Motor torque

[Charles scozzari]

Hello,I read a question this morning and cannot find it again but it was about belt pulley sizing and how it effects motor torque that being a mill lath or whatever. I'm not being sarcastic or a wise guy I just don't understand (many things) how changing a pulley size can change the motors torque. Thanks.

 

[SouthernChap]

Hello,I read a question this morning and cannot find it again but it was about belt pulley sizing and how it effects motor torque that being a mill lath or whatever. I'm not being sarcastic or a wise guy I just don't understand (many things) how changing a pulley size can change the motors torque. Thanks.

Was that the thread from the OP thinking about removing the middle pulley in a three pulley set up (those are probably not the right terms but I think people should know what I'm talking about)?

I don't think people were saying the pulley sizing affects the innate torque of the motor itself, it affects the torque available at the site of the work being done.

If I understand it right, when you increase the size of the driven pulley (or decrease the size of the driving pulley) you're effectively increasing the size of the 'lever', thus allowing more work to be done at the work site, for a given amount of input energy.

 

[682bear]

Please correct me if I am wrong

so I am building a new belt and pulley housing/motor mount and I have all the speeds and torque I could ever need covered by two pulley steps and dialing my vfd between 30-90hrtz. My question is when I make my new setup can I do away with the countershaft design that I’m currently using since...

www.hobby-machinist.com

It doesn't change the motors torque... the motor still makes the same torque, but any time you have a mechanical speed reduction... whether by gearing, pulleys, or chain and sprockets... there is a torque multiplication effect on the driven mechanism that is equal to the ratio of the speed reduction.

If you have a 10:1 speed reduction between a motor and spindle, you will have 1/10 the speed but 10 times the torque available at the spindle.

-Bear

 

[GrifterGuru]

Was that the thread from the OP thinking about removing the middle pulley in a three pulley set up (those are probably not the right terms but I think people should know what I'm talking about)?

I don't think people were saying the pulley sizing affects the innate torque of the motor itself, it affects the torque available at the site of the work being done.

If I understand it right, when you increase the size of the driven pulley (or decrease the size of the driving pulley) you're effectively increasing the size of the 'lever', thus allowing more work to be done at the work site, for a given amount of input energy.

Think of it like the gearing of a bike that has a deraileur system.

3 ring chainwheel, 5 gear freewheel on the rear wheel..

on the smalllest chainwheel and the biggest sprocket on the rear freewheel, it is VERY easy to pedal the bike uphill.

On the Biggest chainwheel and the smallest rear sprocket, the expenditure of torque required to make the same uphill journey is vastly greater.

With that analogy, it is easier to understand what difference changing one or other pulley size will do to available torque from a motor which has a specific torque curve, leveraged through use of different sized pulleys which, effectively, in consideration of the above analogy, act as gearing between motor pulley and driven pulley.

@682bear is also correct about torque multiplication.

 

[682bear]

Just as an example, several years ago I bought a cheap hand cranked sheet metal bead roller, fabricated a stand for it, and powered it through a gear reduction box and sprockets and chain...



The gear reduction box provides an 18:1 speed reduction and the sprockets another 8:1 reduction... together that gives a (18x8=144) 144:1 speed reduction and a 144:1 torque multiplication.

I powered it with a cheap harbor freight drill motor that was switched through a momentary foot pedal. The drill could NEVER have enough power to turn the bead roller on it's own... but with the 144:1 torque multiplication, it has a lot more power than it will ever need...

-Bear

 

[Charles scozzari]

Just as an example, several years ago I bought a cheap hand cranked sheet metal bead roller, fabricated a stand for it, and powered it through a gear reduction box and sprockets and chain...

View attachment 489216

The gear reduction box provides an 18:1 speed reduction and the sprockets another 8:1 reduction... together that gives a (18x8=144) 144:1 speed reduction and a 144:1 torque multiplication.

I powered it with a cheap harbor freight drill motor that was switched through a momentary foot pedal. The drill could NEVER have enough power to turn the bead roller on it's own... but with the 144:1 torque multiplication, it has a lot more power than it will ever need...

-Bear

Thanks to all for the answers / explanations

 

[MikeInOr]

Actually I think the thread was talking about a 3 phase motor in the mill head being run on a VFD (variable frequency drive). A bigger pulley on the motor means that the poster would run the motor at a lower RPM via the VFD for a particular spindle RPM. As the frequency of the VFD goes down the motor runs at a lower RPM and the motor does actually have less torque.

The bigger pulley on the motor is why the motor would be run at a lower frequency and lower RPM with less torque for any particular spindle RPM.

On a side note many VFD's have circuitry to boost the torque of the motor when running at a lower RPM reducing the amount of torque lost at lower RPM's.

 

[jwmay]

This may not be a great example, but it works for me.
Think of levers. A regular 9” ratchet can loosen a bolt ok, and you can spin it pretty fast after it’s loose. But sometimes you need a 3 foot pipe wrench to loosen a bolt. You can’t spin it nearly as fast. You didn’t get any stronger by using the 3 footer. But you were able to use your strength to greater effect. That’s the torque advantage being applied with a larger pulley.

 

[Charles scozzari]

Thank you for all the reply's. I was unable to find the original post but what stuck in my mind is the member changed the diameter of the mills spindle pulley and in doing so was told it would alter the motors torque value. I thought about that trying to understand how that was going to effect the torque unless a V.F.D. was involved. In my head all the pulley change was going to effect was the R.P.M. at the spindle. When I worked and there was a need to change an elevators speed on a A.C. driven hoist Motor we changed the diameter of the drive pulley ( before the VFDs ) but the motor remained as is. Thats actually what stuck in my head. Thanks again.

 

[MikeInOr]

Please correct me if I am wrong

so I am building a new belt and pulley housing/motor mount and I have all the speeds and torque I could ever need covered by two pulley steps and dialing my vfd between 30-90hrtz. My question is when I make my new setup can I do away with the countershaft design that I’m currently using since...

www.hobby-machinist.com