How to Select a Precision Ball Nut for Low-Speed, High-Torque Applications

For a low-speed, high-torque ball screw system, the ball nut should be selected according to axial load, screw lead, preload, required rigidity, lubrication condition and mounting accuracy-not motor torque alone. Low speed does not automatically make the application easy. Heavy loading, frequent starts, short reciprocating strokes and long holding periods can all increase local contact stress and lubrication demands.

The complete ball screw assembly must also be checked. A high-capacity nut cannot compensate for an undersized screw shaft, insufficient bearing support, incorrect preload or poor installation alignment.

High Torque Is Not the Same as Ball Nut Load

Motor torque is converted into axial thrust through the ball screw. The relationship depends mainly on the screw lead and mechanical efficiency. For a driving condition, the approximate relationship is:

This means that the same motor torque can produce different axial forces when different screw leads are used. A smaller lead generally produces more linear thrust per revolution, while a larger lead produces more linear travel per revolution.

The formula is useful for preliminary selection, but it does not include acceleration, impact, guideway friction, external cutting force, seals, preload drag or transmission losses. These factors must be considered separately.

1. Check Both Dynamic and Static Load Conditions

The ball nut should be evaluated using the actual axial load applied during operation, stopping and holding.

• Basic dynamic load rating is used when estimating fatigue life under repeated movement.
• Basic static load rating is important when the system experiences heavy stationary loads or short peak loads.
• Peak axial load may occur during acceleration, impact, clamping or an emergency stop.
• Equivalent operating load should reflect the different loads and durations within the machine cycle.

A low average speed does not justify ignoring dynamic loading. A press adjustment axis, lifting mechanism or clamping unit may move slowly but experience substantial force every time it starts, stops or reverses.

The selected assembly should include an appropriate safety margin based on the severity of impact, load uncertainty and required service life. The screw shaft must also be checked for buckling when it is subjected to compression.

2. Select the Screw Lead According to Thrust and Resolution

A smaller lead can generate greater axial thrust from the same input torque and provides finer linear travel per motor revolution. It is often useful in slow positioning, lifting, clamping and precision adjustment applications.

However, selecting the smallest available lead is not always the best choice. The designer should also consider:

• Required travel speed
• Motor rotational speed
• Positioning resolution
• Required thrust
• Screw diameter and rigidity
• Available installation space

If the required linear speed is very low, motor control quality also matters. Cogging, servo tuning, coupling compliance and external friction can affect low-speed motion even when the ball screw and nut are correctly selected.

3. Choose Preload for Rigidity, Not Simply for "Zero Backlash"

Preload reduces axial clearance and improves rigidity during direction reversal. It is useful in applications where positioning stability and repeatability are important. However, preload does not make the complete machine absolutely free of lost motion because deformation can still occur in the screw, support bearings, nut housing, coupling and machine structure.

Excessive preload can create several problems in a low-speed system:

• Higher starting and running torque
• Increased internal contact load
• More sensitivity to mounting misalignment
• Higher heat generation during repeated movement
• Shorter service life if the total load is underestimated

A standard single nut may be sufficient when compact size and normal positioning accuracy are required. Where better backlash control and axial rigidity are needed, a DFU double-nut ball screw can provide more controlled preload, subject to the actual load, installation space and drive torque.

4. Include Starting Torque in Motor Selection

The motor must overcome more than the torque required to move the external axial load. The total drive torque can include:

• Torque required to generate the axial thrust
• Internal torque caused by ball nut preload
• Seal and lubricant resistance
• Support-bearing friction
• Guideway and machine-table resistance
• Acceleration torque for rotating and moving components

This is especially important when the axis repeatedly starts from rest. If the motor is selected from running torque alone, the system may hesitate, alarm or lose positioning stability during startup.

Before attributing high starting torque to the nut, disconnect the drive if the machine permits and check the assembly manually. Torque that changes sharply along the stroke may indicate contamination, screw bending or installation misalignment rather than normal preload.

5. Pay Attention to Lubrication at Low Speed and Short Stroke

Low speed generally produces less frictional heat, but it can make lubricant distribution less effective-particularly when the nut moves repeatedly over only a short section of the screw.

In a short-stroke cycle, the balls may not distribute grease evenly through the complete recirculation path. The same raceway area is repeatedly loaded, which can increase local wear if lubrication is insufficient.

For this operating condition:

• Use a lubricant approved for the ball screw and operating environment.
• Apply the correct quantity; excessive grease can increase low-speed resistance.
• Provide periodic longer travel where the machine design permits, helping redistribute lubricant.
• Shorten the inspection interval when the axis carries high loads or works in a contaminated environment.
• Check for hardened grease if the machine remains unused for long periods.

Grease is frequently used in low-speed industrial systems because it remains near the contact area, but the final choice should also consider load, temperature, stroke, seals and maintenance method.

6. Confirm the Nut and Screw as a Matched Assembly

A ball nut must match the screw diameter, lead, raceway geometry, circulation structure and accuracy requirement. A nut with the same nominal model designation is not automatically suitable for every existing screw shaft.

This is particularly important for a ground, preloaded or replacement assembly. Changing only the nut may alter axial clearance, preload and running torque. When replacement accuracy is critical, provide the complete screw model, original nut model, shaft dimensions, application and sample photos for confirmation.

DLY's ball nut range includes different flange forms, circulation structures and single- or double-nut configurations. Selection should be based on the matched screw and machine requirements, rather than flange appearance alone.

7. Control Mounting Alignment

The nut housing, screw shaft and support bearings must share a consistent motion axis. If the nut housing is forced sideways or vertically during tightening, the nut carries an additional radial load that it was not intended to support.

A practical installation procedure is:

1. Clean the nut flange, housing surface and mounting holes.
2. Install the fixed and supported ends without forcing the shaft into position.
3. Attach the nut housing or moving table with the bolts initially loose.
4. Move the assembly slowly through the available stroke.
5. Allow the nut housing to find its natural aligned position.
6. Tighten the bolts gradually and recheck the running torque.

If the running resistance increases immediately after the nut-housing bolts are tightened, recheck the height and lateral alignment instead of adding more lubricant.

8. Do Not Use the Ball Screw as a Holding Brake

Ball screws have high mechanical efficiency and can normally be back-driven by an external axial load. In a vertical or suspended-load application, the nut may move when motor torque is removed.

A holding brake, counterbalance, safety nut or another suitable load-retaining device should be considered according to the risk assessment. High motor torque and ball nut preload should not be treated as reliable protection against an uncontrolled falling load.

9. Decide Whether Precision Grinding Is Necessary

Low speed and high load do not automatically require the highest accuracy grade. The accuracy level should be determined by positioning error, repeatability, reversal behavior and machine function.

A rolled ball screw may be sufficient for general pushing, lifting or adjustment where positioning requirements are moderate. A precision ground ball screw is more appropriate when the machine requires controlled lead accuracy, stable repeatability and carefully specified preload.

DLY can provide C7 rolled ball screws for general industrial applications and C5 or higher-precision ground options according to the project requirements. The selection should be confirmed from the travel, load, positioning requirement and drawing.

Low-Speed, High-Torque Selection Checklist

For reliable selection, provide the axial load, speed, stroke, duty cycle, screw orientation, positioning requirement, available installation space and expected service life. These details are more useful than describing the application only as "low speed and high torque."

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