# Converting a screw's linear speed to rotational

Gold Member
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

CWatters
Homework Helper
Gold Member
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?

Gold Member
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

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.

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

Is that right?

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

Is that right?

4500 RPM looks good.

Gold Member
Thank you all :-)