Mount Rack & Pinion Interference in PTC Creo | Avoidance & Solutions

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Discussion Overview

The discussion centers around issues related to mounting a rack and pinion system in PTC Creo, focusing on interference problems and timing discrepancies. Participants explore the implications of mounting heights and the correct definitions of pitch diameter and module in the context of CAD modeling.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant describes interference when mounting a pinion with a pitch diameter of 24 mm and a modulus of 2 mm in PTC Creo, questioning whether they have made an error in their setup.
  • Another participant corrects the first by stating that rack pitch is calculated as pi times the module, not just the module itself, suggesting a potential misunderstanding of the terms used.
  • A later reply clarifies that the distance from the tip of the cog to the pitch line is one module, indicating a possible nomenclature error in the original post.
  • One participant expresses difficulty in downloading the provided CAD files and offers to assist if the original drawings are sent directly.
  • Another participant reports that increasing the mounting distance by half a module alleviates interference but worsens timing issues, indicating a trade-off in the solution.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the cause of the interference or the best mounting solution, with multiple competing views and unresolved issues remaining in the discussion.

Contextual Notes

There are limitations in the discussion regarding the definitions of pitch diameter and module, as well as unresolved mathematical steps related to the mounting configurations in Creo.

Chevreuil
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Hello,

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.
 
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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.

Pinion

20012-N20 from this site

http://www.mekanex.se/produkter/komponenter/kugghjul/cylindriska-kugghjul/

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

Rack

K200500 from this site

http://www.mekanex.se/produkter/komponenter/kuggstanger/

zipped .prt
http://old.solidcomponents.com/company/out/4103932/K200500P_Mekanex.zip
 
Last edited by a moderator:
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 .
 
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.
 

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