Now wait a minute there Dspark, I'm pretty slick when it comes to mechanical things (anyone want to talk aerodynamic theories? Coefficients of drag, Cd 's of lift, Reynolds numbers, gyroscopic precession, etc.?.. I can prove that I know what I'm talking about here...)
Any wobble at all at the end of the bit before it strikes the ground will cause it to self destruct at those RPM's when you're dealing with this kind of mass.
Especially if the point of attachment is a swivel point instead of a solid attachment.
PTO's are typically 540 RPM and post hole diggers are typically a good 4:1 reduced from that. I've seen post hole diggers start to wobble and begin the self-destruct routine even at those low RPM's. Makes a lot of noise...
How did you determine MPH?
MPH compared to RPM is a little vague without something to relate them... Are you measuring the stroke of the piston? The outer circumference of the bit? It has little consequence on the energy built up it a spinning mass. Especially a heavy one.
The spinning mass would cause a gyro effect and everything would be fine until a force acts upon it, (like moving it towards the ground) then this thing called
gyroscopic precession causes the force to act at 90ยบ to the plane of rotation and in the direction of the rotation. Gyroscopic Precession is of extreme importance in aircraft design and useful in aerobatics
(lomcevak) . In other words, the slightest force acted on the bit will cause it to wobble violently (and the forces will be proportional to the square of it's velocity, double the velocity, quadruple the forces).
Also, with the tip spinning at such rpm, the tip would probably just melt away from the friction of the stone. Gotta be a slow grinder, less than 1000 RPM range...
I'd gladly watch from a distance the testing of such a machine... video camera ready...
[This message has been edited by sparky66wv (edited 07-15-2001).]
