Suspend those pesky physics laws!

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.
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Buerste writes:

The flaw of your design is that you do not control acceleration and deceleration, which not only is brutal, but prohibits precision control such as you describe. Curing an excess deceleration problem by braking and adding more deceleration is not a sound approach. You are controlling position mechanically, and perhaps the second derivative of velocity via air pressure, but not the second derivative. You are adding excess energy that is mechanical noise showing up as precision error.
Figure out a way to decelerate evenly at the end of travel, so your mechanism comes to a controlled, smooth stop. You will get better precision, you will use less energy, and your mechanism will wear slower and last longer.
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The obvious method would be to use a servo and program in the ramps. Too expensive at this point, maybe in the future.
I have considered a hydraulic damper near the end of stroke to bleed off the energy and not abruptly shock the system. But you see why if I unclamp the wire, because it has so little mass it's momentum is negligible. The trick I see is unclamping it at the right time so it coasts to a stop at the right place against the steel block at the far end.
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Buerste wrote:

So the system is over constrained, with the wire both being held by the roller and by the steel block? Ooh, bad you.
If the wire's momentum is negligible, can you adjust the tension on the clamping gears so that as soon as it hits the block it stops? Or do the gears fly back a bit after they stop at full extent?
I think a shock absorber on the cylinder (not 'damper', not for this, in the US at least -- believe me, I've been living the terminology for the last few months on That Damned Gate Project) would work quite well, particularly if you could arrange for it to have a bit of a progressive action. You can get little screw-in shock absorbers that are spring loaded, damp when something lands on them, then return to 'out' when you retract whatever it is that is pushing on the thing.
I'll bet you could select one of those that would work quite well, particularly if you put a spring on the end of it so that deceleration force is progressive rather than getting applied suddenly.
Alternately you could valve the cylinder so that as it gets close to full extent the feed gets choked off -- I couldn't tell without doing the math, but it seems like there'd be a 'right' combination of cut off position and residual valve opening that would start bleeding off the speed while still providing enough flow to complete the stroke at an appropriately slow rate. In theory this would act exactly like the shock absorber I'm suggesting, just using the cylinder as the damper and the air as the working fluid.
--
Tim Wescott
Control system and signal processing consulting
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Buerste wrote:

Control the clamping with a spring and a cam so its based on the position of the feed. It sounds like you are having problems controlling the air driven clamping scheme timing.
--
Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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Can you just slow it down a little by restricting the airflow into the cylinders?
----- Regards, Carl Ijames

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Both cylinders have speed controls on both ends. On the bench, at slow speed, there is no over-run. But I have a 300ms window to feed.
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wrote:

Ball screws work very good at this if combined with a PLC and sensor
Gunner
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Gunner Asch wrote:

But how to meet the 300ms travel time with just a PLC for control?
--
Tim Wescott
Control system and signal processing consulting
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Tom
If I understand your application correctly, this just screams for a servo motor.
I used to work with Parker Compumotor products. They have unbelievably top notch customer service. I'd at least give them a call. Programming one of these compumotor units is very simple.
Karl
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I figure about $2500-$3,000. The guys at Carlson Tool (they make brush machines) do a lot of feeds with servos and mechanical methods and they have servo set-ups that could work. You'd be surprized how beefy it has to be. My original design uses a servo, but why stop there...the "X" and "Y" axis on the table might as well be servo driven too. That works great for brush machines that change set-ups often but this machine never will, so cam actuation works just fine and is oh so simple.
I don't mind spending the money but I'll explore the mechanical first.
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Buerste wrote:

If these are 3" cuts, separate the operations. Cut, pieces fall into sorting/stacking bin, a different machine feeds the cuts to your existing machine.
technomaNge
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That's how it's done with round wire. With this flat wire, you can never lose control of it.
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Buerste wrote:

Are you sure about that? I've seen some pretty clever mechanical means for feeding and orienting small parts. I'm guessing there's something proprietary about the process, but if you could reveal a bit more detail, that might be helpful.
One thought I had would be to feed through rollers with one way clutches to prevent the wire from slipping backward. Then feed through a set of jaws that work like those on strapping tape tension tools. These jaws would be on a linear slide, to be reciprocated by a crank. The mechanics of the crank will provide deceleration at both ends of travel. Stroke length easily adjustable in the same fashion used by shapers. Either an electromagnetic clutch or a servo motor to drive the crank, if there is need for dwell between each feed cycle.
Jon
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That basically describes feeder #2 but it's driven by air not a motor. I'll post pix of some of the feeders we still have along with the new one.
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Buerste wrote:

Would love to see a picture of #2. If I get what you're saying above, you were using an air cylinder to advance the one-way jaw in a linear fashion? If so... I'm talking about a crankshaft and connecting rod type setup. Though the crank rotates at a constant velocity, the linear output slows to a smooth stop at each end of travel.
But a picture's worth a thousand words... I'll watch for your posting of them.
Jon
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Bimba makes air cylinders with adjustable internal air cushions. Maybe if you used one of those to cushion the end of the 1.5 x 4" cylinder's stroke, it might be enough to slow the wire feed gears enough (with the one-way clutch still engaged) so they don't freewheel. http://www.bimba.com/Products/OriginalLineCylinders/AirCylinderswithAdjustableCushions / The largest diameter cylinder in the above link is 3", but you might ask a rep if he can supply one similar to the air cylinder you're using.
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wrote:

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.
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Hmmm, the wire needs to be in place and in control to be cut in the 300ms window, that's all. This is the third wire feeder design in ten years. We've had to use different wire manufacturers over the years as some plants have closed and the qualities of the wire change...this or that feed doesn't work on this or that wire too well...etc. This feeder is trying to address the wire that has too much oil, too much scale, lead drag, ect. The wire manufacturer can't fine-tune their quality any better and they are the only domestic supplier. We also use a Taiwanese supplier but their QC and lead-time is worse. Those are the only suppliers in the entire world! And, there are only 4 customers in the world and we are by far the largest user.
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wrote:

If the problem is too much oil and crap on the wire.... Solve it! Don't raise the drawbridge, lower the river.
Howzabout putting a cleaning station on the unspooling device?
Give the machine the despooled wire after you run it over a series of counter-rotating cleaning rollers (short-nap paint roller covers, cheap in bulk and easily replaceable) that are charged (and flushed) with a small pump and a recycled mild solvent.
Think a wire straightener rig with the rollers staggered over-under, and a gear drive to spin the rollers at maybe 5 to 10 RPM. With a catch-pan under the whole thing.
The last few rollers can have a stripper wiper or a spring loaded rubber brayer roller pressing on them to extract the solvent from the roller cover, then the paint rollers absorb the excess solvent from the wire as it goes over them.
You'll need an oscillator motor and eccentric to move the wire route sideways through the rollers during the day to even out the wear.
(You're welcome. ;-P Run with it and see if you can get anywhere. If it works, I'll give you an address for a little "Care Package".)
--<< Bruce >>--
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