How to calculate gear ratio between two timing pulleys

[BrandonClifton]

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!

 

[eromlignod]

Both your calculations are right. You can just use the number of teeth to find the ratio (it's actually the most accurate way).

Don
Kansas City

 

[MikeHalloran]

Like Don said.

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

 

[BrandonClifton]

Ok- now it gets really weird:

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.

 

[dvd]

Every 228 teeth your 57 tooth pulley makes 4 revolutions and your 76 tooth pulley makes 3 revolutions, you can't get to 40 revolutions with multiples of 3 and 4, but you can get to 36.

 

[dvd]

After further thought my comments above strike me as complete bushllit. I am thinking about this some more. Please disregard.

 

[BobM3]

I don't get them lining up after 40 or 36 revolutions (although they're closer at 36).

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.

 

[dvd]

76*39/76 = 39
76*30/57 = 40

One of the heads laps the other...

 

[dvd]

Brandon,

Have you counted teeth on your sprockets to verify they are correct?

 

[hydtools]

See what dvd said. The output shafts line up when there is 1 revolution difference between them. That requires the motor to turn 76 revolutions to cause 39 and 40 revolutions of the output shafts. That is the reverse of what you believe the design to be. 40 revs of one ouput shaft, not 40 revs of the motor shaft.

Ted