Figuring out pulley sizes for certain RPMs

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts
  • hornluv
    Senior Member
    • Oct 2002
    • 899

    Figuring out pulley sizes for certain RPMs

    Hey everyone,

    I am making a buffing machine for polishing up musical instruments. I'd like to make it so I can use three speeds, for 6, 8, and 10 inch buffing wheels. How do I calculate the diameter of the pulleys needed to get certain RPMs, and more importantly, certain surface feet per minute on each of those size buffs. There's nothing that sucks more than having the tuba you just fixed get chucked across the room by a wheel that's too fast.
    I plan on using a 1720 RPM motor and I know the proper RPM for a 6 inch wheel is 3600. I guess if I can figure out the SFPM for that, I should aim for the same amount for the other sized buffs, right?

    Thanks,
    Stuart

    PS Is there anyone in Central Illinois (Champaign/Urbana)?
    Stuart de Haro
  • John Stevenson
    Senior Member
    • Mar 2001
    • 16177

    #2
    Stuart,
    You state 1720 rom motor and 6 " wheel at 3,600 rpm
    To get the right size pulleys for this size you need to increase the speed by a factor of 3600 over 1720 or 2.09:1
    So in this case your motor pully needs to be twice as big as your buffing wheel pulley.

    To move onto the next sizes and keeping the sfpm the same at 3600 a 6" wheel is moving at 6/12 [ to get feet ] x pi [3.1416 to get circunference ]x 3600 [ to get sfpm ] =5655 sfpm.

    To work backwards from this an 8" wheel has a circumference of 8/12 x 3.1416 = 2.09
    feet.
    Divide than into 5655 and you get 2705 rpm on the buffing shaft.

    A 10" wheel is 10/12 x 3.1416 = 2.62 feet so 2.62 into 5655 gets you 2158 rpm on the buffing shaft.

    Going back to the first pulley formula we see that it's the speed of the buffing shaft over motor speed and a 6" wheel needs 2.09 :1 [ call it 2:1 ]
    So an 8" wheel is 2705 / 1720 = 1.57:1
    And a 10" wheel is 2158 / 1720 = 1.25:1

    So for a given motor pulley size your buffing shaft pullies need to be twice as small for the 6", one and a half times as small for the 8" and one and a quarter time as as small for the 10"
    HTH

    John S.

    [This message has been edited by John Stevenson (edited 12-23-2002).]
    .

    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



    Comment

    • SGW
      Senior Member
      • Apr 2001
      • 7010

      #3
      Another thing to consider is that you probably want the belt length to be the same, or nearly so, for all pulley positions, so once you figure out the required ratios as per John's explanation, you want to work out the relative diameters for all the pulleys so the belt length comes out the same.
      ----------
      Try to make a living, not a killing. -- Utah Phillips
      Don't believe everything you know. -- Bumper sticker
      Everybody is ignorant, only on different subjects. -- Will Rogers
      There are lots of people who mistake their imagination for their memory. - Josh Billings
      Law of Logical Argument - Anything is possible if you don't know what you are talking about.
      Don't own anything you have to feed or paint. - Hood River Blackie

      Comment

      • Oso
        Senior Member
        • Nov 2001
        • 1241

        #4
        There's a formula for that in my Goodyear book, I'll post it when I get home.

        Comment

        • hornluv
          Senior Member
          • Oct 2002
          • 899

          #5
          Thanks fellas. Very helpful John. I was thinking I'd turn the pulleys myself, since I'm doing this project for $#!+s and giggles as much as to have the best damn buffer around. About materials, is Aluminum alright for the pulleys? The light weight would mean less energy from the motor to turn it, but would wear be an issue? Would cast iron be better? Any other suggestions? I don't have foundry capabilities, so I'll need to be able to find it in sufficiently large diameters. I'm going to use stress proof steel for the shaft, probably 5/8ths diameter x 3 feet, so I can have at least a foot sticking out on both sides to give me room to move an instrument around. Where is a good supplier of belts? Would an auto belt work? Thanks again guys.

          Happy Ho Ho Ho's and stuff,
          Stuart
          Stuart de Haro

          Comment

          • JCHannum
            Senior Member
            • Nov 2001
            • 10091

            #6
            5/8" diameter x 1 foot on a buffing wheel at 3600 RPM is kind of scary. It might bend easily, especially as buffing wheels are not too well balanced. Then it kinks, and bends 90 degrees and the whole works takes off across the room.
            Jim H.

            Comment

            • mbensema
              Senior Member
              • Jun 2002
              • 440

              #7
              You are going to be very close to the natural frequency of the shaft if you use 5/8". The vibration will be pretty bad and the least bit of deflection will be very dangerous as was already mentioned. A 1" shaft and preferably shorter would be much better.

              Mike

              Comment

              • Oso
                Senior Member
                • Nov 2001
                • 1241

                #8
                Belt formula. This is the simpler one accurate enough for most work.

                length=

                2C +1.57(D+d) + (D+d)/4C

                where C is pulley center distance
                D is larger pulley diameter
                d is smaller pulley diameter

                Somewhere I diddled with this equation to give me the pulley size based on the other data. I misplaced that paper, and I'm too lazy to do it over right now.

                But you can just try sizes and bang on the calculator until the length turns out sufficiently close for each ratio you want.

                Pick your first pulley ratio, and get the length, then try values to match it with a second pulley ratio. If you start with obtainable pulley sizes, it is a lot easier than trying to figure the perfect special.

                You math and engineering types can solve it like I did if you want.

                Comment

                • hornluv
                  Senior Member
                  • Oct 2002
                  • 899

                  #9
                  Thanks Oso. JC and Mike, do you think I could get away with a 1" shaft and perhaps 8" of length on each side? I'd like to have a little room to move around. That should be plenty.

                  Thanks,
                  Stuart
                  Stuart de Haro

                  Comment

                  • jh
                    Member
                    • Nov 2002
                    • 55

                    #10
                    How about 1" id bearings inside a heavywall tube right next to the buff but have the tube hanging out of a secure mounting the distance you want. That way you don't have a unprotected shaft next to what ever you are doing. Some custom machine work would be needed but that would be fun and then you would have the best. Maybe a place for needle bearings?

                    Comment

                    • mbensema
                      Senior Member
                      • Jun 2002
                      • 440

                      #11
                      The 1" shaft 8" long should be fine, but when you first start it up, try a low speed first to make sure things are ok, better to be safe then sorry. You should also keep the weight of the buffing wheels to a minimum, you don't want too much weight on the end of a long shaft. The idea of using the tube to place the bearings near the wheel is a good one, provided you can keep everything rigid.

                      Mike

                      Comment


                      • #12
                        Just thought I would update this one...those formulae only work depending on the saturation of compound and force applied to the wheel, and the material being buffed. In other words..you're on your own. Just use common sense, you can tell by looking at the wheel if it is safe. Always stand to the side when firing up a wheel of any type, then move in. Sometimes buffing wheels are more dangerous than grinding, but you already know that.

                        Comment

                        • lugnut
                          Senior Member
                          • Sep 2004
                          • 1896

                          #13
                          How about VFD!

                          Hornluv, when I saw your post my first thought was that I had seen some industrial buffers on the Grizzly site. Check out http://www.grizzly.com/catalog/2006/Main/166
                          The president of the company is a guitar making person and they have all sorts of buffing equipment. They sell a Variable speed buffer! Now there is a challange for you, build a buffer with a VFD then you wouldn't have to figure pulley sizes
                          Mel
                          _____________________________________________

                          I would rather have tools that I never use, than not have a tool I need.
                          Oregon Coast

                          Comment

                          • Wareagle
                            Senior Member
                            • Nov 2005
                            • 241

                            #14
                            About the use of aluminum for pulleys, my milling machine uses aluminum pulleys, and there are numerous automotive aftermarket manufacturers that offer aluminum pulleys for high perfromance applications; so, my wisdom says (not claiming much on the wisdom part) the aluminum would be fine as long as it is a harder alloy.

                            Let us see it when you are done!
                            Why buy it for $2 when you can make it for $20

                            Comment

                            • ProGunOne
                              Senior Member
                              • Jun 2006
                              • 435

                              #15
                              I found the following robotics website interesting. I stumbled across it while searching for information on converting a wood bandsaw to cutting steel.
                              "The men the American people admire most extravagantly are the greatest liars; the men they detest most violently are those who try to tell them the truth." H. L. Mencken

                              "All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident."

                              "When fear rules, reason and logic are ruled out."

                              Comment

                              Working...
                              X