Wedging

[dgallup]

I'm sure this has come up before but I can't find it. I'm looking at a short cylindrical piston in a bore and trying to determine at what combination of clearance (tilt) and friction coefficient the piston may become wedged in the bore. Also, does anyone have info on steel on steel friction coefficient as a function of surface roughness? Everything I've found just gives a range of values with no mention of quality of the surfaces.


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[BrianE22]

If you are referring to "cocked", keep the L/D greater than the coefficient of friction. "L" being the length of the cylinder and "D" being the diameter. I was told this many years ago and was able to derive it at one point (but probably not anymore).

 

[hydtools]

Wouldn't the L be the length of the piston, not the cylinder when considering the piston tendency to cock in the cylinder.

Ted

 

[dvd]

The ratio of connecting rod length to crankshaft swing will have a big effect on the sidewall forces, as well.

 

[hydtools]

L/D = tan of the piston maximum tilt angle

Friction force/Normal force = tan of angle between applied force and Normal force = mu, coefficient of friction.

Ted

 

[Tmoose]

If the cylinder's OD profile can deviate from a simple cylinder it can be made more jam-proof.

To start, sharp corners could be trouble, so chamfers top and bottom can help some.
To reduce the contact pressure (and thus jamming friction) on the piston upper and lower edges a low angle chamfer with some length

I submit since a piston shaped like this can not jam in the bore, a "short" piston with the same OD profile would be jam-proof.
0:50 here -

https://www.youtube.com/watch?v=umBNA74g0a4

Although if this is a piston as I think of a piston, pivoting on a connecting rod, with some sealing rings, allowing it to tip even a few degrees would destroy the rings' seal.
In the limit the "piston" could just tip 90° if it felt like it.

https://www.youtube.com/watch?time_continue=14&v=eu1NrdFEL60&feature=emb_logo

 

[BrianE22]

Wouldn't the L be the length of the piston, not the cylinder when considering the piston tendency to cock in the cylinder.

Yes, you are correct.

 

[dgallup]

Thanks guys, I like the shallow chamfer idea. This is an element in a switch mechanism, there is no connecting rod or piston rings.

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[dgallup]

It's more complicated than just L/D because the amount of tilt possible depends on the clearance between the piston and cylinder in addition to the length and diameter.

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[hydtools]

sqrt(L*L + D*D) > D + clearance

Ted

 

[drawoh]

dgallup,

Can you find a copy of Product Design for Manufacture and Assembly, by Geoffrey Boothroyd, Peter Dewhurst and Winston Knight? They have tables on what washers and other cylindrical objects will drop cleanly down a round hole.

--
JHG

 

[dgallup]

Can you find a copy of Product Design for Manufacture and Assembly, by Geoffrey Boothroyd, Peter Dewhurst and Winston Knight? They have tables on what washers and other cylindrical objects will drop cleanly down a round hole.

Do you know what page or chapter that is in? Thanks

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[drawoh]

dgallup,

I have the Second Edition here. Section[ ]3.15 Reducing Disc-Assembly Problems should be good for you. See page[ ]111.

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JHG

 

[dgallup]

Got it, thanks.

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