Servo brakes and constant torque heat losses

Generic Default · May 22, 2019

It seems like on most newer (not ancient) cnc machines, there are no weight counterbalances for vertical axes and the servos for these axes just use a constant torque output to keep the ballscrews from back-driving, which would crash the spindle or turret into the table or mechanical stop. The servos have brakes on them to hold stuff in place when machines are off or E stops are active.

I'm curious about how much energy is used since the brakes are apparently solenoid driven and must be energized in order for axes to move. Does anybody know the wattage these servos use when the brakes are not engaged? And how much power does it take for the Z servo to hold up a typical VMC headstock?

Ziggy2 · May 22, 2019

The power required to hold the servo on position is a function of the ball screw pitch and the mass of the head assembly if you are talking Z axis.

The reason that machine tools have gotten rid of the counter weight is that the counter weight inertia limited maximum acceleration rates.

The power the servo brake consumes in holding the break off is basically minor and of little consequence. Eliminating the counterweight and using the servo motor to hold position is uses less total energy while in motion since the total mass being accelerated is significantly reduced.

Milland · May 23, 2019

Ziggy2 said:
The power required to hold the servo on position is a function of the ball screw pitch and the mass of the head assembly if you are talking Z axis.

A third, smaller determinant is the screw pitch diameter. That, along with pitch determines the helix angle, which sets how easily the screw will back drive. A still smaller factor is system friction.

number 2 · May 25, 2019

The Mazak VMC at my last job had about 50% Z axis load just sitting there. I kind of felt Mazak could have done better there as when hard drilling load come to near Zero telling you nothing about how the drill is performing.

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Servo brakes and constant torque heat losses

Generic Default

Aluminum

Ziggy2

Stainless

Milland

Diamond

number 2

Hot Rolled

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