity of grease that is required to guarantee a good lubrication. The quantity of grease in a bearing design loses its lubricating properties as a result of mechanical load, ageing and increasing levels of contaminants. The grease in open bearings must, therefore, be supplemented or changed from time to time. An excess of grease, however, will make the operational temperature of the bearing rise quickly, especially with high rotational speeds. It can be stated as a general rule, that only the bearing and the free clearance in the bearing house must be partially filled with grease (between 30 and 50%).

 

Service life calculation

If the axial and/or radial shaft loads (from the machine) are known, Rotor B.V. can combine them with the data of the electric motor type and make a theoretical service life calculation by using computer software. This, however, only provides an indication since the attainable practical service life will greatly depend on the above mentioned factors as well as other factors. Rotor nl® electric motors can also be supplied with a special bearing design that has been tailored for specific application and/or operational conditions (also see “Maintenance and operating instructions”).



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