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

Dear John2005:

"John2005" <johnjmechanical@xxxxxxxxx> wrote in message
news:1137559029.229780.320570@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Hi David,
>
> Thanks for your reply.
>
> How would what you suggest compare as far as
> cost, to threading ?

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.

> 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,

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.

> 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.

OK.

> 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.

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.

> 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.

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


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