Today is...
Tuesday, September 17, 2019
Welcome to, the global online
community of automation professionals.
Featured Video...
Featured Video
A tutorial introduction to programming using the QuickBuilder Programming Environment.
Our Advertisers
Help keep our servers running...
Patronize our advertisers!
Visit our Post Archive
Ball screw Gear Ratio?
Does anyone knows about gear ratio of Ball-Screw?

Does anyone knows about gear ratio of Ball-Screw?
When I'm modeling the motor & transmission dynamics, there I must calculate effective mass of motor and there is ball-screw in transmission.

'motor' - gear(1:4) - 'ball-screw'

To know about the effective mass, I must know the gear ratio of ball screw. But through ball-screw, torque is converted into force which tansformation I can not calculate gear ratio.
Is it the (rev/m)? or (ball-screw diameter/lead)?

I wish somebody can help me.

Been dealing with ballscrews a long time and have never heard it refered to as a gear ratio. The load to torque calculation is Load (pounds) times lead (inches) divided by 5.65 gives you torque in inch-pounds. The diameter of the screw has no bearing on it's mechanical advantage. One rev of a 1" dia with a 1/4" lead screw is going to take the same torque as a 2" dia with a 1/4" lead (not considering added mass of the larger screw).

I hope this helps because I don't know of a "gear-ratio" to throw at you.

Good Luck.


By George Younkin on 18 July, 2002 - 3:48 pm

In working with ball screws you refer to the lead of the screw as [in/rev].

To reflect the machine slide inertia to the motor use:
J of slide @ motor=W * (L/N)^2 x 0.0000656= (lb-in-sec^2)
W= Total load weight [lbs]
L= Ball screw lead [in/rev]
N= Drive ratio

To calculate the reflected inertia of the ball screw to the motor use:
J of the screw @ mot.=[DIA^4xLGTHx7.2x10^-5]/N^2 = [lb-in-sec^2]
DIA= Screw diameter [in]
LGTH= Screw length [in]
N= Drvie ratio
L= Screw lead [in/rev]

If you want more information, contact me with your FAX number.
George Younkin, P.E.

By Martin Davenport on 5 February, 2019 - 4:41 pm

I think "gear ratio" could mean that if you have a pitch of 5mm of a single start screw on a thread diameter of 25mm would give a ratio or mechanical advantage of (25xPi)/5 so 15.71:1

This could be useful to calculate the conversion of say motor torque referred at the nut and also to work out referred inertia (15.71)^2.

Or am I missing a trick and there is an easier way to arrive at these useful values amongst others.