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flat belt pulleys

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  • flat belt pulleys

    are the steps on flat belt pulleys supposed to be convex or flat?


  • #2
    They should have a slight crown (convex). It makes the belt stay on the pulley, although it's slightly counter-intuitive.
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    • #3
      my logan 920 has a flat belt and the pulleys have a slight crown convex


      • #4
        Page 834 Machinery's Handbook 12th edition. 1/20 of pulley width for leather belts and 1/150 of pulley width for cotton belts. It also says 1/16 to 1/8 inch per foot of width for high speed and 1/4 inch per foot for low speeds. It then says a slipping belt will run off a crowned pulley faster than a flat pulley.


        • #5
          Slight crown convex on the pulleys on my pedestal drill.
          Last edited by R W; 03-12-2008, 01:53 AM.


          • #6
            Thanks guys. I thought they were convex. I am repairing a band saw that is driven by a flat pulley. The shop wanted me to machine the pulley steps flat, but i wasn't sure if i should. I will check the deviation and then decide on what to do.



            • #7
              You don't have to go to the trouble of making a smooth even radius across the pulley face. The center third can be flat as long as the belt is wider than that and then a very slight slope machined on the outer thirds blended in with a file or some emery. Belts always climb to the largest diameter because the belt is stretched slightly more there and the stretch pulls the lesser stretched part with it since the lesser stretched part has less tension.

              It's basically impossible to make a flat belt run on flat pulleys unless they are flanged. You may be able to make it run for a little while but it will soon start to track off.

              This principle is universal for flat belt systems. Flat belts are still very common in many applications such as this paper transport from a photocopier. As you can see the pulleys are crowned.

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              • #8
                Thanks Evan. Out of curiousity....what is that a picture of?



                • #9
                  Like Evan said.

                  I used to make pulleys for box machines about 10" diameter and 6" long.

                  These were drivers and had a 2 degree taper on the corners about 1/3 the way back each side.

                  They were also knurled. I knurled the center section, then followed the 2 degree contour off both ends.

                  Who says a knurl has to have be the perfect diameter to work?



                  • #10
                    Out of curiousity....what is that a picture of?
                    It's a vacuum paper transport assembly from a photocopier. It moves the copy paper from one place to another using the belts. The transport has a small vacuum fan inside that sucks the paper against the belts via the visible holes in the metal. The fingers between the belts guide the curled down edge of the paper onto the transport.

                    I worked on this stuff for over 20 years. It always amazed me at how complex these machines are. Although the actual process is pretty straighforward, at least for monochrome copies, the real life implementation is incredibly complicated. It is a merging of a number of contradicatory hardware requirements. You have an accurate wide field folded optical system with first surface mirrors running a few inches away from a development system filled with micron sized particles of black dust. That development system is filled not only with black themoplastic dust but with very fine granular steel powder that must be constantly mixed and moved around using mechanisms that cannot be lubricated. These are not simple mechanisms and in some machines the weight of the developer powder exceeds 25 lbs.

                    Then there is the actual imaging unit that uses very high voltages, high enough to ionize air, in the neighbourhood of 2 to 3000 volts. These tend to arc in dry weather when the voltage climbs and this must be tolerated by the control computers(s) located in the same chassis a few inches away. Just to make matters worse some of the big machines use exposure flash lamps a foot long that can melt your retina in one exposure. One model series used four of these lamps flashing simultaneously with enough intensity to vaporize small insects. The interlock system was both mechanical and electrical and designed so that even the most careless (or ingenious) technician couldn't easily defeat it. Another series of very large printers used a class 3B argon laser with about 1/10 watt power output, enough to melt your retina and cornea.

                    Then there is the fuser system that melts the thermoplastic powder into the paper in conjuction with high pressure. It must melt what amounts to a very sticky black or colored glue and yet not have it stick to the rolls that apply the pressure, even when the entire sheet has a thick coating of this "glue". The fuser system uses a temperature above the ignition point of the paper because of the very rapid heating that must take place. One series of machines actually had an inboard CO2 extinguisher to put out the flames if paper became stuck in the fuser.

                    The White house had one of these machines installed in an anteroom to the oval office. One day it caught fire inside and the extinguisher failed to operate. The secret service promptly backed up a pickup to the window, threw in a chain that was looped around the machine (about the size of a 20 cubic foot freezer) and yanked it out through the wall. I don't think that made the news at all back in the early 70s.

                    Working on these machines requires a broad spectrum of knowledge of electronic, computer, network and mechanical/electromechanical and pneumatic systems. Some even have hydraulic systems too. Most service reps specialize in two or three products since that is about as much as most people can handle.

                    However, myself and a few others in the company like me had full product line territories in remote locations. I worked on the entire spectrum of products from copiers and laser printers to faxes and multifunction network connected high speed document systems weighing over a ton, over 50 different products in all. We had to be able to walk up to a machine and fix it even though the last time we saw one like it might have been 6 months ago. It was a very difficult job but it paid well. When I finally quit ten years ago I was making around 45K with very complete benefits, and pension on top of that and a remote living allowance with new vehicle every couple of years.

                    I don't miss it a bit.
                    Last edited by Evan; 03-13-2008, 08:31 AM.
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