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  1. #1
    jkilroy is offline Diamond
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    Default How to size V-belts vs. HP

    I am working on a new project. What is the reference for designing V-belt drives? In my case we are talking about a 3hp 1725rpm motor controlled with a VFD. The machine will, at times use 2 to 2.5hp, but probably not for more than 30 seconds. I think a double belt A size setup would be fine, any reasons to think otherwise? I am fresh out of "rules of thumb" when it comes to this kind of thing.

  2. #2
    Mike C. is offline Diamond
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    The Van Norman 22L of the old museum shop and my Steptoe shaper both have 2hp motors and use two matched A belts. Might want to go up a size or add another groove on the sheave.

  3. #3
    Benesesso is offline Hot Rolled
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    A lot depends on the pulley sizes and belt wraps. You can easily transmit 5 HP with one A belt using 5"+ pulleys.

  4. #4
    Forrest Addy is online now Diamond
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    The V belt manufacturers have done a brilliant job of confusing the issue of simple V belt drive engineering. It should boil down to a few formula and some guidelines but no. They publish acres of tables with little reference to drive design considerations like: low cost, low weight, low bulk. The presumption seems to be every V belt drive will be targeted for an industrial application where there is endless room, limitless budget, and a need for generations of trouble-free service.

    That said Gates has several good catalogs and design guides. Trouble is you need most of them to cook up a design that balances all the factors you need to address.

    I once worked up from the catalogs constants for acceptable belt tension for the various standard sections. One with a calculator can reconstruct these figures working from HP transmitted to driving pulley diameter and RPM.

    That said: The Gates Heavy Duty V Belt Drive Design manual shows that a "A" V-belt section will transmit about 2.2 HP per belt for a minimum pulley OD of 3" at 1750 RPM. Derate non-linearly for smaller diameter. By non-linear, I mean a 2" pulley will transmit about 1 HP and a 1 1/2" pulley will barely transmit 1/3 HP. A belt with an AX designation has a notched inner surface that greatly enhances belt flexibility and improves performance over smaller pulleys.

    If your design will accept a 3" dia motor pulley and you can tolerate slighly reduced belt life you can increase the belt tension slightly and reliably transmit 3 HP. If the drive is subject to overloads or shock I'd reccommend a double "A" section V belt. Add machined cast iron pulleys for greater drive performance and reliability. Die cast and aluminum pulley's don't do well in max performance drives.

  5. #5
    Mike C. is offline Diamond
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    Yep, both the drives I mentioned have pretty small drive pulleys and no idler to increase wrap. My bridge mill has a 3hp bridgeport style head with a single belt, but the pulleys are very large in diam.

  6. #6
    jkilroy is offline Diamond
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    Default Thanks...

    Thanks for the pointers gentlemen. I am am in a situation where diameter is easier than width, and I'm pretty sure I can fit a single groove pulley of 4" diameter so I am going to give that a go.

  7. #7
    Benesesso is offline Hot Rolled
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    How much RPM change do you require? If the pulley diameters differ a lot, you need to move them away from each other more to avoid belt-wrap problems, if possible.

  8. #8
    BobH is offline Aluminum
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    I am also trying to design a belt drive using a micro-V belt similar to the serpentine belt they use on cars now. The Gates design book mentioned above is a great reference and free too! What they don’t address, and my problem, is when you have two driven loads from one driven sheave, something like an engine driving the alternator and the fan with one belt. If I take Gates rating for power transmission through a system using like size sheaves and adjust it for the difference in contact area for my set-up will I be ok? Equal sheaves would be 180° of contact; if I had 90° of contact would the power transmission be 1/2? I think this would be a linear change if the tension stayed the same. I don’t have enough space to go overboard with “more is better” engineering but It better work or I'll have to fix it. Bob

  9. #9
    Mike C. is offline Diamond
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    Another way to get more wrap and power transfer from a single belt is to put a tensioner/idler on the loose side, between the pulleys, that pushes the belt inward with pressure via a flat pulley on the back of the belt. Put that more near the smaller pulley for best results.

  10. #10
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    SteveinAZ is online now Stainless
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    Hey guys, if you don't overwhelm me, I can use either Woods or Dodge software to design belts and sheaves...do it most every day. What I would need is center to center distance (minimum), driver speed (motor), driven speed, what belt style you prefer (A, B, etc.), HP, shaft sizes, and service factor (1.4 is std).

    The software will kick out sheaves, bushings, belts, c/c distance, actual speed, service factor and list pricing in a few seconds. They are quite accurate in the speed calc...usually with 1-2 rpm of design with a 1,750 motor and ~600 driven rpm.

    PM me if you would like a selection, and I'll see what I can do.

    Steve

  11. #11
    BobH is offline Aluminum
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    Well, I solved my own problem. Buried in the idler section of Gates handbook is a chart with the correction factors for belt wrap. Told you it was a good book! Thanks for the input. Bob

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