Interference Fit, Rotor Core and Shaft

Intereference fit, rotor core & shaft

As it is already noted in previous posts, the rotor core is interference fitted to the shaft. This radial interference fit value is optimized in order to provide adequate torque to transmit torque under full rotating speed.

In general the interference fit analysis should be carefully performed since it can affect the performance of the e-motor significantly. Too small interference fit values can cause loosening of the rotor core and heavy NVH issues. Too big interference fit value on the other hand can cause microcracks and other abnormalities on the rotor core and on the shaft.

 

There are two basic interference fit methods:

  • Shrink fit
  • Press-fit

The shrink fit is applied with the used of temperature, e.g. induction heating and the pressfit is applied with the use of special machines that apply large forces on one of the mating parts.

The analyst should bear in mind that the interference fit behavior is not something constant and can change due the environment, loading conditions, material, etc. Interference fit preload is also a significant factor that should be taken into account for other subsequent analyses.

During press-fit large forces can act on the mating parts. Some common issues are related with the bending of the shaft. Hence, carefull analysis is required for both the method and the value of the press-fit that is going to be used. In addition, parts should be thoroughly cleaned.

 

FEA analysis of an interference fit

It should be noted again that the proper analysis of an interference fit for e-motor applications is very significant for the robustness and error-free operation of an e-motor.

A symmetric approach is used for the following example. It is reccommended that the mating parts on the region of interference to be modeled with hex elements and node to node coincidence, otherwise there are significant chances that the results are wrong.

Below are some simulation results of a simplified case for the interference fit of a rotor core and a shaft. Symmetry is used.

Hex and node coincidence mesh of an interference fit between rotor core and shaft.

 

Stress results of press-fit between shaft and rotor

Results of contact pressure between shaft an rotor.

 

E-motor has a plethora of interference fits between the components. Study of interference fit should start from initial stage in the most simplified version possible. When the design stages develop, the simulation of interference fit should increase the level of detail and capture as many phenomena as possible. For a full analysis and robustness of interference fitted parts various phenomena/loads/conditions should also be taken into account:

  • Temperature
  • Other preload effects
  • Rotational velocity
  • Materials
  • surface finish
  • method of interference, e.g. shrink fit or press-fit
  • etc…