designing amoving gantry

[veteq]

Hello people,

on a balscrew there may not be radial forces, if i design it like the thumbnail below.
will there be a moment or radial force on the ballscrew or will the closest rail take the biggest moment and force.

Other machines hav the ballscrew below the table with a construction that connect to both rails. In my opinion there is also a moment on the rail only then on both rails.

would love some feedback.

Kind regards,

Roy Bakker.

[kaye7877]

It looks like your carriage is pretty short in the direction of travel. I assume you have two THK/NSK/INA linear bearing pillow blocks on the carriage? I've designed some machinery like this in the past, for special applications. None of them were machining applications. All of them were very low force applications and I took special precautions to insure a crash did not destroy the system. It will in this config if you are not careful.

Note that come to mind.
You will need to keep the forces pretty low in the middle of the carriage.
A crash will put huge forces on the bearings.
Linear bearings work surprisingly well this way if the forces are low!!! But, you MUST protect the system. That ballscrew with minimal torque will put huge forces on the bearings.
When figuring allowable carriage forces pretend the far bearing is held in place and apply motor torque to the ball screw. Maybe motor torque x 2 or 3.

[veteq]

Hello Kaye,

The drawing is just a set up to show the construction type, i drawed it in a few minuts. The dimensions are not real.
The machine is going to be a mill only for mill aluminium.
I want to make a high speed machine working with passes that take off a real small volume every pass. (max 0.5mm by 0.5mm)
Offcourse there going to be 2 wagons per rail and 2 rails, i want to use the NSK astro 500z for spindel (picture), it puts out 50.000 rpm and has 270 watts.
So the milling forces are real low.
The forces that the ballscrew cann deliver when a servo motor is applied on its are max. 5500 N (peak- 1400N rms). So the distance off the force to the rail is 0,1 meter.
Thus the torque applied on the rail is max 550 Nm

When i look at the specs from a THK 20L rail with 2 wagons mounted it cann take a torque on that axle from 1400Nm.

Notes:
If i understand it right the rail closest to the ballscrew will take the most beating from the forces/torque. (could over design it??!!??)

When there is a lot off torque on a rail is there still the smooth gliding?
what happens with the friction coefficient. Is there a way to calculate the resistence force.

Are there more people here thinking off making a machine like this??

Kind regards,
Roy Bakker.

[kaye7877]

I don't know that I would put the screw outside the carriage rails if I were planning to do machining. Of course if the actual geometry could make a huge difference.

In the systems I have experience with, there is still smooth gliding, but the deflection is quite large.
The friction coefficient is very low, something less than .01. Make sure to consider the bearing seals as they are significan in a system with a light carriage.

I wouldn't put the ballscrew outside the rails and my bearing spacing would no less than a ratio of 2:1 carriage width to bearing spacing.

Are there more people here thinking off making a machine like this??