For a 45 kg vertical table, 15 mm/s travel speed and 5 mm screw lead, the calculated load torque is 0.426 N·m, or 0.86 N·m with a safety factor of 2 [S1].
This selection applies to screw-driven vertical axes where power loss can cause back-driving, covering the ball screw, motor brake, support arrangement, screw accuracy, critical speed and service-life checks.
Define the Vertical-Axis Duty and Failure Exposure
A vertical axis must preserve its position during a power failure, so the mechanical design must combine a suitable screw and drive train with a positive power-off holding brake.
Counterweights, mechanical hard stops, lock springs and caliper brakes can supplement the system, but they should not replace controlled power-off holding. Hard stops can deform threads or damage ball bearings and the return system, while a dog stop is suitable only as redundant end-of-travel protection [S3] (2016-08-02). A vertical-axis design should therefore specify controlled deceleration, electrical release of the brake, brake feedback, and redundant travel limits.
Record the total moving mass, external force, travel speed and tolerance, stroke, screw length, lead, efficiency, duty cycle and required holding mode before selecting commercial components. Oriental Motor’s example uses a 45 kg table, zero external force, 15 ± 2 mm/s speed, 800 mm overall screw length, 20 mm shaft diameter, 5 mm lead and 0.9 screw efficiency [S1].
Calculate Axial Load, Torque and Motor Duty
The governing load is the suspended mass multiplied by gravitational acceleration, plus any external axial force and the friction contribution dictated by the axis orientation. The Oriental Motor example specifies a 45 kg table-and-load mass, a 90° ball-screw inclination, a 5 mm lead, and a ball-screw efficiency of 0.9 [S1].
The resulting 0.426 N·m load torque is then multiplied by a safety factor of 2 to produce 0.86 N·m [S1]. This calculation is an example, not a universal margin: the final value must reflect the actual mass, lead, efficiency, preload, duty cycle, shock load and permitted drop tolerance.
At 5 mm lead and 15 mm/s, the screw requires 180 r/min. A 1,450–1,550 r/min four-pole motor therefore needs a calculated gear ratio of 7.1–9.9, with ratio 9 selected in the example.
Compare Lead, Diameter and Drive Arrangement
A long lead improves motor-side speed and torque requirements but usually reduces axial force capacity and can worsen back-driving. A short lead increases axial force per motor torque but demands more motor speed for the same table speed. Ball-screw diameter and end-bearing arrangement must therefore be checked together, not selected from lead alone. [S1]
The comparison below separates four practical options for a vertical load. The figures are decision limits, not a substitute for manufacturer life calculations.
THK’s horizontal high-speed example specifies a maximum speed of 1 m/s, a servo-motor speed of 3,000 min⁻¹, and direct coupling without a reduction gear [S2]. THK’s horizontal high-speed example specifies a minimum feed amount of 0.02 mm/pulse [S2]. Those encoder relationships remain useful for a vertical servo axis, but the final lead must also satisfy axial-force and critical-speed limits.
Set Accuracy, Backlash and Support Requirements
Vertical-axis accuracy is a system requirement because screw deflection, bearing play, nut preload and coupling errors all contribute to positioning error. THK’s horizontal high-speed example specifies 0.15 mm backlash, ±0.3 mm/1,000 mm positioning accuracy, and ±0.1 mm positioning-accuracy repeatability, and states that lead-angle accuracy must be ±0.09 mm/300 mm or higher to achieve that positioning accuracy [S2].
Backlash should be specified as the permitted lost motion under the specified load and direction. A preloaded nut may reduce backlash, but the selected nut and screw pair must retain the specified preload through the operating load and temperature range. Do not infer vertical performance from nominal unloaded accuracy [S2].
Support arrangement affects effective length, allowable speed and load capacity. The 800 mm overall screw in the vertical example must be evaluated with the actual supports, overhang, stroke, bearing life and installation orientation. If an end-mounted motor brake or shaft brake changes bearing loading, include its thrust load in the support calculation. Coupling selection also needs to account for the motor and screw alignment requirements; the comparison in Ball Screw Coupling Selection: Bellows vs Jaw provides a framework for that mechanical interface.
Size the Power-Off Brake for Holding Torque
The brake must be sized for the worst credible downward load, including the static load plus applicable external force and safety margin, not merely the steady running torque. For the 45 kg example, 441 N is the calculated moving force before the 2:1 torque safety factor is applied [S1]. Any imbalance, tooling weight, cable pull or process force must also be included.
A spring-engaged, electrically released brake is the normal power-failure holding arrangement because it remains engaged when control power is removed. The cited motor example uses an electromagnetic brake motor [S1], while dedicated ball-screw brakes are described as spring-engaged and air-released [S5]. A dedicated shaft brake can reduce the load carried by the motor bearings, but it introduces pneumatic or electrical release hardware and a mechanical coupling that must tolerate alignment error [S3].
Check three separate ratings: continuous or holding torque, peak brake torque during an emergency stop, and the axial force permitted through the brake shaft or coupling. Brake response time must be compatible with controlled deceleration and the drop distance allowed by the machine risk assessment. Do not use a hard stop to conceal an undersized or late-acting brake [S3] (2016-08-02).
Verify Life, Speed and Application Suitability
Ball-screw life depends on the actual dynamic load, screw diameter, end supports, lubrication, speed, load direction and operating temperature. Obtain a life rating for the selected diameter, length, support method and load spectrum rather than extrapolating from a catalog diameter alone. For the cited vertical duty of 5 hours per day, the screw must be checked against the full intermittent operating schedule [S1].
Critical speed is a separate constraint from motor braking. Increasing lead reduces screw rpm at a given table speed, but a longer or unsupported screw can reach its allowable speed before the motor does. Check the fixed-end, free-end and support-bearing arrangement for the actual stroke. High-speed resonance selection is covered in Ball Screw Maximum Speed Before Critical Resonance.
The mainstream choice is a preloaded ball screw with a spring-engaged motor brake for compact, low-speed vertical positioning with moderate load. Do not select that arrangement for a very long stroke or high-speed duty without confirming critical speed and bending load, or for a process requiring zero controlled movement while the brake is released. A belt, linear motor, counterweighted mechanism or shaft-mounted brake may be better when travel length, shock loading or regenerative duty dominates.
Issue the Selection as a Traceable Engineering Specification
The procurement specification should define the screw model, nominal diameter, lead, overall length, accuracy grade, axial clearance, preload method, support method, lubrication, allowable speed and calculated life. It should also define the motor power, rated speed, gearhead ratio, rated torque, service factor and brake type [S1][S2].
For the worked example, record a 20 mm diameter, 5 mm lead, 800 mm length, 0.9 efficiency, 441 N axial force, 0.426 N·m load torque, 0.86 N·m factored torque, 9:1 gear ratio and 0.118 N·m motor torque [S1]. Add the brake’s holding and peak torque, release voltage or air pressure, response time and feedback contact to the same specification.
Factory acceptance should verify low-speed holding after brake release, power-loss retention, controlled stop distance, reversal backlash, positioning repeatability and thermal stability over the rated duty. Track the selected screw diameter, lead, calculated life, critical speed, brake holding torque and measured drop distance as the primary procurement and test records.