How do anti backlash nuts work

Axial anti-backlash nuts consist of a main nut body and a secondary ring that share the same thread form. 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. 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 more significant load, you need a stronger spring to keep backlash to a minimum. However, a more robust spring means more friction between the nut and screw which requires more drive torque to overcome. Radial anti-backlash nuts consist of a nut body with flexible "fingers. 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 amplify the force of the spring. Applications which need linear positioning in machines that will see heavy and continuous duty will generally make use of Acme threaded rod and Acme nuts.

The threads of Acme rod provide more contact area on the side walls of the threads and so do not wear as quickly as plain threaded rod.

Tolerance specs are better and initial backlash specs are better too. There are two Acme thread standards. The 2G class is the most common and the least expensive. The higher classes offer reduced backlash. The other Acme thread standard is called Centralizing C Acme thread, and is often referred to as precision Acme thread.

It too is available in three classes — 2C, 3C, and 4C. But the point here is that it is possible to buy Acme threaded rod and nuts that offer very good backlash specifications, and that these specifications may be good enough so that backlash can simply be ignored. In the introductory paragraphs it was mentioned that there are cases where the amount of backlash is known and it is always known which side of the screw thread is contacting the nut thread, and in these cases it is possible to simply subtract out the backlash where appropriate.

This is the essence of software backlash compensation, which is offered by some of the computer software available to drive Computer Numerical Control CNC machines. If the machine you are designing is not a CNC machine than this would not be an option of course. The software works like this. When it is configured you specify the backlash of each of the linear positioning devices of your machine.

The software knows which side of the nut thread that the screw thread is in contact with at all times. If it turns the screw in the same direction as it was last turned it knows there will be no backlash and so does no compensation. Whenever it turns the screw in the opposite way from the way it was last turned it knows it has to add enough turn of the screw to take up the backlash before it will actually start moving the nut and the carriage to which it is attached.

Although such a scheme works fine in theory, movement of the carriage and nut that is not done explicitly by the positioning software can render this unworkable in actual practice. The conditions under which software backlash compensation will work are enumerated below. Note that all conditions must be met for it to work. Most backlash reduction schemes involve mechanical pre-loading of the nut for movement in both directions.

In the examples above when the screw was actually driving the nut and carriage, the screw was driving the load of the carriage assembly. If it starts turning the other way it is unloaded until the backlash is taken up, at which point it begins driving the load of the carriage assembly the other way. Preloading, that is, imposing a load on both sides of the screw thread simultaneously even while it is not moving means there is never backlash that needs to be taken up.

The generalized way to preload is to make sure the nut is in contact with both sides of the screw threads at all times. One of the most common ways to do this is to use two nuts, each in contact with one of the surfaces of the screw thread, as in figure 3. One way to do this is to secure both nuts to the carriage in some way that allows both adjustment of their positions along the thread and some way to secure them once properly positioned.

In practice such an arrangement has some problems when metal nuts are used with metal screw, as tolerances of the screw and nuts, small positioning irregularities and expansion and contraction of the metal components with changes in temperature tend to make things bind up. But this can work surprisingly well when plastic nuts are used on a metal screw.

Another common way to preload the screw is to mount only one of the nuts to the carriage and have the other secured to the mounted nut by some linear slide mechanism that keeps it from spinning on the screw. The two nuts are then pushed apart or pulled together by a spring. Again, this is less of an issue with some plastic nuts. Domande Frequenti. When the screw 1 is under a compression load, the bottom of its thread surfaces are supported by the top thread surfaces of the worm gear 2 at point A.

The anti-backlash nut 3 , being pinned to the worm gear and floating on these pins and being adjusted downward by the shell cap, forces its bottom thread surfaces against the upper thread surfaces of the lifting screw at point B. Thus, backlash between worm gear threads is reduced to a regulated minimum for values refer to E-Series Screw Jacks brochure P When wear occurs in the worm gear threads and on the load carrying surfaces of the lifting screw thread, the load carrying thickness of the worm gear thread will be reduced.

This wear will create a gap at point B and provide backlash equal to the wear on the threads. Under compression load, the lifting screw will no longer be in contact with the lower thread surface of the anti-backlash nut.