Flow control in pneumatics

Flow control valves control only speed. Positioning is a difficult job. Describe the application more thoroughly.

A similar question has already been asked. Check out http://cr4.globalspec.com/thread/5018/Pneumatic-Cylinder-Control

The application is wrong for lifting heavy loads to specific positions, you should use hydraulics.

its a weight of 400 kg

Variable positioning with pneumatics is frustrating and impractical. Multiple hard stops for pre-set positions can work though. Fail safe would be done with a rod lock.

1. A pilot operated check valve which should be connected directly to the cylinder port with a metal pipe nipple. The check function holds the fluid air or oil in the cylinder if the hose breaks or a power failure interrupts the source of pressure.

2.Air over Oil will tame any tendency of the cylinder to hunt, rise or sink when the oil is trapped within the cylinder by the pilot operated check valve.

To position:

• Manually - A person could stop the cylinder with a manual valve at any time.
• Sensors - Limit valve, limit switch, electric eye, proximity sensor etc. can be adjusted to the desired position and stop the cylinder with a solinoid or pilot operated valve. Multiple positions can be controled with a PLC or manually.
• Positioner - A Linear Displacement Transducer, Temposonic or other electronic feed back device including one that measures the amount of oil that has been forced into the cylinder. Most of these are analog positioning techniques where the transducer sends more or less electric current or voltage to a PLC.
• Mechanically - One or more mechanical stops. These can be moved in and out so that multiple positions may be pre-selected.

If you tell your local pneumatic/hydraulic/automation/controls supplier what you want to do and how you want to do it they can provide an answer.

Air is compresseble. When the pneumatic cylinder stops through control valve, because of area difference the cylinder will still move till the air pressure drops. The good idea is to use a double ended rod type cylinder or a rod less cylinder. However 400 kgs lifting that too pneumatic is not so practicle

Agreed. A check valve is okay in the horizontal position but is not considered safe in the vertical (especially with that size of mass). No matter what is used to actuate it, A lock/brake or possibly a shotpin, if constructed right, are the best methods.

400 kilos? I even wouldn't try pneumatics. It is very difficult to control speed of displacement this mass with pneumatic actuator. Use motor - it will be cheaper and fail safe. Or may be hybrid actuator - but it can be more expensive than elecric motor with all mechanical system.

We've used pneumatics for lifting 600kg test weights for weigh scales. We don't do positioning. We just lift the weights, calibrated the scale, and let them down. They're large things, the cylinders but, yes, they can lift 400kg.

The speed? No problem, the cylinders have built-in speed regulators. The lifting and decending is very smooth.