Telescopic Overlap

[markja]

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

 

[GregLocock]

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

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

 

[ccw]

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.

 

[markja]

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


Cheers

Mark

 

[IRstuff]

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

TTFN

 

[mrMikee]

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

 

[Tmoose]

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.