How to change the VSR（vibratory stress relief) motor tachometer board?-relieving residual stress

It is a parts composed by a light-emitting diode to feedback the motor speed.
Beam is interrupted uniformly by the embedded motor(of vibratory stress relief) gear when the motor rotates.
It reflects the motor speed one minute correctly.
When power is opened and motor safely shut down after high- speed run , it shows speed board is damaged and doesn’t work .