36" bandsaw speed reduction
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  1. #1
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    Default 36" bandsaw speed reduction

    Hello. I'm new here, first post.

    I have an old 36" L Power & Co bandsaw I'm trying to get up and running again. it has wooden rimmed wheels with metal spokes. When I bought the saw years ago the motor was 3 hp 1750 rpm. The sheave on the motor was 3.5" outside diameter and the sheave on the driveshaft of the saw is 15.5". I believe this means the 36" wheel is turning about 395 rpm or less.
    That motor has been repurposed over the years and I have a 7.5 hp 3 phase motor which runs at 3500 rpm I'd like to use on the saw. I'm trying to determine the sheaves I will need to reduce the 36" wheel to 300 - 350 rpm.

    So my real question is, can someone tell me what diameter of the sheave I should be measuring for doing the calculations? From what I can tell, it seems as though the pitch diameter is the relevant number.

    Thanks in advance for any help!
    Pete

  2. #2
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    Whatever diameters you like as far as OD or Pitch - as long as you are applying one or the other to both pulleys


    10 to 1 gets you about 350

    As in 3" pulley and 30" pulley.

    As you might agree, impractically fast motor without additional jack shaft and other pulleys

    Jack shaft rig could be 3 driving 10, then another 3 driving 9

    3.333 X 3 = 10 - the overall ratio wanted

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    I am surprised no one asked the material that was to be cut. It makes a big difference in the drive ratio needed.

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    Thanks for the responses. I will be cutting wood with this saw. I mainly want it for resawing. From what I've read, it seems like 3000 sfm or less is a good speed to be at. I'm inclined to go a little slower yet because of the wooden rimmed wheels.
    I had not thought of it in terms of ratios. But then when I tried playing with the ratios, they don't exactly match up. For example: a 4.75" od sheave with a pitch diameter of 4.4" on the jack shaft to a 2.95" od sheave with a pitch diameter of 2.6" yields a ratio of 1.61:1 for od and a ratio of 1.69:1 for the pitch diameter...... perhaps I'm splitting hairs? The discrepancy adds up when extrapolated to the 15.5" sheave.

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    Gates and others selling sheaves/pulleys have all the available pitch diameters in their sales literature.

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    Once you have variable speed to tune to material anything else is settling. With the effort going into the cool saw you can go with reduction and a vfd, your blades, saw, and material will thank you.

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    I'll throw in a few notes:

    While the shaft RPM for your wheels is important, do your calcs based on SFPM, rather than shaft RPM. THEN... look at the bearings of the shafts, and compare that to the original configuration for that saw. Realize that back when bandsaws were first being built, they were driven by lineshafts, and the lineshaft speed was a fairly universal thing... the overhead hangers would allow for a few different sheave sizes, but because of hanging distance from the ceiling, and flat belt traction surface limitations, the variation in sheave diameter wasn't huge. Engineers had to balance those factors against the bearing surface speeds, surface areas, and belt tension limitations. The wheel diameter, therefore, was just as important for managing blade SFPM, as it was throat clearance!

    Will your saw's original bearings withstand the RPM and sideload (blade tension and belt tension vectors) with the lubrication that you're planning on using? Are the bearings original babbit, with oil wick, or have they been changed?

    Will your drive belt sheaves have enough surface area, at the ratios you desire, to transmit the power you need, and if so, will the tension on the belts need to be higher than what the bearings can handle?

    Expanding on John's note about ratio... going to a 3500rpm motor means you'll need a greater differential in sheave size, which means reduced tractive surface on the belt. If you want to run that motor, you'd need to use a layer of additional reduction to get within reach of a proper blade speed without being limited in belt surface contact.

    An 1800rpm 3phase motor with a VFD would work well, and grant you substantial flexibility not only in speed, but control.

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    Pete, I always recommend looking at the OWWM and Vintage Machinery sites, plenty of information and experienced members there. The latter has publication reprints you can download free as pdf's. Looking at one, they show a 36" bandsaw with wooden rim wheels and babbitt bearings , and recommend a speed of 530 rpm, so it looks like you're fine with 3000 sfm.

    Old Woodworking Machines - Index page
    L. Power & Co. - Publication Reprints | VintageMachinery.org

  9. #9
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    From Gates Heavy Duty V-Belt Drive Design Manual

    These are A/B sheaves - up to 6 grooves each
    Meaning usable with either belt section

    These are the listed pitch diameters set up with jack shaft - and using the A belts and their listed pitch dias.

    3 turning 9 = 3 to 1

    3.6 turning 12 = 3.333 to 1

    3 times 3.333 = 10 to 1


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