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Flat Belt Drive

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  • Flat Belt Drive

    I have an old-style flat belt floor drill press that I am getting back into running condition but am having trouble with the belt that drives the upper cross shaft slipping under load. I added an adjustable idler near the lower pulley on the side of the belt that moves down as it is running, but now the belt wants to slip sideways off the lower pulley when the idler is on. The belt tracks ok when the idler is out of contact and I tried shimming the idler shaft mounts at both the left side and then the right, but this only seemed to make it worse. As far as I can tell the idler shaft is square to the other shafts and everything checks level. The original belt was missing so I substituted one made from what looks like black rubber. The belt is 1 3/4" wide and runs at about 300 rpm. Does it make a difference which side of the belt the idler is on, or is a rubber belt just not workable? Any advice appreciated.

  • #2
    Does the idler have a slight crown?
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    • #3
      Yup. Crown the idler. If the idler is on the outside of the belt you might even have to use a flanged pulley. Also, is the idler on the slack side of the belts? It should be as it's much more effecive there.


      • #4
        The idler is not crowned, it is a straight aluminum cylinder about 3 5/8" O.D. and as long as the cone pulley, the idea being that whatever step on the cone that the belt was on, it would still be contacting the idler. Are you saying that the idler should be only as wide as the belt or slightly wider and be able to move sideways on its axis as the belt is shifted to the different steps? The belt runs vertically and I'm not actually sure which is the slack side. Looking endwise at it, the driving pulley turns clockwise and the idler is immediately above and just to the right of it, should it be moved to the left side, and does the diameter of the idler make a difference?


        • #5
          It is near impossible to run a flat belt on a straight pulley. If it is off by even the tiniest amount the belt will walk to the high end. Make several crowned profiles on the idler to match the different belt positions.

          [This message has been edited by Evan (edited 12-07-2004).]
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          • #6
            Think of a tank tread, when a tank is moving forward. If the drive pulley is at the rear, when moving forward, the bottom of the track will be pulling, while the top of the tread is being pushed, the pushed side is the slack side.


            • #7
              It sounds like what you actually need is rosin belt dressing rather than an idler. I used to own a SB10" lathe with a flat belt to drive the spindle. When it started slipping, out comes the rosin stick and the slipping goes away.

              Adding the idler is most likely to just goof up the belt tracking. That is why on most of the old installations where a belt length accomodation was required to account for speed changes, the solution was to add an idler shaft. The input belt would drive the idler shaft and the idles shaft would drive the load. The idler shaft could then be floated, tensioned or spring loaded to accommodate center distance changes.

              Last thing is that drive horsepower on flat belts was designed by the width of the belt. If your setup happens to have more space on the pulley than the belt takes up, then you may need a wider belt. In any case, the belt width and rosin should make everything run well.


              • #8
                If this is the old style stiff arm drill press that has the step cones top and bottom right?
                If it is then the slack side of the belt will be the righthand side of the machine facing the table.

                Like said above the idler needs to be crowned

                The amount of crowning differs with the running conditions,the type belt used and the speed of the pulley.
                1/16-1/8" per foot for high speeds 400sfpm> and 1/4" per foot for low speeds <350sfpm

                Okay with that said you also have to consider belt thickness.Too thick a belt and the belt will not conform to the radius of the pulley and slippage will occur.
                The maximum thickness for the belt width you are running should be 3/16",anymore than that and it won't work.

                Is this an endless belt or a laced belt? If its laced you might try setting it up as a Mobeus strip,unpin the lacing and rotate oneside of the belt 180* and repin.This may solve the slippage problem without the need for the idler.
                I just need one more tool,just one!


                • #9
                  Thanks for the replies everyone.


                  • #10
                    On my old drill press, I was thinking of removing the belt, cutting grooves in it as per John S: helpful hits and installing a auto serpentine belt.

                    I want a steam like engine running mine off air.

                    It'll never happen. I got a dc motor/drive/gearbox sitting on the base right now. Pretty cool old stuff. You know them old flat belts are dangerous? Lots of one armed men learned a lesson from them on the farms.