What pitch is possible for mechanical gears?

Discussion in 'Software and Applications' started by EMHmark7, May 16, 2013.

  1. EMHmark7
    EMHmark7 New Member

    Anybody knows with strong and flexible plastic:

    1) At up to what pitch we can make gears (such as a pitch of 32, 48, etc. being the amount of teeths per diameter inches)?
    2) Precision gap between each gear?
    3) So about dimensionning of parts?
    4) Other gear parameters that make a difference for using strong and flexible plastic in precision mechanisms?

  2. 1. The limitation will be the wall thickness of the material chosen as it applies to the pitch line tooth thickness of the gear. Keep in mind also that WSF is not especially accurate, nor particularly smooth, so don't expect the equivalent of machine cut gears. You may have better strength in the gears if you model them with a positive profile shift - it thins the tips a bit but reduces the undercut at the root. Don't forget to adjust the center distance to suit and modify the mating gear as necessary.

    2. The center distance between gears should allow for the errors mentioned above, and for a bit of backlash. If you mount them too close you'll either not be able to turn them, or will wear them out prematurely.

    3. Dimension the parts as you would for any other standard gear. KHK has some very handy information in a PDF - it's mostly metric module stuff, but the principles are the same.

    4. Define "precision." If you're trying to make watch parts, you'll be disappointed. If you're trying to make a timing belt sprocket, you'll do better. Keep in mind this material is somewhat weaker than bar stock nylon - it's porous, after all. I would expect that all the factors that influence the AGMA grade will hold sway here too.

    I'm in the midst of doing a planetary gear arrangement as we speak. It'll be a concept demonstrator model so high accuracy isn't really be necessary, but I'm modelling it as if it was. Once I get the parts I'll post the results. (I'm going with 32 pitch, 20° pressure angle, and as few as 12 teeth.)
  3. EMHmark7
    EMHmark7 New Member
    Thanks for your reply,

    But I do not catch all your suggestions. Yout talk about:
    -"model them with a positive profile shift- it thins the tips a bit but reduces the undercut at the root."
    -"Don't forget to adjust the center distance to suit and modify the mating gear as necessary"
    -"factors that influence the AGMA grade will"

    I want to do something like this http://www.shapeways.com/model/296214/70mm-lens-gear-pitch-0 -8-for-follow-focus.html?li=productBox-search
    It is made of a pitch 31.75.

    A pitch 28 as well as 32 give a teeth tickness of about 1.5mm at the base and 1mm at top, although pitch 28 allows deeper teeth.
    I do not need much strenght and wont prit it with gears already put side-by-side. So the problem is to know if such gear workswell for a slow motion
    and if it breaks easily.

    And relative to teeth corner curves, etc, I don't know that stuff.

    1) When do you think you wil have a result?

    If anybody in the forum already knows good or bad experiences relative to specific pitches and other parameter, please let us know! For shure will help many people.

  4. Sorry. I guess I pitched that one a little tall. Take a browse through this:

    http://www.khkgears.co.jp/en/gear_technology/pdf/gear_guide1 .pdf

    Be warned, it's a big download! (20-ish Megs.) There's pretty much all you need to know, and potentially too much. If you get stumped, or even just need confirmation, feel free to ask. :)

    Something low- or no-power like a lens drive should be ok, assuming the lens is reasonably free to move. WSF's flexibility may actually help in such an application, cushioning any drive motor vibration.

    Having the same module (metric) or pitch (imperial) for both gear and pinion is critical, much as having the same pitch for both chain and sprocket. Keep the face width (length of tooth) fairly narrow. Large face widths require MUCH closer parallelism of the gear shafts, lest the gears start binding at their ends. Higher strength is better achieved by changing to a larger tooth profile (smaller diametral pitch number or higher module number), so using a wider faced gear should be as a last resort where available space limits the design.

    Currently, I'm about 1/4 of the way through my design. Getting it printed will be a bit of a financial hit - probably on the order of 50cc or more of print volume. Stay tuned, sports fans!