I thought the forces would be pretty small given the small distance traveled etc. Currently my biggest obstacle is finding the right size spring give 8' blades.
When things are in balance, yes - they are small, but when they aren't - Length is squared when moment of inertia comes into play.
Let's say you have a 20 pound 8.5' blade spinning at 200rpm (pretty standard for homebrew machines) and one blade extends just .2 inches out-of-balance...
No big deal right?
Wrong.
if we then calculate the amount of opposing energies in that scenario:
Moment of inertia is T(energy) = 1/2 * m(mass - kg) * l^2 (length to the center of gravity - meters) * w^2 (angular velocity - rad/s)
So the energy difference is
"big blade" - "small blade"
Which after some conversion turns out to be around 26 joules!
That's not all - if that 26 joules is able to bend the structure your mill sits on (this is usually the case as there is usually some give in the rotating structure the mill sits on in order to enable it to rotate) and it's as small as .2 inches - that actually adds .2 inches to one side and takes it away from the other.
Now you're dealing with around 92 Joules which will contort your machine more. That 92 joules will increase the offset thus giving you more mechanical energy to dissipate in the structure which will again increase the contortion and the energy.
because length is squared, this is a runaway scenario. Once it reaches a critical point, you will no longer have a machine as it will have ripped itself apart.
To add to this, the vibration you'll deal with will constantly wear your mill mechanically and that critical point will slowly be lowered as your materials weaken.
Yes, there *may* be people that are flying their mill with these types of balance problems, but the overall life of their mill will be reduced based on the severity of the balance issue. I strongly reccommend not overlooking this unless you're ok with just having a mill for a couple of months instead of several years.
