Piston Ring End Gap: Are We Doing It Right?

[nbaggs]

Im an intern at a manufacturing company looking at whether our end gaps are too small on piston rings. Can someone take a look at this and tell me if I'm doing it right?

We have a piston ring that goes into a 9 5/8" bore. The upper limit of the end gap is 0.136. Max temperature is 300°F. For the ring material the radial CTE is 4.4*10^-5 and axial CTE is 8.9*10^-5.

I said circumference c=∏*d=30.237 (assuming d=9.625)
length=circumference-max end gap=30.237-0.136=30.101
ΔT=300-70=230°F (assuming tolerances given at 70°F)
I assumed you use axial CTE α=8.9*10^-5

ΔL=α*ΔT*L=(8.9*10^-5 /°F)(230°F)(30.101")=0.616"

Since ΔL > max end gap, do I assume our piston rings close up?

That seems like a pretty drastic design error with the piston ring trying to close almost a half inch greater than the end gap, I wanted to see if I'm doing this right before I take it to my boss.

Thanks for any help.

 

[SteamKing]

Although the ring is supposed to fit into a bore of 9.625", have you actually measured the diameter of the cold ring?

It would seem that an expansion of 0.616" would close a gap of 0.136" and then some.

 

[nbaggs]

We don't have tooling to accurately measure the piston rings. We machine the cylinders ourselves but we purchase the piston rings from another manufacturer. I'm taking dimensions off of our drawings. One thing I didn't take into account is the thermal expansion of the cylinder, but that's ductile iron with a CTE of 6.25*10^-6 so I think the ring is still going to close up. I think it might be time to call the ring manufacturer and see if I'm missing something.

Also, we're looking at this because of broken rings, so your statement that it seems like it would close the gap is supported by what were seeing.

 

[nbaggs]

Okay, I think I figured out what they did. With a little bit of algebra, you can turn

ΔL=αΔTL

into

End Gap=L2(1-1/(αΔT+1))

Then, if you assume a ΔT of 150°F and a L2 of 8.495 (DIAMETER of the piston, not the bore) you get within .004" of our end gap low tolerance.

But, that's usind diameter, not circumference. Now, I'm not saying engineers make mistakes, but is it possible someone overlooked turning the diameter into a circumference and didn't bother to actually check the design temperature of the cylinder? Especially as an intern, I don't want to bring something like that up unless I'm really sure that's what happened.

 

[pbuk]

Why are you taking the diameter of the piston? The ring should be compressed against the sides of the bore so the circumference of the ring at working temperature (including the gap) is the inner circumference of the bore at working temperature, not the outer circumference of the piston. Bear in mind that the compression ring temperature is higher than the cylinder liner temperature, and the piston temperature (at the top) is higher still.

Have you looked at the specification/tolerances of the grooves?

 

[SteamKing]

As an intern, you can always ask someone about something you don't quite understand.

 

[AlephZero]

Just out of curiosity, what material are these rings? The expansion coefficients seem huge (10 times steel), and why is the "axial" coefficient twice the "radial"? If "radial" and "axial" are relative to the piston, the most interesting direction would seem to be circumferential, not radial or axial.

Maybe "CTE" doesn't mean what you think it does.

 

[nbaggs]

I did bring these questions up to one of our design guys. Turns out the piston ring gaps really were too small. The material is proprietary to the company we purchase them from, but they are a compression molded glass and molybdenum disulphide filled PTFE material. There was a different CTE than what I was using, but it was close enough that it didn't change the result that much. Also, bore diameter was the correct measurement to use.

It's interesting to see these kind of things in real design issues rather than a homework problem.

 

[pbuk]

As a matter of interest, what is the application of this engine? It sounds like a really interesting place for an internship.

 

[nbaggs]

Reciprocating gas compressors, mostly used for upstream, midstream, and downstream natural gas applications, as well as some processing and CNG applications.It is a pretty interesting place for an internship.

 

[AlephZero]

It's interesting to see these kind of things in real design issues rather than a homework problem.

Yup. You remember the experience of fishing a pile of broken bits out of a machine for longer than getting 0 for a wrong answer to your homework

 

[nbaggs]

Sorry to bring up an old post, but I was thinking about this the other night and came up with a theory.

These are glass and molybdenum disulphide filled PTFE material piston rings in a hydrogen application (specific gravity is approx. 0.07). Suction pressure is approx. 740, discharge pressure is approx. 1470. This is a reciprocating compressor.

Is it possible for the rings to "soak up" any hydrogen and could this cause the ring to swell? About 0.020" of swelling lengthwise on a 9.625" diameter ring would explain the rings closing up.

Am I off base here, or is that possible?