Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mounting a rack and pinion -- Distance to avoid interference in PTC Creo CAD design

  1. Jun 7, 2016 #1

    I have downloaded CAD models of a rack and pinion, and I don't seem to be able to mount them up in PTC Creo without quite obvious interference.

    The pinion has a pitch diameter of 24 mm, and a modulus 2. The "pitch diameter" of the rack is exactly one modulus lower than the tip of the cogs on the rack. The modulus is, of course, 2 mm. Measurements on the CAD-models suggest that I haven't had a mixup of some kind.

    In Creo, if I mount the pinion exactly 12 mm above the "pitch diameter" of the rack, it's quickly obvious that something is wrong. If I put a cog from the pinion facing straight down between two cogs of the rack, I get interference on both sides. If I rotate the pinion, after a few turns, the timing between the rack and the pinion is off. Why is this? Creo says that it is moving the rack pi*24mm per turn of the pinion, and I can't believe that Creo could mess that up. Have I done something wrong?

    Increasing the mount height if the pinion to 12.5 mm, everything looks a lot better. The timing problem is still weird, though.

    A) Why is this such a mystery?

    B) Is it horrible to mount a pinion slightly off from a gear? This is for a feeding mechanism of a disposable item. The pinion stays in the machine, but the rack is supposed to be soft plastic, and it is discarded after it has passed the pinion.
  2. jcsd
  3. Jun 8, 2016 #2


    User Avatar
    Science Advisor
    Gold Member

    Last edited: Jun 8, 2016
  4. Jun 8, 2016 #3
    Oh, sorry, nomenclature error, I meant the distance from the tip of the cog to the pitch line, which according to the roymech document is 1*module.


    20012-N20 from this site


    If you have trouble navigating and you are brave enough, use this direct link to zipped proE file.
    http://old.solidcomponents.com/company/out/4103919/20012-N20_Mekanex.zip [Broken]


    K200500 from this site


    zipped .prt
    http://old.solidcomponents.com/company/out/4103932/K200500P_Mekanex.zip [Broken]
    Last edited by a moderator: May 7, 2017
  5. Jun 10, 2016 #4


    User Avatar
    Science Advisor
    Gold Member

    I'm having problems downloading those files . If you still want some help send me your drawings directly . Any common CAD file format is ok .

    See conversation for my Email address .
  6. Jun 10, 2016 #5
    Thank you!

    I have sent the original .prt files from the Mekanex source. Mounting them so that the pitch diameter of the pinion is in contact with the "pitch diameter" of the rack (which is exactly 1 module from the tip of the cogs of the rack) give me serious interference issues. I also have trouble getting the translation of the rack right in Creo.

    If I increase the mounting distance by half a module I get rid of the interference, but this sloppy solution messes up the timing even more.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted