Pneumatic Piston Problems...

We are building a pneumatic claw with both sides being controlled by a piston each. We are having problems with the double-acting cylinders. The cylinders only fire when the “T-thing” (the piece that connects the solenoid to the cylinder) is loosened, and when they are loosened the piston fires and air leaks. Teflon tape is put on the threads there. See below for the picture. Is there something that is set up incorrectly?

First off – do you really want a powered open and close? Consider rubber bands to open and only run a powered close to conserve air.

Instead of using the white flow control valves, use the gold pneumatic fittings. We had some issues with the white ones not releasing air properly and the gold ones work much better.

If I am remembering correctly, the small screw inside the “T-thing” must be loosened a little bit to fire.

Correct. The “T-thing” on the pneumatic cylinder is an “exhaust flow control valve”. These valves are adjustable, and allow air to flow into the cylinder freely, but restrict air flow on the exhaust side, which slows the action of the cylinder without affecting the force generated.

Controlling the exhaust flow provides more precise speed control of the cylinder than restricting the incoming air would do.

If the flow control valve is working properly and not leaking, it will not affect air consumption; however, if you don’t need flow control, I would agree with @mwang17 to just use a plain fitting.

are pistons stronger when pushing rather than pulling?

They should have the same force going in both directions.

Only for the nitpicky:

For a double acting cylinder, the extend is a bit stronger than the retract. When determining the cross sectional area based on the bore, you need to subtract the cross sectional area of the cylinder rod, which affects the retract force. For the small bore cylinders of VEX, it doesn’t rally matter, but when we design large fluid-flow assemblies (either hydraulic or pneumatic) using larger bore cylinders, it can be significant to the design.

For single acting cylinders, the calculated extend force must be reduced by the spring return, and the retract force is based on that spring return only.

We changed the fittings to the gold ones, but the regulator valve doesn’t work unless its loose. When it’s loose, we have air leaking out.

Is it a constant leek or just after it fires?

When the piston isn’t extended

Use a bit of dishwashing detergent in water to hunt for leaks. We use this at work:, but it’s really just soapy water in a convenient container.

Yeah, sounds like a leak somewhere. We use a silent room and our ears…