Question about using rack and pinion drive

[kpoindext]

I have a question about rack and pinion drive systems.

The mount seems overly complex to me. let me explain. A gantry typically rides on a set of upper and lower rails via ball bearings. There is almost no play between the rails, so the gantry has no room to move, except along the axis. So if a rack is securely mounted to the rail, and the motor has the drive gear directly mounted to the motor shaft, there is no place for the rack and pinion to separate.

So why have a special plate with a spring to keep tension on the rack. I realize there is a gear ration change using the belt, but I would think I could do the same thing with the stepper motor driver control.

So the question is , Would my idea for directly connecting the motor shat gear to the rack work? Why or why not? Pros and Cons?

Thanks,

Kim

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[coherent]

I have seen machines with the stepper mounted pinion gear directly connected to the rack driving the machine. I have also seen the motor driving an "axle" with a pinon gear on each end connected to/driving the axis using 2 racks. They work ok with machines that don't have a need for lots of cutting torque requirements like a spindle would have. I'm referring to a machine like a plasma or laser that doesn't actually contact the material. The belt drive rack and pinion systems normally have a pulley ratio (like 3:1) that increases the torque considerably. Also the inclusion of a belt affords something to "give" (i.e. the timing belt breaking) instead of other damage that may occur with direct drive system in the event of a crash.... but torque or machine speeds is the common reason. The spring mounted setup also offers some give and take regarding alignment or rack precision issues, whereas a solid, direct drive has little room for misalignment or variations of gear fitment or variations along the travel/length of the rack mating surface. It's also something else that will "give" (for example debree on the rack) instead of damaging the machine components.

[kpoindext]

Thanks, I hadn't even considered the debris issue. I just need to figure out if there is a way I can mount the pulley system on my machine.

[ger21]

So why have a special plate with a spring to keep tension on the rack. I realize there is a gear ration change using the belt, but I would think I could do the same thing with the stepper motor driver control.

Because without a spring, the rack would need to be mounted parallel to the linear rails withing about .001". Any less, and it binds. And more, and you have backlash. Commercial machines use a fixed pinion, but it generally requires machines mounting surfaces to get the rack parallel.

And changing microstepping on a drive is not the same as a gear or belt reduction. Direct driving a pinion on a stepper is a bad idea.

If you can get the rack mounted perfectly, not using a spring is a better way to do it. Preferably mounting the stepper to a gearbox, and the pinion on the gearbox.