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Metal structure sound test

  1. Dec 12, 2013 #1
    Hi there ,

    I have two cast iron crankshafts for an engine i'm building , now even though they are similar in every way , made ofr the same engine etc etc, there is one thing I noticed different and it got my attention ,
    I took a stainless steel screwdriver and hit each one of the cranks at their " cheeks" or other parts that aren't asociated with sleeve bearing mounts.
    Now the interesting thing here is that they both have this ring like a bell but a more precise tone.Now that doesn't surprise me but what surprises me is that one crank has a higher pitch while the other has a lower one , and the difference is pretty audible , with nothing but a human ear i can clearly tell the one crank sounds higher in pitch while the other lower.

    Does somebody know what does that mean in terms of the cast itself and maybe the material used and how pure it is and other factors related to structural things of cast iron or metal in general?

    thanks.
     
  2. jcsd
  3. Dec 13, 2013 #2
    Are they on the same surface, with the same contact points touching other objects?

    The tone changes, largely based on braced/unbraced area.
    Make sure you put both of them on the same object and turned the same way, etc.


    I believe (forgive me if I'm wrong) that denser materials naturally ring higher. Check the weight, perhaps one is heavier.
     
  4. Dec 13, 2013 #3
    I think that these changes in sound have something common with the wheel tapping
    Maybe there is some deffect in the lower pitch crankshaft.

    http://en.wikipedia.org/wiki/Wheeltapper
     
  5. Dec 13, 2013 #4

    AlephZero

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    A complicated shape like a crankshaft will have many different vibration frequencies, and they will depend on exactly how it is supported. I wouldn't be too bothered about this. If one rings for much longer than the other, that would be more worrying, suggesting there were defects like cracks (maybe internal and invisible) or porosity in the one that doesn't ring.

    If you want to investigate this further, try suspending the crank in the air using something as flexible as possible (e.g. large elastic bands, or stretchy coil springs) and find the best positions to suspend it and tap it to make the tone sound as "pure" as possible. That is one way that "professional" vibration testing is done, to minimize the effects of poorly defined restraints like the object just resting on a bench. The flexible supports take the weight of the object but they don't restrain its vibration, whatever direction it wants to move in.
     
  6. Dec 13, 2013 #5
    well there is one ting that may or may not cause this difference , I took my scales out and weighed the cranks , and the lower pitched one is heavier by about a 1kg than the other higher pitched one, maybe there is the reason for the difference.

    Speaking about the surfaces on which they were standing , do you really think that could play that big of a role? I put them in many different places on many different surfaces and the higher /lower pitch diference was clearly audible and the same in all cases.

    A soft rubber belt would be a good idea to hand them on and then measure the sound , well they both as of yet have this clear high pitched bell kinda noise, like a small bell would ring. For cast iron I guess it's good , a dull and short sound would indicate problems is that correct?

    I read somewhere that the bell noise can be different depending on either the metal is forged or cast as in my case it is cast iron.
     
  7. Dec 13, 2013 #6
    The frequency will be generally a function of the length and the stiffness of the material. The higher sounding one will be made of a stiffer alloy is my guess.
     
  8. Dec 13, 2013 #7

    cjl

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    1kg difference? Between 2 crankshafts made for the same engine? That's a pretty large difference - I'd be inclined to look into why there was such a large difference in weight before proceeding with the engine build.
     
  9. Dec 13, 2013 #8

    SteamKing

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    I think there is some difference in all crankshafts to some extent. Have these cranks already been balanced yet?
     
  10. Dec 13, 2013 #9
    ups sorry I forgot to tell you that the heavier , lower pitched one is a newer model with some little adjustments , it fits the same engine model but has a larger stroke from 80 to 84mm as compared to the previous original one , so the difference in weight is not the problem hat concers me , rather the difference in the sound they make as I am thinking about the durability of the material and other factors.

    By the way can anyone give me a straigth answer to the question are higher pitched metals more dense and what other specific factors may come into play of this difference in pitch, thank you :)

    @Steamking yes the crank is balanced as can also be indicated from the drill holes in the crank it has been balanced out.
     
  11. Dec 13, 2013 #10

    cjl

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    I would tend to think the pitch of the sound would be inversely proportional to density (higher pitch = less dense), but directly proportional to stiffness (higher pitch = stiffer).
     
  12. Dec 13, 2013 #11

    sophiecentaur

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    The resonant frequency of a bar depends upon its density and also its diameter and length. They tune glockenspiel bars by shaving off the length to go up and thinning down the waist to go down in frequency. Likewise for bells, they thin the edge to go up and the waist to go down. (You can't put material back, of course)
    If one crank is ground differently or has thinner webs, that will make a massive difference to the resonance because it alters the stiffness. Do they measure the same?
     
  13. Dec 14, 2013 #12
    @Sophie , yes I kinda thought about it too , well the length is the same as it fits the same engine, the pins or how do you call the places where the rod attaches the crank are also the same diameter so are the main bearings.

    The difference is in the counterweights , the original crank has kinda thicker counterweights , this new one has them a little bit thinner and the shape is alos a little different , they are thinner because I assume that the stroke is longer so to equal out in weight they made a thinner but longer counterweigth.
    I can post pictures later maybe then someone will be able to say a more in depth opinion , as for now this is all I know.

    Oh also the oil channels inside the crank which let the oil flow from the central bearing towards the rod bearings , the channels are different , they are less but each of them are with a bigger diameter , the original crank had more channels but with smaller diameter.As they are on the inside of the crank as you know maybe that could affect the sound it makes?
     
  14. Dec 14, 2013 #13

    SteamKing

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    Since the stroke is different, it's reasonable that the counterweights would be different. So what appeared to be two identical cranks initially has now turned into two not-so-identical crankshafts. Your ping test worked.
     
  15. Dec 14, 2013 #14
    the larger stroke is okay , I was looking for this it's not a surprise , it's just rather that I know there have been some doubtful quality parts from this manufacturer before to some people , that's why before assembling the crank into the engine I want to make damn sure it will do it's job properly , so I'm trying to get as much info about the crank as possible.

    Well as for now it looks as it should be fine , I even tried a miniscule hand drill (1 mm) and with a magnifying glass i did several one and a half turn drills into different non bearing parts of the crank and the same to the original crank to see the how would the iron perform and it did similar.

    I really do hope that the lower pitch is because of the shape of the crank which is a little different and the mass which is roughly 1kg more.
     
  16. Dec 14, 2013 #15

    SteamKing

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    If you know the manufacturer has a doubtful reputation for quality, why take a chance on using them for such a critical part?
     
  17. Dec 14, 2013 #16
    because it's their engine also and their car , made back in the day , and if you want to stay authentic you have to use it no matter what.

    It's an old communist car called Vaz.Back in the days of soviet union they made it actually quite good , then the politicians decided that they need more bombs and planes and stuff so the build quality of parts declined.Then after the collapse of the union mafia took over the plant and used it to earn money and now teir back again producing mainly the Niva which is a 4x4 lightweight jeep.
    Since it's very good at offroad conditions it has a followerbase around the world and many parts from it are 1:1 to the car that i'm working on so some of them I am using.

    Well it's a very complex and long story.
     
  18. Dec 14, 2013 #17

    AlephZero

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    That sounds like "similar" as in "the Party's 5 year improvement plan for the motor industry was successfully completed." :biggrin:

    If the masses differ by that much, it's hardly surprising they have different frequencies!
     
  19. Dec 14, 2013 #18
    now the fact that the mass differs is nbormal because the newer crank is with a larger stroke and the crank itself is a little different , so the weigth difference is a factory thing , the only thing I was afraid a little was the quality since this newer crank is an experimental one but it seems it will be okay.

    Yeah the party was full of paradoxes but out of those paradoxes some very interesting things and sometimes even extremely good ones were born also.
    Well definately not the cars they made:D
     
  20. Dec 15, 2013 #19

    sophiecentaur

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    Wasn't it Pythagoras who noticed the tones that a blacksmith was producing whilst bashing pieces of metal of different sizes and wrote about it? It was a lot easier to be smart in those days - especially if you happened to be as smart as he was.
     
  21. Dec 15, 2013 #20
    yeah it was a lot easier to be smart even 100 years ago , as the last 100+- years have like went into geometrical progression in terms of technological advancement , sadly this goes only about few percent of the population who made those achievements while the majority is just trying to cach up in the level of everyday life to these things and quite many seem to actually have gotten dumber :D
     
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