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  • Hole size in relation to axle size in design.

    Discussion in 'The main mechanical design forum' started by omidontop, Jul 11, 2013.

    1. omidontop

      omidontop New Member

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      Hello Guys!

      I have a very basic and simple question which might sound very stupid but forgive me as I'm not a Mechanical engineer but an Electrical one. I am however trying to design something all by myself!

      I need to know when I design something - say a gearbox - what should I choose as the diameters both for the axles and the holes in the gears. I mean should I choose exactly the same diameters for both? Or if not, what's the rule of thumb in choosing one in relation to the other? I present the following cases :

      1. If an axle is supposed to be coupled with a gear
      2. If a gear is to be able to rotate freely around an axle
      3. If a hole is supposed to have an axle (of a gear?) rotate inside of it freely as in the housing of the gears.

      Thank you very much in advance.

      Best Regards,
      Omid
       
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    3. MichaelCreighton

      MichaelCreighton Member

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      This depends on what material your gears, axles, and gear box are made of. Most of my experience is with plastic gears on metal axles in plastic gear boxes. And they are usually always small, axles around 1/16" to 1/8" and gears typically not much larger than 1" in diameter.

      So with that in mind, if a metal axle is supposed to be coupled with a plastic gear, go with the same dimension (for example, a 1/8" axle in a 1/8" hole in the gear). Or you could even go with a slightly smaller hole in the gear, maybe by .005" (for example, a 1/8" axle forced into a .120" hole). Ideally, you can use a set screw on the shoulder of the gear to tighten down on the axle.

      If you want a plastic gear to rotate freely around a metal axle, make the hole in the gear about .005" larger than the axle (for example, a .130" hole for a .125" axle).

      Same thing goes for the plastic gear box, if you want the metal axle to rotate inside of it, make the hole about .005" larger than the axle.

      If you find your gears are not spinning freely, and a little bit of slop in your gear box is OK, then consider adding about .005" between your gear centers. That will space your gears out slightly, and should help loosen up the gear box.
       
    4. Michael Ross

      Michael Ross Well-Known Member

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      http://en.wikipedia.org/wiki/Interference_fit

      That is a start.

      The questions arise - do the shaft and collar have to stay together, or can the rotate WRT each other?
      Then how much force do want to withstand before they start to rotate?
      If you do not want them ever to rotate how are you going to hold them together?
      Welding (electrically, by spinning, by ultrasonics, by explosive impact or percussion, etc) , pressing, shrinking, bolting, set crews, broaches and keys, adhesives, and so on are all useful was to attach a gear to a shaft.

      You need to provide a bit more information to get a sensible answer for your application.
       
    5. John Nurre

      John Nurre New Member

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      Hi Omid

      Classic question. Engineers classically use the term "fit" for this subject... you're asking about the "fit" between the gear and the shaft.

      To fully answer your question, you need to know how much play you need AND the shaft diameter. They go together. As the shaft gets bigger, the amount of clearance-interference will also grow.

      When I started, there were many-many classes of fits. ANSI B4.1 condenses these. It's a big document, but you can find summaries on the web. Find a summary. I see shopfloortalk.com has a good package originally published by Purdue University.

      If you want your shaft to turn freely in a hole, you want to aim toward a "running fit." Those are designated "RC1" through "RC9," with RC1 having the least clearance. There is some art here... an RC1 will have less rattle, but an RC9 will have less friction. As a general hint, a narrow gear will need a tighter fit (to avoid leaning), and a wider gear (hole) will need more clearance (to avoid binding).

      Choose a fit-class, get your approximate shaft diameter, and you'll find the precise hole and shaft diameter from the tables in ANSI 4.1.

      If you want NO turning between shaft and hole (or shaft and gear), you can use the same method and look for a force fit. These are FN1 through FN5. FN5 has the most interference. With metals, you'll probably want to use thermal expansion to join the parts (heat the hole, cool the shaft).

      How much press to guarantee there's no motion? That varies. Steel-on-steel will create a good joint with relatively little interference, but a delrin gear will probably always spin on a steel shaft regardless of the initial press. You can use textbook equations to check the pressure at the joint between shaft and hole. (Beer and Johnson has it, I believe Roarke and Young has it too.) From the pressure, you can get radial force. Force times radius times friction coefficient will give holding torque, inch-pounds or Newton-meters.
      If this makes your head spin, consider a "shaft key." (Look that up in wikipedia.) You'll find off-the-shelf options from most gear suppliers.

      Have fun!
      John
       
    6. omidontop

      omidontop New Member

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      Dear Michael, thank you very very much, that is indeed a great starting point for me. I will take your advices into account and I will see how it goes. Cheers! :)
       
      Last edited: Jul 16, 2013
    7. omidontop

      omidontop New Member

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      Thank you so much Michael, something that had never even crossed my mind! The slippage of course! I specifically appreciate your list of ways to attach the gear to the shaft. I now have a clue to search for and learn more at least. Thank you and cheers!
       
    8. omidontop

      omidontop New Member

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      Dear John,

      A thorough crash course on the "fitting" which thanks to you guys I added to my keywords. I will look through ANSI B4.1 but I think I will try to use a shaft key as you suggested just to make sure and ease the calculations a bit. It might save me some time and money by decreasing the trial/error amount needed. Huge thanks to you.

      Cheers,
      Omid
       

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