• Skip to primary navigation
  • Skip to main content
  • Skip to primary sidebar
  • Skip to footer

Linear Motion Tips

Covering Linear Motion Systems, Components and Linear Motion Resources

  • New
    • Editor’s blog
    • Industry news
    • Motion Casebook
    • Video
  • Applications
  • Slides + guides
    • Ball + roller guides
    • Track roller (cam + wheel) guides
    • Crossed-roller slides
    • Linear bearings
    • Plastic + composite guides
  • Drives
    • Ball + lead + roller screws
    • Belt + chain drives for linear
    • Rack + pinion sets
  • Actuators
    • Ball + leadscrew driven
    • Belt + chain driven
    • Linear motors
    • Mini + piezo + voice coil
    • Rack + pinion driven
    • Rigid-chain actuators
  • Encoders + sensors (linear) + I/O
  • Stages + gantries
  • Suppliers

Ball screw preload: What you need to know

May 11, 2018 By Danielle Collins 2 Comments

When a linear motion application requires high thrust force and rigidity along with good positioning accuracy, a preloaded ball screw assembly is often the best choice. Manufacturers offer ball screw preload amounts ranging from just 1 or 2 percent for applications where backlash would be detrimental, up to 10 percent or greater for applications that require very high stiffness.

While preload is beneficial for some applications, it produces additional loads that must be accounted for when sizing the screw assembly, motor, and other related components.

Methods for achieving ball screw preload

There are several ways to preload a ball screw, but the three most common methods use oversized balls in the ball nut, an adjustable-diameter ball nut, or a double nut system. Creating preload by using oversized balls allows the manufacturer to set the preload to a precise amount, although this technique is generally used for preload amounts of 5 percent or less. However, this is the most common method for preloading because it can be used with a variety of nut styles and is relatively low-cost.

ball screw preload
Axial play, or backlash, is caused by clearance between the balls and raceways. Using oversized balls in the ball nut can reduce or eliminate backlash.
Image credit: NOOK Industries, Inc.

Adjustable-diameter ball nuts allow the user to increase or reduce the preload manually, via an adjustment screw that changes the inner diameter of the nut. This method is the preferred option when the required preload may change over the life of the screw assembly – for example, to compensate for increased wear or clearance in other machine components. Although it doesn’t facilitate setting the preload to a specific level, the benefit of the adjustable nut is that if a ball nut needs to be replaced or interchanged (on the same screw), the preload of the new ball nut can easily be set to the required level.

ball screw preload
One way to produce preload in a ball screw assembly is to use a double ball nut, with either a spacer or spring between the two nuts.
Image credit: THK

Double ball nut designs produce preload in one of two ways: through tension between two single nuts, or through an offset in the lead of a single nut. The first method uses a spring or a spacer to create tension between two nuts that are mechanically coupled. In the second method, during machining of the ball nut raceways, an offset is created in the lead, approximately halfway down the length of the nut body. This lead offset creates preload within the ball nut. Because there are no spacers or springs required, the lead offset method is more compact than a double nut design, but it reduces load capacity. With greater complexity and higher cost, double nut designs are typically used when high preload (7 percent or greater) is required.

Ball screw preload reduces axial play and increases rigidity

In many applications, the primary reason for using a preloaded ball screw is to reduce internal clearance between the raceways of the nut and screw, which causes axial play (backlash). Reducing or eliminating backlash in turns improves the repeatability of the screw assembly, since there is no “lost motion” when the direction of travel is reversed. (Note that preload does not affect lead error or positioning accuracy.)

ball screw
Preload reduces or eliminates axial play (backlash) between the nut and the screw shaft, which improves repeatability and rigidity .
Image credit: Bosch Rexroth

Higher preload also correlates with higher nut rigidity, which improves the rigidity of the complete screw assembly. But because the ball nut is only one component in the ball screw assembly, and its rigidity is relatively high compared to the other components, increasing the preload — from 3 percent to 5 percent, for example — has a minimal effect on the rigidity of the assembly. The most noticeable increase in rigidity of a ball screw assembly occurs when a non-preloaded ball nut is replaced with a preloaded version.

Ball screw rigidity
The ball nut rigidity (RN) is one component of the total rigidity of the screw assembly, along with the screw shaft, end bearings, and housing.

Preload also increases motor drive torque and decreases life

Another effect of ball screw preload is that it induces an internal load on the ball nut, which needs to be considered during sizing to ensure the motor can produce sufficient torque for the application. Preload is specified as a percentage of the ball nut’s dynamic load capacity, so in order to determine the internal load due to preload, the preload amount (0.02 for 2 percent, or 0.07 for 7 percent, for example) is multiplied by the dynamic load capacity.

The load due to preload must also be accounted for in the applied load (F) component of the bearing life equation, as shown below. Because of the cubic nature of the equation, even a small internal load due to preload can have a significant negative effect on the life of the ball screw. (Note: When the external load meets or exceeds 2.8 times the internal load due to preload, it counteracts the internal load, and the internal load can be disregarded.)

Bearing life preload
When calculating the applied force (F) for the bearing life equation, the internal load on the ball nut due to preload should be considered.

You may also like:

  • ball screw
    When do you need a ball screw?

  • How to calculate motor drive torque for ball screws
  • Double Ball Nut
    What’s the purpose of a double ball nut?

  • How to calculate ball screw rigidity

  • What is preload?

Filed Under: Applications, Ball + lead + roller screws, Ball + leadscrew driven, FAQs + basics, Featured, Linear drives (all)

Reader Interactions

Comments

  1. Te3d Harris says

    November 7, 2019 at 1:04 pm

    Here is a new one for me. I have play in, let’s say 0 to 90 degrees and absolutely no play for the next 270 degrees. Brand new ball screw on one side and the other side is perfect.

    I have a video of it if you are interested.

Leave a Reply

You must be logged in to post a comment.

Primary Sidebar

DESIGN GUIDE LIBRARY

“motion
Subscribe Today

RSS Featured White Papers

  • Evaluating actuators for washdown in food & beverage applications
  • Identifying Best-Value Linear Motion Technologies
  • Introduction to accuracy and repeatability in linear motion systems

RSS Motion Control Tips

  • Schneider Electric launches Universal Automation Discovery Packs to foster industrial innovation
  • Encoders from SIKO support Industrial Ethernet
  • Draw-wire encoders from SIKO measure position, speed and inclination
  • Incremental encoders configurable via NFC (near-field communication)
  • Low-cost motion control offered with CLICK PLUS PLCs from AutomationDirect

Footer

Linear Motion Tips

Design World Network

Design World Online
The Robot Report
Coupling Tips
Motion Control Tips
Bearing Tips
Fastener Engineering

Linear Motion Tips

Subscribe to our newsletter
Advertise with us
Contact us
About us

Follow us on TwitterAdd us on FacebookAdd us on LinkedInAdd us on YouTubeAdd us on Instagram

Copyright © 2022 · WTWH Media LLC and its licensors. All rights reserved.
The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of WTWH Media.

Privacy Policy