friction on a threaded rod

[Crawler]

Hey guys, i'm working on a little project to use for training members on different ways of doing things, my goal is to use a window motor to drive an 8mm threaded rod, the rod having a nut which will move an object back and forth. Now if i did my calculations right then our 9.1 N*M window motor will be able to deliver 113.75 N of force to overcome friction and get the thing moving (if im wrong then please tell me what to do because im new to this).

Now this is neglecting the force of friction from the threaded rod to the nut, so the question is, how do i calculate the coeficient of static friction between the 2.

Also, if i use multiple points where the object is connected then being that its still the same normal force, that shouldnt factor in right?

[Tom Line]

This is actually a fairly complicated question that you are asking. You're making a leadscrew.

You can probably get a first level approximation treating the thing as a ball screw with a very very low efficiency (50%?)

Driving torque is in Newtons
Thrust Force in Newtons
Screw Lead in meters (distance the nut travels along the rod during one revolution)

Driving Torque = (Thrust Force * Screw Lead ) / ( 2 * pi * efficiency)

So for you,

Thrust Force = (Driving Torque * (2 * pi * .5) ) / Screw Lead

For a rough approximation.

This will give you a more accurate answer:

http://en.wikipedia.org/wiki/Leadscrew

[PAR_WIG1350]

Originally Posted by Crawler

Hey guys, i'm working on a little project to use for training members on different ways of doing things, my goal is to use a window motor to drive an 8mm threaded rod, the rod having a nut which will move an object back and forth. Now if i did my calculations right then our 9.1 N*M window motor will be able to deliver 113.75 N of force to overcome friction and get the thing moving (if im wrong then please tell me what to do because im new to this).

Now this is neglecting the force of friction from the threaded rod to the nut, so the question is, how do i calculate the coeficient of static friction between the 2.

Also, if i use multiple points where the object is connected then being that its still the same normal force, that shouldnt factor in right?

Before I worried about the friction I would look again at what I was actually calculating. 113.75 N would be the force tangential to a disk 16 cm in diameter coupled to the window motors shaft.

Even if you were calculating the tangential force directly, you divided by .08 when you should have divided by .004. One millimeter is one thousandth of a meter, or .001, so the diameter of the rod is .008 m. Additionally, you should be using the radius, not the diameter, so that would be .004 m.

However, the axial force of a threaded rod on a nut is not the same as its tangential force at the outside of the rod (unless the thread angle is 45 degrees). The tangential force is applied at an angle to the thread of the nut which results in a normal force perpendicular to the threads surface with a tangential component and an axial component. The axial component of this force is what determines the force the nut can exert along the length of the rod.

That being said, you could always disconnect the motor, turn it by hand, and guess whether or not there is too much friction.

[Tom Line]

At a glance, I'm not sure if you are using the motors stall torque. That would be a bad idea. Motors are happiest around half stall.

[Chris is me]

I would use an actual leadscrew for such a project and not simply threaded rod. They're not quite interchangeable - if memory serves the leadscrew thread profile and material would make it more accurate than simple threaded rod, which might be thinner and looser.

[Crawler]

you guys are amazing, thanks everyone. BTW sorry about the .08-.008 diameter, ill chop that up to me being derpy, havent read through all of this yet but the theme im getting is I should use threaded rod thats actually ment for this rather than some home depot grade.

[Tom Ore]

Here's an example of a lead screw driven by a window motor. We used it to adjust our hood angle last year.

[TheMadCADer]

Ah, how convenient! Lead screw friction was one of my projects this summer at my internship...

I would second the use of a purpose designed lead screw. For one, a threaded rod of any standard size has so many threads per inch that it can be quite slow. Also, UN style threads and ACME lead screw threads are different, UN threads will be prone to binding up as they are suited for use as fasteners.

Now, on to the friction problem. If you look at it the right way it's one of the simplest physics problems around. First, examine what a screw really is. It's an inclined plane wrapped around an axis. Basically what you have here is the old "box on ramp" problem from the beginning of every physics class! Your torque becomes a force acting on your "block" (the nut), and you can determine the angle of this "ramp" from the thread data on your screw and a little basic trigonometry.

Set this problem up, and you can determine the maximum coefficient of friction that your system could handle. Lead screw manufacturers almost always have CoF data available on their screw+nut systems (it's easiest to get the nut with the screw). If they don't you can call them, someone there knows it. Find a screw that fits your pitch, diameter, and length needs with an appropriate nut and you're all set!

Any more questions, feel free to ask. If I messed something up, call me out on it!

[Dick Linn]

This might be useful: www.eng.auburn.edu/users/marghitu/Screws.pdf

For other than a square thread, "The angle increases the frictional force by the wedging action of the threads".

Also see "Friction coefficient Bolted Joints" partway down here: http://www.roymech.co.uk/Useful_Tabl...o_of_frict.htm