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Need some help increasing rpms

  1. Dec 23, 2011 #1
    Hello everyone! I just want to start out by saying that i really don't know very much about physics. That is why I am coming to you for help. I am making a motorized grain mill for brewing beer. Most people start buy using a 1hp electric motor and a large pulley attached to the mill to reduce the speed. This is a project that i started a few week ago an I have come the a halt. My question is can i increase the rpm of a motor (which i already own) from 53rpm to 300rpm.

    Here is an example of what im trying to make:

    Thank you for any help you provide
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Dec 23, 2011 #2


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    Hi calisto737, welcome to the board. In general, yes you can change the RPM of the mill by changing the sheave (pulley) size. Note however that power required for the mill is a function of a number of things and one of them is RPM. A second factor is feed rate. Your motor will only have sufficient power to provide some RPM and some feed rate. If you exceed one of those two parameters, the motor will stall and you'll trip a motor breaker assuming you have one. Otherwise, it'll just catch fire. <just kidding. I'm sure you have a breaker somewhere>

    Mills generally have a mechanism to adjust feed rate, or at least the industrial ones I've worked on do. From watching your video, I see there's a slot in the bottom of the hopper but it isn't clear to me whether or not that slot width can be adjusted to control feed rate.

    A secondary problem is that the machine you have on YouTube has no way to adjust the belt tension. No big deal I suppose, but it would be better to have belt tension adjustment. You'll need to do something if you change a sheave size to increase RPM.

    So what are you looking for? Are you looking for a method to determine sheave size? Or just general information around grinding mills?
  4. Dec 23, 2011 #3
    ok...i guess ill give you some more info. the you tube video was not mine, i just wanted to give people an idea of want i am building. most people start with a motor that has a rpm in the thousands and they slow it down with a large sheave attache to the mill shaft. i want to do the opposite. i need to figure out what size sheaves to but to do this (if its even possible). also I am going to follow the mill manufacture recommendation for the feed slot. i plan on mounting the motor on a set of rails to ajust the tension (haven't got that far yet)

    here is the motor:

    http://monsterguts.com/index.php?act=viewProd&productId=4 [Broken]

    the mill:


    this is the hopper and base i built:

    http://i1189.photobucket.com/albums/z425/bhartbeck/IMG_20111222_230800.jpg [Broken]
    Last edited by a moderator: May 5, 2017
  5. Dec 24, 2011 #4


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    The motor you have is only 0.03 hp. I see other people are using a 1/2" drill to drive the mill but the manufacturer doesn't state what power is required. Knowing other folks are using a 1/2" drill and they're using it to directly drive the mill at lower RPM, I'd guess the power is on the order of 1/4 to 1/2 hp so the motor you have is woefully underpowered.

    Electric motors are not that expensive, though they will be a bit more expensive than a $100 drill so your least expensive option might be to purchase a 1/2" drill and drive it directly. They produce about 1 hp at 1200 RPM. If you want to go with an electric motor, note that a 1/4 hp motor is only a little bit less expensive than a 1/2 hp motor so I'd suggest going with 1/2 hp. Take a look online at McMaster Carr or Grainger for motors. They'll be about $200. These companies also sell sheaves and belts but you can probably get those at your local hardware store. Another option might be to pull a used motor out of an old appliance such as a garage door opener or washing machine. You might try Craigslist.

    Regarding size of the sheaves, the speed of the mill can be determined as follows:
    Grpm = Mrpm (Md / Gd)
    Grpm = RPM of Grinder (mill)
    Gd = Diameter of Grinder (mill) sheave
    Mrpm = RPM of motor
    Md = Diameter of Motor sheave
  6. Dec 24, 2011 #5
  7. Dec 24, 2011 #6


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    Wow! Great price. Yea, that'll work. RPM's are high but that just means there will be a big difference in the size of your sheaves.
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