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Unread 26-01-2004, 22:26
Gary Bonner Gary Bonner is offline
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Re: Mulit-postioning with pnuematics this year?

It seems to me that the pressure in these setups can go much higher than 60 psi, and the proposed remedies would not necessarily work.

Take the 2” cylinder example from before to somewhat of an extreme, but with no change in the load the piston is lifting. The surface area of the extend side of the piston is 3.14 in^2, and the retract side is 2.83 in^2. Suppose we are using the cylinder to lift 180 lbs. The extend side is pressurized to 60psi and the retract side is vented to the atmosphere. This produces a lifting force of 188lbs (60 psi X 3.14 in^2). When the piston gets to the height we want, we close the retract port. The extend side remains at 60 psi and exerting a 188 lb force on the piston. This is balanced by the 180 lb load on the piston plus the equilibrium pressure reached in the exhaust side, in this case, 2.8 psi ( 8 lb / 2.83 in^2).

Now, suppose we want to lower the piston a little. We pressurize the retract side to 60 psi and vent the extend side to the atmosphere. When it gets to the desired position we close the extend port. Now the retracting force on the piston is 350 lbs [180 lb load + (60 psi * 2.83 in^2)]. To counteract this force, the extend side becomes pressurized to 111 psi (350 lbs / 3.14 in^2).

In this situation, a venting regulator on the exhaust would merely act as a flow control valve and allow the piston to slowly move. Reducing the supply pressure wouldn’t necessarily help either.

If the load on the cylinder was in the other direction (e.g. we are pulling our robot up on the bar) the pressure would be greater. Also, any change in the load after we stop the piston (e.g. a robot hanging above slips down on us) would change the pressure on the exhaust side.