Can Ball Screws Carry Radial Loads? Design Limits Explained

Ball screws are designed primarily to carry axial loads, not radial loads. There is normally no universal maximum radial-load rating that can be applied to every ball screw. Moving-table weight, side forces and overturning moments should be supported by linear guides, linear bearings or another dedicated guide mechanism.

A small unintended radial force may occur because of shaft weight, installation error or operating deflection, but this should not be treated as a normal working load. If the ball nut is deliberately used to guide the moving component, side loading can create uneven ball contact, higher running torque, heat and premature raceway wear.

Axial Load vs. Radial Load on a Ball Screw

The difference is determined by the direction in which the force acts.

Ball screw catalog values such as basic dynamic load rating and basic static load rating generally describe axial loading of the ball screw assembly. They should not be reinterpreted as allowable radial-load values.

Why Is There No Single Maximum Radial-Load Value?

A number such as "100 N maximum radial load" cannot be applied to all ball screws because the resulting stress depends on the complete mechanical arrangement.

Important variables include:

• Screw-shaft diameter and unsupported length;
• Position at which the side force is applied;
• Fixed-supported, fixed-fixed or another bearing arrangement;
• Distance between the screw supports;
• Ball nut length and internal structure;
• Shaft straightness and end-journal runout;
• Installation alignment and nut-bracket rigidity;
• Screw speed, acceleration and vibration;
• Required accuracy and service life.

The same radial force may produce very different shaft deflection and ball-nut contact conditions on a short, large-diameter screw and a long, small-diameter screw. For that reason, radial loading should be analysed as an abnormal side force or a shaft-deflection problem rather than selected from a universal ball-nut rating.

Where Do Unwanted Radial Loads Come From?

The Moving Table Is Not Properly Guided

If the moving table is connected to the ball nut without adequate linear guides, its weight and external forces may act directly on the nut. The ball screw is then forced to perform two different jobs: driving the axis and guiding the moving mass.

This arrangement may appear to work in a small, lightly loaded mechanism, but it does not provide the same rigidity, life or positioning stability as a properly guided axis.

The Screw and Linear Guides Are Not Parallel

When the screw axis is not parallel to the guideway travel, the nut bracket pulls the ball nut sideways as the carriage moves. The side force may change along the stroke, producing a tight point at one position and smoother movement at another.

Typical signs include:

• Running torque that changes along the stroke;
• Local heating near the ball nut;
• Noise or vibration at repeatable positions;
• Coupling movement or motor-load variation;
• Uneven seal or raceway wear;
• Binding after all mounting bolts are tightened.

The Nut Bracket Forces the Nut into Position

A nut bracket with an incorrect center height, poor squareness or insufficient rigidity can introduce side load. Tightening the mounting bolts should not be used to pull a misaligned nut into the required position.

A practical installation check is to leave the nut-bracket connection slightly loose during initial alignment. If the bracket visibly shifts when brought into contact with the nut flange, the mounting geometry should be corrected before final tightening.

The Screw Shaft Is Bent or Insufficiently Supported

A long or slender screw can deflect under its own weight. Additional side force, poor transportation support or a bent shaft can increase radial movement during rotation.

This condition should be checked separately from the axial load rating of the ball nut. Increasing the nut load rating does not correct excessive shaft deflection.

Does a Larger Ball Screw Tolerate More Radial Load?

A larger screw diameter normally provides greater bending stiffness under the same length and support condition. However, this does not mean that the ball nut should be selected to carry a continuous radial working load.

A larger diameter may help control:

• Shaft deflection caused by self-weight;
• Buckling under compressive axial load;
• Critical-speed limitations;
• Torsional deformation;
• Vibration on longer screw assemblies.

The correct design still uses the ball screw for axial drive and the guide system for radial and moment loads. Diameter should be selected from axial load, screw length, speed, buckling, critical speed and rigidity requirements-not from an assumed radial load rating.

Does Preload Increase Radial-Load Capacity?

No. Ball screw preload reduces axial clearance and increases axial rigidity. It does not convert the ball nut into a radial bearing or guide block.

Increasing preload while side loading is present may make the problem worse because the balls already carry an internal load before the external misalignment force is added. Possible results include higher torque, additional heat and shorter fatigue life.

If a ball screw runs tightly, do not automatically increase the preload. First check the screw alignment, nut-bracket position, support bearings, lubrication and shaft straightness.

How Should the Machine Carry Radial Loads?

Use a linear guide system sized for the moving-table weight, machining force, acceleration and overturning moment. In a typical CNC or automation axis:

• The ball screw generates axial driving force;
• The ball nut transfers that force to the moving carriage;
• The linear guide blocks support the carriage weight and side forces;
• The rail arrangement resists roll, pitch and yaw moments;
• The screw support bearings locate the rotating shaft and carry the loads required by their bearing arrangement.

The screw axis and guideway travel must then be aligned so that the ball nut can move through the complete stroke without being pulled sideways.

For system-level design, read Ball Screw and Linear Guide Coordination. For installation checks, see How to Align a Ball Screw During Installation.

How to Evaluate an Existing Side Load

If an existing machine applies a radial force to the ball screw, record the following information before deciding whether the system is acceptable:

1. Identify the magnitude and direction of the side force.
2. Measure where the force acts relative to the support bearings and ball nut.
3. Confirm the screw diameter, lead, total length and unsupported length.
4. Check the fixed and floating support arrangement.
5. Inspect the linear guide layout and carriage rigidity.
6. Measure screw runout and installation parallelism.
7. Check whether running torque changes along the stroke.
8. Calculate shaft deflection and bearing reactions where necessary.

If the machine depends on the ball screw itself to guide a substantial moving load, the more reliable correction is usually to add or redesign the guide mechanism rather than search for a ball screw with a larger supposed radial-load rating.

Final Answer

There is no universal maximum radial load that all ball screws can tolerate. Ball screws and ball nuts are primarily rated for axial load. Continuous radial loading is generally a sign that the moving structure is not adequately guided or that the screw, nut bracket and linear guides are misaligned.

A larger screw diameter may reduce shaft bending, but it does not make radial loading a normal ball-nut operating condition. The moving mass, lateral force and overturning moment should be supported by properly selected linear guides, while the ball screw provides axial drive.

For the meaning of catalog load values, read What Is the Static Load Rating of a Ball Screw?.

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