# calculating ballast needed to offset wobble of a 1g unbalanced centerfuge spinning at 20000G's

So this seemed pretty straight forward at first, but then I wasnt too sure about gravity forces...

So.

• centerfuge
• 2 samples
• 1 sample is 1gram heavier than the other
• centerfuge at top speed is exerting 20000 gravitys of force on the samples

What kind of weight / ballast is needed to keep the centerfuge from wobbling off the table?

Id imagine this is a linear equation ... So if i could get the equation that would be TOPS!

Ballast should be placed on 4 corners on the outside of the sample

• At what radius and angle relative to the samples will the ballast be placed? – Alan Rominger Aug 23 '11 at 21:32
• description updated – carl crott Aug 23 '11 at 21:38
• Are you saying that it's like a hexagonal arrangement with the 2 samples opposite each other and the balancing weights in the other positions? In that case I would place a calculated weight in the 2 locations closest to the lighter sample. Is this correct? – Alan Rominger Aug 23 '11 at 21:43
• @Zass no im only looking at 2 samples .. each opposing each other. – carl crott Aug 23 '11 at 22:04
• @delinquentme Then I have no idea what the 4 corners specification means. In order to solve this problem people need to have the geometry clear. If you can put something right next to the lighter sample, then you would place the difference in masses between the light and heavy sample. I'm pretty sure you're looking for $a=v^2/r$ type of math for this, but the force balancing depends on the geometry witch seems to be either trivial or unclear. – Alan Rominger Aug 23 '11 at 22:25

## 1 Answer

the force caused by this disbalance (1 gram at 20 000 g) is the weight of 20 kilograms, assuming coefficient of friction of 0.5 you need at least 40kg to stop it from wobbling around. Somewhat more depending to the exact geometry (the centrifuge would act as lever).

It really sounds very unsafe. High speed centrifuges need to be shielded so that the fragments of failed rotor couldn't kill you.

• could i get the equation you used? =] – carl crott Aug 23 '11 at 22:04
• 1 gram, at 20 000 gravities, is 20 000 grams of force, that's why they call it gravities you know. That gives you the maximum shaking force (assuming no suspension coz you didnt specify any). Then, to make it not move when its dragged with 20 000 grams of force, you need to ensure that friction is larger than this force. The friction is given by coefficient of friction. There may be other strong vibrations though, or you may have suspension that decreases the shaking force, none of that you specified. – Dmytry Aug 24 '11 at 1:06