Critical speed was mentioned earlier in this thread, so let’s dive in a bit deeper.
Here’s a nice definition:
Critical speed is the first resonant frequency (speed) of the rotating shaft. Resonance in a rotating shaft can be catastrophic and even break the shaft.
Here’s a nice example:
Here’s the formula for calculating leadscrew critical speed:
N=(4.7610^6) dr*C/L^2
where
N: speed (RPM)
dr: root diameter (in)
L: shaft length (in)
C: constant (0.36 for fixed-free, 1.00 for simple-simple, 1.47 fixed-simple, 2.23 fixed-fixed)
Notice the inverse square (L^2)? Doubling the length drops the critical speed by 4.
EDIT: And another interpretation of the above. Assume you use a certain length and certain thickness at a certain speed. If you wish to maintain the same speed and try 2x the length, you need to compensate with 4x the thickness.
Out of the three:
- thin
- long
- fast
you can only pick two.
In our case, it’s a Tr8*8 leadscrew with a 6.20mm root diameter, 1000mm long in a simple-simple bearing configuration. Therefore
N=(4.7610^6) (6.2/25.4)*1/(1000/25.4)^2=750RPM
Manufacturers recommend speeds between 60% (conservative) and 80% (aggressive) of critical speed. The aggressive value would be
0.8*750=600RPM
Multiplying with 8mm/rev lead:
600*8=4800mm/min
If you run at 9000mm/min you are 87.5% higher than the max recommendation, 50% higher than the critical speed.
The bottom line is everyone is free to experiment, and most of us have learned great lessons by breaking stuff. But, please do not encourage others to do so without some sort of warning or “at your own risk” note 