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snowman
03-07-2008, 11:20 AM
Ok, this is a retarded question, not because of the content...but because I'm having trouble with remembering BASIC design rules.

Proper length of thread engagement in jig and fixture design, is it three FULL threads, or 2 x thread diameter or something else.

jimmstruk
03-07-2008, 11:23 AM
My book says 1&1/2 diameters in good steel, more in softer or lower strength materials. JIM

winchman
03-07-2008, 11:43 AM
I doubt there's any meaningful "rule" for this, since there are so many variables. See: http://www.tribology-abc.com/calculators/e3_6e.htm

I'd look at the nuts available for different types/grades of fasteners to get an idea of what's required.

Roger

Fasttrack
03-07-2008, 12:14 PM
The general rule of thumb I always heard was three full threads or 1 and 1/2 times the diameter of the fastener - which ever engages the most threads. In some cases, three threads is actually more than 1 and 1/2 times the diameter so then three thread rule would apply, otherwise 1 and 1/2 times the diameter. This is just what i hear in the shop, no idea how good that rule of thumb is.

kf2qd
03-07-2008, 12:37 PM
Cut threads - ie Tapped holes - a good rule of thumb is 1 1/2 times the OD - MINIMUM. Most purchased fasteners are rolled threads which are much stronger that are CUT threads.

snowman
03-07-2008, 01:42 PM
thank you...i don't know why that was slipping my mind

Michael Moore
03-07-2008, 02:52 PM
It seems like you've got to factor in both the TPI and diameter. If you've got a 1" x 20TPI thread does it really need 1.5" of engagement (30 threads)? When a rule of thumb can give a range of 3 to 30 threads engagement it seems like a pretty big thumb is being used. :)

cheers,
Michael

snowman
03-07-2008, 03:23 PM
If you need a 1" thread, I'd hope you'd either err on the cautious side and use the 1.5 or do the calculation :)

But it answers my question. I mostly wanted to know for #6, #8, #10, 1/4-20, 5/16...small stuff.

Michael Moore
03-07-2008, 03:30 PM
Carroll Smith says that "AN bolts have a consistent length of thread for each diameter (about two diameters) and is calculated to accept a standard AN washer, a standard AN full-height tension nut and leave 3.5 threads exposed after the nut."

cheers,
Michael

Sprocket
03-09-2008, 10:02 PM
Or try this:

http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement.htm

I've used this before when I wondered...

Doug

Rich Carlstedt
03-09-2008, 11:05 PM
Most folks get too complicated here because there are several issues with fasteners. If you follow the comments made on one of the above URL's they look for "screw" failure (ie stripped treads) as a resolution..
this may not be what you want
It is simple to figure out.
Take the strength (PSI ) of the materal of the screw, and divide it by the strength of the material of the hole ( threads)....thats it !
If you use a screw with 180,000 tensile (Allen ,Camcar etc) and put it in a steel hole made with 60,000 tensile (mild steel i.e.) you need 3 times the diameter of the screw to get a maximum strength joint..simple !
If you use a mild steel screw, and a mild steel nut, Its one diameter because they are the same .

be aware "Maximum joint strength" and "Thread Shearing" (stripping)
are not the same

Rich

J Tiers
03-09-2008, 11:48 PM
Rich.....

I believe that with fine threads (the stronger type) and long engagements, there is a sort of progressive failure mode where stretch of the screw partly fails the threads closest to the screw head..... Obviously the stretch isn't enough to strip them, but it points up the fact that there is a limit to the improvement.

As far as AN bolts, that is a totally different deal. In general they can NEVER have threads within the part for shear loading, only un-threaded shank. They are made the way they are made so as to be able to get a correct bolt for just about any thickness of materials. Sometimes a washer may be needed as a spacing device to allow tightening a nut when the next longer bolt is used, if a shorter one would have threads in shear.

oldtiffie
03-09-2008, 11:55 PM
Carroll Smith says that "AN bolts have a consistent length of thread for each diameter (about two diameters) and is calculated to accept a standard AN washer, a standard AN full-height tension nut and leave 3.5 threads exposed after the nut."

cheers,
Michael

Thanks Michael.

What you say is probably very close to the requirements of the "Steel Structures Code/s" in most countries - as it is here in OZ.

I'd suggest asking a competent practicing professional Structural Engineer.

Or just look up the Code for yourself.