Rotating Ballnut - design ideas

[JAZZCNC]

I've read this recommendation several times but doesn't the increase in pitch cancel out the increase in torque so that the maximum linear force that can be applied to the gantry without stalling the motor remains the same? Or are you more concerned about the moment of inertia of the ballscrew itself?

Kit

Kind of yes but no, or should say maybe your misunderstanding what's being said or maybe I'm not putting it over clear enough..

Yes, 32mm pitch with 2:1 ratio will give the same linear torque as 16mm pitch 1:1 so torque is canceled by pitch difference like you say.
However, when I say Torque is doubled so the smaller motor can be used, what I'm referring to is that if 1:1 the 32mm pitch would require a larger motor to give the same linear force as 16mm pitch. So the ratio doubles the torque and allows the smaller motor to have the same linear force as 16mm pitch.

The trades off's and gains are that 32mm linear movement halves but so does rotation speed of the nut. So the pay off is that you get the same feed and torque as 16mm at 1;1 but with half the rotational speed of the nut or screw. This is the main goal to reduce whip and stress on the ballnut.

Hope that makes more sense.!

[teamloks]

HI JAZZ thanks for your insight regarding my question just talk with my ball screw supplier he suggest me to use 4040 for 3.2meter axis and 3232 for 2 meter axis..and i might follow their suggestion regarding that if budget is not constraint..anyway maybe i opt for 3232 all axis 4040 is really expensive

[JAZZCNC]

HI JAZZ thanks for your insight regarding my question just talk with my ball screw supplier he suggest me to use 4040 for 3.2meter axis and 3232 for 2 meter axis..and i might follow their suggestion regarding that if budget is not constraint..anyway maybe i opt for 3232 all axis 4040 is really expensive

Do they know you are rotating the nut and tensioning the screw.? Makes big difference.
This is the first 10x5 machine using rotating ball-nut I've built, I've built plenty of 8x4 and the difference between them really surprised me.

The secret is keeping the nut rotation speed down and getting a good tension on the screw with nice straight pull.

Good luck

[teamloks]

Do they know you are rotating the nut and tensioning the screw.? Makes big difference.
This is the first 10x5 machine using rotating ball-nut I've built, I've built plenty of 8x4 and the difference between them really surprised me.

The secret is keeping the nut rotation speed down and getting a good tension on the screw with nice straight pull.

Good luck

do you use BK/Fix unit on both end side?

[JAZZCNC]

do you use BK/Fix unit on both end side?

Don't need end bearings because not spinning the screw. The ends need to be either externally threaded or internally-threaded so you can tension the screws. I've used both methods on different machines and prefer externally threaded as can get more torque.
The force required to take Sag out over 10ft is considerable so make sure the brackets holding the ends are substantial and adjustable so can align screw in two planes.

[Kitwn]

Yes, 32mm pitch with 2:1 ratio will give the same linear torque as 16mm pitch 1:1 so torque is canceled by pitch difference like you say.
However, when I say Torque is doubled so the smaller motor can be used, what I'm referring to is that if 1:1 the 32mm pitch would require a larger motor to give the same linear force as 16mm pitch. So the ratio doubles the torque and allows the smaller motor to have the same linear force as 16mm pitch.

Aha! The point I had missed is that your 'smaller motor' required to drive a 32mm pitch screw with 2:1 pulleys is NOT smaller than the motor required to directly drive the 16mm pitch screw. In fact it may well be the same motor, which is at the heart of what I was trying to say. Thanks for clearing that up.

Is there a case for using the direct driven screw with a bigger motor? You are saving the cost and complexity of the 2:1 drive and halving the speed required from the motor, so running it in it's higher torque region anyway?

Kit

[JAZZCNC]

Is there a case for using the direct driven screw with a bigger motor? You are saving the cost and complexity of the 2:1 drive and halving the speed required from the motor, so running it in it's higher torque region anyway?

Kit

Can do either but using a larger motor usually doesn't cost less because requires larger drive and often more volts so larger PSU.
However, on long machines rotating nut wins hands down and requires a belt connection, so the ratio is no extra cost. Rotating the screw on the long machine means much larger ballscrew is required and this really starts to ramp up the costs because everything scales up with it, Bearings, couplers, motors, drives, PSU
Even then a rotating the screw system cannot achieve the feeds a rotating nut can.