Calculate the torque needed to drive a lead screw from axial force, screw lead and efficiency, or solve for the force, lead or efficiency instead.

Lead-screw model T = F ร— L รท (2ฯ€ ร— ฮท). Blank efficiency means an ideal screw (ฮท = 1); real screws need more torque.

Load force along the screw axis. Changing units converts the value.

Travel per revolution. For a multi-start screw, lead = pitch ร— number of starts.

Screw efficiency (optional)
Blank = 1 (ideal). Enter 0.30 for 30%. Acme screws are typically 0.2โ€“0.5, ball screws about 0.9.

Breakaway Torque Formula

The following example problem outlines the steps and information needed to calculate the Breakaway Torque.

BT = (BF ร— SL) / (2ฯ€)

Note: The formula above is the ideal (100% efficient, frictionless) lead screw relationship based on work per revolution, where SL is the screw lead (linear travel per revolution). For a more realistic estimate that accounts for friction and other losses, include an overall efficiency factor ฮท (greater than 0 and at most 1). Blank optional efficiency uses 1. This does not separately account for bearing/nut drag, inertia or static starting friction:

BT = (BF ร— SL) / (2ฯ€ eta)

Variables:

  • BT is the estimated screw drive torque (lbf-in)
  • BF is the axial load force (lbf)
  • SL is the screw lead (in)
  • ฮท is the overall screw efficiency (unitless, greater than 0 and at most 1)

To calculate a breakaway torque (ideal), multiply the breakaway force by the screw lead, then divide by 2 times pi. To include losses, divide by (2ฯ€ ร— ฮท) instead.

How to Calculate Breakaway Torque?

The following steps outline how to calculate the Breakaway Torque.

  1. First, determine the breakaway force (lbf). 
  2. Next, determine the screw lead (in). 
  3. Next, gather the formula from above = BT = BF * SL / (2*pi).
  4. Select Drive torque, enter the force and lead, optionally enter a verified screw efficiency, then select Calculate.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

breakaway force (lbf) = 300

screw lead (in) = 5

BT = BF * SL / (2*pi) = ?

BT = (300 ร— 5) / (2ฯ€) = 1500 / 6.283185… โ‰ˆ 238.7324 lbf-in