Grain mills, rollers, & RPMs

[Alemental]

I am currently working on a home made grain mill. The rollers are 4 3/8" diameter & 6" long. I will be driving it with a 3/4 hp motor, but could up that to a 1 hp if need be. The drive sheaves that I can find at McMaster Carr will give me about 400 rpm. Does anybody have any idea if this will be too fast? I can slow it down a little more by substituting a 9 or 10 inch sheave for the 8" one that I'm looking at, but the price more than doubles at that point. How fast do you think these rollers will be able to go without running into problems?

 

[Evan!]

I bought my mill so I'm no expert...and maybe you've done more research than I think...but my first thought is that 4-3/8" dia. rollers is a bit overkill. All the mills I've seen have 2"dia rollers at the most. What's your reasoning?

PS, I have no answer for you actual question

 

[beergears]

Maybe the 2 in. rollers crowd can give you their rpm setings and you can calculate the "surface speed" or actual velocity as experienced by the grain (in./sec?), and calculate required speed to your setup to match...?

 

[Alemental]

I bought my mill so I'm no expert...and maybe you've done more research than I think...but my first thought is that 4-3/8" dia. rollers is a bit overkill. All the mills I've seen have 2"dia rollers at the most. What's your reasoning?

PS, I have no answer for you actual question

Larger rollers are more gentle to the grain as it is crushed. (I know, oxymoron ) The larger, the better. Commercial homebrew mills have 1 1/2 or 2-inch rollers for the sole reason for the small size is that it is far cheaper. In order to function at all, they need to knurled to give them enough gripping power to grab the grains. Rollers the size of mine should easily grab the grains without knurling because of the much lower tangential angle of a larger diameter. One of the homebrew books that I have (can't remember which one) states that 8' is the proper minimum size. I'm not so sure that I agree, but it would be nice.

 

[beergears]

Maybe the 2 in. rollers crowd can give you their rpm setings and you can calculate the "surface speed" or actual velocity as experienced by the grain (in./sec?), and calculate required speed to your setup to match...?

Countinuing with that thought, (if valid idea) that works out to be:

2 in. roller 200 rpm -> 20 in./sec

 

[fifelee]

This article says 200 rpm for the standard hombrew mill. That would be less than 100 rpm if you where trying to keep the same surface speed.

2in*3.14*200rpm= 1256in/min
4.375in*3.14*90 rpm= 1250in/min

 

[abracadabra]

A 1/2" driver sheave @ 1750 RPMs + a 10" driven sheave = 87.5 RPM output.

 

[abracadabra]

I bought my mill so I'm no expert...and maybe you've done more research than I think...but my first thought is that 4-3/8" dia. rollers is a bit overkill. All the mills I've seen have 2"dia rollers at the most. What's your reasoning?

PS, I have no answer for you actual question

The reason we buy 2" rollers is the same reason many of us buy keggles.

They aren't better they are cheaper.

 

[HenryHill]

A 1/2" driver sheave @ 1750 RPMs + a 10" driven sheave = 87.5 RPM output.

A 3/4 hp motor will most probably have a large outside diameter shaft. Even if it were a small fractional hp motor, it's shaft would be at least 3/8" and I can't imagine a 1/2" belt funning diameter sheave being physically possible for any size shaft (as well as a having a 3/8"ID?) as the v-belt would have to do an instant 180...perhaps as a timing belt, but that is still pretty small of a usable pulley size.


A 3/4 hp motor is way big for this need, 1 hp is just crazy.

Large diameter rolls are an excellent idea, however.

You need to be looking at AC gearmotors, and direct coupling.

 

[abracadabra]

A 3/4 hp motor will most probably have a large outside diameter shaft. Even if it were a small fractional hp motor, it's shaft would be at least 3/8" and I can't imagine a 1/2" belt funning diameter sheave being physically possible for any size shaft (as well as a having a 3/8"ID?) as the v-belt would have to do an instant 180...perhaps as a timing belt, but that is still pretty small of a usable pulley size.


A 3/4 hp motor is way big for this need, 1 hp is just crazy.

Large diameter rolls are an excellent idea, however.

You need to be looking at AC gearmotors, and direct coupling.

You are absolutely right that a 3/4 hp motor will have a shaft size of at least 3/8" more likely it'll be 1/2".

My calculations were simply an example using the OP's 10" sheave as the driven sheave. Then I got busy and didn't EDIT it.

Using a 4L pulley/sheave 1.5" OD and a 3L belt it'll still takes a 15" sheave/pulley to get you down in the range where you need to be
122.5 RPM assuming a 1750 RPM motor.

I also don't know the RPM speed of the OP's motor. The post I made was simply an example to show that a 10" sheave was not realistic. But I didn't have the time to elaborate. Thanks for bring it to my attention.

I completely disagree that a 3/4 HP motor is way to big. You want to worry about having a motor that's too small. Having a motor that can easily do the job is not a problem.

Use what you've got! The OP stated he had a 3/4 hp motor he didn't ask what size motor he should buy.

Beside that I'd think a 4 3/8 diameter X 6" roller could stand a little extra HP.

I'm a poor technical writer. You'll notice that most all of my posts have been edited. But I like doing it and also like trying to help out DIYer's.

 

[Alemental]

Well,
I contacted someone on another forum who has built a mill with a bit larger rollers than mine (6"). In fact, I am basing my mill on his design. He assures me that 500 - 750 RPMs will be just what I want. He uses 500 with a large drill motor. This allows me plenty of freedom in sheave choice, without having to go to extremes with either one. Nice!

I know 3/4 or 1 hp is much bigger than most people use, but with the larger the roller diameter, the more power the mill will require. And, as abracadabra says, use what you have. And I have both. And I am cheap.
Thanks for the input.