Rack and Pinion Spring Return Actuator Torque

[Sid89]

Why the torque output in the rack and pinion actuator during air stroke (when springs are being compressed) is less than the torque generated by the spring return stoke? As per Hooke's law applied force on the spring should be equal to the restoring force of the spring so the torques should be the same! F=kx should be equal to F=-kx. Why this theory is not valid in spring return Rack and Pinion actuators? Am I missing something here?

 

[Compositepro]

Your logic is flawed. Air pressure compresses the spring and supplies torque in one direction. The return torque is supplied by the spring only.

 

[Sid89]

Yes. That's correct but why does the magnitude of both torques vary? Why they are not the same ... torque generated during spring compression stroke should be equal to the torque generated during spring expansion stroke!

 

[Compositepro]

Springs follow Hook's law, so spring force is not constant though it's stroke. Air pressure can be varied over a wide range. So why would you expect the torque ratio to be a constant, let alone a specific ratio of "one".

 

[3DDave]

A free-body diagram including the pinion will answer this question when comparing what is required to provide torque and overcome the spring as opposed to the spring applying torque on its own. You'll see that varying the air pressure changes the available torque from not enough to turn the valve to any amount required, indpendent of the spring, hence they are unlikely to be equal.