Essential Tips for Using Bearings in External Rotor Motors

Outer-rotor motors are a common form in motor technology. Most discussions in bearing application technology focus on inner-rotor motors, with outer-rotor motor applications often only mentioned in passing without detailed analysis.

Frequent inquiries about the selection and application of bearings in outer-rotor motors make it necessary to provide some basic explanation.

As the name suggests, an outer-rotor motor has a stator and shaft that remain stationary, while the outer iron core rotates. The operating conditions of the bearings in this setup differ from those in inner-rotor motors.

1. From the Perspective of Load

In an inner-rotor motor, the shaft and the inner ring of the bearing rotate, while the motor base and the outer ring of the bearing remain stationary. The rolling elements rotate within the outer ring of the bearing, driven by the inner ring, primarily bearing the load in the outer ring’s load zone, with the inner ring bearing the load all around.

For outer-rotor motors, the shaft and the inner ring of the bearing are fixed, while the outer rotor iron core and the outer ring of the bearing rotate. The rolling elements rotate on the inner ring, driven by the outer ring, bearing the load in the inner ring’s load zone, with the outer ring bearing the load all around.

Consequently, when we disassemble an outer-rotor motor, the load traces on the outer ring of the bearing should be distributed across the entire raceway, while the inner ring of the bearing only has load traces in some areas.

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2. From the Perspective of Tolerance Matching

From the previous load analysis, we can observe that when the motor bears a static load, the inner ring of the bearing in the outer-rotor motor bears a fixed load, meaning the load is carried in the fixed part of the bearing’s inner ring, with no relative rotation. As for the outer ring of the bearing, the load rotates all around the entire raceway.

Therefore, at this time, the inner ring of the motor bearing uses clearance fit, and the outer ring of the bearing chooses transition fit. The principle of matching between the bearing and the shaft, and the bearing housing, is opposite to that of the inner-rotor motor. This can be referred to in the third line of the form in the tolerance matching selection principles below:

LegendBearing Rotational Conditions Load CharacteristicsFitting
Static Load 
Inner Ring Rotation
Outer Ring Stationary
Inner Ring Rotational LoadInner Ring: Interference Fit
Rotational Load 
Inner Ring Stationary
Outer Ring Rotation
Outer Ring Static LoadOuter Ring: Clearance Fit 
Static Load
Inner Ring Stationary
Outer Ring Rotation
Inner Ring Static LoadInner Ring: Clearance Fit
Rotational Load
Inner Ring Rotation
Outer Ring Stationary
Outer Ring Rotational LoadOuter Ring: Interference Fit 

According to the principle of tolerance fit, an appropriate tolerance recommendation can be selected from the tolerance fit chart. A simple method would be to choose based on the tolerance fit of the internal rotor motor, retaining the tolerance bandwidth and swapping tolerance band grades.

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3. Bearing Configuration of Outer Rotor Motors

Like inner rotor motors, the iron cores inside outer rotor motors do not dissipate heat as well as the exterior. In this case, the stator of the inner rotor motor doesn’t cool as efficiently as the rotor, leading to significant axial and radial expansion of the stator.

Therefore, the bearing configuration of outer rotor motors is largely similar to that of inner rotor motors, requiring one fixed end and one floating end.

If preload is needed on the floating end bearing, it is generally applied on the outer ring of the bearing in inner rotor motors; for outer rotor motors, the opposite is true – the preload should be applied on the inner ring of the bearing. Hence, wavy springs typically used for outer ring diameters, whose spring force will be offset by a tight fit, cannot serve the purpose of preloading.

4. Lubrication of Outer Rotor Motor Bearings

In outer rotor motors, the outer ring of the bearing rotates, driving the grease to revolve, which, compared to the inner ring rotation, generates a larger centrifugal force. This increases the circulation speed of the grease within the bearing.

Therefore, when choosing a dust cover style bearing, outer rotor motor bearings are prone to grease leakage. As a result, it is generally not recommended to use dust cover (-2Z) bearings for outer rotor motors.

Regarding the selection of bearing lubricant, the radial diameter of the load zone in outer rotor motors is relatively smaller, with a smaller load zone area compared to the same bearing in outer rotor motors. Therefore, it is advisable to use a lubricant with a higher viscosity than that used in inner rotor motors.

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On the other hand, with respect to lubrication intervals, halving the lubrication time intervals for outer rotor motors compared to inner rotor motors will be more beneficial for bearing operation.

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