• 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

How to avoid ball screw buckling

May 21, 2015 By Danielle Collins 6 Comments

Imagine two columns—one short and wide and the other long and narrow. As you begin to put weight on them, the long, narrow column gradually begins to bend, to the point that it becomes permanently deformed and eventually breaks, while the short, wide column handles the weight with ease.

In the same way the long, narrow column reaches its buckling point earlier than the short, wide column, a long ball screw with a small diameter will deform and buckle under a smaller axial load than a shorter screw with a larger diameter.

Determining the buckling load is an important step in ball screw sizing. The buckling (compressive) load can easily be calculated based on the screw’s root diameter, unsupported length, and end bearing configuration.

Ball Screw Buckling

where:

Fc = maximum compressive load (N)

fb = end bearing factor

d1 = root diameter of screw (mm)

L = unsupported length (mm)

Application Tip: The end bearing factor, fb, is dependent on the end fixity of the screw. Ball screw manufacturers recommend a safety factor of at least 2 for the maximum applied buckling load, and some incorporate this into this published end bearing factors. To avoid over- or under-sizing, be sure to check whether the manufacturer has built a safety factor into the published fb values.

As shown in the equation, for a given diameter and length ball screw, the permissible buckling load can be increased by changing the support bearing arrangement. The more rigid the support bearings, the higher the bearing factor—ranging from approximately 2.5 for a fixed-free arrangement to 40 for a fixed-fixed arrangement. This means that a fixed-fixed bearing arrangement will result in approximately sixteen times the permissible buckling load than a fixed-free arrangement on the same screw assembly.

In a horizontal application, the axial force is generally equal to the load times the screw assembly’s coefficient of friction, so buckling is typically less problematic than other performance factors, such as critical speed or drive torque. But some horizontal applications, such as pressing and injecting, introduce an additional axial load on the screw assembly, and the buckling load can easily become a constraining factor in screw selection.

When a ball screw is used in a vertical application, it sees the full load as an axial force, which makes buckling a more important concern. Mounting the screw assembly with the most rigid support (typically the fixed bearing) at the top puts the screw in tension rather than compression and counters the effects of the axial load. In the unusual case of a high reversing force—for example, pushing a plunger into a housing on the screw’s upward stroke—the most beneficial bearing arrangement is to orient the fixed bearing at the bottom of the screw assembly.

Ball Screw Buckling
Orienting the fixed bearing on top puts the ball screw in tension.
Image credit: NOOK Industries, Inc.

Application Tip: Most, but not all, screw driven linear actuators use a fixed bearing on the motor side and a floating bearing on the non-driven end. For vertical applications, be sure to check which end of the actuator the fixed bearing is on, in order to avoid excess compressive forces on the screw.

Buckling is a relatively straightforward, but sometimes overlooked, factor in ball screw sizing. The important things to remember are to check the process forces in horizontal applications, and to check the location of the fixed bearing in vertical applications.

Filed Under: Applications, Ball + lead + roller screws, Ball + leadscrew driven, Linear actuators (all), Linear drives (all) Tagged With: Ball Screw, buckling, sizing

Reader Interactions

Comments

  1. daniel says

    December 5, 2019 at 12:41 pm

    In what units is Fc calculated?
    I solved Fc=231, will it be pounds or kg, newtons?

  2. Danielle Collins says

    December 13, 2019 at 12:02 pm

    Hi Daniel – I apologize, I forgot to put the units on the compressive load, Fc. This is in Newtons, N.

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

  • Bush and shaft assemblies feature 4-micron clearance
  • New gearboxes for electric motors in mobile machines
  • Leading motion-control and actuation options for robotics
  • Basics of wave and cycloidal gearing for robotics and servo designs
  • New micro servo-drive system for extra-low-voltage manufacturing applications

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