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rpm to fpm on a sawmill

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  • rpm to fpm on a sawmill

    hi guys, I am building a homemade bandsaw mill , would like to get some help . gas motor turns 3100rpm with a 6" pulley on it go's to a sec 6" pulley and on the same
    shaft I need to added a pulley [size ?] , and that go's to the main shaft i need to know that pulley also and on that shaft it has a 19" pulley that I need to turn the saw blade that is 1 1/4 "wide x 144" long the blade needs to go at 4200 ft per min.any help would be helpful, thank you

  • #2
    Originally posted by sawman View Post
    hi guys, I am building a homemade bandsaw mill , would like to get some help . gas motor turns 3100rpm with a 6" pulley on it go's to a sec 6" pulley and on the same
    shaft I need to added a pulley [size ?] , and that go's to the main shaft i need to know that pulley also and on that shaft it has a 19" pulley that I need to turn the saw blade that is 1 1/4 "wide x 144" long the blade needs to go at 4200 ft per min.any help would be helpful, thank you
    We need the diameter of the wheel that turns the bandsaw blade.

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    • #3
      a 6 inch diameter pulley at 3100 RPM will be about 4800-4900 FPM.
      a 19 inch diameter pulley at 3100 RPM will be about 15400 FPM (! ! !)
      if you reduce RPM to 850, the 19 inch will be about 4200 FPM

      if you're unable to reduce RPM (which sounds like the case here) then a pulley of 5.175 inches diameter will provide a speed of 4200 FPM

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      • #4
        hi guys thank you for replying I added a pic that will be eze to see what I am doing

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        • #5
          the picture you uploaded isn't available, can you upload it to http://postimage.org/ and send us that link?

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          • #6
            hi guys the pic. was a diagram and I see that it was posted on the last post any question leave me a note thank you

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            • #7
              Originally posted by sawman View Post
              hi guys the pic. was a diagram and I see that it was posted on the last post any question leave me a note thank you
              I think there is a problem. I see a small icon in your post, but when I click on it to view image, I don't see anything. Lets try in words.

              The motor shaft drives an intermediate shaft with 6" drive and driven pulleys, so it turns at the same 3100 rpm.

              You have a choice of drive pulley on the intermediate shaft to drive the output shaft via a 19" driven pulley.

              The output shaft also has a wheel which the band runs over. You want 4200 ft per minute, or 50400 inches per minute. To convert that to an rpm, we need to divide by the circumference of the wheel (pi x diameter). If the wheel is the same diameter as the 19" pulley, then we need 844 rpm. If it is NOT 19" you need to tell us or work through the rpm yourself.

              We set up a proportion x inches/19 inches = 844 rpm/3100 rpm, so x = 5.175 inches. You would then come as close as you could, 5 " or 5.25". Remember this is just an example predicated on the band wheel being 19", sames as the pulley on that shaft. If not true, the problem must be reworked with the correct data.

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              • #8

                hi guys I thank this will work , http://s6.postimg.org/6sytznwrl/IMG_...NEW_Page_1.jpg

                this is the a diagram and I see that it was posted on the last post any question leave me a note thank you

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                • #9
                  that picture worked perfectly. from the drawing I see:
                  • motor at 3200 RPM
                  • 6 inch pulley to ??? pulley
                  • connected to 7.5 inch to 14 inch pulley
                  • connected to 19 inch pulley

                  I think I have the answer, however I'm going to put my math down as well because I'm not entirely sure I'm correct, and I'd like to learn if I've done something incorrectly

                  6/? = unknown ratio
                  7.5/14 = 0.5357 ratio
                  compound ratio = ? (due to ?)

                  if I plug 12 into the ? then 6/12 = 0.5
                  0.5 * 0.5357 = 0.26785 compound ratio
                  3200 RPM * 0.26785 ratio = 857.1428 RPM at the final 19 inch pulley
                  19 inch pulley has circumference of 59.6902 inches
                  59.6902 inches * 857.1428 RPM = 51163.08036 inches per minute
                  51163.08036 inches per minute = 4263.5900 Feet per minute

                  using that, I can whittle it down to 12.18 inches for the ??? pulley to get 4200 Foot per minute on the sawblade

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                  • #10
                    Originally posted by sawman View Post
                    hi guys I thank this will work , http://s6.postimg.org/6sytznwrl/IMG_...NEW_Page_1.jpg

                    this is the a diagram and I see that it was posted on the last post any question leave me a note thank you
                    Great. OK, the band runs on 19" wheel, so we want the output shaft turning about 844 rpm (844 x 19" x pi = 50378 in/min or 4198.2 fpm)

                    If motor shaft is at 3200 rpm with 6" pulley and intermediate shaft has D inch driven pulley, intermediate shaft turns 3200 rpm * (6"/D) rpm

                    7.5" drive pulley drives 14" driven pulley on output shaft so

                    3200 x (6/D) x (7.5/14) = 844 rpm, leading to D = 12.2"

                    A 12" or 12.25" pulley on the intermediate shaft will give you approximately the drive ratio you need for.
                    A larger pulley will turn the intermediate shaft slower.

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                    • #11
                      hi guys thank you for the input I guess it is true when you get 3 or more smart guys and 1 redneck you can make any thing run,

                      like my friend told me when I was 9 yrs. old , "you can turn a power plant tribune with a 5 hp Briggs & Stratton gas motor with
                      the right pulley system."

                      I have never tried it but that is one to get your brain a round,
                      thank you guys
                      I will try this 12" pulley and try to put some real pic of the sawmill up. thank you
                      Last edited by sawman; 03-23-2015, 04:05 AM.

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