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Telescopic Overlap

Telescopic Overlap

Telescopic Overlap

(OP)
Readers,

I am in the middle of designing a telescopic support. It is simply a cylinder inside a cylinder. The base cylinders is 300mm OD x 274ID and 800mm long. The second (Sliding) is 273.8OD x 254ID and 700mm - Material is ASTM A316. I have a 120mm overlap at full extension.

There is a 15,000 Nm moment being applied to the top cylinder.

I am trying to determine the if the overlap length is sufficient to prevent jamming and also failure. I have completed this using FEA, however would like some possible hand calculations to verify the FEA Solutions.

Any help would be appreciated

RE: Telescopic Overlap

15e3/.12=1.25e5 N at each contact point.

spread that out over 13*75 (WAG) mm,=1000 mm^2 gives a stress of 125 n/mm^2

I'd say you've got a problem with plastic deformation. 120mm overlap is ridiculously little given your tube geometry

Cheers

Greg Locock

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RE: Telescopic Overlap

Hi markja,
   
Also, with 316 running against 316, there could be some galling problems.  Fairly high contact pressures around the openings of both tubes in the overlap under this load, as I am sure your FEA has shown.

RE: Telescopic Overlap

(OP)
What does the term (WAG) mean this is the first time that I have seent this abb.


Cheers

Mark

RE: Telescopic Overlap

wild-a**-guess
or possibly
wildly-accurate guess

TTFN



RE: Telescopic Overlap

This sounds like it would be the same issue as with stop tubes in cylinders.  Some manufacturers have rules of thumb in their catalogs, but nothing I can find right now.  It impressed me as being a lot however, possibly 20 to 25% of the stroke, but this is really a big guess.  Maybe someone else has more info.  I tried to come up with a rational basis to calculate the overlap but wasn't successful.

Regards,
-Mike

RE: Telescopic Overlap

If it has to slide while any moment is applied I'd want engagement = at least 4 diameters, with lubricated  surfaces made of real bearing materials. Stress/deflection results might dictate more engagement and wall thickness.

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