calculate width of Teflon lead nut for silent actuator 1

[learningallthetime]

I've a short bar of teflon and made a 15mm wide lead nut for a M12 steel rod (standard thread) to build a silent linear actuator mechanism.

The nut raises a platform with a load of up to 250N.

Have limited material to experiment, so don't want to proceed entirely by trial and error.

Can anyone say whether that's an appropriate width of nut - whether it should be more or less? How to estimate?

Anecdotally, when the platform is fully loaded, the lead screw seems to become rather more difficult to turn than I would have hoped (having played with this calculator:

http://www.daycounter.com/Calculators/Lead-Screw-Force-Torque-Calculator.phtml

): which left me wondering if this was perhaps due to some kind of overload at the lead nut?

 

[israelkk]

What did you expect to get and what you got?
The nut width has nothing to do with the friction it will only need to be able to carry the 250N load. I.e. the number of threads has to be strong enough to work with the 250N load as long as needed. I assume you used the 0.1 friction coefficient. But the real life will be at lease 0.2. The 0.1 is for light loads and high speed and usually received in lab tests and samples but, for working conditions in real life the reality is 0.2.

 

[learningallthetime]

Unfortunately I don't have a force-gauge to hand but, indeed, using the calculator with a 0.1 coefficient of friction was expecting circa 20Ncm-1 torque to raise and in reality it "felt" like quite a bit more when turning by hand. My NEMA 17 stepper was unable to cope.

However it did all work quite well at lower loads.

If, as you suggest, the observed coefficient of friction increases at higher loadings, that would perhaps explain it.

In which case, if I double the width of the lead nut will that not halve the loading per unit area of teflon and thence reduce the coefficient of friction at higher loads? Hopefully back from 0.2->0.1.

 

[israelkk]

From you questions I have doubt you are in a position to handle this task. I do not know your experience and skills but your question indicate that you either not a professional engineer or very inexperience. What kind of M12 rod are you using? If it just standard that the thread is not optimized for the task regarding surface finish. How did you manufactured the Teflon nut and measured it? All these factor will affect the smooth movement of the nut over the rod. Usually, this is not the right approach. The standard thread roads are for clamping and contacting parts and not for linear actuators. I recommend you look at the Nook company products.

http://www.nookindustries.com/Product/ProductLine/Acme-and-Lead-Screw-Products

 

[learningallthetime]

Thank you for the help.

 

[dvd]

Teflon has a much larger coefficient of thermal expansion than does steel. Due to friction, the nut gets warm/hot and the teflon expands, thus binding the threads with the cool steel screw. The nut gets hotter than the steel screw because the nut is continuously bearing load on the same threads, whereas the steel screw is only seeing a few turns as the nut passes. Probably need to make a loose nut thread to accommodate material differences.