Z-Axis and Ball Screws

[samualt]

I would like to use Ball Screws for my Z-Axis, mainly because of less backlash. However, unlike Acme Screws the Ball Screws will evidently let force on the nut turn the screw! (Backdriving).

Question:
So, if you mount a Ball Screw on the Z-Axis won't the weight of the router and other parts cause a continuous motion downwards because of gravity and try to turn the screw? Wouldn't your motor always be having to apply power upwards to counteract that?



From Nook Industries:
-----------------------------------------
BACKDRIVING
Normally, ball screws are used to
convert rotary motion into linear
motion. Backdriving is the result of
the load pushing axially on the
screw or nut to create rotary motion.
All ball screws, due to their high
efficiency, will backdrive. The
resulting torque is known as
“backdriving torque” and is the
torque required to hold a load
in position.
CAUTION - When using ball screws,
applications should be analyzed to
determine the necessity of a brake,
especially when the possibility of
injury may occur.

[High Seas]

Ahhh - thats where the "holding force" on your stepper motor comes in! At least thats what I figure - thought I'd have that problem too and was going to add a pin to "set" the router in place - but haven't done that yet.
Jim

[balsaman]

The motor will keep the screw from backdriving. Yes there will be power at the motor at all times.

Eric

[limbo]

Your stepper needs to be powerfull enough to lift the Z axis, so it will certainly be powerfull enough to hold it under the force of backdrive. The way to use smaller steppers is to counterbalance or use a spring, not by adding static friction as in acme thread.

greetings,
John

[limbo]

Your stepper needs to be powerfull enough to lift the Z axis, so it will certainly be powerfull enough to hold it under the force of backdrive. The way to use smaller steppers is to counterbalance or use a spring, not by adding static friction (as in acme thread).

greetings,
John

[limbo]

Something went wrong, accidentely posted twice but it doesn't let me delete the second one.
Admin, help??

[pack rat]

I am not shure, but may be he is asking when the power is off. That was my first impression, I am curious too.

[anoel]

The "ballscrew lover's" solution is to use a motor with a brake. That way when the power is off, the brake is engaged. If you'll pay attention to a lot of commercial designs, they will use a precision acme screw on the Z to keep it from free wheeling downward when not powered. If they use a Ball screw, the motor will have a brake.

[limbo]

Ah! Now I understand. Didn't think about power off. In my Zaxis theres a spring that is stronger than the weight of Z. Would be a funny sight having the Zaxis fall upwards.

John

[HomeCNC]

My Z axis on my router is powered with a .2 pitch ball screw and a servo is running it. I have the ratio of 2:1 and with the 3-1/4 HP Porter Cable router I just have enough mechanical advantage to hold the weight when the power is off of the servo motor.

[pack rat]

Thank God that I cannot afford a ballscrew, it would be an additional problem with design and I already have a bagful.

[steveald]

Hi,

One of the wonderful features of a vertical Z axis is that the weight of the spindle eliminates the backlash in the screw. If the detent torque of the motor isn't enough to prevent backdriving, I would use just enough spring to hold the axis in place, but I wouldn't balance it perfectly as that would introduce backlash back into the system.

Steve