Shaft Loads & Design Criteria, e-Motor

Overview

Shaft material and optimum design are important for the efficient and trouble-free operation of an e-motor. Figure below illustrates an example of shaft for e-motor.

A keyed shaft for e-motor applications

 

Some criteria on the shaft material selection are the following:

 

  • mechanical properties
  • machinability
  • wear resistance
  • hardness
  • cost
  • rigidity
  • thermal properties
  • etc..

 

The most common material for shafts is the carbon steel. The amount of carbon determines the mechanical and thermal properties of the shaft. When carbpon is increased there is an increase in hardnsess, yield and ultimate strength strength but the material becomes more brittle.

Heat treatment is beneficial for the shaft since it helps on the following:

  • Hardness
  • Yield strength
  • Ultimate tensile strength
  • fatigue strength

 

Cold rolled or hot rolled carbon steel can be used for manufacturing motor shafts. The advantages of cold rolled method are fine surface finish, improved strength properties and high dimensional tolerances. However with the cold process residual stresses can be created.

Special steel alloys are also used for very demanding applications.

The following list is an example of some important loads than shaft experiences:

  • torsional loads
  • transverse forces, e.g bending
  • Axial loads, for special applications
  • Overhanging part, bending moment for transferring of torque to GB
  • Motor resonance, shear stresses
  • Preload forces
  • Shock loads

Various phenomena

Dynamic Shaft Runout

Dynamic shaft runout is when a shaft does not rotate around its true center. This may be caused due to the following

  • shaft deflection
  • shaft imbalance
  • machining tolerance
  • bearing misalignment
  • vibrations
  • etc.

Special machines such as coordinate measuring machines can be used which can measure this phenomenon for specific applications

Shaft eccentricity

Various factors can contribute to the shaft eccentricity such as:

  • shaft deformation
  • unbalanced forces
  • shaft to bore missalignment
  • bearing clearances
  • dynamic ran out of shaft

Shaft eccentricity should be kept to the minimum to optimize motor performance and general NVH behavior. Some remedies to minimize shaft eccentricity include:

  • balancing of rotor
  • inspection of interference fitted parts
  • minimize overloading
  • use of flexible couplings

Simulation

An FEA analysis of a simplified keyed shaft for e-motor applications is shown below. The response of the shaft on the loads applied onto it is a very complicated mechanism and FEA tools are strongly recommended to drive the design process, especially for keyed shaft connections.

 

 

In the early design phase it is important to perform various FEA simulations for the shaft and the interacting components. Loading conditions can include

  • rpm
  • pre-stress effects
  • interference fitted parts
  • thermal loading
  • dynamic loanding (random and shock loading)
  • etc.

The following topics are important for the analysis:

  • keyed shafts and stress concentration
  • complicated shear mechanisms on key slots
  • Spline shafts stress concentrations
  • heat treatment process
  • tapered shafts and keyless motor designs
  • Shaft manufacturing methods
  • Hollow shafts and connection techniques on sub-parts of shaft