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.

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


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