curved lead screw
curved lead screw
(OP)
Hi All,
Has anyone seen a custom thread design that allows a threaded nut to follow a curved threaded rod (true arc trajectory), like bending a lead screw. can it be done?
I think it might be possible if the nut only has one thread inside.
has eveyone seen something like that? and where?
thanks,
Lee
Has anyone seen a custom thread design that allows a threaded nut to follow a curved threaded rod (true arc trajectory), like bending a lead screw. can it be done?
I think it might be possible if the nut only has one thread inside.
has eveyone seen something like that? and where?
thanks,
Lee





RE: curved lead screw
Can you cut the nut with threads that grossly clear the threaded rod? If you use a thin ACME thread, it will have a full thread profile, and enough clearance to handle the curved rod.
JHG
RE: curved lead screw
The nut could follow a curved screw if it has only one internal thread, as you say, or if the internal thread actually comprises two wheels with thread- profile shaped tires. But there's not much load capacity either way because of the small pressure area.
If the nut is relatively long and relatively thin and of uniform thickness, you could bend the nut to the same radius as the screw is bent after the nut is threaded. That should be fairly strong.
There's also no reason you couldn't make the nut in multiple pieces, with axial splits, sort of like the half- nuts used for threading on lathes. You could probably EDM the working faces of the nut segments, using a piece of bent leadscrew as an electrode.
Recognize that the leadscrew has to be skinny and spring- tempered in order to allow any rotation, and the curvature and slender geometry greatly reduces its column stiffness, so that may limit the load you can transmit with it. You also should expect vibration resulting from coupling between the windup of the leadscrew, extra bending and lost displacement from the 'axial' load, and stick- slip in the nut and whatever you use for thrust bearings.
Find a chassis service manual for a '60..63 Pontiac Tempest for an example of a (not threaded) shaft designed to carry considerable torque (and no thrust) and be rotated while bent.
-Mike-
RE: curved lead screw
-Mike-
RE: curved lead screw
I was not referring to a thin nut. I meant a regular shaped nut with a thin thread profile. You are correct though that the load is concentrated and the stresses are high.
Bending the nut could not work because you would be unable to rotate it around hte curved shaft. On the other hand, if the nut were made of some flexible plastic, you would have full thread contact. The stress analysis for this would be a whole lot of fun.
JHG
RE: curved lead screw
RE: curved lead screw
Don't ask me how to machine it, that's not my area.
Jeff
RE: curved lead screw
If you're intending to rotate the nut instead, then the shaft could be made with an interrupted thread, i.e., with 'teeth' on top and bottom, and the remainder relieved below the minor diameter of the thread, or milled to a diamond shape like locator pins, or made from bent sheet.
-Mike-
RE: curved lead screw
Barry1961
RE: curved lead screw
I like the idea of a flexible nut and would offer the suggestion that operation be similar to the rotating ball screw type design. Then you would have an outside change of providing the necessary clearances since the teeth don't converge towards eachother going around the bend, if you know what I mean. The profiles must definitely be small otherwise the crests of the teeth converge and the nut would not pass. A ball screw would solve this issue.
I can't seem to wrap my brain around usage. Why would a guy need a "curved screw"? Also, I would hate to be the machinist trying to cut or roll the product. The setup(s) would truly be a novel manufacturing concept!
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: curved lead screw
RE: curved lead screw
- - -Dennyd, P.E.
RE: curved lead screw
RE: curved lead screw
A star for EnglishMuffin for having saved us from roting in hell.
Cheers
SACEM1
RE: curved lead screw
Thanks for the replies. Long story short. I was challenged by an industrial designer to come out with a working plastic curved lead screw design for a medical device project (apparently, designers prefer arc over line). I agreed with him because of the fact that the payoff will be big, it will make the device follow human shape better. The nut needs to be rotated. load is not a major concern.
Initially, I was looking for some mathematical equations for the threads so that I can model it in CAD, make a prototype and try it out. After doing some research, I was surprised to find out that there are little information about this subject. maybe it is an opportunity.
Due to high volume, my client doesn't want to use any flexible parts or springs. Everything has to be rigid plastic for low cost. He would prefer to pay for 300 thousand dollars injection molding tool over springs that might only cost a few pennies each.
I still believe it is possible to do it. I think the main problem is the fact that the pitch will not be constant between the inner radius and outer radius along the rod. If the clearance is big, the nut might rock forward and backward which I don't like. To simplfy the problem, I have decided to consider true arc trajectory (constant radius) instead of random curves (splines or conics). The rod will have partial threads on opposite sides (no full rotation). I'm thinking of using a straight thread profile (constant pitch)....I'm thinking of using a collar (no threads inside) which can move smoothly along the curved rod to guide the nut......
Thanks for all the responses, it is nice to learn a few possible creative ways from you. I didn't respond because I don't have anything to contribute so far. I will definitely share my experiences after I have time to work on it a little bit more and have something to contribute.
It turns out to be a fun problem.
thanks,
Lee
RE: curved lead screw
Ray Reynolds
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RE: curved lead screw
Is this lead screw or guide?
If you turn a curved lead screw by one end it swing in an orbit equal to the radius of curvature, unless the lead screw is . A guide could be used say with belt or chain driver.
RE: curved lead screw
On the other hand, a steady feed might be developed but the machining of the mold would be interesting to say the least. The rights to the drive might be worth more than the device it goes on.
Does this device look or operate anything like an orbital welding head? Is the radius fixed or variable?
Griffy
RE: curved lead screw
RE: curved lead screw
I'll be damned, the spring nut could indeed travel the entire length of bent screw, back and forth many times! I agree with Dennyd, you get the Star Award on this one!
Out of curiosity...with a name like "EnglishMuffin", any coaching suggestions for England in the European Cup 2004? I'm pretty sure after that game against France, beyond the box thinking would be welcomed!
Absolutely excellent idea, don't know what I can use this for, but excellent idea. Congratulations!
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: curved lead screw
RE: curved lead screw
I got it. The implementation of the curved lead screw is almost identical to the straight screw, except that the teeth are circular patterned on the single ruled arc (the tooth is radially spaced / duplicated about the same arc). I made an SLA protoype and it worked.
Thanks,
lee
RE: curved lead screw
I don't know if this helps and it does have its limits. But check out a Patent I have regarding linear leadscrews moving curved members. The main feature is buried in the entire assy. It is the leadscrew nut & interfacing slider.
Patent # 6,038,127
RE: curved lead screw
<nbucska@pcperipherals DOT com> subj: eng-tips
RE: curved lead screw
Tim, the turboengineer
RE: curved lead screw