Questions regarding thread diameter and pitch for special design case wtih limited thread length

Dear John2005:

Can you: precision bore the base plate, turn the rod end to 0.0005" larger, heat the base plate, cool the rod in liquid nitrogen, and simply jam the rod in the hole?

Do both ends need threads?

David A. Smith

Hi David,

Thanks for your reply.

How would what you suggest compare as far as cost, to threading ? If I were not going to use threads, perhaps a slip fit between the rod and the hole and the use of loc-tite at the joint would be the way to go, but I think threads are probably my best bet, it's just a matter of what thread diameter and thread pitch are best.

The rod will have threads on each end.

Also, If I use the method you suggest, where the rod goes into the hole, I may not be able to put the full 3/8" OD in the hole, and may still have to neck the rod down, where it goes in the hole, due to design constraints. Necking the end of the rod down, may also help it assemble more straight, since the hole does not have much depth.

Also, the base plate has bearings in it, and I cannot deform the bearing bore with heat, since the bearing bore sizes the bearings and bearing radial internal clearances. The rod also has a close tolerance hole machined in it center, along it's longitudinal axis.

Thanks again, John

You should make the threaded part a couple threads longer than mating hole depth to allow for tolerances and so the necked area won't jam in the hole.

Finer thread would be better, but with decent strength material you'll have plenty of capability for most applications. 5/16-24 thread with 50 ksi material can carry over 2000 lb in shear.

It would be easier to cross thread the smaller threads.

The threaded part is in bending and shear either way.

That's not a good idea to try and count on that extra surface area. Depending on fit and tolerances, it may not be there. And if the rod does bottom out, you won't be able to remove it easily if needed.

I agree that threads are probably best IF the motion of the rod is cyclical. Eventually it will work its way out of the hole if press fitted. Some mechanism to prevent unthreaded is also recommended or the same could happen with screw threads. See my replies in "alt.mechanical.engineering" regarding thread type.

This doesn't make sense, except that a shoulder at the neck-down will help RETURN the rod to perpendicular. Is that important?

Does that mean that the hole is used in such a manner that would prevent the rod from rotating? If that is the case then a shoulder on the thread does not serve to prevent unthreading (again, see my other posts) and may not be necessary.

Mark 'Sporky' Stapleton Watermark Design, LLC

"N:dlzc D:aol T:com \(dlzc\)" wrote in news:Wzhzf.9591$jR.9192@fed1read01:

How do shrink fits cope with 'writhing' loads?

Cheers

Greg Locock

Dear John2005:

Not less than double, since two precision operations are involved. All it gets you is lower stress. Since you have a bearing bore already, negligible additional cost for the base.

Don't use loc-tite, as this only provides a mechanism for failure. If the parts are really that tight, "diffusion" or "galling" will prevent slipage. Don't want that stuff in your bearings either.

OK.

Yes, but the minimum area is/can be closer to 3/8" than with a fine pitch 3/8" thread. Which has to be formed with very expensive tooling.

Press fit is out then, since it will slightly compress the tube, and shrink your ID. Unless you machine/ream the ID after assembly. Room temperature will be OK for the base plate. Heating to less than 100 C is usually sufficient.

Welding is out, since you cannot heat it without heat treatment and machining afterwards.

David A. Smith

Dear Greg Locock:

Used in many places in older gyroscope gear trains and such. If the materials have same/identical Young's/bulk modulus, I don't see how they would fail.

David A. Smith

"John2005" wrote

Conventional constant taper threads aren't particularly good in this application, especially with the low thread depth you're specifying. There are a variety of tapered threads (tapered profile and tapered tooth-form) used in oilfield applications that have significantly higher moment capacity than constant pitch threads...they have to be machined though, not tapped, so they're expensive.

If you want more info, see

Tom.

Dear John2005:

An additional question: Are you intending a particular relationship between the precision bore in the rod and the centerline of the bearing race? A threaded connection won't do this well at all.

David A. Smith

John: No, the threaded connection does not have to provide any type of alignment between the two bores, it's just a method to connect the two parts and it's not a high precision connection.

John