# Converting a screw's linear speed to rotational

1. Jun 23, 2013

### Femme_physics

Suppose I have a screw that makes some sort of frictionless ball bearing move at 0.3 m/sec

That means that to find the RPM of the screw I do

pi x mean diameter of the screw x rotation per minutes / 60000

2. Jun 23, 2013

### CWatters

You need to know the pitch of the screw thread. eg how many turns per meter.

If the thread has a very coarse pitch the ball will move a long way for each revolution. If the thread has a very fine pitch it will move only a short distance for each revolution.

EDIT: I assume by "linear speed" you mean the speed along the threaded rod parallel to it's axis not the velocity around the rod? Perhaps I misunderstand what you mean?

3. Jun 23, 2013

### Femme_physics

By linear speed I do mean axial speed, yes.

I know the pitch (it's 4mm)

I just can't find the formula that relates it to rotational speed, taking the axial speed of the nut into consideration

4. Jun 23, 2013

### vk6kro

The pitch is the distance the nut will move if the screw rotates once.

So you can work out how many times it rotates for the nut to travel 300 mm.

This gives revs per second, so you can work out how many revs per minute that is.

5. Jun 23, 2013

### Femme_physics

4mm x RPS = 300mm
RPS = 75
RPM = 75 x 60 = 4500 [RPM]

Is that right?

6. Jun 23, 2013

### Staff: Mentor

4500 RPM looks good.

7. Jun 23, 2013

### Femme_physics

Thank you all :-)