ThaDawg Posted February 5, 2022 Posted February 5, 2022 Hey all, i'm builing a new ball mill but im stuck on calculating the pulley on the axle dat will drive the jar.. Motor:1410 rpmPulley diameter on motor 5.8cmJar diameter: 20.5cmAxle diameter: 2.5cm I want the jar to spin around 60rpm I googled a lot but i still don't get it, drive technology is hard for me and in english it's even worse.. So i'm wondering if someone can help me out?
SharkWhisperer Posted February 5, 2022 Posted February 5, 2022 Hey all, i'm builing a new ball mill but im stuck on calculating the pulley on the axle dat will drive the jar.. Motor:1410 rpmPulley diameter on motor 5.8cmJar diameter: 20.5cmAxle diameter: 2.5cm I want the jar to spin around 60rpm I googled a lot but i still don't get it, drive technology is hard for me and in english it's even worse.. So i'm wondering if someone can help me out?Pulley diameter vs RPM calculator: https://www.blocklayer.com/pulley-belteng.aspx
SharkWhisperer Posted February 6, 2022 Posted February 6, 2022 Thanks Shark i think i got it now!Cool. Keep in mind that if you are using the axle to rotate the jar by friction, the axle and the jar will not be turning at the same rpm. You probably know this, but sometimes this is difficult to visualize. The basic relationship between two pulleys is that rotation speed x pulley diameter is always going to be proportional when pulleys are connected to each other. D1N1=D2N2. So in your case, the motor (and drive pulley) rpm (N1) is 1410 and the motor wheel (D1) is 5.8 cm in diameter. The axle (second pulley/wheel, D2) is 2.5 cm in diameter. So, the formula to calculate how fast the axle that turns the jar is rotating (N2), is: D1N1/D2=N2, or 5.8x1410/2.5 = 3271 rpm. That's super high-speed. Whenever you transfer rotation from a bigger wheel to a smaller wheel, the speed is going to be greater on the smaller wheel (in this case the axle). But there is a second calculation that you need to do. If your axle was passed through the long axis of your mill jar, then of course they would turn at the same rpm. But most mills are going to have the axle rolling along the outside surface of the larger-diameter jar, in direct contact. This is essentially a second pulley system--if the axle and jar were connected by a belt instead of touching, the rotation speed relationship would stay the same as if they were touching. So...you have a 2.5 cm axle spinning at 3271 rpm (this seems really really fast), and it touches, and rotates, the larger 20.5 cm milling jar. So you just do the D1N1=D2N2 calculation again. Now, 2.5x3271/20.5 = mill jar rpm (N2), which in this case would be 399 rpm (let's just say 400 rpm). Obviously this is faster than you want your jar to spin. If you can't slow down your motor, then you will need to attach a larger-diameter wheel onto your axle to slow the axle rotation speed and jar rotation. How big of a wheel do you need with your current set-up? Well, you want your jar to spin 60 rpm and you've got a calculated jar spin of 400 rpm to start with, before modifying your axle. So it's spinning 400/60=6.67 times too fast. That means you need your D2 on the axle to increase from the 2.5 cm axle diameter by a factor of 6.67. 2.5 x 6.67 = 16.67 cm diameter wheel needs to be attached to your axle and connected to the motor pulley. This is 6.56 inches for Americans. This also seems like a really large wheel on your axle. If you can slow your motor speed by half, then your axle-connected wheel (D2) only needs to be half the diameter to keep the same jar rotation speed. Another way to slow your jar is to reduce the size of the pulley on the motor. It helps to visualize this if you draw out the pulleys on a piece of paper. In general, if you connect two equal-diameter wheels together with a belt, they will turn at the same speed. If you connect a large drive wheel to a smaller driven (mill) wheel, the smaller wheel will rotate faster than the drive wheel. If you connect a small drive wheel to a larger mill wheel, the larger wheel will rotate slower than the drive wheel (this is what you want with your mill). Hope this was more useful than confusing...
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