Step pulley design (sizes)?

It is a real bad idea to make a belt drive setup with no adjustment to take up slack. Reason I mention this is it sounds like it's possible you are thinking of a system with fixed center distance between pulleys - this would require constant belt length moving from step to step.

That having been said, there isn't really any good data I found that is complete enough and current enough to answer your question. The two resources I found that were the best were "Machinery's Handbook" and the Browning Morse catalog titled "Components" (available at your local Browning stocking distributor or probably by request from Browning). However, the formulas and data in both were approximate and *very* confusing. Here's the deal: Say you have an A sized belt, and 2 pulleys with 6" OD. If you layed a steel tape out on a bench and made a mark on the outside of the belt and rolled the belt along the tape to measure its circumference, the first thing is that the circumference you measure is NOT the stated length of the belt. And the amount it differs from stated length will vary both between manufacturers and between individual belts (have you seen the thread on matching belts recently?). So even in this ideal situation it is very difficult to calculate what the center-to-center distance of THIS belt over your 2 6" pulleys will be. The references I cite make use of a concept called the "pitch diameter" of a pulley, but after doing some measurements I just junked the whole concept and worked with the measured circumference of the belts and the OD of the pulleys, figuring if you make the pulleys right the outside of the belt will be coincident with the outside of the pulleys.

Figuring this way, then to first order you can figure your step sizes. Suppose for convenience you have a step pulley 6", 5" and 4". If you assume the belt will go 180 degrees around the pulley then the length around the biggest step is PI*6", the length around the middle is PI*5", and the smallest is PI*4". The difference in length between any 2 steps is e.g. PI(6-5) or PI" or 3.14". Thus if you drove this pulley with another 4-5-6" pulley (or any pulley with step size 1") then to first order your center distance would be constant. Why first order? Because the belt won't actually contact the pulleys for exactly 180 degrees. Think about it. See my problem:

http://www.t> Is ther a system or formula or tables or software somewhere that'll tell me

I knew that - but wanted to have a small range of adjustment, just enough to loosen the belt, change it, and set it tight again. Don't want to have to use a different belt because I made the sheaves too big/small.

The easy way is to use two identical pulleys, inverting one, like most drill presses. In other words, if pulley #1 has steps of 1", 2", 3" then pulley #2 would have steps of 3", 2", 1".

Bob

Of course you mean: R1*2*pi/2 + R2*2*pi/2.

More easily, if the sums of 1/2 the two circumferences must be the same, then the sum of the diameters must also be the same (use diameter since pulleys are sized by diameter). With the sum of the diameters in all pairs the same: 6" & 2", 5" & 3", 4" & 4" is one possible set (any pair adding to 8").

Note that the "diameter" should be the _pitch_ diameter. I'm not sure if pulleys are spec'ed by pitch diameter. An easy way to measure pitch diam is to put a belt on the pulley and make a chalk mark on both the belt and pulley at the same point. With the belt tight to the pulley, revolve the pulley 360 degrees and make a another mark on the belt where the mark on the pulley is. The distance between the marks on the belt is the pitch circumference.

Bob

"jt" snipped-for-privacy@spamkiller.hfx.andara.com

jt

For old flat-belt machinery, you would work center-to-center of the driver and driven shafts. How you adjust is another issue.

Without trying to go to a formula, if you match the sum of the driven shafting pulley's steps and those of the drive shafting pulley's steps (1, 2, 3, 4 to 4,

3, 2, 1, etc.) then the belt length that is correct for one set is correct for all.

The pitch diameter issue comes in when you are trying to figure the *speed* transmitted. For vee belting it gets tricky, because belts lay differently in the vee--so the speed of the driven will be different, depending on how far into the vee the belt contacts. But for sizing countershafting pulley steps, same sum for diameter to diameter solves all but the adjustment and speed issues. Frank Morrison

That is no way guarantees that it will work right unless the pulley manufacturer decides to make the right ratios. Your example works because the sum of each pair it the same ie

1"+3"=4", 2"+2"=4", and 3"+1"=4"

Bob Rob>

[ that was me ]

Machinery's handbook (1946), had the answer:

Graphical method...

Construct two circles on the centres (to scale) for two known pulleys.

Draw a vertical from the midpoint of the line connecting the centres.

On the mid-point vertical, at a height of the centre-distance divided by pi, draw a circle that is tangent to the line tangent to both pulley circles (the belt line).

Any other pair of pulley circles that have a connecting tangent which is also tangent to the mid-point circle will work.