Electrical Machines
The armature shaft must be able to with stand

 Any unbalanced magnetic pull on the armature core.
All of these
 Bending moment due to weight of the armature.
 Twisting stains due to transmission of torque.

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Electrical Machines
The armature mmf of a DC machine has

 stepped distribution and rotates at the speed of armature.
  triangular space distribution and is stationary in space.
  triangular space distribution and rotates at the speed of armature.
  trapezoidal space distribution and is stationary in space.

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