Gearhead selecting

[bullethead67]

I intend to buy the kelling 495oz nema 23 stepper with a 3/8 shaft. How do I go about buying a gear head. Not the ratio but how do i know it will bolt up without some kind of adapter. I have read a little about GN style replacing GK style and that i should make sure that the motor and gearhead match. So are steppers GN? Do steppers used with gearboxes have a special shaft? can i just buy a gearbox somewhere that will fit onto a nema 23 stepper??

[jcc3inc]

Sir,

Gearboxes are expensive, at least the good low backlash ones. Have you calculated the force you need as well as the max speed at which you want to travel? I used a direct drive off my small steppers (10X microstepping) and they work fairly well. My pinions are 1/10 inch circular pitch, 2.000" circumference. If I needed a lot more force, I'd consider belt reduction as a simple way to get what you need.

One might be able to purchase decent gears and make a single stage gear reduction. Of course you have sealing problems and need careful design of the gearbox to minimize backlash. We did make a fairly large gearbox where we had the motor on a slotted plate to adjust out the backlash.

Regards,
Jack C.

[Torchhead]

The problem with direct drive to a pinion gear is that you sacrifice both torque and resolution. Given that microstepping is not true resolution gain (it quits being effective at higher rpms) you are stuck with the 200 steps per rev of the motor. With a 1" DP pinion you move 3.14" per rev. Your effective resolution is then 3.14/200 or about .016". With a 2.000 pinion it's a lot worse. You can never be more accurate than the resolution and usually other factors like backlash and mechanical issues, make the accuracy a lot less. You also end up with 1/3 the torque so now your 495 oz-in motor is down to 165 oz-in. Considering steppers lose torque at RPM you are operating at double digit torque numbers. Why worry about torque? You need torque for low end acceleration...you need acceleration for sharp corners. The heavier the gantry the more torque you need. Starting and stopping the mass of a gantry even with a plasma torch takes torque (not speed). Instantly changing direction or cutting small circles requires more torque than straight cuts.

We find that a 3:1 belt reduction (easy to do with existing toothed pulleys and belts) gives you back the torque and resolution you lose with the pinion drive. You can also fashion a motor/drive pinion mount that can swivel and be spring loaded to keep the pinion better seated in the rack.

With the motor directly to the pinion you will get astounding rapids but you will stall the motor before you get there. Direct coupled you should be able to achieve 2400 IPM rapids! Speed you can't use is wasted. Better to trade it for torque and resolution you can use. If you can rapid at 300 IPM and cut at 200 or less with decent torque you have the makings of a good plasma cutter.

I know all the arguments about belts wearing out (they cost a few bucks) but it's over stated. Modern belts are tough and last a long time.

Gearboxes that have minimal backlash are pricey. If you have no way to build a belt reduction mount then even the gearbox approach may prove to be a challenge.

Tom CAUDLE
www.CandCNC.com

[jcc3inc]

Torchhead,

"You also end up with 1/3 the torque so now your 495 oz-in motor is down to 165 oz-in.-"

I don't think the motor torque has changed because of the pinion size. Did you mean FORCE?

Most of my experience has been with DC servos; however my small test bed machine with direct drive off the stepper motor pinion, 200 step, 10x microstepping, does well in terms of resolution (.001"/step) and acceleration. One can compare torque at 3X speed (3:1 belt ratio) with the available torque using direct drive. Also, one loses some torque in belt drive arrangements. If I had my druthers, I'd take a gearbox as the preferred method of reduction.

Regards,
Jack C.

[bullethead67]

ok i found this http://www.robotics.com/pr23.html and i like it. I am going to use kellings 425 oz motors. I think the ipm with a 2:1 reducer would be 274.75 using 200 max rpm and a .875 dia pulley. resolution would be .0068 i think. is that an acceptable res?

[Torchhead]

Originally Posted by jcc3inc Torchhead, "You also end up with 1/3 the torque so now your 495 oz-in motor is down to 165 oz-in.-" I don't think the motor torque has changed because of the pinion size. Did you mean FORCE? Regards, Jack C.

I misquoted myself....I should have said "effective torque" Since Stepper torque is at holding torque (pretty useless number when the objective is to move the load) the pinion gearing causes 1/3 of that to be converted to speed. So it gets faster but with less mechanical force to move the gantry.