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View Full Version : Plain bushings OK for CNC stepper reduction unit?



alsinaj
12-09-2010, 05:25 PM
I'm designing a simple single-state belt speed reduction unit for a CNC router. Steppers will be in the neighborhood of 400 oz-in, maximum speed about 300 - 400 rpm before reduction, 75 - 100 after reduction. I'm thinking of a simple sandwich of two aluminum plates separated by spacers, with the driving and driven pulleys pinned to their shafts and captured between the plates. The machine will not be heavily used or abused. Can I get away with flanged bushings (bronze or plastic), or should I use ball bearings for the shafts?

macona
12-09-2010, 06:01 PM
They will be fine. A lot of commercial stepper gear boxes use bushings.

squirrel
12-09-2010, 06:04 PM
Bushings are sometimes the best option for an application, its hard to say. Personally I favor the simplest solution because it can be the most robust and cost effective.

If you have dirt a double sealed bearing would be the wise choice.

whitis
12-09-2010, 07:36 PM
I'm designing a simple single-state belt speed reduction unit for a CNC router. Steppers will be in the neighborhood of 400 oz-in, maximum speed about 300 - 400 rpm before reduction, 75 - 100 after reduction. I'm thinking of a simple sandwich of two aluminum plates separated by spacers, with the driving and driven pulleys pinned to their shafts and captured between the plates. The machine will not be heavily used or abused. Can I get away with flanged bushings (bronze or plastic), or should I use ball bearings for the shafts?


Bear in mind that a belt or gear reduction unit not only adds play to the system but may not give the desired results as far as torque. In some cases a 4:1 reduction produces zero additional torque compared to running the stepper motors at 1/4 speed. More typically, you may only get half the torque you expected (i.e. a 2:1 vs 4:1 torque increase). Of course, you are talking about very slow speeds; these speeds are rather dubious (i.e. likely to be inadequate) for a machine tool or CNC router. Also, the oz-in rating on a stepper is normally holding torque, not at operating speed.

If you are trying to use an old surplus stepper (or new with old design), bear in mind that these may be dogs compared to newer models. You can get steppers with considerably more torque and speed, now. Here is one supplier: http://www.kelinginc.net/StepperMotor.html

Also, why are you asking about bushings bearings on your belt drive system? The motor should have ball bearings built in, as should your leadscrew support or pinion support. Surely you are not trying to build a separate gear reduction unit that has to be connected via couplings to the leadscrew and motor.

alsinaj
12-09-2010, 10:01 PM
Whitis:

The drives are rack and pinion, the pinion DP being about 1", resulting in about 3" of travel per pinion revolution, hence the need for speed reduction/torque multiplication. Some designers have fudged this by using up to x16 microstepping. I mistrust high-multiple microstepping, and would rather get my reduction mechanically. But I'm open to being educated.

macona
12-09-2010, 11:48 PM
You are going about it the right way. Microstepping increases the amount of steps but is not accurate. It is primarily used to get smoother motion at the cost of reduced torque. We run 10x micro on the camera gantry rigs at work.

whitis
12-09-2010, 11:50 PM
Ok, that would give you a fast traverse speed. Nothing wrong with 16x microstepping (and it reduces resonances), however, there will be some pure magnetic elasticity in the motor (which will be the same whether you microstep or not) which you are reducing in the belt drive while at the same time adding some not-so-pure elasticity and backlash in the belt drive.

At 400oz*in geared 4:1, you would have about 200lbs of force. However, at 400rpm, or 2667 half steps per second, one random stepper motor might have about 1/3 of its holding torque or 66lbs of driving force. If you have no friction on the slides, belt drive, or rack and pinion, and no mass to accellerate, this is just barely enough to overcome the typical cutting forces from a 1/4HP spindle. Another random motor might have 2/3 of its holding torque at that speed.

Another way to provide a similar gear ratio is with a ball screw.