Piston Ring Rotation

[dougk]

Hi,

Everyone knows rings rotate, does anyone know what forces cause this? It has been observed in Pyrex cylinders, so that rules out cross hatch as the reason. Thanks.

 

[ivymike]

I can think of a few possibilities:
* asymmetric gas flow thru the end gaps producing thrust
* "lumps" on the bore that allow the ring to find "low energy" positions at various orientations as the piston moves down the bore (bore distortion due to cyl pres and/or thermal effects)
* piston rocking combined with uneven ring-piston contact friction about the circumference of the ring grooves
* coriolis effect (a stretch, I know, but pistons do undergo significant acceleration, and I don't remember my kinematics very well)

 

[ivymike]

I still think that crosshatching probably plays a big role in "real" cylinders, btw...

 

[franzh]

There are a couple of studies that show that rotating rings create friction, and that the piston forces required to start and stop the ring movement at TDC and BDC are considerable. One company is working on a rotating cylinder which turns a couple of degrees at TDC and BDC which in effect keeps the rings from seeing a full stop condition.
Franz

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[DesignOutsourcing]

With so many forces at play on the rings, what are the chances that they (the forces) will be in balance and the rings will not rotate? Put something light on a vibrating surface and it moves.

Franz, I'm having trouble imagining how it would take less energy to rotate a heavy cylinder than that lost by the change in rotation of light rings. If we are talking about the vertical movement I can easily see that: "forces required to start and stop the ring movement at TDC and BDC are considerable." Perhaps they are trying to avoid static friction which is much higher than kinetic friction and hope to maintain the kinetic state through rotating the cylinder? cheers, derek


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[strokersix]

My bet is on piston vibration about the cylinder axis. This is not widely discussed, at least not in the general hot rod population. Torsional stiffness of the connecting rod will affect the characteristics of this vibration. I understand that some large engines have been built with tubular connecting rods to provide axial, bending, and torsional stiffness simultaneously.

If the piston is vibrating in this manner it will obviously cause the rings to rotate in their grooves.

 

[franzh]

Opensourcecar:
You are correct about static friction versus kinetic friction. When a piston ring comes to rest at top and bottom, there is considerable force required to "break them free".
Take a look at this, I still have some questions about sealing:

http://www.utexas.edu/opa/pubs/oncampus/01oc_issues/oc010426/oc_fuel.html

The student has since obtained his Ph.D, partially based on this hypothesis.
Franz

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

 

[icrman]

I would guess it could be due to a combination of various vibrations that exist in any running recip engine and the couples. Kinda like when you have a loose bolt with a washer under it. And the washer will start turning with the vibratory motions.

 

[DesignOutsourcing]

Regarding the rotating liners, I looked at their site

http://www.rotatingliner.com

and it looks like the info is from 2002. A google search threw up this report from February:

http://www.rotatingliner.com/RLE-Article-Final-022404.pdf

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