Penumatic Flow Control Valve

[Jon Lawton]

Okay... the pneumatic manual clearly states that the flow control valves should be put *directly* on the cylenders. However, our mentor from Festo last year told us to put the flow control valves on the output ports of the solinoids. This set-up works... but... is there some reason why the manual says we should put them on the cylenders?

[Lloyd Burns]

From the good sense standpoint, putting two flow controls directly on a cylinder defies reason, which is not altogether out of the question under the circumstances.

A flow control on end A controls flow into A, and slows the piston movement away from A. However, when air goes in B, the flow control on A slows the escape of the air in end A, and this slows the movement of the piston toward end A.

Having another flow control on B is redundant, which you can say again, because it is useless. You will spend all day adjusting B, only to find A is still controlling it.

The only good way to control motion towards A is to have the flow control in the exhaust port connected through the valve to A. It will act only when A is exhausting, and won't throttle air flow in and out of B. Another flow control on the other exhaust port will not come into play until the motion is towards B, and this control will not affect A.

The pneumatics manual says on page 5 at the bottom "Always mount these into the ports of the cylinders before you hook up the tubing." This line is more intelligent if it is read, always screw these valves in before attaching the tubing(, because the don't swivel like the "L" push-in connectors).

Or perhaps the ports of the 'cylinder' is a transposition for 'valves'.

[Gary Bonner]

The flow control valves only limit flow in one direction. They're kind of like a one way valve, but but instead of completely stopping the flow in one direction, you can adjust it. When you mount them directly on the cylinders, flow into the cylinder is not restricted, only the exhaust. The speed of the piston is then limited by how fast the air exhausts, and extend and retract speeds can be individually adjusted. They would work attached to the valve, but the action of the piston under load would be more uniform if the valves are on the cylinder.

[Lloyd Burns]

Thanks for supplying missing information.

[Jon Lawton]

Originally posted by Gary Bonner
The flow control valves only limit flow in one direction. They're kind of like a one way valve, but but instead of completely stopping the flow in one direction, you can adjust it. When you mount them directly on the cylinders, flow into the cylinder is not restricted, only the exhaust. The speed of the piston is then limited by how fast the air exhausts, and extend and retract speeds can be individually adjusted. They would work attached to the valve, but the action of the piston under load would be more uniform if the valves are on the cylinder.

Okay... this makes sense... but why is the action of the piston under load more uniform if the valves are on the cylinder?

[Lloyd Burns]

I made a demo board up in December that used a flow control from a previous year. I attached the flow control to one exhaust port on a 5-port 2-way single solenoid valve, rather than directly onto the cylinder. The valve slo-o-o-wly extended, then snapped shut like a mouse trap.

If the flow control only acts as a control for air going from the push connection to the screw in, and acts as an open tube for air coming in the screw in, and flowing out the push in, then the above could never have worked: the air was going from screw in to push in, and was flow controlled.

Obviously, this valve didn't work the way someone has suggested earlier in this thread: are this year's flow restrictor valves different from those issued in the past ?

[Gary Bonner]

I haven’t hooked them up yet, but this year’s flow control valves look the same as last year. They limit flow going in the treaded end of the valve. If they’re mounted on the cylinder, they limit exhaust from the cylinder; if they’re mounted on the solenoid valve, they limit airflow into the cylinder.

When there’s no load on the piston, the only force acting to move it is the airflow into the cylinder. If you regulate this flow (flow valves mounted on the solenoid), you regulate the piston speed. However, if there is a load on the piston, both the force of the load and the inflowing air are acting on the piston. If the exhaust is not limited, there is nothing to keep the added force of the load from accelerating the piston. Mounting the flow valves on the cylinder limits how fast the air can escape, and maintains a higher pressure on the exhaust side of the piston. The speed of the piston is then limited by the how fast the exhaust air can escape, regardless of load.

[Lloyd Burns]

So what is the difference between mounting them on the cylinder exhaust and mounting them on the valve exhaust ? They're in the same leg of the circuit. and have the same direction of flow through the valve.

And they seem quite similar to last year's. and you cannot blow (ie at less than 2- 3 psi)either way. The checkvalve action you described (closed in the direction of flow control, open in the opposite direction) ought, IMO, to introduce no great amount of back pressure; yet I was able to operate the demo board (see above) at 30 psi with my circuit.

I don't need to worry about pressurized tubing being stiff, but I do worry about losing a piston when a vandal breaks the flow control off in an exposed position.

Anyway, we're both close to having the circuits on our machines ready to test the ACTUAL operation of the valves under higher operating pressures.

[Lloyd Burns]

To clear up any misconceptions, I put a push-on and a tube directly on the compressor, and connected the flow control, which was turned completely SHUT. I started the compressor, and the (obviously unbypassed) pressure blew the flow control off the end of the tube. P o o o h !

Having previously screwed the flow control into the A port on the compressor, and turning the Thomas on, I fiddled with the knob of the still quite effective needle valve. The pressure bled out (unbypassed) through the valve.

I conclude that the flow restriction is bidirectional, not only in one direction.

[David M Johnson]

Flow controls have a restricted flow path and a "better" flow path. Technically, both are restricted... but one direction is adjustable by means of the adjustment knob. The can be used to meter the exhaust flow out of the cylinder or to meter the supply flow into the cylinder. Usually, the meter out direction is selected. They can be placed nearest the valves or at the cylinder... but there may be a difference. If placed at the valve, the flow control will meter all the air in the tubing as well as the volume of the cylinder. If the tubing volume is substantial, the cylinder may reach end of stroke without any slowing at all. If used in a vertical application, I would suggest trying meter in for a slow extension. Flow controls are very sensitive and can provide slow extensions if used properly... but will, in all cases, serve to slow the cylinder (acting as a resistance to flow). Observe the arrow symbol on the side of the component --- larger arrow points to the better flow path.