On Thu, 4 Mar 2010 14:31:33 -0500, "Buerste"
wrote:

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.

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.
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.

I'd say you need to do better at defining your objective without
trying to preserve ego invested in design thus far. The only
objective I see stated here is to move wire 3" in 300 milliseconds. I
suspect that there are further unstated constraints or strong wants.