Pulley Ratios and Torque

[dusty]

I have a motor driven pulley system. Think Shopsmith standalone bandsaw or Craftsman table saw while thinking variable speed dc motor (treadmill motor).

Is the available torque different if the pulley ratio is changed from 1:1 to say 3:1 or even 1:3 ? I know that there are speed changes but let us ignore that.

Does pulley ratio change torque characteristics?

[WileyCoyote]

Yes, if I remember my physics class many years ago correctly, a smaller drive pully will decrease RPM but will increase torque. I do not believe the two are proportional.

[dusty]

Yup but there is a lot more to this than I had bargained for. I went looking for assurance before I began cutting metal and found this which I thought was worthy of passing on. It answers a lot of questions I had not even thought to ask.

The direct answer to my initial question seems to be that torque does not change as long as the pulley ratio does not change.

http://www.gizmology.net/pulleysbelts.htm

[garys]

Yes, remember 7th grade science class? That is what they taught using levers and pulleys. It is called mechanical advantage and it hasn't changed since we were in 7th grade.

"Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force. The model for this is the law of the lever."

[dusty]

Yes, I do remember (the class). What I have trouble with these days is remembering what I was taught in those classes. Fortunately, math and science were my favorites and I still have most of the books and some of the work books that were used.

[garys]

The basics of mechanical advantage taught in 7th grade science was to me one of the most useful pieces of information I learned in school. It is the most basic and simple rule of mechanics and I have used it all my life in my jobs and also in my projects at home. It is one I never forget.

Input force x distance = output force x distance. (less of course any losses due to friction.)

[JPG]

So when does the 'peak' left side of the equation equal the 'peak' right side.

A spill over from another thread.

Add time to the lever and you get horsepower.

[dusty]

So, I can transfer power from the motor pulley to the load pulley without loosing torque so long as I do not change pulley ratio.

Is that the consensus? This is NOT a loaded question.

[wa2crk]

I think of this as related to a car transmission. Taking off from a traffic light in first gear is better that starting off in second gear. The horse power of the engine doesn't change but the load is easier to move. Torque amplification?
Bill V

[JPG]

'You' said previously - "The direct answer to my initial question seems to be that torque does not change as long as the pulley ratio does not change."

Yes it is!(also correct)