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Piston ring end gaps

  1. Sep 17, 2013 #1
    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.
  2. jcsd
  3. Sep 17, 2013 #2


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    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.
  4. Sep 17, 2013 #3
    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 gonna 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.
    Last edited: Sep 17, 2013
  5. Sep 17, 2013 #4
    Okay, I think I figured out what they did. With a little bit of algebra, you can turn



    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, thats 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 wanna bring something like that up unless I'm really sure that's what happened.
  6. Sep 17, 2013 #5
    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?
  7. Sep 17, 2013 #6


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    As an intern, you can always ask someone about something you don't quite understand.
  8. Sep 17, 2013 #7


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    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. :confused:

    Maybe "CTE" doesn't mean what you think it does.
  9. Sep 18, 2013 #8
    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.
  10. Sep 18, 2013 #9
    As a matter of interest, what is the application of this engine? It sounds like a really interesting place for an internship.
  11. Sep 19, 2013 #10
    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.
  12. Sep 19, 2013 #11


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    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 :smile:
  13. Oct 10, 2013 #12
    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?
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