140 replies to this topic

[cooperman435]

option 5 is to have a driven roller with a larger/smaller diameter drive surface on one end for different mill jar sizes. it effectively slows or speeds up the surface in contact with the jar.



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Like that sort of!

[mike_au]

I vaguely recall someone (possiby Lloyd Sponenburgh) explaining that as you change jar size (keeping all other variables the same) two things happen:

1) the optimum RPM decreases (i.e. you want the jar to turn slower)
2) the gear ratio between the rollers and the jar changes

Either by lucky coincidence or some engineering magic, it just so happens that the change in gearing gives pretty close to the correct change in RPM as long as you are only talking about a few inches variation in jar size.

So if you optimise your mill for a 4" jar, and then chuck a 6" on (or vise versa), it won't be perfect, but it won't be too far out.

Also remember that your media diameter has some effect on optimal and critical speed. If you had different sized media for your different sized jars, you could work it so that they are both perfectly optimised running off the same roller.

Personally, I would optimise everything around the BP milling jar and just live with the fact that the other jar might not be at it's perfect speed. If the stuff in the small jar is just going to be BP ingredients then it doesn't matter if they aren't super fine before they go in to the main jar.

[digger]

I vaguely recall someone (possiby Lloyd Sponenburgh) explaining that as you change jar size (keeping all other variables the same) two things happen:

1) the optimum RPM decreases (i.e. you want the jar to turn slower)
2) the gear ratio between the rollers and the jar changes

Either by lucky coincidence or some engineering magic, it just so happens that the change in gearing gives pretty close to the correct change in RPM as long as you are only talking about a few inches variation in jar size.

So if you optimise your mill for a 4" jar, and then chuck a 6" on (or vise versa), it won't be perfect, but it won't be too far out.

Also remember that your media diameter has some effect on optimal and critical speed. If you had different sized media for your different sized jars, you could work it so that they are both perfectly optimised running off the same roller.

Personally, I would optimise everything around the BP milling jar and just live with the fact that the other jar might not be at it's perfect speed. If the stuff in the small jar is just going to be BP ingredients then it doesn't matter if they aren't super fine before they go in to the main jar.

OK then I have had this conversation before. It cetainly sounds sensible. However I have a variable speed drive on my mill so whenever I chuck a jar on there I tune the speed by turning it up slowly to critical speed then back it off untill the balls start crashing about (usually 0.6 to 0.7 of critical).

Anyhow I thought I should do the proof of the maths to show what the surface velocity should be for a mill jar at critical speed to see if different jar sizes do or don't have different surface velocity requirements.

So here we go:- (assume centre of mass is at the wall of the jar for simple proof)

Force on the media due to gravity: F=mg (Force (N) = m (kg) x g (m/s^2))
Centripetal force: F=mv^2/2 (v = velocity m/s)

Therefore equating the two and canceling gives the following expression for v at critical speed v=(gr)^0.5

So we can now make a table for the required surface velocity for for different mill jar sizes (given that optimum speed is 0.6 times the critical speed).

10 cm jar : Surface velocity 0.70 m/s
15 cm jar : Surface velocity 0.86 m/s
20 cm jar : Surface velocity 0.99 m/s
25 cm jar : Surface velocity 1.11 m/s
30 cm jar : Surface velocity 1.21 m/s
35 cm jar : Surface velocity 1.31 m/s
40 cm jar : Surface velocity 1.40 m/s

So it would appear that the size of the jar will effect the milling efficiency if the roller speed remains constant for different sized jars. Ok if there is a small variation in jar size 10% - 15% of diameter then it should not have a great effect as it will stay in the 0.6 to 0.7 of critical speed (given that you were somewhere towards the top end of that range) however larger variations will start to have an effect on efficiency.

So if you double the jar size you need to speed the rollers up by 40% (rpm)

Hope this helps

Edited by digger, 24 September 2009 - 09:23 PM.

[Vic]

Nice work PyroPDC

There is nothing worse than a crap ballmill to make you annoyed enough to make one that is the biz.
.
Some day I will post mine for you to see and you all can have a good laugh, but it works well and doubles up as a star roller as well .

[cooperman435]

yeah looks very nice mate. where did the rubber rollers originate?

[Vic]

I would call that blatant advertising Mr Cooperman shame on you.

[cooperman435]

It would be excellent advertising if I got him them.

I need some new ones so am currently looking for suitable bits

[dr thrust]

i found this novel design for a mill thats different, what's yer thoughts ?

[matchkatt]

Brilliant concept! It solves the slip problem of conventional design mills, and i think that is also creates less vibration. On the other side its bigger.

[cooperman435]

very ingenous thinking

[BrightStar]

Very original, I'll give it that.

It saves the cost of another axle and a couple of bearings at the price of two big pulleys and two more belts, so probably costs marginally more.

I'd be worried that the jar would walk itself off one of the belts if the composition clumped at one end. Dropping a jar of BP from a height next to a running motor.. not great. Oh, and only one jar at a time of course.

It's probably nice and quiet though and another option to consider if you have the parts already.

[johnheritage]

For all HAZMAT ball milling requirement, please be speaking with us at the Legoverya processing facility for low cost solution

Is featuring variable geared drive from high torque mains power plant, slip clutching, all glass / teflon mill construction. Makes possible the handling of highly corrosive / neurotoxic chemical component. Here see processing of semiconductor for electroluminescent display

Vibration make noisy. But also make superior the mixing capabilities

Edited by johnheritage, 29 September 2010 - 10:20 PM.

[digger]