Lead Screw Torque Calculator

Inputs

Axial load (F)

Working thrust load the screw must move before service factor.

Screw lead (L)

Linear travel per screw revolution. For single-start screws, lead equals pitch.

Screw efficiency (eta)

Use an estimated mechanical efficiency. Acme screws are often much lower than ball screws.

Service factor (SF)

Multiplier applied to the working load before sizing motor torque.

Available motor torque (T_m)

Usable screw torque after couplings, gearboxes and speed derating.

N·m

Screw speed (n)

Rotational speed of the screw.

rpm

Pitch diameter (d_p)

Approximate screw pitch diameter used for lead-angle calculation.

Root diameter (d_r)

Smallest screw diameter, used for critical-speed screening.

Unsupported length (L_s)

Distance between effective bearing supports.

End support

Bearing support condition for the critical-speed estimate.

Screw material

Material used for the critical-speed estimate.

Default result

Required motor torque(T_req)

2.842N·m

Pass

Design load * lead / (2*pi*efficiency).

Also computed

Available thrust(F_avail)1,759N

395.5 lbf

Motor torque converted back to linear force through the screw.

Torque utilization(U_T)0.7105

Required torque divided by available motor torque.

Linear speed(v)50mm/s

Screw lead times RPM.

Speed utilization(U_n)0.1577

Operating RPM divided by 80% of calculated critical speed.

Recommended max speed(0.8 n_cr)3,806rpm

Critical speed(n_cr)4,757rpm

Method notes 4 notes

Torque uses T = F*lead/(2*pi*efficiency), with the entered service factor applied to the working axial load.
Available thrust reverses the same relation from motor torque to linear force. Use usable running torque, not only stepper holding torque.
Critical speed uses a uniform circular screw-shaft bending model from root diameter, unsupported length, material stiffness and bearing support condition.
The recommended speed limit is 80% of calculated critical speed. Manufacturer charts should control final screw selection.