Pneumatic parts catcher moving too jerky: small parts can shake out

[MadMax]

I've just installed a parts catcher in my Puma 200. I've fiddled around with the little needle valve adjustments to try to tune the motion. My problem is that my parts catcher jerks downward a little too fast when it's retracting which can cause the part to tumble out. Even with the needle valve fully cranked down, the arm drops too fast before the backpressure on the opposing side bounces against the powered side.

Has anyone else had to deal with this? I'm thinking about adding an oil damping piston to the rack and pinion assembly to deal with this issue, but I'm hoping there are easier fixes out there.

[MadMax]

Actually, now that I think of it, I think I could add a needle valve in the inlet side which pushes the arm down to slow the pressure buildup when the arm starts driving downwards. The check valve on that side only restricts when the arm is going upwards.

I just had an epiphany. The upper side of the piston is the side which drives the arm in the retracting direction. The lower side has the rod throughbore so it has a lower effective area which I think is what is causing my jerky drop. Even if I fully crank down the lower end needle valve, I still see a rapid start to the retract before the arm pushes against the backpressure. I think it's because of the difference in piston area on the top and bottom side. Throttling the inlet pressure on the top side should fix this I think!

[JLSD]

Hi,
Never had anything to do with parts catchers, but plenty with pneumatics.
Conventional practice is to throttle at the exhaust side, and the restrictors are usually one-way (free flow in, metered out)
Normally, on a direction change, the exhaust side should still be at pressure from last move, and the restricted outlet prevents a rapid take-off.
Hope this helps,
John.

[MadMax]

Yeah, the cylinder has the check valves and restrictors, but even with the throttles cranked down on the exhaust side, the cylinder still lurches down. It doesn't bottom out and slam down. Instead the arm jerks downwards and bounces against the throttled exhaust like an overdamped system.

I think it's because pneumatic cylinders are not balanced. The back side of the piston has a larger effective area than the side which pushes on the rod. Basically the rod side of a piston has less area because the air pressure does not act against the piston where the rod sits. The cross sectional area of a piston rod is subtracted from the piston (on the rod side) which results in a lower net force acting on the rod side of a piston.

If you pressurize both ends of a double acting cylinder to the same pressure you get a undamped extension. Basically the rod extends at a force determined by the rod area X pressure when both sides are pushed to equal pressure. With check/throttle valve assemblies, air is trapped in the depowered side instead of escaping freely. In the situation in my part catcher, I think the force imbalance on the non rod (NR) side of the piston is compressing the air in the rod side to higher than the inlet pressure on the NR side. Basically the pressure-area difference is balancing out to equal force by compressing the R side which results in an initial jerk.

Neat thing about the conventional throttle/check valve arrangement is that the piston acts in an approximate equal force mode which results in no acceleration while the arm is moving. Unfortunately, when the arm starts dropping, the area difference between R and NR side results in a net force so the arm accelerates until the pressure increases in the R side which is probably what's causing the bounce.

Ideally I need to restrict the inlet flow in the NR side so the restriction in the NR input is a bit more restrictive than the exhaust restriction on the R side which will prevent the bounce altogether.

Not hard really. I think I can do this with another throttle/check assembly inline with the existing one on the piston NR inlet.

[BAD DOG]

JUST HAD THE SAME PROBLEM,,,,, SAME MACHNE,,,,, SLACK IN THE CHAIN,,,,,

BAD DOG