Double Acting Cylinders Lesson 4

The air that surrounds us on planet earth is comprised of 78% nitrogen, 21% oxygen and some trace amount of other types gases. Pressure caused by the weight of the atmosphere is called atmospheric pressure, also referred to as reference pressure. Atmospheric pressure varies depending on the weather and geographical location at the time of the measurement. At sea level, air exerts 14.7 pounds per square inch of surface area, or in abbreviated form 14.7 psi. A tool called a barometer is used to measure atmospheric pressure. A barometer indicates, on a inch scale, the height of a column of mercury held in a glass tube exposed to the weight of the atmosphere. The equalization point of mercury is 14.7psi of atmospheric pressure is approximately 29.92 inches.

Gauge Pressure Since we live in a pressurized world, we generally use pressure gauges that measure pressure beyond that of our atmospheric pressure to monitor a pneumatic system. Gauge pressure is designed for calculation purposes as psig. The letter g is used to designate that the pressure reading has been taken from a gauge that has compensated for the pressure of the atmosphere.

Absolute Pressure Pressure readings above zero are referred to absolute pressure. Absolute pressure is designated for calculation purposes as psia. The letter a is used to designate that the pressure reading is taken from a perfect vacuum, i.e. zero pressure. Another way to think about absolute pressure is with an equation: Gauge pressure + atmospheric pressure = absolute pressure Absolute pressure = PSIG + Atmospheric Pressure 90 + 14.7 = 104.7 PSIA

Atmospheric Pressure II As mentioned before, atmospheric pressure is not constant. The pressure at 1,000 ft. of altitude is 14.2 psi. You can calculate the pressure for just about any distance above sea level by subtracting ½ lb. per 1,000 ft. For instance, at 5,000 feet above sea level atmospheric pressure is only 12.2 psi. The difference in pressure explains why your ears need to pop to compensate for the pressure difference within your inner ear as you drive over a mountain pass. Unless stated otherwise, always use 14.7 psi as atmospheric pressure for calculating an pneumatic problems.

Double Acting Cylinder I Double acting cylinders do not have an internal return spring. The piston will be pushed in either one direction or the other because the air pressure will be directed into a cylinder on one side of the piston or the other. Note that a double acting cylinder has two connection ports to facilitate pressurizing and exhausting air. We have one double acting cylinder in the kit.

Double Acting Cylinder II The advantages of using a double acting cylinder is that work can be performed in both directions of rod travel. The most force is created in the forward stroke of the piston. The cross-sectional area taken up by the rod reduces the relative force that can be created when retracting, 5/2 way valves are used to control this device because the valve has two exhaust ports built into the design, which will allow either cylinder connection port to exhaust air as needed. Beyond this lesson is a vast array of double acting cylinders that range from linear actuators with no piston rod, to cylinders that produce high speed impact, to piston rods that turn linear motion into rotational movement.