[pointcloud]

Does anybody have an explaination for why the screw always turns and not the nut? Question for both single and doubles?

[SORCHEROR]

there are some rotating ballnuts designs out there,extra long screws benifit from rotating nut,eliminates screw whip at high speed,the reason most rotate the screw is easy,lower cost,easier to design and build
steve

[pointcloud]

So you can not just use any ball nut ?

[SORCHEROR]

most any screw and nut can be used in a rotating setup,some require more work,depending on how they mount.
the nut must match the screw of course,you cant mix up nuts with different type or pitch

[pointcloud]

You ever trued this?

[RICHARD ZASTROW]

pointcloud, There are applications where a rotating nut is less expensive. If the screw is stationary & secured, you eliminate the need for the high cost of the ballscrew support bearings. However, the nut must be able to rotate in good bearings as well. Also, the motor driving the nut may be fairly difficult to access also.
If the application allows, a hollow shaft motor (large hole usually "pancake" or "Torque-motor") can have the rotating nut attached internally with keyless clamping sleeves. This way the ballscrew can have simple ends and be shorter than one with bearing seats and lockscrew threads or keyways. This will eliminate the need for coupling at the end of the ballscrew.
By judicous selection, motor bearings should be able to support the ballnut radially. Then the axial load (the principal force) can be taken by thrust bearings.
Again all the above depends on the application allowing its use.
As SORCHEROR said, there are complications etc. but when appropriate, why not?

[pointcloud]

Great thanks... ESP for the hollow motor, I have never heard of one, well not with a nut but a pump.
I am building a plasma table, and I want a longer Z than most desire. I have a router that has a 24 inch Z and the motor is mounted in top of the Z, the machine is light weight. I think it is just to much weight on that lever.

Thanks for the input
I got it all worked out...