Larry Jaques wrote:
On Thu, 4 Mar 2010 14:31:33 -0500, the infamous "Buerste"
scrawled the following:

Damn inertia!

I have an air cylinder, 1.5" x 4" stroke, that pushes a crank on a shaft to
rotate the shaft about 90 deg. On the shaft is a gear with a one-way
clutch. The 4" x 1/2" gear turns about 3" at the perimeter per stroke of
the cylinder. By moving the link to the air cylinder up or down on the
crank the gear will rotate more or less per stroke. A second gear is meshed
into the first gear with pressure from another air cylinder. Wire is fed
between the gears in a grove cut in the face of each gear. The idea is to
feed wire 3" per stroke.

What kind? Mangrove? Orange grove?


Is works perfectly and fast! By varying the pressure in the cylinder that
presses the gears together, the wire is gripped very well at 60 lbs air
pressure and will slip through the gears easily at no pressure.

The only problem I have is that the gears over-run and feed too much wire.

That doesn't sound quite like "perfectly" to me. g


I've considered lightening the gears by drilling a bunch of holes. Also, a
drag brake on one or both gears with screws pressing brass pucks against it.
Another idea is an air limit switch that is struck at the end of the stroke
by the crank. This switch would unload the pressure on the clamping
cylinder and let the wire slip between the gears when they over-run.

Would the air limit switch work fast enough to unclamp the wire in time?
Would I be better off with an electric switch and valve? I might have to
lighten the gears, brake them and switch off the air clamp The feed has to
take place in 300ms.

Why not feed 'em with stepper motors turning the gears, Tawm? That
would be much more precise than having to maintain exact feed tension
and maintain split-second air timing unless you're actually ratcheting
the gears. That design would make for a high-maintenance situ, tho.

This is kinda sorta the absolutely worst sort of application for
steppers. High acceleration = high torque; steppers don't produce much
torque for their size or power input compared to DC motors, and when you
ask for more torque than they can deliver they lose their place
completely. So pushing a stepper for more torque leads to reliability
issues -- you can sacrifice great chunks of an otherwise enjoyable
engineering career to trying to make an undersized stepper reliably
accelerate and decelerate.

Besides, by the time you get a stepper and a stepper controller designed
in, you could well have designed in a brushless motor and a servo
controller -- and with careful design, the only thing that happens if
your brushless motor lacks oomph is that things slow down a bit.

There are applications for which you'd be crazy to use anything other
than a stepper, by the way -- if you've got space and power to burn, you
only need moderate precision for the speed, and tuning a servo motor may
be a questionable exercise, steppers are superlative little gems. But
"go from point A to point B damn fast and stop" is not a strength of a
stepper, in my experience.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com