[SSfab]

Any body here know how to figure out RPM using different size pulleys and a given motor speed?

Formula?

[Al_The_Man]

Direct product of the ratio!
With timing belt drive the number of teeth of each is used for the ratio.

Al.

[brunog]

If v belts used:

motor RPM X motor pulley dia/ last pulley dia, there is no need to calculate intermediate pulleys.

Best regards

Bruno

[Al_The_Man]

If v belts used:

motor RPM X motor pulley dia/ last pulley dia, there is no need to calculate intermediate pulleys.

Bruno

Only if the intermediate pulleys both driver and driven are the same size, often the intermediate driven (input) is larger than the driver (output), in this case the gearing ratio applies.
Al.

[SSfab]

thanks a bunch!

[Al_The_Man]

If you go to the Stock Drive Products page they have belt formula's.
The problem with belt reduction, it does not need much reduction before the driven pulley get large in diameter, anything over 5:1 with 1 reduction is about the practical limit, the other problem is that the larger the ratio, the farther distance has to be observed from each axis in order to have the proper amount of belt engagement with the pulley, you cannot arbitrarily keep both as close as possible if it violates the minimum engagement.
This is why as I mentioned earlier, you can gear down with intermediate stages and if these double back on themselves it makes for a more compact set up for a large reduction.
Al.

[Cardamom]

OK - I'm wondering if y'all can help me with this! I'm still trying to do the calculations... I have a grain mill attached to a 1725 rpm motor with v-belt pulleys gearing it down. There is a 2" diameter pulley on the 5/8" motor shaft driving a 2.5" pulley on an intermediate 3/4" shaft. Also on the intermediate shaft is a 6" pulley driving the 12" flywheel of the mill.

Can someone walk me through the calculations to get to the speed of my mill? I did it once and came out with 119.79 rpm. Another time, came out with 95.83. Still another time, and came out with 690! I think I'm mixing up formulas... I'm so confused.

I want the mill to run between 50 and 60 rpm, so I'm trying to come up with what pulley sizes I would need.

Thanks for any help!

[Al_The_Man]

Your number of 690 looks right, you need an overall reduction of around 30:1, that quite a bit to expect out of belt reduction.
A 2" on the motor to a 6" intermediate, then a 2" on the intermediate to a 12" on the mill would still be 95rpm.
You might want to look at a gear box.
The larger the ratio, the farther the pulleys spacing has to be to retain belt engagement.
Al.

[Geof]

2" driving 2.5" means that the rpm of the 2.5" pulley is 2/2.5 = 0.8 of the rpm of the 2"pulley.

6" driving 12" means the rpm of the 12" pulley is half that of the 6".

The 2.5" pulley and the 6" pulley are on the same shaft so they have the same rpm so the overall reduction is 0.8 * 0.5 = 0.40

So if you motor rpm is 1725 your mill rpm is 0.4 * 1725 = 690

At least one of your answers is correct.

You want the mill to run at 60 rpm so you need an overall reduction of 60/1725 = 0.035

This is difficult with belt drives and I think almost impossible with only a two stage reduction using belts. The reason I say this is that each stage would need a reduction of 0.187. If your motor pulley is 2" and the small pulley on the intermediate shaft is 2" this means the large pulley on the intermediate shaft is 2"/0.187 = 10.7" and the pulley on the mill is also 10.7".

This may be feasible if you have space for this size of pulley.

[Geof]

I think Al_The_Man should stop reading my mind and simply typing faster (and rephrasing things).

[Cardamom]

Thank you so much!

I made a mistake in how I described the mill - the 2" motor pulley goes to a 6" intermediate, then a 2.5" intermediate goes to a 12" mill pulley. We disassembled the pulley setup a year ago to build a new cabinet, and I forgot how it was... :withstupi

Am I right in assuming a gear ratio of the intermediate pulleys 6 / 2.5 = 2.4?

If so, I calculate that 2" motor pulley x 1725 rpm is 3450.
3450 / 12" mill pulley = 287.5

Then 287.5 / 2.4 gear ratio = 119.8 rpm mill pulley speed.

The absolute maximum recommended rpm for the mill is 120 rpm. The best speed for grinding flour is 50-60 rpm so the flour remains as cool as possible; and this speed reduces the strain on the mill bearings and grinding plates.

We are redesigning our mill setup, and building a cabinet to house it, so we have space to put in another shaft, if necessary.

Can I reduce the size of the pulley on the motor? Or is a 2" v-belt pulley the smallest I can go with a 5/8" shaft?

[Al_The_Man]

Thank you so much!

I made a mistake in how I described the mill - the 2" motor pulley goes to a 6" intermediate, then a 2.5" intermediate goes to a 12" mill pulley. We disassembled the pulley setup a year ago to build a new cabinet, and I forgot how it was...
Can I reduce the size of the pulley on the motor? Or is a 2" v-belt pulley the smallest I can go with a 5/8" shaft?

If you notice, that is around the example I showed, which gave around 95rpm.
If you go to one of the sites like Stock Drive Products, they have sizing program, you just punch the numbers in and see the resultant reduction and the distance you must observe between centres.
Al.

[Cardamom]

Yes, I definitely noticed! Thanks again for your help and patience, and for the website.

[Geof]

You have it a bit upside down.

Al_The_Man and I described things equivalently but differently which could have generated some confusion.

I would calculate the ratio as 2/6 = 0.333 so the rpm of the intermediate shaft is 0.333 * 1725 = 575

Similarly I would calculate the second ratio as 2.5/12 = 0.208 so the final rpm is 575 * 0.208 = 120


If you have the space you could exchange a 12" pulley for the 6" on the intermediate shaft and that will divide your final rpm by 2.

There is the possibility you could get belt slip because when a small pulley drives a large pulley it reduces the angular contact of the belt on the small pulley.

[Cardamom]

Thanks again! This explains it better for me.

Here's a link to what I'm doing.

http://www.countrylivinggrainmills.c...ction=motorize

We used to have the mill with a horizontal setup, and we had to use a "tensioner" pulley when it was in use. Ran fine. Belt only slipped when I forgot to lock the tension pulley in place. Took up a lot of table space, though.

Now we're going to mount it vertically, and that should eliminate the tension problem.

Funny how my backwards calculation method still comes out with the same answers as the way you showed me...saves a step, too

Thank you all!

[Geof]

....Funny how my backwards calculation method still comes out with the same answers as the way you showed me...saves a step, too

Thank you all!

I didn't look at it too closely but I think it is a case of two wrongs making a right; not a reliable way to solve problems because if you get an odd number of wrongs you are wrong.

[Cardamom]

You're right! Definitely not a reliable way to solve problems.

My nuke husband laughed when he saw all this... I could've just been patient and asked him to show me the math - duh! He had showed me the gear ratios and all that, but I just wasn't getting the calculation parts...

[toddwinnburns]

I have the exact same situation. I have the same grain mill and the same motor, but I don't have any pulley yet. It sounds like you would need very large pulleys to accomplish turning the grain mill pulley. Someone mentioned a gear box and it was also suggested that you could use more Pulleys? If anyone has and suggestions concerning the best setup to use I would LOVE to have a workable solution. I have a 12" pulley on the grain mill and I have a 1725 RPM motor with no pulley yet installed (it has a 1/2" shank) I have to get the RPM on the mill to between 55 and 60 RPM. Thanks in advance for any help you can give me. I have been working on this project for some time and I would love to find a workable solution.
Todd

[toddwinnburns]

To clarify part of my previous question. I wanted to see if it would be possible to get to 55 or 60 RPM by using more than two pulleys or can I use pulleys on pillow blocks to get to this rate (from the 1725 shaft) in a combination of two pulleys in the middle and the one I will put on the motor. Unlike the origional person that asked this question I have not purchased any pulleys yet! Any suggestions would be greatly appreciated.

[Geof]

You need a reduction ratio of 29 to 1; three sets of pulleys each at a 3 to 1 ratio, that is a 3" pullley driving a 9" pulley, will give a combined ratio of 27 to 1. You could easily make the first set of pulleys a 2-3/4" pulley driving the 9" and your overall ratio is then slightly more than 29 to 1.

The setup would be:

2-3/4" pulley on motor driving 9" pulley on first shaft; ratio 3.27 : 1

3" pulley on first shaft driving 9" pulley on second shaft; ratio 3 : 1

3" pulley on second shaft driving 9" pulley on mill; ratio 3 : 1

Overall ratio is 3*3*3.27 = 29.43

Your ratio will probably not be exactly 29.43 to 1 because the actual contact diameter between a belt and a pulley is not easy to predict, but it will be close.

You will also lose a lot of power in this three stage reduction because belt drives are not efficient. My guess is that the overall efficiency of this drive will be less than 50%.