Using a ballscrew on the Z axis

[uspl]

Hi all

I am about to start on a new build, previous units have been threaded rod, then ACME, now I have the budget to go with ball screws.

From my reading I find that ball screws are not self locking, so if used vertically gravity will pull the router head down and the screw will free-wheel (assuming of course the motor is not powered)

Am I correct in this understanding and if so what can I do about it ?

My thoughts are:

• Dont use ball screws, use ACME for Z
• Make sure the motor is always powered, a bit hard
• Use some sort of electric clutch or free wheel inhibit

Any advice please ?

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[hub]

I have a 5mm pitch ballscrew on my Z axis. No problems when powerd off, the stepper has some resistance which is enough to keep it from moving, at least in my case. The weight of my Z axis is a bit over 10kg.
I think it might be a problem if the pitch was 10mm for example..

[RICHARD ZASTROW]

Motor with a brake?

Dick Z

[Bear5k]

From my reading I find that ball screws are not self locking, so if used vertically gravity will pull the router head down and the screw will free-wheel (assuming of course the motor is not powered)

Am I correct in this understanding and if so what can I do about it ?

Things influencing the ability to back-drive or "free wheel":
- Preload on the ball nut
- Lead length
- Weight of the moving assembly on the Z-axis
- Inductance of the servo or stepper motor
- Brakes/Springs designed to counteract gravity

Personally, I'm taking a "build it and see" approach. I've got a fair bit of preload on my Z-axis ball nut and will probably go with a pretty beefy motor, so I may get away without having to add springs. Even if I have to add them, I prefer that given the other advantages I have (like going with a pre-built Parker Daedal actuator).