AN EXAMPLE OF LOOSE FIT EFFECT BETWEEN ROTOR AND SHAFT OF AN E-MOTOR

Interference fit method is commonly used on electrical machines for the assembly of rotor core and shaft. Proper selection of clearance values is of vital importance for the rotor dynamic stability. There are two common methods that are used for securing the rotor core to the shaft, one is with the use of temperature and is called shrink fit and the other with the application of force from e.g. hydraulic machines, known as press-fit.

A proper value of interference fit should take into account the following:

 

  • ¬†stress state when the machine is idle
  • stress state at different duty operations (e.g. values of rpm)
  • temperature limits
  • surface finish
  • materials
  • friction coefficient
  • steady and transient effects
  • perturbations of interference fit on shape of mating parts (e.g. buckling)
  • non-linear effects
  • etc..

 

One of the most important parameters for this analysis is the contact pressure Pc. This value give us significant information regarding the security of the interference fit against various loads and machine operations. It can be stated that Pc is a function of many parameters and also on time. One simple case study is shortly examined below where rpm variations are examined against contact pressure.

For the following example two subcases are involved

  • interference fit
  • rpm load

 

The simplified rotor assembly is shown below:

Rotor shaft assembly case study

 

 

 

 

 

 

 

 

 

 

A symmetric analysis is performed (which can be simplified even more)

 

 

 

Picture below depicts two cases. Case on the left is with PF only, and case on the right is with PF and max RPM. As it is shown on the right picture, there is no contact anymore between the rotor pack and the shaft.

Comparison of contact pressure between two subcases: Case on the left is with PF only and case on the right is with PF and RPM. Zero contact pressure values on maximum RPM.

 

Graph below illustrates the contact pressure vs RPM values. X axis is the RPM axis and Y axis is the contact pressure

Pc vs RPM. Higher values of RPM lower the contact pressure up to the point where the contact pressure becomes zero.

 

The above phenomena can have significant effect on the NVH performance of e-motor and eventually can lead to machine breakdown.