Math Question: Drum Size/Spin RPM's/G Forces

bestf100July 27, 2011

Here's a math question for anyone sharp enough to know the answer.

I have seen a front load washer(Speed Queen Horizon), with a 1000 rpm spin speed claim, it has a 305G force. It has a drum volume of 2.84 cubic ft. I want to know, what happen, when you move up to a washer with a drum volume of 4.5 cubic ft, with a same 1000 rpm's (LG Waveforce).

Are the G Force and centrifugal numbers A) the same, B) higher or C) lower?

For extra credit, show your math work, to back up your answer.

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Volume figures will not help you. You need to know the radius of the drum involved in order to properly calculate G-force. The two are completely different and an increase in one (volume, for instance) does not necessarily equate to an increase in the other (radius, for instance).

    Bookmark   July 27, 2011 at 4:08PM
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I'm not so sure about that. It is my understanding that the larger the tub (volume), the higher the centrifugal force, relative to the rpm. Someone correct me if I am wrong here.

    Bookmark   July 29, 2011 at 10:18AM
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Diameter of the spinning basket is directly related to G-forces. A tall, narrow basket may have the same capacity (cubic feet of space) as a short, wide basket ... but the wider basket will have higher G-force at a given RPM.

    Bookmark   July 29, 2011 at 5:52PM
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dadoes.. and shriv.. are correct. G force a is directly related to diameter or circumference. The larger the radius the higher the g force. Not a physics major so couldn't really intrepret what 305 Gs means in the practical laundry experience. A few years ago spin speeds were a major big deal to laundry enthusiasts here as the US was moving to front loaders, but honestly I think that we may have given too much weight to this metric. My experience with front loaders tells me that so long as a machine can spin in the 1200-1400, regardless of drum size, it will yield pretty good extraction. Anything faster and the layman launderer will end up with over wrinkled clothes. machines in Europe go up to 1800 rpms, but Europeans tend to be more knowledgeable in laundering where as Americans tend to throw everything in one load, not paying much attention to the different cycle options, temps (washing in cold is good,washing in hot is bad to many Americans), etc.

    Bookmark   July 29, 2011 at 9:02PM
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Here is an equation I found online, but consider the source (LOL):

g = 0.56 x D (n/1000)^2, where D is the diameter of the drum in mm and n is the RPM of the spin cycle.

Without knowing the drum diameter measurement, you cannot calculate G-force. Drum volume says nothing about the diameter, since a small diameter but long drum can have a higher volume than a large diameter but narrow drum. However, the larger diameter drum will always produce higher G-forces than the smaller diameter drum at the same RPM, regardless of volume. Make sense?

Using the above equation and plugging in the OP's Speed Queen numbers yields:

305 = 0.56 x D (1000/1000)^2

Solving for D gives us a diameter of 544 mm or approx. 21 inches.

Qualify all of the above with the fact that I got C's in math all through high school... :)

    Bookmark   July 30, 2011 at 12:46PM
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Isn't the rpm rated is based at the center of rotation? e.g. the shaft that is driving the drum.

The rotation at the rim would be different than at the shaft so that the rpm variable must be the rpm at the rim of the drum.

I remember that the ice skaters in the finale had a "demo" of the chorus where a couple would be rotating slowly & the laughs startup at the end when the last 2 or 4 skaters had to skate very fast to try catching up to the linked chorus line.

Perhaps the 1000 rpm at the center translates to a different value at the rim?

    Bookmark   July 30, 2011 at 4:03PM
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I get it....really, I do. However, I wouldn't base any buy/don't-buy decisions on it. I can't imagine any machine you might buy disappointing you because of it's g-forces during the spin. Your clothes will come out damp and dryer-ready no matter what you buy. This is a purely intellectual exercise, IMHO.

Even the highest-spin machines available have a number of cycles that specifically AVOID their highest spin-speeds just as they are able to reduce or increase temperatures.

Beyond the interest of the calculation for the fun of it, I regard as non-issue.

    Bookmark   July 30, 2011 at 8:07PM
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@ vintage36

If the outside of the drum rotated at a different rate than the shaft, you would very quickly have a broken drum. The RPM at the center of the drum is exactly the same as at the perimeter, but the angular velocities are very different. But one full revolution of the drive shaft is equal to one full revolution at the outer edge of the drum as well. No difference. However, the G-force would be substantially higher at the drum's edge (large diameter) versus the shaft (small diameter).

    Bookmark   July 31, 2011 at 1:10PM
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