Yes in theory one could design a 300:1 belt reduction in one stage. But according to the belt manufacturer, you do need to have a minimum number of belt teeth engaged. I think about 11 teeth. If only 180 degrees of the small pulley is engaged with the belt then the minim pulley size is 22. So to get 300:1 you use a 6600 tooth pulley. But this is a 6 meter diameter pulley. The worm gear works so much better because it is in effect a "one tooth" gear and so for the same pitch reduces the diameter of the larger gear by a factor of 22.
There is also a minimum amount number of degrees of contact required with the small pulley. The pulley must be far enough away so the minimum number of teeth are engaged. Getts 180 degrees of corverage is impossible if one pulley is larger then the other So in order to get (say) 120 degrees of bend around the smaller pulley, it must be pretty darn far away from the 6 meter diameter pulley. But now you need 33 teeth on the small one and the big one is up to 9 meters diameter. With pulleys, if the reduction ratio is 100:1 the larger pully must be literally 100X larger than the smaller pulley and there is always a minimum size for a small pulley. So typically high reduction ratio belt drives use two or three stages. 6:1 is a reasonable reduction, 10:1 gets hard to fit into many spaces. With these high rations you use worm gears, multiple stages or cycloid reductions (or silly things like a 1/2 inch pulley paired with a 50-inch pulley) On Fri, Jul 10, 2020 at 2:39 AM andy pugh <bodge...@gmail.com> wrote: > On Fri, 10 Jul 2020 at 02:00, Chris Albertson <albertson.ch...@gmail.com> > wrote: > > > Belts can go to about 6:1 at best a double belt reduction only goes to > 36:1 > > I don't see why there is any limit to the possible reduction of a belt > drive, if idlers are added to increase the wrap. > If I was doing this (and this particular application is something that > I have considered) I think that I would use a rotary variant of the > "servobelt" system: Glue a length of T-section beliting to a circular > component with the teeth pointing out to make a sprocket, and then > have a close-spaced pair of rollers to keep the belt wrapped round > the sprocket except for a short loop that pops out and wraps round the > stepper pulley. > > > > He is doing only > > about 10 Hz. > > In that case I think that a user-space loop in something like Python > would be fine, just monitor the time using an accurate RTC (GPS > module?) and have the loop say "have I made as many pulses as I should > have for this time of day?" and either make a pulse on a GPIO pin, or > not, accordingly. > > -- > atp > "A motorcycle is a bicycle with a pandemonium attachment and is > designed for the especial use of mechanical geniuses, daredevils and > lunatics." > — George Fitch, Atlanta Constitution Newspaper, 1912 > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Chris Albertson Redondo Beach, California _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
