What is the change in diameter of a solid piston ring under pressure? 1

[tribosonics]

Hi,

Just wondering if anyone can point me in the right direction to work out what the change in diameter of a piston ring under a internal pressure?

thnaks

 

[ivymike]

under what conditions? When it's installed on a piston within a cylinder bore?

 

[tribosonics]

A solid ring (no splits or breaks), 1.5mm thick in the radial direction, 3mm wide, 68mm diameter, with a 20MPa pressure from the inside forcing it out (i.e. the pressure increases it's size).

Thanks

 

[richdubbya]

It expands to the hot and running bore size, no more no less.

 

[NickE]

>>>It expands to the hot and running bore size, no more no less.<<< - richdubbya
Best answer evar, however I dont know if you're right, no star today.

 

[EngJW]

Well, I'll give him one! Gas pressure on the ID forces it out to the wall where an equal and opposite force acts on the OD. Now, if you want to get down to the micron level, there could be some localized compression of the ring, cylinder wall, and oil film.

 

[NickE]

I'm sure you could do a hoop stress caluculation, then using modulus find the strain developed, then using poissons ratio to find deflection. (Thats proabably wrong, but it is close.)

nick

 

[ivymike]

I'm wondering whether this is really a piston ring - are there many piston rings w/o gaps? If this is a piston ring with no gap, then how can you be sure it expands to the bore diameter?

 

[ivymike]

if this is a piston ring with a gap, then hoop stress won't work (you need curved beam bending formulae). If this truly does not have a gap, and is not restrained from the OD, then you're basically talking about a very loose press fit calculation (see other threads on press fits).

 

[patprimmer]

Is this a real question, or some hypothetical question set as homework for a student?

I have seen gap less rings, but never rings with no split. How would you install them?

Real rings are restrained by the bore which as previously stated, are stable to within microns.

This has the benifit of a very small doubt at this stage, but unless I see a real reason for the question, I see a red flag appearing very soon, that is unless someone else has already done it.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ivymike]

all "gapless" rings that I've ever seen for automotive pistons would be more accurately called "obstructed gap" rings. I use the terms "gap" and "split" interchangeably, though (prefer gap). If you defined gap as an unobstructed split, then you could have true "gapless" rings.

 

[EngJW]

Even a gapless ring is free to conform to the bore diameter. If the question is truly about a continuous ring, then it cannot be a piston ring. It is either a thick or thin walled cylinder under internal pressure and any college machine design textbook (or Roark) will have all the equations needed.

 

[strokersix]

Perhaps I'm thinking too much but it is Friday afternoon. It appears most responses are assuming this ring must be part of a reciprocating engine of some type. While that's certainly my first thought too, I don't see anywhere in the question details about the application. tribosonics: If this is a real question, please post more details!

jlwoodward: As I recall, the thick and thin walled cylinder equations apply to a theoretically infinite length cylinder. This isn't infinite length, which means it's a three dimensional problem instead of two. Someone please correct me if I'm wrong. Also "it cannot be a piston ring" more accurately could be stated "it cannot be a piston ring in a conventional reciprocating engine design". One example is polymer sealing rings used in automotive automatic transmissions. They are often solid rings without gaps and I would consider them "piston rings" in a more general sense.

 

[Warpspeed]

I would worry about heat flow and differential expansion between the bore and piston ring. It the thing is completely solid, why even fit a ring, why not just a ring-less piston ?

The whole idea behind a ring is to allow it to expand to exactly fill the bore, especially if gas pressure can get behind the ring to aid in the process. A completely solid ring is really a non functioning part.

 

[icrman]

<<<<<<Hi,

Just wondering if anyone can point me in the right direction to work out what the change in diameter of a piston ring under a internal pressure?

thnaks>>>>>>

Just so we can see the question again.
First what is the material of the ring?????
Second what is causing the internal pressure?????
Third what is the internal pressure????
Forth what are the dimensions of said piston ring????

If it is a plastic or teflon or rubber, it will expand
till it pops out of the piston ring land and or breaks. That is if there is no cylinder that the piston is fitted into, to restrain the ring from popping out. That is if it is a solid gapless ring.
If your talking a regular piston ring in the average piston engine. The diameter follows what ever the diameter of the cylinder is. So if heat or gas pressure expands the cylinder ever so slightly the ring follows it. And of course there is some leakage of gas past the ring as well as through the gap.
Or is the question really, what will be the expansive force of the ring against the cylinder under the influence of gas pressure?????