Bearing Design

The bearings in an electric motor are used to support the rotor, to bear the forces that are created in the motor and to position the rotor in the stator. The accuracy of the bearing design must meet high criteria to guarantee trouble free electric motor operation. If an electric motor is connected directly to the driven equipment, the bearing of the electric motor often absorb forces created in by the driven equipment. Rotor nl® electric motors are equipped with axially pre loaded and located bearings as standard.


Bearings are the only components in electric motors that are subject to wear. Standard bearing arrangement is usually sufficient to achieve the required service life where one bearing is located and the other bearing can axially slide in the bearing housing (loose fit). The service life of the bearings can, however, be shortened significantly by excessive load coming from the driven equipment or due to bearing damage resulting from operational conditions.

The damage that occurs when pumps or fans are the application and the motors have “floating” bearing arrangement, is referred to as “Brinell effect”. The use of “pre-loaded bearings” on both sides can sometimes eliminate the damage. The transferred force is effecting the outer ring of the motor’s bearing where the pump or fan has been mounted and the outer bearing ring of the axially “floating” bearing will start turning inside the bearing housing. This occurs at pumps or fans with a large moment of inertia (J fan ≥ 5 x J rotor) or when the fan is unbalanced or as a result of vibrations. The bearinghousing will wear out and also frictional corrosion between the outer ring and the bearing housing will occur, which may lead to the jamming of the effected bearing. The rotation of the bearing in the housing will be reduced by applying an axial pre-loading. Frequent bearing damage occurs when motors are stationary and subject to vibrations. When stationary, there is no lubrication film between the balls and the bearing races. The rotor moves as a result of external vibrations. This then leads to impact damage (a Brinell effect) of the balls in the races. The play can be reduced by applying axial pre-loading, which can limit the damage significantly. Pre-loaded bearing design is a good solution for specific application cases. There are many more bearing designs available, so a consultation with the electric motor manufacturer is recommended in order to establish the correct solution for specific applications or operating conditions.


Floating bearings

This bearing type is used, for example, for electric motors with sleeve bearings. There is (nearly) no mechanical axial force limitation in the bearings, which means that the shaft has free axial movement. Often the rotor centralizes in the stator finding its position through the electromagnetic forces. This position will be ideally determined when the motor is running at no load. A care must be taken when mounting a clutch as no axial forces must be applied on the shaft. Another example of floating bearing design is when the outer rings of both ball bearings slide in the bearing housings and are not axially located.

 


Located bearings

The outer ring of at least one ball bearing have been fixed in the bearing housing by use of a bearing cap or bearing retainer plate. The located (fixed) bearing cannot move in the axial direction. In comparison with the floating bearing design, the located bearing design prevents the rotor to move axially in relation to the stator. The only possible axial movement under normal circumstances is through the axial play in the ball bearings.

 


ROTOR STANDARD EXECUTION Enclosed + located bearings axially preloaded

The outer ring of NDE bearing is “located” in the bearing housing and on the shaft through the shaft circlip. The outer ring can not move axially in relation to the shaft. The located bearing is usually used in combination with an enclosed bearing design and when large axial loads are involved.

 


Enclosed + located bearings with increased axial pre-loading

Only one ball bearing can be located at a two ball bearing motor design. The other bearing should be able to slide axially in the bearing housing to compensate for any dilatation. Standard wavy washers are used for axial pre-loading or sometimes strengthened wavy washers and/or cup springs are used for increased axial pre-loading.

 



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