# Centrifical Force ratings misleading ?

Discussion in 'Hardscaping' started by xtreem3d, Oct 2, 2012.

1. ### xtreem3dLawnSite Senior Memberfrom St. LouisPosts: 801

Been shopping for another compactor, a big one with alot of CF. Came across this article (scroll down to Centrifical Force) makes me wonder what is really true ? http://www.mbw.com/pdf/Word2Wise.pdf

Steve

3. ### blakescapeLawnSite MemberPosts: 108

That is a good article and they are right. These plate compactors have a mass that is accelerated in a circle around an axis of rotation. Kinda like you would spin a ball and string around in a circle. There is no "centrifugal" force acting on the ball, which is a term used to explain the absence of centripetal force. The only forces acting on the ball are acceleration towards the center and a constant velocity tangent to the circle. The acceleration towards the center is called centripetal force, which keeps the ball rotating in a circle. Without centripetal force, the string would break and the ball would go on a straight path tangent to the circle.
http://www.regentsprep.org/regents/physics/phys06/bcentrif/centrif.htm

Due to the heavy mass in motion inside the plate compactor and the speed at which it is spun, the weight of compactor isn't enough to hold the spinning mass still, which in turn causes a lack of centripetal force a.k.a "centrifugal" force. Since there is no string to break and release the mass tangent to the circle, the mass pulls the axis of rotation (which in this case in the spinning shaft) tangent to the circle it is spinning on. That "centrifugal" force causes the plate to move up and down. Centrifugal force doesn't accurately quantify how much force the plate is hitting the ground. That is where amplitude comes in. As they said in the article, amplitude is a term used to "quantify the motion of a mass subject to energy inputs." So the calculator you have takes into account the effecting mass(the amount of weight being put into motion, not the total shipping weight), the frequency(which is how fast the plate moves up and down), and the centrifugal force rating on the machine.

Luckily this company give the specs on their unit:http://www.webermt.us/uploads/Downloads/CR1_GB_2011.pdf So lets find the amplitude for the cr1. In this case the operating weight is 260 lbs, the centrifugal force is 4500 lbs, and the frequency is 6000 rpm or vpm (revolutions per minute or vibrations per minute). The amplitude is 1.32mm, which is the measurement of how much the plate moves up and down. Plug in the specs for the cr3 and the amplitude is 2.06mm, so the higher the amplitude the better. A compactor with a higher centrifugal force rating is going to obviously be more powerful than one with a lesser one, but this calculator would be great to compare two different compactors with the same advertised centrifugal force rating. The centrifugal rating may be the same between two different manufacturers, but the operating weight and the frequency will be different. The amplitude will tell you which one is better, just pick the one with the higher amplitude. I'm going to use this calculator when buying my next compactor. Thanks for sharing those links!

5. ### blakescapeLawnSite MemberPosts: 108

yeah I think they made a mistake with the vpm. Hertz is the number of cycles per second. The number they give is 90Hz or (90 cycles/sec) which you need to convert to cycles per minute or vibrations per minute. There are 60 seconds in a minute so (90 cycles/sec)*(60 sec/min) = 5400 cycles/min or vibrations per minute. So when you plug in CF 8317 lbs, Operating weight of 529 lbs, and 5400 vpm, you get 1.48 mm.
Amplitude is measured from the equilibrium point to the peak of the wave (picture below). That means for each cycle, the plate is traveling up 1.48mm from the equilibrium point 0, back down 1.48mm to 0, then 1.48mm below 0 and then back up to the equilibrium point. So the total distance the plate travels from highest point to lowest point (peak to peak) is double the amplitude (2*1.98 mm = 2.96 mm total).

6. ### xtreem3dLawnSite Senior Memberfrom St. LouisPosts: 801

That makes more sense...I didn't know to convert Hz from seconds to minute.
Thanks again,
Steve