How to calculate gear ratio between two timing pulleys
How to calculate gear ratio between two timing pulleys
(OP)
I have a machine with some sets of timing pulleys and I need to calculate the gear ratios. All are H-series, 3/4" wide belts.
Set #1 has a driving pulley with 39 teeth and a driven pulley with 76 teeth.
Set #2 has a driving pulley with 30 teeth and a driven pulley with 57 teeth.
I get these ratios but something doesn't seem right
Set #1: 76/39 = 1.948718
Set #2: 57/30 = 1.9
I seem to remember something about using the pitch diameter to calculate the ratios, but I can't seem to find anything to give me a concrete answer.
Thanks in advance for any help!
Set #1 has a driving pulley with 39 teeth and a driven pulley with 76 teeth.
Set #2 has a driving pulley with 30 teeth and a driven pulley with 57 teeth.
I get these ratios but something doesn't seem right
Set #1: 76/39 = 1.948718
Set #2: 57/30 = 1.9
I seem to remember something about using the pitch diameter to calculate the ratios, but I can't seem to find anything to give me a concrete answer.
Thanks in advance for any help!





RE: How to calculate gear ratio between two timing pulleys
Don
Kansas City
RE: How to calculate gear ratio between two timing pulleys
The odd thing about timing pulleys is that the PD is larger than any physical diameter of the teeth; it's roughly halfway through the belt web, where the cords are. So it's not something you can measure on a pulley in isolation. Just count the teeth.
Mike Halloran
Pembroke Pines, FL, USA
RE: How to calculate gear ratio between two timing pulleys
The driving pulleys are on the same shaft; the motor. The other pulleys are on two heads (small pulley on inner head, large pulley on outer head). The ratio is designed so if the heads start with their 12 o'clock positions lined up, then each revolution the outer turns a little farther than the inner, so they become offset. Theoretically after 40 revolutions of the motor they are supposed to line up again:
1.948718 - 1.90 = .048718
1.948718/.048718 = 39.99
Unfortunately with my system they line up again at 36 revs, not at 40.
RE: How to calculate gear ratio between two timing pulleys
RE: How to calculate gear ratio between two timing pulleys
RE: How to calculate gear ratio between two timing pulleys
For 40 revs of the motor,
one head will rotate 40(39/76)=20.526 revs.
other head will rotate 40(30/57) = 21.052 revs.
For 36 revs of the motor,
Head 1 goes 36(39/76) = 18.47 revs.
Head 2 goes 36(30/57) = 18.947 revs.
RE: How to calculate gear ratio between two timing pulleys
76*30/57 = 40
One of the heads laps the other...
RE: How to calculate gear ratio between two timing pulleys
Have you counted teeth on your sprockets to verify they are correct?
RE: How to calculate gear ratio between two timing pulleys
Ted