# How can I couple two shafts along the same axis so that they rotate together, but only one shaft is allowed to drive the rotation?

This question is going to have an application in aviation (to avoid doubts and relocation to other forum) but let's concentrate on theoretical mechanics here.

If I want to allow a shaft to rotate in one direction and not the other, then a ratchet would be a solution.

But what I need is a base/platform with shaft M (master) and shaft S (slave) connected to each other the way that if I rotate shaft M (both clockwise or counter-clockwise) the shaft S will rotate with the same speed and direction. But if I try to rotate shaft S in any direction it should not be possible the same way as if I tried to rotate a ratchet in blocked direction.

What is the name of a device I described?

Shaft M --> ===[Device in]=== <-- Shaft S
_______________[question ]________ <-- platform

• Do you need to avoid back-rotation while the M-shaft is driving (i.e. the S shaft is trying to fight the M-shaft), or just avoid driving the M-shaft when it's not active? – Carl Witthoft Nov 15 '16 at 14:42
• Are you okay with activating the device in order to couple the master to slave? Or are you looking for a passive device? – Chuck Nov 15 '16 at 20:21
• @CarlWitthoft - Any "back-rotation" where the slave shaft "fights" the master shaft is the load torque. The rotational inertia attached to the slave shaft will mean that the slave-shafted system "wants" to stay at zero rpm. That is, a driving torque from the master side could also be viewed as an attempt to back-rotate the master. – Chuck Nov 15 '16 at 20:24