How to Match a Rotating Ball Nut With a Servo Motor

A rotating ball nut system can be useful when a long ball screw must operate at high linear speed. Instead of rotating the entire screw shaft, the servo motor drives the ball nut through a belt, pulley, gear or other transmission mechanism. The screw normally remains non-rotating, and relative movement between the nut and screw produces linear motion.

Matching the rotating ball nut with a servo motor requires more than selecting a motor with sufficient rated torque. Axial load, screw lead, target speed, acceleration time, transmission ratio, rotating inertia, bearing arrangement and duty cycle must be evaluated together.

How Does a Rotating Ball Nut System Work?

A rotating ball nut contains the same basic rolling-contact mechanism used in a conventional ball screw assembly. Steel balls circulate between the screw raceway and the nut raceway, converting rotational motion into linear motion with relatively low friction.

The main difference is which component rotates. In a rotating nut design, the ball nut is supported by bearings inside a rotating housing. The servo motor drives this housing or the nut flange, commonly through a timing belt and pulley. The screw shaft is restrained from rotating.

Depending on the machine design, either the rotating nut assembly travels along the stationary screw or the screw moves axially through a nut assembly mounted on the machine frame. The exact structure should be confirmed before motor sizing because the moving mass and rotating inertia will differ.

Why Use a Rotating Nut Instead of a Rotating Screw?

When a conventional long screw rotates at high speed, shaft deflection may cause vibration or whipping as the speed approaches the screw's critical speed. Increasing the screw diameter can improve rigidity, but it also increases rotational inertia and motor requirements.

A rotating nut design keeps the long screw from rotating. This can reduce the critical-speed limitation associated with a rotating shaft and make the arrangement suitable for long-stroke, high-speed motion. However, the design still has limits related to ball circulation speed, bearing speed, heat generation, lubrication and structural rigidity.

A rotating nut structure is generally considered when the application has one or more of the following requirements:

• Long travel combined with relatively high linear speed;
• A slender screw that would have a restrictive critical speed if rotated;
• High acceleration with a need to reduce the inertia of the rotating member;
• Limited space for a large drive motor at the end of the screw;
• A machine structure designed to place the drive near the moving nut assembly.

The rotating nut design is not automatically better for every application. It requires a suitable bearing housing, transmission mechanism, lubrication arrangement and protection system. A conventional ball screw assembly may remain simpler and more economical for shorter strokes or moderate operating speeds.

Information Required Before Selecting the Servo Motor

The motor should not be selected from the ball screw diameter alone. At minimum, the following application information is required:

• Ball screw nominal diameter and lead;
• Total stroke and installation length;
• Maximum linear speed;
• Acceleration and deceleration time;
• Moving mass and external axial force;
• Horizontal, vertical or inclined mounting direction;
• Ball nut preload and expected transmission efficiency;
• Direct drive, belt drive or gear drive arrangement;
• Pulley diameters or transmission ratio;
• Operating cycle and continuous running time;
• Required positioning accuracy and repeatability;
• Environmental temperature and contamination conditions.

Without these values, it is possible to estimate a motor size, but it is not possible to verify whether the system will meet its actual speed, acceleration and service-life requirements.

Step 1: Determine the Required Axial Thrust

The axial force acting on the ball screw depends on the mounting direction and machine operation.

For a horizontal axis, the required thrust may include guide friction, process force, seal resistance and other external resistance. For a vertical axis, gravity must also be included. During acceleration, the force needed to accelerate the moving mass must be added.

Where:

• F_a = acceleration force in newtons;
• m = moving mass in kilograms;
• a = linear acceleration in metres per second squared.

The total design thrust should include acceleration force, external process force, friction and gravity where applicable. Shock or impact loads should be evaluated separately rather than treated as normal continuous load.

Step 2: Calculate the Torque Required at the Ball Nut

The torque needed to generate axial thrust can be estimated using the axial force, screw lead and transmission efficiency:

Where:

• T = torque at the rotating nut in N·m;
• F = total axial force in newtons;
• P = ball screw lead in metres per revolution;
• η = mechanical efficiency expressed as a decimal.

This calculation represents the torque needed to overcome the axial load. The motor must also provide torque to accelerate the rotating ball nut, pulley, coupling, bearing components and other rotating parts.

Starting torque, seal resistance, bearing friction and preload can further increase the actual requirement. Therefore, the thrust torque alone should not be used as the final servo motor rating.

Step 3: Calculate the Required Rotational Speed

The required nut speed is determined by the target linear speed and ball screw lead:

Where:

• n = nut rotational speed in revolutions per minute;
• v = required linear speed in metres per second;
• P = screw lead in metres per revolution.

For the same linear speed, a larger lead requires a lower nut rotational speed. However, increasing the lead also increases the torque required to generate the same axial thrust and may affect positioning resolution. Lead selection is therefore a balance between speed, torque and positioning requirements.

The calculated rotational speed must remain within the allowable speed of the ball nut, recirculation system, bearings and belt transmission. The lowest permissible speed among these components becomes the system limit.

Step 4: Select the Transmission Ratio

Rotating ball nuts are commonly driven by timing belts because a belt arrangement allows the motor to be mounted parallel to the screw and provides flexibility in transmission ratio.

If the motor and ball nut use different pulley diameters, the motor speed and torque at the nut will change according to the selected ratio. A reduction ratio can increase available torque at the nut but requires a higher motor speed. A speed-increasing ratio has the opposite effect.

The following items should be checked when selecting the pulley ratio:

• Maximum and continuous motor speed;
• Required nut speed;
• Motor rated and peak torque;
• Belt tooth load and allowable belt speed;
• Pulley inertia;
• Belt tension and bearing radial load;
• Available installation space.

A timing belt should be tensioned correctly. Insufficient tension may cause positioning errors or tooth jumping, while excessive tension increases radial load on the rotating nut bearings and motor bearings.

Step 5: Check Acceleration Torque and Inertia

A servo motor must accelerate all rotating components as well as the moving linear mass. Even when steady-running torque is low, a short acceleration time can produce a high peak torque requirement.

The rotating inertia may include:

• Ball nut and rotating housing;
• Support bearings and bearing spacers;
• Driven pulley and motor pulley;
• Motor coupling or gear components;
• Reflected inertia of the linearly moving mass.

The motor supplier's sizing software can be used after the ball screw and transmission parameters are confirmed. The resulting inertia ratio should remain within the motor and servo-drive manufacturer's recommended range for the required response and control stability.

A very high load-to-motor inertia ratio may cause slow response, overshoot, vibration or difficulty tuning the servo system. Changing the transmission ratio, increasing the screw lead, extending acceleration time or selecting a motor with different inertia may improve the result.

Step 6: Compare Rated Torque and Peak Torque

Servo motors normally have both a rated torque and a peak torque. These values serve different purposes:

The maximum calculated torque should remain below the motor's allowable peak torque. The root-mean-square torque over the complete cycle should remain below rated torque. A reasonable margin should also be retained for changes in friction, load, temperature and operating conditions.

Step 7: Check the Rotating Nut Bearing Arrangement

A rotating ball nut cannot be supported in the same way as an ordinary nut that is bolted directly to a moving table. The nut or nut housing must rotate while carrying axial load, so the bearing arrangement is a critical part of the design.

The bearings must provide sufficient axial rigidity and support the required rotational speed. Their load rating, preload, lubrication and installation accuracy must be considered. Belt tension may also apply radial load to the bearing system.

An unsuitable bearing arrangement can cause axial play, heat, vibration and positioning errors even when the ball screw itself is correctly selected. The housing should also allow accurate alignment between the rotating nut, screw shaft and machine travel.

Step 8: Verify Positioning Accuracy

The final positioning accuracy is influenced by more than the servo encoder resolution. Important mechanical factors include:

• Ball screw lead accuracy;
• Ball nut axial clearance or preload;
• Axial rigidity of the rotating nut bearings;
• Belt elasticity and transmission error;
• Thermal expansion of the screw;
• Machine-frame and guideway rigidity;
• Servo tuning and feedback arrangement.

For high-accuracy applications, a linear encoder can measure the actual table position and reduce errors that cannot be detected by a motor encoder alone. Whether this is necessary depends on the required accuracy, stroke, temperature variation and machine structure.

Example of the Matching Process

Consider an application with a specified moving mass, axial process force, target linear speed and acceleration time. The matching process should proceed in the following order:

1. Calculate the acceleration force from the moving mass and required acceleration.
2. Add process force, friction and gravity where applicable to obtain the design axial thrust.
3. Use the ball screw lead and efficiency to calculate thrust torque at the nut.
4. Calculate the nut speed required to achieve the target linear speed.
5. Select a pulley ratio that keeps both motor and nut within their permitted speed ranges.
6. Calculate the inertia of the nut housing, pulleys and reflected moving mass.
7. Calculate peak acceleration torque and RMS torque over the full cycle.
8. Check motor peak torque, rated torque, rated speed and maximum speed.
9. Verify bearing load, belt tension, ball nut speed and thermal conditions.
10. Confirm positioning accuracy and rigidity before finalising the design.

Because the actual motor size depends on all of these inputs, selecting a servo motor only from screw diameter and lead can produce an undersized or unnecessarily large drive.

Common Matching Mistakes

• Confusing a rotating nut with a conventional ball nut: A standard nut model is not automatically a complete rotating nut assembly.
• Checking only rated torque: Acceleration may require peak torque much higher than steady-running torque.
• Ignoring inertia: A system may have enough torque but still be difficult to tune because of an unsuitable inertia ratio.
• Using motor speed as nut speed: The pulley or gear ratio must be included.
• Ignoring bearing and belt limits: Motor capacity does not determine the allowable speed of the complete mechanical system.
• Using an excessively small safety margin: Actual friction, load and temperature may differ from the initial calculation.
• Increasing lead only to gain speed: A larger lead changes torque demand and positioning resolution.
• Overlooking vertical-axis safety: A brake or counterbalance may be required to prevent uncontrolled movement.

What Should Be Confirmed With the Ball Screw Supplier?

Before selecting the final servo motor, confirm that the proposed screw and nut arrangement is suitable for rotating-nut operation. A complete rotating nut unit requires more than an ordinary ball nut.

The following information should be confirmed:

• Whether the selected nut is designed for use in a rotating assembly;
• Allowable rotational speed of the nut and recirculation system;
• Dynamic and static load ratings;
• Available preload and accuracy grades;
• Nut mounting interface and bearing-seat dimensions;
• Lubrication method and maintenance interval;
• Required screw-end machining and anti-rotation structure;
• Whether the supplier provides only the screw and nut or a complete rotating nut unit.

DLY supplies multiple ball nut configurations, but the required structure should be reviewed against the drawing and operating conditions. A conventional ball nut should not be identified as a rotating ball nut without the necessary bearing housing and drive design.

Conclusion

Matching a rotating ball nut with a servo motor requires coordinated mechanical and electrical calculations. The process begins with axial thrust and linear speed, followed by nut torque, rotational speed, transmission ratio, inertia, acceleration torque and RMS torque.

The ball nut, bearings, pulley system, screw shaft and servo motor should be treated as one motion system. A motor with sufficient torque may still be unsuitable if its speed, inertia or duty-cycle performance does not match the mechanical design.

For a preliminary review, prepare the ball screw diameter and lead, stroke, moving mass, external force, maximum speed, acceleration time, installation direction and proposed transmission arrangement. These details make it possible to evaluate the screw specification and provide the correct mechanical parameters for servo motor selection.

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