YEP Dan has it right. Torque is the measure of FORCE. HP is a measure of work over time. HP is a calculation and torque is a true measurment.

This is semantics. You are just saying the same thing in a different way. I don't know how you define a 'true' measurment. If you put a motor on a dyno, you are measuring toque and you are also measuring speed (time in the equation).

However I will add that torque is also a static force. I doubt you have a way of directly measuring torque in a practical way with a machine spindle, However, you CAN use the motors HP power curve to calculate the torque at any point. Since torque is what generates the stress on the mechanism (which I think was your point), then you can assess the effect, but you would get the torque out of the HP calculation . . which is linear.

If the manuf provides HP curve and also the torque curve, then there is no need to know the formula and the relationship is irrelevant, but inquiring minds may still want to know how it all fits together.

Another part of the spindle equation is RPM and balance. The inertial value of the imbalance squares as the rpm doubles making balance very important for high speed spindles. It can add to the drawbar pressure equation.

Agreed. Certainly the lateral forces rise exponentially (as opposed to linear) as the speed rises. Recall that I found two separate drawbar specs for BT30 differentiated by RPM. Incidentally I would point out that the drawbar tension is force and not pressure, but that would be nit picking, don't you think?