On Jun 26, 11:14 am, "Tom Gardner"
wrote:
"Ned Simmons" wrote in message

...



On Thu, 26 Jun 2008 00:37:41 -0400, "Tom Gardner"
wrote:

I want to turn a 4" OD x 3/8" thick gear 80 to 110 degrees in 100 ms. I'm
thinking of a 4" stroke by 1-1/4" bore air cylinder attached to a rack gear
that
would engage the round gear. No, I won't do it with a servo! I'm thinking a
1/2" round rack gear in a tube with Thomson linear bearings and a scallop cut
in
the middle of the tube to expose teeth to the round gear. This seems simple
and
cheap. In fact, it's so simple it would seem that it should be an
off-the-shelf
item! I just assume it would be easier to do with round rack than square
rack.
Is this available or did I invent a new wheel?

I've seen a mechanism like that; my friend Jim invented it to swing
the head on a die cutting machine in order to nest the cuts and save
material. The guy who invented the rack-type rotary actuator invented
it before him. Jim had cam followers supporting the rack where it
meshed with the gear.

The gear would just spin back and forth and the air cylinder/rack will have an
adjustable stop to get my 80 to 100 degrees rotation. The round gear will
have
a one-way clutch installed because I only want the one direction of shaft
rotation. The shaft will power a wire feed mechanism to feed 3" to 3-1/4" of
wire.

100ms is pretty fast for an air cylinder which, in this setup, will
probably never stop accelerating 'til it hits the endstop . Make sure
you install bumpers or hydraulic shocks (Ace, Enidine) at the ends of
the stroke.

Presumably there's enough friction in the wire feed to prevent
excessive overrun when the gear slams to a stop.

--
Ned Simmons

We currently use a rather large urethane bumpers and they get the crap beat out
of them even though the striker is 1" in diameter.



Sounds like a need for a valve that will give a burst to get the
mechanism moving, let it coast most of the way (or continue feeding
air somewhat more slowly than full on), then once it hits the stop
give full pressure to hold it against the stop.

Maybe use a couple of valves actuated off the moving parts.


Dave