Ultra Fine Threads

[drawoh]

Does anybody have experience analyzing ultra fine threads for shear?

I have specified a 1/4-127UN thread screw that is to compress a die-spring with a force of around 250N (60lb). Someone is sure to ask me how strong this is. There is not chance of compression failure, but I would wonder how accurate the threads are, relative to the nominal profiles.

--
JHG

 

[Ron]

Drawoh...I would do some pull-out tests on these fasteners. I don't know that any other premise is accurately calculable.

 

[tbuelna]

There are ANSI standards that provide all of the information you need for calculating the fits/tolerances of a UN threadform. However, I would say that accurately calculating the minimum root shear capacity of your proposed 1/4-127 UN thread joint poses quite a difficult problem due to its extremely small size and the significant effect even tiny tolerances will have.

 

[MikeHalloran]

Given an assumed short stroke, consider some sort of differential jacking mechanism, where a nut that's constrained to slide pushes on the die spring, and is in turn driven by a double threaded stud that's also threaded into the 'ground'. The trick being that the two ends of the stud have slightly differing pitch, neither being super fine, and the downside being that the stud has to translate axially quite a bit in order to effect the differential jacking.



Mike Halloran
Pembroke Pines, FL, USA

 

[drawoh]

MikeHalloran,

You mean differential threads. M5[×]0.8 works very well against 10-32UNF.

I have tried that. They did not like it. This is an optical fixture, and all sorts of ultra-precision threads are available off the shelf. My problem is that I need a pre-load spring. The extension springs that fit in your system are not very stiff. I have figured out how to install medium strength stripper plate die-springs, which are stiff, and provide quite a lot of travel for their height. The 1/4-127UN screws should work okay, but I am wondering about their shear strength.

--
JHG

 

[3DDave]

Shear should be OK - what's the engagement length? The usual reason for not using ultra-fine threads is the difficulty in making them right, not potential weakness.

Analytically, the finer the thread, the closer the pitch diameter is to the outer diameter, so the larger it is. The bending moment is similar because teeth shorten in proportion to the pitch. Presumably the tolerance is proportional to the pitch, so the loss due to clearance is the same overall regardless of size.

What was unliked about the differential screw setup?