What Is Ball Screw Preload?

Introduction to Ball Screw Preload

Ball screws are precision mechanical components that convert rotational motion into linear motion. Ball screw preload refers to the intentional application of internal axial force to reduce clearance or backlash within the ball screw assembly. This is done to enhance the rigidity and accuracy of the system, particularly in precision applications like CNC machines, robotics, and other high-precision machinery.

To achieve load bearing and repeatability characteristics, a key design feature of ball screws is the ability to preload the ball nut. By preloading the ball nut, manufacturers can eliminate backlash and improve rigidity.

Manufacturers offer ball screw preload amounts ranging from just 1% or 2%, all the way up to 10%. Generally, manufacturers will recommend a preload amount of 4-5% for general automation machinery, with a preload of around 9-10% for machine tools or machinery requiring very high stiffness.

Kiwi Motion are UK & Ireland linear motion specialists supplying a quality range of linear motion products including ball screws. As linear experts, we can help to specify the correct ball screw for your application.

2 key advantages of ball screw preload

Eliminate backlash.

By preloading the ball nut, the internal clearance between the raceways of the nut and the screw is reduced. By reducing this internal clearance, backlash (axial play) can be eliminated, meaning no lost motion and in turn improving the repeatability of the screw assembly.

Improved rigidity.

Using a higher preload – bigger ball bearings so greater contact between the ball screw and the raceway – the rigidity of the whole ball screw system can be increased. If the application requires high stiffness (machine tools) or will be subject to vibration, high rigidity can be a prerequisite to a successful linear system.

Ways to achieve ball screw preload

Oversized ball bearings

There are two main methods to achieve ball screw preload. The first and most common is to use oversized balls in the nut. This low-cost method can be used with many ball nut styles and allows the manufacturer to set the preload precisely to the desired amount.

Using a double-nut setup

The second way to achieve ball screw preload is to use a double nut system, with either a spacer or spring between the two nuts.

A double-nut design is inherently more rigid than that of a single-nut counterpart. The design uses a spacer or spring between the two nuts which are mechanically coupled, to create tension. A double nut will generally only be used when a high preload of around 7% or greater is required.

With the higher cost and design complexity (much longer in length) to be considered, an application with vibration or very high stiffness requirements would normally only be suitable.

Motor drive torque and lifetime considerations

Ball screw preload is specified as a percentage of the nut’s dynamic load capacity. To determine the internal load, due to preloading, the preload amount (0.04 for 4%, or 0.08 for 8%) is multiplied by the dynamic load capacity. When selecting a ball screw system this internal load that preload induces should be considered. As a motor will need to be specified to power the ball screw system, the motor should be sized so it can produce sufficient torque for the application.

Using a higher preload increases the friction and heat produced between the ball bearings and the bearing raceway. This will require greater torque requirements for the motor and could also reduce the lifetime of the ball screw. As such careful consideration should be taken when choosing a higher preload.

Kiwi Motion preloaded ball screws use gothic arch groove profiles to achieve zero backlash, and smooth operation. Using this system low torque and high rigidity can both be achieved.

Kiwi Motion supplies rolled ball screws and ground ball screw systems to the whole of the UK & Ireland. Available in a vast range of diameters, pitches, and types on quick delivery times. Ballscrew end supports can also be supplied with custom machining available for a complete ball screw linear motion system. 

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