Ball Mill Speed?

[ribrown]

Hi guys,

 

I am a veteran of Class C fireworks but a stone cold "Green" newbie to the hobby of making fireworks. I am in the process of building a ball mill. I have all the components needed including the media and have the cabinet built with 2 axles, but can not for the life of me figure out what the proper RPM of my jar should be. My motor came from a rather large Craftsman drill press which I found at a salvage yard for 25 bucks. (1/2 HP 1750 RPM.) I first tried a 2 inch pulley on the motors shaft and a 5 inch pulley on the drive axle. My jar is a slightly larger then 1 gal design modled off of the Sponenburgh design (6in Sch 40 PVC with 8- 1/2" pipe). When I ran the thing empty with these pulleys, I counted a little under 60 turns per minute. So next I tried changing the 5in pulley to a 3in and filled the jar up with 25lbs of 1/2" hardened lead balls. This time when I ran the thing, I got 104 RPMs, which I assume to be too fast. This time when I ran it, I had tighter belt tension then in the first run.

 

So anyway, I can play with trying out different sized pulleys on my own, but what I can't seem to find after much searching, is the optimal RPM for my set up. I'm not exactly dumb by any means (mabey I am), but the few calculator web pages I've seen I just couldn't understand.

 

So is there anyway you guys can help me out with figuring out this one problem of figuring out the correct RPMs I should be seeking?

BTW, I got all the ideas for building my machine from information on this fourm and with some advice from Ned G. So I greatly appreciate the advice and direction of everyone on this fourm.

 

THank You Kindly, Rick Brown

[oldguy]

Hope your motor is a TEFC type?

 

This might help you on RPM.

 

http://www.benchnote...ycalculator.htm

 

It differs for everybody as everbody has differing HP & RPM motors, pulley sizes, drive roller diameter, jar diameter & media diameter.

 

Optimal RPM is 65% of critical speed.

 

My Big Bertha mill runs about 66 to 68 RPM running two 12X12 inch PVC jars

 

Edited January 24, 2012 by oldguy

[Col]

I aim for 65% of the critical speed. Critical speed = 265.45 / SQRT (jar ID in inches - media diameter in inches).

For an 8.5" ID jar with 1/2" lead media, the critical speed is 93.82rpm, 65% of that is 60.98 rpm.

For a 6" ID jar, the critical speed is 113rpm, 65% of that is 73.5rpm.

[Mumbles]

I'd stick with the 5" pully. The "optimal" speed is around 70 as Col pointed out, but honestly as long as you're in the ball park you should be churning out hot BP in a few hours.

 

There is an excellent calculator on Passfire if you're a member there. If not, if you tell us the outer diameter of your roller including any tubing or anything that is over it, I can calculate the proper diameter of pullys if you'd like

[ribrown]

Thanks for the fast responses guys!

 

Yes Old Guy, it is a TEFC with a capacitor. Going from a recommendation by you, I returned a 5" grinder that I bought from Harbor Freight and went out and found the drill press motor. The salvage yard was trying to sell the whole drill press which had the mounting plate for the motor busted off. So the motor was dangling there and I asked if I could buy just the motor. They first told me $30, so I offered them $20, and finaly settled on $25. I was prepared to buy the 3/4 HP motor that Harbor freight sells for $89 before I found this one.

 

But in response to the Woodworking calculator, I have no idea how to use it. I'd like to use it to calculate the size pulley I need for my drive axle, but I've tried and tried to figure it out and have had no luck.

 

So to Col. That is exactly what I was looking for! I saw that formula a few times online but didn't realize what "SQRT" stood for. "SQUARE ROOT dummy", I just said to myself. I thought it was an abreviation for the anwser to "Jar diameter minus Media diameter". SO I kept coming up with a Critical speed of 48.26 RPMs. "DUHHH". So thatnks for getting my brain into gear.

 

Now to Mumbles. I am not yet a member of Passfire, but I will join very shortly since I'm starting to get really into this hobby. But my 2 rollers are both lentghts of 5/8" diameter steel rods with pieces of 5/8" ID rubber automotive hoses wraped around them. Upon measuring them with a small gauge, I get an outer diameter of 7/8". The current pulley on the motor is 2" diameter and on the drive axle is 3" diameter. With this setup, I'm coming up with exactly 104 revolutions per minute. I put a 1" square of red duct tape on the jar and counted how many times it passed while timing out 60 seconds. So that's a pretty accurate figure. My jar is a 8.5" long piece of 6" sc40 PVC, with a sc40 6" to 4" reducing coupling and a sc40 end cap on the other end. I have a 2" long section of 4' sc40 PVC in the open end with a rubber hose clamp type cap. Any info on what size pulleys I should try next would be awesome.

 

Thanks Again my friends!

Edited January 25, 2012 by ribrown

[Col]

With your current pulleys i make the theoretical (no loss) speed 118rpm.. A 5" (127mm) pulley on the drive axle should give you a realworld speed around 60rpm

 

I had a look at the spec for sch 40 pipe and it appears to be 6.025" id and 6.625" od. The motor rpm should be in the region of 152rpm with the 2" and 3" pulleys which suggests the motor is losing a fair few rpm turning the heavy jar...keep an eye on the motor temperature, it could be getting pretty hot The ideal speed for a 6" sch 40 pipe is 73rpm, i`d go for a 4.5" pulley on the drive axle which should give you ~70rpm after accounting for the losses.

Edited January 25, 2012 by Col

[ribrown]

Excellent! THank you kindly for the help Col! That sounds perfect!

[Mumbles]

I did the same calculations and came up with similar values as Col. I did use 7.2" as the OD however, though I am unsure of the real value. Col used the OD of the pipe itself, but you also have to account for the thickness of the fittings you have on there. There does seem to be quite a bit of slip on there. At high speeds it is to be expected, but around normal operating speeds it shouldn't be that high. You may find that your jar starts to travel around the theoretical speed as the hoses roughen up a bit. The hose may also be slipping on the shaft, though that isn't as likely.

 

Yours is a pretty typical setup as far as jar, motor, and shafts go. Many people use a 2" pully on the motor and 6" on the shaft to give right around 70 rpm (in theory).

[ribrown]

Thanks Mumbles. I believe the problem I had with my first, empty jar test run was that I did not have the belt tight enough. This is why I only got about 60 RPMs out of the jar with a 5" pulley. When I tested the jar full of 1000 - 1/2" lead balls and a 3" pulley, I had the belt a bit tighter and thats probably why I got a much faster speed. According to the calculator that Col used, I should get about 140 RPMs with a 3" shaft pulley.

 

But the hardware store I purchased the pulleys from has either 5" or 6" available, but no 5.5". A 5" will supposedly give me 84.5 RPMs where a 6" will give me 70.4 RPMs. If i had to choose between the 5 or 6" pulley for my drive shaft, which one would be the best choice? I want to get the shortest mill time out of the machine, but ofcourse the more important factor is the most effective grind.

 

Thanks for the advice!

[Col]

An empty jar is more likely to slip than one thats half full of lead

Its a good idea to account for the motor slowing down under load, if you use the no-load motor rpm (1750) in the calculations you may find the real world rpm is lower than expected.

[Essohbe]

You mean you guys don't just throw it in an old washing machine?