How to Choose an Anti-backlash Lead Screw Nut

Christopher Nook by Christopher Nook

Lead screws are the norm for today's linear motion applications but despite the lead screw's advantages over competing technologies (i.e. affordability, precision, the ability to design with freedom, reduced maintenance, etc.), lead screw systems do have a certain amount of “play"

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Lead screws are the norm for today's linear motion applications but despite the lead screw's advantages over competing technologies (i.e. affordability, precision, the ability to design with freedom, reduced maintenance, etc.), lead screw systems do have a certain amount of “play". An important design consideration for utilizing a lead screw is the amount of backlash your system can tolerate and still achieve it's goal. The big question is - which lead screw nut will help you in the battle against backlash?

All lead screws need clearance between the screw and the nut to operate. Backlash is the relative axial movement between the nut and the screw without rotation of either one. Backlash can increase with wear and can be detrimental to your application. If your system has no way to compensate for backlash, you could experience inaccurate positioning. Syringe pumps and robots are two examples of applications where there's an emphasis on power, positioning, and accuracy in lead screw assemblies. These two applications are also a good example of where excessive backlash can really become a negative factor.

Thankfully, compensating for backlash is fairly simple and Helix offers a variety of anti-backlash lead screw nuts to ensure that your application performs as expected. There are different styles of anti-backlash lead screw nuts but two of the most common available from Helix are Axial Anti-backlash nuts (AAB) and Radial Anti-backlash (RAB) nuts. Let's take a closer look at each.

Axial Anti-backlash (AAB) nuts consist of a main nut body and a secondary ring that share the same threadform. There is a spring between the two components that force each part against opposing flanks of the screw thread. This biasing takes up the clearance between all the components i.e. gets rid of the backlash. The drawback to this design is that if the load is greater than the spring force, you can still get backlash in the system. For a greater load you need a stronger spring to keep backlash to a minimum, however, a stronger spring means more friction between the nut and screw which requires more drive torque to overcome.

Radial Anti-backlash (RAB) nuts consist of a nut body with flexible “fingers”. There is an axial spring that pushes a ring along the nut body. There are ramp features on the ends of the fingers that the ring uses to push the fingers inward towards the screw. The threads on the fingers are pushed into the valleys of the screw thread to take up any clearance. There is a mechanical advantage provided by the ramp features between the fingers and the collar which amplifies the force of the spring. Unlike the AAB design the RAB design can handle loads greater than the spring force. There is also less of an increase in required drive torque compared to an AAB nut.

Both styles serve the same function of preventing backlash while maintaining high efficiency. Both are available from Helix in standard and metric thread forms with flanged, threaded or threaded with a screw-on flange nut bodies.

Like lead screws themselves, the type of anti-backlash nut that is selected depends on the application. Some key factors to consider are the level of repeatability and precision required, the magnitude of the load, and the amount of drive torque available in your system. If you are unsure which style of anti-backlash nut is right for your application, contact one of our sales engineers for assistance. We would be glad to put our expertise to work for you to help determine the best anti-backlash solution for your application.

We hope that you've found this information about controlling backlash useful. To assist you in selecting an appropriate lead screw assembly for your next project, we have also assembled some additional useful info and expert tips in our Engineer's Guide to Lead Screw selection. Feel free to download your free copy below.