Back Drive Ball Screw: What It Is and How to Prevent It | Ball Screw Guide

Ball screw back driving occurs when an external axial force pushes the nut or screw and causes the screw system to rotate. Instead of the motor converting rotary motion into linear travel, the load converts linear motion back into rotation.

This behavior is especially important in vertical axes, lifting equipment and mechanisms exposed to external thrust. If power is removed and no suitable holding device is available, the load may move downward and rotate the ball screw, motor and transmission components.

Because a ball screw uses rolling contact and has relatively low friction, it should not normally be treated as a self-locking component. The practical engineering question is therefore not only whether back driving is possible, but whether the torque generated by the load is greater than the resistance and available holding torque of the complete drive system.

Important safety note: Friction, preload or motor detent torque should not be used as the only load-holding measure in a safety-critical vertical axis. A suitable brake, mechanical lock, counterbalance or other independently verified safety device may be required.

During normal operation, motor torque rotates the screw and produces axial movement. During back driving, the direction of energy transfer is reversed. An axial force acts through the ball contact path and generates rotational torque at the screw shaft.

In a horizontal machine, this axial force may come from a spring, process force, pressure or external impact. In a vertical axis, gravity continuously applies axial force through the moving mass. Once the drive torque or brake is removed, this force may cause the screw to rotate.

The approximate torque generated at the ball screw shaft by an external axial load can be calculated as follows:

T_b = F × L × η_b ÷ (2π × 1000)

Use this form when the lead is entered in millimetres and the result is required in N·m.

T_b = torque generated by back driving, in N·m
F = axial force acting on the ball screw, in N
L = ball screw lead, in mm/rev
η_b = estimated back-driving efficiency
2π = conversion between one revolution and angular motion
1000 = conversion from millimetres to metres

Back-driving efficiency is not automatically identical to forward efficiency. It depends on the ball screw design, preload, lubrication, seals, installation condition and operating state. For final brake or safety-system selection, use verified product data and consider the resistance of the complete transmission rather than relying on an assumed efficiency value alone.

Consider a ball screw exposed to an axial load of 1,000 N. The screw lead is 10 mm, and an estimated back-driving efficiency of 0.80 is used for an initial engineering calculation.

Given values:

F = 1,000 N
L = 10 mm/rev
η_b = 0.80

T_b = 1,000 × 10 × 0.80 ÷ (2π × 1000)

T_b ≈ 1.27 N·m

In this example, the axial load can generate approximately 1.27 N·m at the screw shaft before other system losses are considered. This value is not automatically the final brake specification. The brake selection must also account for load variation, acceleration, transmission ratio, brake location, allowable stopping distance, wear, temperature, mounting orientation and the required safety margin.

A simplified preliminary comparison can be expressed as:

Comparison	Likely result	Required action
Load-generated torque exceeds system resistance	The axis can back drive	Provide controlled holding or braking
Holding torque is only slightly above calculated back-drive torque	Small changes may cause movement	Review the safety margin and worst-case load
Verified brake torque exceeds the design requirement	The brake may hold the axis as designed	Confirm brake response, wear and failure behavior

System resistance may include ball screw drag torque, seal resistance, support-bearing friction, gearbox resistance and motor-related torque. However, these values can change with lubrication, temperature, wear and assembly condition. For this reason, measured friction should not replace a positively acting holding device where uncontrolled movement could create a hazard.

Back-drive torque increases in direct proportion to axial force. In a vertical system, the calculation should include the moving mass and other forces acting in the load direction. Dynamic loads, process forces and unexpected impacts may be higher than the normal static load.

A larger lead produces more shaft torque from the same axial load. For example, if the load and efficiency remain unchanged, changing from a 5 mm lead to a 10 mm lead approximately doubles the calculated back-drive torque.

Lead selection must still consider speed, motor torque and positioning requirements. A smaller lead may reduce the torque generated by back driving, but it should not be treated as an independent safety lock.

Efficient lubrication, low-resistance seals and correctly aligned bearings reduce the torque required to move the assembly. These are desirable operating characteristics, but they also mean that less axial force may be required to back drive the screw.

A powered motor may resist load movement during normal operation. During a power failure, emergency stop, drive alarm or cable fault, this active holding torque may disappear. The machine design must define what happens to the load under each of these conditions.

A brake-equipped servo or stepper motor is commonly used to hold a stationary vertical axis. The brake is generally intended for holding rather than repeated dynamic stopping unless it is specifically rated for that duty. Brake torque should be checked at the actual shaft where the brake is installed.

A mechanical lock, safety nut, clamp or locking pin can provide an independent holding function. This may be necessary where personnel could enter the hazardous area or where a dropped load would cause serious equipment damage.

A counterweight, gas spring, pneumatic cylinder or other balancing mechanism can reduce the net axial force acting on the ball screw. Reducing the effective load also reduces the torque that the holding system must resist.

A belt or gearbox changes the torque and speed seen by the motor-side brake. When a reduction mechanism is used, calculate the torque at the brake shaft and include the transmission efficiency. Do not compare screw-shaft back-drive torque directly with motor-brake torque without accounting for the transmission ratio.

Selecting a smaller lead reduces calculated back-drive torque for the same axial load. However, it also changes the required screw speed, motor torque and linear travel per revolution. Lead should therefore be selected as part of the complete motion design rather than as a substitute for a brake.

Increasing nut preload raises internal resistance and may reduce the tendency to move under a small external force. However, preload also increases drive torque, temperature and sensitivity to alignment errors. Its main purposes are backlash control and axial rigidity, not fail-safe load holding.

Preload and lubricant condition can change during service. A vertical axis that remains stationary during an initial test may begin to move after running-in, relubrication or wear. The holding strategy should therefore be based on a verified brake or mechanical safety device rather than uncertain internal friction.

Before calculating the back-driving torque or selecting a holding system, confirm the following data:

Maximum axial load, including moving mass and process force
Ball screw diameter and lead
Vertical, inclined or horizontal mounting direction
Moving screw or moving nut structure
Direct drive, belt drive or gearbox ratio
Motor and brake holding torque
Required stopping and holding behavior during power loss
Need for a counterbalance or independent mechanical lock

For a broader explanation of load behavior in an upright mechanism, see using a ball screw to hold a vertical load. You can also review the relationship between lead and operating speed in high-speed ball screw selection.

Ball screw back driving is the reverse conversion of axial movement into screw rotation. Its likelihood and severity depend mainly on the axial force, screw lead, back-driving efficiency and resistance of the complete drive system.

Calculating the load-generated torque provides a useful starting point, but it does not complete the safety assessment. Vertical and safety-critical axes should include a properly selected holding device and should be evaluated under maximum load, power-loss and component-failure conditions.

DLY supplies rolled C7 and ground C5 ball screws with different diameters, leads, nut structures and shaft-end machining options. Load direction, speed, lead, accuracy and holding requirements should be confirmed before selection.

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