The impact of one-sided magnetic pull on motor bearings
One-sided magnetic pull is a common concept in motors, specifically referring to the magnetic pull generated by an unbalanced motor air gap due to misalignment of the motor stator and rotor centers.
Even for typical horizontal motors, the possibility of unilateral magnetic pull may arise, even without considering process deviations.
The most common reason is deflection caused by the weight of the motor shaft itself, which leads to a deviation in the air gap above and below the center of the stator and rotor, as shown in the figure below:
At this point, the upper air gap of the engine will increase by 𝛅1, while the lower air gap will decrease by the same amount. When the motor is operating, the upper and lower air gaps are not equal, therefore the magnetic flux densities are different, leading to unequal magnetic pull forces acting up and down the rotor. This difference in magnetic pulling force constitutes what we call unilateral magnetic pulling force.
In fact, the engine rotor itself experiences a magnetic attraction force around the entire air gap. However, when the air gap is consistent, the magnitudes of these forces are equal in the circumferential direction, but have opposite directions, canceling each other out.
For horizontal motors, the bearings at both ends must support not only the weight of the motor rotor (assuming no other additional axial and radial loads), but also an additional radial load – the one-way magnetic pulling force.
Therefore, when engineers perform bearing life verification calculations, they must take the one-sided magnetic pulling force into consideration, incorporating it into the radial load of the motor bearings to make an accurate selection.
Does the influence of unilateral magnetic pull force need to be considered for all horizontal motors?
From the above analysis, it can be seen that even without considering the process deviation, the unilateral magnetic pull force can still occur in horizontal motors.
So, this means that we can draw a conclusion: when carrying out bearing life check of all horizontal motors, should the influence of unilateral magnetic pulling force be taken into account?
From a design perspective:
The cause of one-sided magnetic pull is closely linked to the bending of the motor shaft, which results in uneven air gaps and subsequently leads to pull. Engineers need to be aware that for engines prone to flexing, this effect should not be neglected, especially in cases where the rotor has substantial weight or features a slender structure.
On the other hand, for compact or lightweight motors where shaft bending is minimal, the unilateral magnetic force due to bending can be practically disregarded. Therefore, when checking bearing life for these types of motors, one-way magnetic pull does not need to be taken into consideration.
From a process and manufacturing deviation angle:
Despite manufacturing deviations and process inaccuracies, motors can still produce one-way magnetic pull. The degree and location of such discrepancies are key areas of control for engineers.
Therefore, if there are tolerance controls for uneven air gaps and the maximum tolerance value results in a one-sided magnetic pull significant enough to affect the weight of the rotor, engineers must incorporate this factor when checking motor bearing life. On the other hand, if the attraction is insignificant, it need not be considered.
