Lead Screw - Jam Condition

[freerangequark]

I have a 1/4 x 16 Acme thread (1/16" lead) .... if an acme nut on this rod hits a stop, the nut will jam and lock against the stop.

I have a 1/4 X 1" Lead Screw (1" lead) ... if a leadscrew nut on this rod hits a stop, the nut does not jam or lock against the stop.

My question is, what is the minimum lead/angle/pitch that will not easily jam or lock up?

Short of experimentation and a lot of prototyping, I am hoping someone here can make a general statement on the subject or perhaps state a rule of thumb.

Thanks,
Glenn

 

[desertfox]

Hi

Have a look at this link and towards the end there is a graph relating to self locking and friction coefficient, lead screw angle.

http://www-mdp.eng.cam.ac.uk/web/li...dvd_only/DAN/threads/mechanics/mechanics.html

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[GregLocock]

The answer has a great deal to do with boring old ramps and friction. Since mu is less than 0.3 for a maintained system, a ramp angle of atan .3, or say 20 degrees, is an upper limit, and you are unlikely to see less than atan 0.1, 6 degrees as a lower limit. Turning a ramp angle into a pitch is an exercise for the less easily bored.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[ornerynorsk]

One thing you need to keep in mind is that this also directly relates to whether your lead screw assembly (and whatever it is driving) needs to be self-holding or not. Multi-start threads with a very high helix angle (IE: greater lead) are generally not self-holding, and you can end up with a runaway condition or unintended movement of your machinery, depending on how it's used.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[freerangequark]

Thank you for the replies.

Just to clarify the application a little... on the motorized lead screw shaft is a fixed collar/stop. The lead screw nut is permitted to travel axially until it hits the collar/stop at which point it stalls the motor. I don't want the lead screw nut to be able to jam against the stop/collar and I'm sure this will require a high helix. Self holding and backdriving the system are not concerns.

Thanks,
-Glenn

 

[btrueblood]

" on the motorized lead screw shaft is a fixed collar/stop."
"The lead screw nut is permitted to travel axially until it hits the collar/stop at which point it stalls the motor."
"I don't want the lead screw nut to be able to jam against the stop/collar and I'm sure this will require a high helix."

I don't think the helix angle does diddly in this case - if you don't want the lead screw nut to jam on the stop collar, then
A) remove the stop collar, or
2) don't keep turning the screw when the nut approaches the stop, or
iii) put a torque-limiting clutch between the motor and the shaft, or
7) use a ball screw shaft/nut combo., with an overrunning or freewheeling capability.

 

[freerangequark]

I want the lead nut to stop against the collar... because once it stops, the leadscrew keeps rotating an additional 90 degrees and rotates the nut with it before hitting a hard rotational stop.

The 1/4" x 1" lead screw I have here is of such a high pitch that there is no way that the nut would jam (and lock) up against the collar. I need to find the lower pitch limit for a non-jamming condition.

 

[ornerynorsk]

What are the materials leadscrew and nut? Ball or plain?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[freerangequark]

Plain.
The leadscrew is a mild steel.
The nut is acetal.

Thanks,
Glenn

 

[btrueblood]

Let me get this straight - the nut is not restrained from rotating with the leadscrew? Then how does it advance along the screw? Or does the nut restraint have a spring or other mechanism that allows for some rotation?

 

[ornerynorsk]

Do you have any force requirement for reversal of the leadscrew to extricate itself from the stopped condition? There will inherently be some peak force of sorts upon reversal regardless of pitch due to the fact that you are essentially creating a torqued assembly upon stall. Granted, it's not much with a 1 inch pitch, but is the objective to create a travel as slow as possible without undue reversal torque required?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[freerangequark]

I'm going to post a few video clips to show the motion I am trying to achieve.

 

[freerangequark]

Video 1 shows the motion I am trying to reproduce (although with a lower pitch lead). Notice that once the nut reaches the stop, the torque flips it over and I can then reverse the screw and the nut stays flipped on the other side. The nut never locks up against the stop.

 

[freerangequark]

Video 2 shows an attempt to achieve the same motion as video 1, however due to the low pitch of the acme thread, the nut(arm) readily locks up against the stop.

 

[ornerynorsk]

I can no longer access videos at work, but I'll try to remember to look tonight.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[tbuelna]

With plain lead screws you must also consider any prevailing torque present due to interferences between the mating thread surface contacts.

There is also usually a significant difference between static and dynamic frictions. Static (breakaway) friction coefficients are usually much higher than dynamic (sliding) friction coefficients.

 

[ornerynorsk]

How many choices of lead screw do you have to choose from, in terms of lead/pitch, starts? I took a quick look in McMaster, which is certainly not an exhaustive source, and found only several. There is a 16 TPI with 4 starts that gives you 1/4 inch travel per revolution, putting the helix angle at just over 17 degrees. Maybe? I believe Greg is right with his figure at around 20 degrees being the upper limit for self-locking, after which the thread is essentially non-fixing.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[freerangequark]

The second video is using 1/4 X 16 Acme thread (from McMaster).

Some other 1/4 leads I can choose from include .333", .500", and 1.00" leads. I know the 1.00" lead will not jam. I have some .333" lead on the way to test next. I might have that as soon as today.

Thanks,
Glenn

 

[ornerynorsk]

I hope you'll let us know how that works. Good luck on the project!

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[freerangequark]

I received the 1/4" x .333 (4 starts) lead screw material today along with the nuts. I put a collar at the end of the rod and ran the nut hard up against it. No jamming whatsoever

Thanks for all the input.
-Glenn