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  • Help regarding Timing Pulley selection

    Discussion in 'The main mechanical design forum' started by yashu1991, Mar 28, 2014.

    1. yashu1991

      yashu1991 Member

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      Hello all,

      I am a student working on a project in which I plan to use timing pulley and timing belt for the power transmission and speed reduction. I want the speed reduction to be 7:1. The diameter of the motor shaft (driver) is 6 mm and the driven shaft diameter is 1 inch. Can any one suggest me if I can achieve it in single step or I have to opt for compound pulleys ? I found the smaller pulley (type MXL, anodized aluminum) number of teeth: 20, OD: 0.685 in, Bore size: 0.25 in, Pitch dia: 0.509 in., 0.25 inch wide belt can fit in www.mcmaster.com. Please suggest me the suitable larger pulley and pitch of the belt that correctly meshes with both the pulleys so that I can achieve the desirable output speed and also source where you found. If this cannot be done in single stage what ratios should I use for intermediate stages? Help will be greatly appreciated. Thanks in advance.

      Cheers !!!
       
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    3. bdeuell

      bdeuell Member

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      A 7:1 ratio is achievable using timing belts at least in the smaller pitch belts. I would recommend going to SDP-SI (https://sdp-si.com/eStore/CoverPg/Drive_Components.htm) for your belts and pulleys they have a much greater selection than mcmaster-carr. They also have excellent design guidelines/documentation, they do have a drive design program to help you chose components but I fount it easiest to just look through their catalog. I recently designed a timing belt transmission using components from them and was able to achieve a 8:1 ratio on my first stage which coincidentally was driven from a 6mm motor shaft. however, you may have difficulty finding a pulley with a 1" bore in the smaller pitch sizes you could always modify a pulley for your shaft tho. The 1" shaft makes me question what kind of torques you expect to have, 6mm to 1" seems like a big jump in shaft size for a gear ratio of 7:1. If you do have large torques you may have to limit your selecction to larger belt pitch sizes.
       
    4. yashu1991

      yashu1991 Member

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      Hello bdeuell,


      [​IMG][/URL][/IMG]
       
    5. bdeuell

      bdeuell Member

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      looking at your application I can see the need for a large shaft diameter to handle the impact load. It would probably help if you can support your load with a bearing closer to the bar. I would put a bearing between the pulley and the bar and put it as close to the bar as possible, this will reduce the bending stress on the shaft. Even better would be to place a bearing on each side of the bar and cantilever the pulley but it is probably difficult to place a support above the bar.

      A few years back I was working on a battle bot with a spinning bar setup similar to this. We did eventually shear all of the teeth of the belt. I would guess we were using a L series belt 3/4" wide. Output shaft was 7/8" and had no issues with it, I would guess it was made from 1018 steel but I am not sure as I did not design it.

      I would stick with the large output shaft you have in your design but recommend placing a bearing as close to the bar as possible. Assuming you have a low inertia motor (i.e. not a gear-head motor) your motor shaft will probably be strong enough. You may have to replace the belt every so often as the shock loads will be particularly harsh on it. I would use the largest belt you can (both in pitch and width) that will allow you to achieve the desired gear ratio.

      Quickly looking through SDP-SI's catalog I think the XL pitch belts will be your bet bet. I found a 10 tooth and 72 tooth aluminum pulley in that size.

      Pitch diameter is different from pitch. For example an XL pulley has a .2" pitch, so be sure this matches on your belts and pulleys. You will also need to make sure that the pulleys you select will be able to fit your belt width. There are several different pitches and tooth profiles available but the L, XL, and MXL series belts are the most common and traditional so buying replacement belts would probably be the easiest. Other tooth profiles to offer better tooth shear strength so they might be worth a look.
       
    6. bdeuell

      bdeuell Member

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      Does the bar pivot on the bolt holding it onto the hub?

      If it does it would help a lot the reduce the impact loads seen by the shaft and drive system.
       
    7. Dana

      Dana Well-Known Member

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      I'm not sure why none of your images are displaying (at least for me), but:

      Ratio is a function only of number of teeth. If your smaller pulley has 20 teeth then the larger pulley needs to have 140 teeth for a 7:1 ratio. Whether these sizes are available from stock is another matter, 140 teeth in an XL (1/5" pitch) is nearly 9" pitch diameter.

      17500 rpm is really fast for any kind of belt drive. I don't know if a standard timing belt can take that but I doubt it; you will need to look at the manufacturer's published information. HTD (round tooth) timing belts are cable of higher speeds, but I don't know how high off the top of my head. HTD belts are typically in metric, not inch, sizes.

      You will also need to know what kind of torque you are talking about and size the belt drive accordingly... XL belts are for relatively low loads. As bduell said, the 6mm motor shaft seems mismatched with a 1" diameter output shaft.
       
    8. yashu1991

      yashu1991 Member

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      Hello Dana,

      So what you trying to point is I cannot find a suitable large timing pulley with appropriate pitch diameter and bore of 1 inch ? There will not be too much load on the pulleys though.
       
    9. yashu1991

      yashu1991 Member

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      Hi bdeuell,

      I searched many places but cannot find a timing pulley with 1" bore so do you suggest me to go for little smaller size say like you people have tried 7/8" ? The maximum bore size available in https://sdp-si.com/eStore/CoverPg/Drive_Components.htm is 0.625" so do I need to make changes of my output shaft accordingly to make it a good fit with the pulley ?
       
    10. Dana

      Dana Well-Known Member

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      No, without looking, I don't know how big a pulley you can getin that pitch. I've never needed one that big.

      SDP has a large selection, but other manufacturers (Boston Gear, Browning, others) have more big stuff, SDP focuses on the smaller stuff.

      Another thing to consider is if you have a large pulley and a small one close together, there might not be enough wrap on the small pulley... there's a "teeth in mesh" factor; the ratings are reduced if there aren't enough teeth in mesh. Again, the manufacturer's data will have guidance on that.
       
    11. bdeuell

      bdeuell Member

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      You can bore out a pulley to fit on your shaft just make sure that the hub is large enough to still provide sufficient strength.

      To really be able to spec out the components and design your parts you need to have numbers for all the forces acting on your system. There are only two ways I know of two get these numbers and that is through calculations of the physics involved or through experimentation/experience. I gather that you have not done the calculations to determine all of the forces, if this is beyond your skill level currently that is OK. Design engineering draws on knowledge from many different areas and it is to be expected are still learning (and hopefully always will be :)). While not always the most ideal engineering approach your best bet might be to build the thing as beefy as possible and see how it works. You will learn a lot form the experience. On your next project and throughout your education you can continue to incorporate more engineering and optimize your designs better.
       

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