On my steam plants, I use some flat belting, and think I have discovered maybe the best material to make these belts from. I have tried belting from PM Research which works, but is too expensive. I have successfully made belts from thin leather, but they stretch with use, and have poor pulley traction, which means you need some kind of tensioning device. I was in my shop thinking about what I could make a belt from when I spied an old bicycle inner tube in the trash can. I cut this tube into strips about 1/2 in. wide, and of sufficient length. I then placed the strip around my pulleys and added some stretch. I marked the length, cut the ends at 45 degree angle, and super glued the overlap. If you use the inner surface of the tube, it is naturally concaved, which matches well with crown of flat pulleys. This gives extremely good belt/pulley traction, and needs no kind of tensioner. So far, mine is running quite true with lots of grip, with no tendency to jump off the pulleys. This may not work for long belt runs, but for shorter belts, it works well.
D.E. Jones