Ball Screw Lead Selection: Balance Travel Speed and Drive Torque

Increasing ball screw lead can reduce the rotational speed needed for a specified linear movement. It can also change the torque required to produce the same axial force. For machinery buyers and engineers, ball screw lead selection is therefore a system decision involving the screw, motor, support arrangement and motion profile.

A catalogue description such as high speed or large lead identifies a design direction. It does not prove that the assembly suits a particular machine cycle. A useful enquiry begins with the travel distance, working force, acceleration and positioning requirements, then compares candidate leads using consistent assumptions.

Distinguish lead from pitch

Lead is the axial movement produced by one revolution of the screw. Pitch describes the spacing between adjacent thread forms. The terms may coincide for some geometries, but they are not universally interchangeable. Use the manufacturer's lead designation when calculating motion and identifying the order.

Write the required units clearly. A lead in millimetres per revolution and a linear speed in millimetres per second can be related directly after accounting for seconds and minutes. Confusing these units can create a motor speed requirement that is wrong by a large factor.

Retain the drawing and complete model designation with the calculation. Two screw assemblies of the same nominal diameter can have different leads, nut constructions and ratings. The outside diameter alone is not sufficient to establish equivalent performance.

Compare leads with a simple speed calculation

For a rotating screw with a nonrotating travelling nut, linear speed equals lead multiplied by rotational speed. When lead is in millimetres per revolution and speed is in revolutions per minute, divide the product by 60 to obtain millimetres per second.

As an illustrative calculation, a target of 200 millimetres per second requires 2,400 revolutions per minute with a 5 millimetre lead, or 1,200 revolutions per minute with a 10 millimetre lead. These are kinematic results, not approved operating limits for a Wangong product.

This explains why large lead ball screws can be relevant to faster travel. It does not finish selection. Compare the calculated speed with the permitted speed for the actual shaft length, support arrangement and nut construction, including the manufacturer's applicable limiting criteria.

Evaluate the full motion cycle

A machine rarely spends its entire cycle at maximum travel speed. Record acceleration, deceleration, constant speed travel, dwell time and any reversal. Identify the working force during each part of the cycle, especially where machining, pressing or lifting loads occur.

A lead that looks attractive for a long rapid move may create a less convenient torque demand during a short loaded move. Conversely, reducing rotational speed can be useful where the shaft's speed limit becomes restrictive. Compare the complete cycle rather than selecting from one headline velocity.

Give the motor supplier the same cycle used for the screw review. Peak torque and sustained heating are different checks, and they should use consistent load and timing assumptions. Leave uncertainty visible when a process force or acceleration is still provisional.

Keep positioning requirements separate from travel rate

A smaller lead produces less axial movement for a given angular increment. That geometric relationship does not guarantee a particular positioning accuracy in the assembled machine. Accuracy, repeatability, stiffness, backlash, thermal effects and control performance are distinct considerations.

Describe the positioning requirement at the point that matters to the machine. A controller's commanded increment should not be presented as demonstrated motion accuracy. The final outcome depends on the screw assembly and the surrounding mechanical and control system.

If a lead is changed during design, review the control parameters and encoder relationship as well as the hardware drawing. A replacement that physically fits can still change travel per motor revolution and therefore the machine's motion scaling.

Turn the comparison into a traceable specification

The final order should identify the selected lead, diameter, nut type, accuracy requirement, preload where applicable and end support arrangement. Attach the agreed drawing and note the operating assumptions used in the review.

For a revision, record why the lead changed and which motor, control and mechanical checks were repeated. This keeps a later purchasing substitution from undoing the reasoning behind the original selection.

Ball screw travel speed is an important requirement, but it belongs beside force, torque and positioning behavior. A selection based on the full cycle gives the engineer a defensible configuration and gives procurement a clear description of the assembly that must be supplied.