Gear Design Problem

  1. Hi,

    I am looking for some help designing a gear for an invention of mine. I know gear design basics, but this is a little beyond what I know. Any help would be greatly appreciated.

    The needed gear (to be made from plastic) is part of a toy and will be under very little stress. For my prototype, I will be making a 3D solid model in AutoCAD and will export it for fabrication via stereo lithography.

    Here are the parameters

    1. The two gears need to be identical.
    2. They need to have a pitch diameter of approximately 2".
    3. 11 Teeth per gear is preferred.
    4. The axis of each gear must be at a 48 degree angle to the other.
    5. The two gears must also mesh at a 12 degree angle to each other on an axis that I can best described as a "twist" in the way the two gears meet up.


    Thanks for your help.

    -Glenn
     
  2. jcsd
  3. FredGarvin

    FredGarvin 5,087
    Science Advisor

    What exactly is it you are asking help with? It looks like you have a pretty good handle on things. If the design worries you, I would suggest picking up a copy of ANSI B92.1 Involute Gears and Splines Inspection. It will give you every bit of information for all standard gear pitches and pressure angles. I highly recommend it.

    By your constraints, are you trying to come up with a cross between a spur gear and a bevel gear?
     
  4. brewnog

    brewnog 2,793
    Science Advisor
    Gold Member

    I don't quite understand point 5. By the 'twist' and 12 degree angle, do you mean that the gears are to be helically cut?
     
  5. If you use a 20([edit] or 14 for that matter[/edit]) degree contact angle you are much more likely to find your gears as an off the shelf item. If you plan on taking this to production then cost should be an issue. Having a custom gear cut or cast for a single product line adds to costs.

    Actually, re-reading your post you are looking for a bevel gear with a helical cut(like the pinion in a rear whell drive differential--- http://www.moserengineering.com/catalog/category_images/RING PINION GEAR.jpg ). If you are looking for software to aid you in your design then try this: http://www.cwattsdesign.com/gearfactory.htm As Fred said you may want to look into standard gear designs because it will be cheaper for you in the end. Custom gears, even little plastic ones, cost orders of magnitude more usually.

    Good luck.
     
  6. brewnog

    brewnog 2,793
    Science Advisor
    Gold Member


    I think the OP's problem is that his axes are at 48 degrees to one another, and as a result I doubt stock gears would be viable (I'd think standard bevel gears are only available for 'sensible' angles, 45 and 90 degrees etc).
     
  7. Hi,
    I got hold of a copy of Machinery's Handbook which has a whole section on gears. After reading up a bit on gears, here is perhaps a better description of the gear set I am trying to come up with a design for.

    Two identical mating helix gears which have a bevel angle 48 of degrees and a shaft angle of 12 degrees. (did I get the terminology correct?)

    I found the equations for designing "Helical Gears for a given shaft angle with equal center distances", and I found equations for designing Bevel Gears. I couldn't find equations that covered a combination of the two.

    So my question is how do I go about designing combination helical/bevel gears?


    Thanks,
    Glenn

    PS - I was unable to find any stock gears out there that will suit my needs, even for my prototype. Approximate angles won't work as the angles are critical to what I am trying to achieve.
     
    Last edited: May 10, 2005
  8. FredGarvin

    FredGarvin 5,087
    Science Advisor

    Are you absolutely sure that you can't simply use a bevel gear at a non custom angle? Do you really require a helical gear? What will you really gain by having a helical gear in your transmission?

    I ask this because that would simplify your design for the gears. Designing a custom angle bevel with a straight spline would not be out of the realm of someone who oes not do gears for a living.
     
  9. Hi Fred,

    I was able to download a gear designer LSP for AutoCAD which allowed me to do what you mentioned; designing a custom angle bevel with a straight spline. But that still leaves me with only meeting the 48 degree requirement. I still have the 12 degree requirement to meet as well. Am I correct that I would only be able to achieve that with a helical gear?

    Thanks,
    Glenn
     
  10. FredGarvin

    FredGarvin 5,087
    Science Advisor

    I really need to understand where this 12° is coming from. Your description isn't clicking with me. I would think that the bevel alone would take care of what you need.

    Also, I just noticed that the number of teeth are identical. Is there any reason why yopu couldn't go with a flex shaft? What will having the two identical gears get you?
     
  11. Hi Fred,

    Here are a couple of drawings that will better describe the geometrical relationship of the two gears....


    http://www.gsg.microminutes.com/images/gear 1 of 2.jpg
    http://www.gsg.microminutes.com/images/gear 2 of 2.jpg

    The angular dimension on the first drawing is 48 degrees.
    The angular dimension on the second drawing is 12 degrees.

    Flex shafting (or any other type of shafting) will not work for this application. The requirement for identical gears is a result of one of the symmetric characteristics which makes this invention unique.

    Thanks,
    Glenn
     
    Last edited: May 10, 2005
  12. FredGarvin

    FredGarvin 5,087
    Science Advisor

    Okey Dokey. Well, it does look like the straight gears will do what you want, correct? In any case, good luck.
     
  13. Nope, straight spline gears won't work. Those drawing I posted the links to were only for illustrating the geometric relationship between the two gears. They would never mesh.
    :frown:

    -Glenn
     
  14. FredGarvin

    FredGarvin 5,087
    Science Advisor

    When I stop and think about it (which I should do from the beginning) you are correct. They won't work. If you are doing an SLA of the models, then it shouldn't be too much to come up with a custom helix angle and make the prototypes yourself. Getting a gear cutter to make some may be a completely different story though.
     
  15. Unfortunately that is the part I don't know how to do.

    I have some postings up on other forums. Hopefully something will pan out. I found a gear designer who will do the design ($$$$). Ouch! Especially considering that I am doing this on my personal time and with my own money.

    -Glenn
     
  16. FredGarvin

    FredGarvin 5,087
    Science Advisor

    Not knowing what you are doing, but I would seriously try to find another way to do it.
     
  17. brewnog

    brewnog 2,793
    Science Advisor
    Gold Member

    I'm afraid Fred might be right.

    I can understand if you're not willing to share details of your design, but I'm finding it hard to see why such a setup would be needed which could not be addressed in a different way. Custom gear hobbing is very specialised, and as we've discovered you have some pretty odd geometry here. Even addressing the manufacturing issues by using SLA or SLS (probably a good call), I reckon attempts at making your CAD model are going to cause you some sleepless nights!

    Perhaps you could get a CV or Universal joint to work?
     
  18. Unfortunately, in this case, that is analagous to saying to find another way to design a Hoberman Sphere. :biggrin:
    The gears are primary to what this invention is all about.

    Luckily I am onto a lead through a friend of a friend who has some rather sophisticated gear design software.

    Thanks,

    -Glenn
     
    Last edited: May 12, 2005
  19. another bevel gear design problem

    I am designing a mechanism which requires two identical 32-48 pitch, straight cut gears of 1 inch pd and having a 45 degree shaft angle. I need ss 303 gears or hardened steel if possible. I would also prefer a gear with no hub although I can machine it off. I am hoping for a standard part but I have not been able to find that so far. If this does not exist, I have access to wire EDM
    and a cad design program. Might anyone have some ideas? Thank you Tom Kasmer 607-7631607 EDT
     
  20. Q_Goest

    Q_Goest 2,949
    Science Advisor
    Homework Helper
    Gold Member

    tkat, welcome to the board. Have you considered simply having one made? Go to Thomas Register, and look for gear manufacturers in your area. Give them a spec on your gear simply giving them the information you've already provided in an official looking format. If you don't make up a spec sheet, many companies won't take you seriously. Send the spec out to a dozen manufacturers and ask for quotes on small quantities (ie 1 to 10) but then ask for a quote on larger quantities (whatever you may need for production). Explain you need a prototype. Most gear manufacturers will sell you a couple at a very reasonable cost if you suggest there is a large production need later on.
     
  21. FredGarvin

    FredGarvin 5,087
    Science Advisor

    I have known a couple of wire EDM shops that I could give a 1:1 drawing of what I wanted and they did the programming and cut one out. Admittedly, you're not going to get the precision most true gear makers would get, but if that's not your end, then that could work for you as well. Make some calls. Like Q already mentioned, make some calls. I think you'll find someone to do it pretty quickly for you.
     
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