# Power to rotate shaft in bending

1. Jul 8, 2009

### Simon Cohen

How do you calculate amount of power needed to slowly rotate shaft under bending moment? This is not related to transmittal of torque.

Given:
shaft size
span between support bearings
bending moment value
rpm

Assume: slow rotation (ignore centrifugal forces caused by deflection)

2. Jul 8, 2009

### vin300

To set a stationary shaft into rotation needs application of torque.
Do you need the shaft to rotate around the central axis along its length or the central axis perpendicular to length?
"Bending moment" is torque.
The moment of Inertia, given by I=M{[R^2]/4 + [L^2]/12) muliplied by angular acceleration gives the torque that you know.
Power is supplied only until it accelerates.Fdx/dt is the power, but to produce the same rpm with the same torque F varies as F=T/r, where r is the distance of point of application of force from the centre, power varies inversely as r for the same torque.

Last edited: Jul 9, 2009
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