Questions regarding thread type and strength when length is limited

[John2004]

Hello everyone,

I would like to ask if anyone could please provide some feedback on the following situation.

I have a 3/8" diameter steel rod that is threaded horizontally into a vertical steel support at one end, and that has a moderate vertical load applied to the rod at the other end, 5" or 6" from the support. The load tends to deform, pry or bend the threads out of the support.

The threaded end of the 3/8" rod necks down to a 5/16 diameter by 24 pitch thread, and the necked down portion that the threads are applied to, can only be .183" long Maximum. The actual threads won't quite be .183" long because they probably cannot go completely flush to where the rod necks down.

The threaded end of the rod is screwed into the steel support until it bottoms out, right where the rod necks down...

(3/8 - 5/16 = .0625 / 2 = .03125) so there is a 1/32" wide portion of the rod OD that bottoms out on the steel support.

I want the threaded connection to be as strong as possible so the rod supports the largest load possible. Would I be better off going with an extra fine 5/16 diameter X 32 pitch thread?

Are there any disadvantages of using extra fine threads that I should be concerned with?

Since the rod is basically cantilever mounted horizontally, and the loads are vertical, it seems that the amount of material that bottoms out on the steel support may effect things.

For example, if I use a 3/16" diameter thread, a 10-32 thread, or a 1/4" diameter thread, then this produces more "neck down" on the 3/8" OD rod than the 5/16" diameter thread, and this provides more surface area to "bottom out" on the steel support.

My guess is that a 5/16-32 thread would stil provide the best holding power.

I would appreciate any opinions on the best thread diameter and pitch to use. Unfortunately, I cannot just go with a longer thread.

Thanks
John

 

[GregLocock]

Sure. Fine threads are easy to cross thread and I doubt they are much stronger in practice. I vaguely remember that if you have more than three full threads engaged you are actually going to lose strength, for a given length of engagement.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ccw]

If you use a high strength screw stock, say Grade 8 or better, then a thread with a pitch = to the soft steel parent material thickness would have the best strength. Obviously that is a theoretical limit and practically silly, so I agree with Greg, --about 3 threads.

This is based on a study that my college did for biomedical research several years back, on the best thread for attaching instrumentation to rat's skulls.

 

[GregLocock]

Yes, I was going to say that for ultimate strength it seemed that just one perfect thread, as large as possible, might be better.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[John2004]

Hi everyone,

Thanks for your replies.

How close can I actually get the threads to the lip where the rod necks down to the threaded portion ? This will tell me how many threads I can actually get along the .183" necked down portion of the rod.

If I go with a 5/16-24 thread, I am guessing I would probably get three threads along the .183" length, since the fourth thread probably won't be able to be machined right up to the lip where the rod necks down.

The rod will probably be 0-1 tool steel (3/8" OD stock) because I need something that can be hardened a little, and not deform too much from hardening. I could make the support out of some other type of tool steel, but prefer something available in 1" round stock (it gets a flat milled on it where the rod threads in).

I guess a 5/16 thread OD would be best and then it's just a matter of what thread pitch is best, perhaps a 24 pitch or even less would be optimal.

I did not know that you can actually loose strength when using more than three full threads for a given length of engagement. Why is this so ?

I have always tried to make the thread length engagement at least 1.5 times the diameter of the screw or bolt, but I never gave consideration to not having more than three full threads engaged.

Thanks again for your feedback guys,
John

 

[diamondjim]

Why not use a 3/8 diameter
thread? Fine pitch is
supposedly stronger.

 

[Tofflemire]

Hi All,

Can you put a bushing in between the 2 items so the bushing takes the load and does on bend the rod?

Tofflemire

 

[MikeHalloran]

You would be better off with Tofflemire's bushing.

Said bushing could have a stepped hole of 3/8 x 5/16 diameter, with the counterbore extending most of the way through the bushing, e.g. leaving a 1/16" thick web. The bushing would have an o.d. of say 1" or so.

The bushing could be tapered on the o.d. The important dimension is the o.d. at the column face. You can make it arbitrarily large to reduce the tension on the threads per unit of applied load.

Think of the problem as a lever, with the fulcrum at the bottom edge of the bushing where it abuts the column, and you can see how it works.



Mike Halloran
Pembroke Pines, FL, USA

 

[Screwman]

Don't worry so much about the threads on the "shaft", you are going to have much worse worries with the cantelevered portion of the assembly. I completely agree that going to something like O1 is overkill for the application. Look at your loading and balance out the nut member strength with the threaded rod strength.
This sounds like a fatigue application where you will spen d more time looking at stress concentration factors than UTS. Make sure that you have good threads and that no stress areas averlap and you should be in good shape.

Dick

 

[unclesyd]

You might want to checkout the different thread engagements.

http://www.hexagon.de/dose/dose1e.htm#2.3.2

 

[DAVIDSTECKER]

"Since the rod is basically cantilever mounted horizontally, and the loads are vertical, it seems that the amount of material that bottoms out on the steel support may effect things"

There is also a moment at work here.
Some of the threads are being pull out of the support and some of the threads are being pushed in. And some of the treads are doing nothing.
You could create more of a pure tenslie load by fixing a washer to the rod so that the pivot point is lower on the support making the loading more tensile and shear.
Another option may be to change the class of fit for your threads so you have more engagement.

 

[GregLocock]

Also, consider using an Acme or square thread.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[WhiteTiger]

you can get full thread engagement and a much stronger threaded joint by making your 5/16 section about 1/4 inch longer than needed and then using a piece of the 3/8 rod to make a threaded collar style spacer to epoxy in place.

in short engagement situations such as yours it's the absent first thread that allows the bending moment to begin the process of thread failure. Bit more work, but it's proven well worth it for me many times.