Fluid power accessories
Some components used in fluid power systems do not necessarily fall into any of the categories discussed in preceding chapters. These accessory items may be used for powering, modifying, monitoring, or connecting in any type circuit, as the system designer deems appropriate.
Quick exhaust valves: The speed at which an air cylinder strokes is determined by how fast compressed air enters it and how fast the air already in the cylinder exhausts to atmosphere. System pressure drives air into the cylinder and this does not pose a speed problem in most circuits. Air leaving the cylinder is different because it was at system pressure when the directional valve shifted. Although the air starts exiting quickly, it still holds the piston back. Speeding up a sluggish air-operated cylinder is best accomplished by dealing with its exhaust air. The cross-sectional view and symbol in Figure 18-1 illustrate a quick exhaust valve, which does just that.
The cylinder in Figure 18-1 delivers high impact from low force . . . stamping parts with steel dies and leaving a lasting impression. Cylinder force alone is not capable of making the desired impression -- if any impression at all. Accelerating piston speed over a few inches of travel makes the weight of the tooling act like a hammer swung through the air.
As the cylinder retracts and is held at rest, the shut-off wafer covers the exhaust port and forces air to the cylinder rod end. When the directional valve shifts to extend the cylinder, pressure drops on the left side of the shut-off wafer and trapped pressure in the cylinder forces the wafer to the left. As the shut-off wafer moves left, it closes off flow to the valve and opens a direct path to atmosphere only a short distance from the cylinder port. The rapid exhaust of air reduces backpressure on the cylinder piston, allowing high-pressure inlet air to accelerate and move the piston very quickly.
Any time slowly exhausting air is a problem, look to a quick exhaust valve to remedy the situation.
Mufflers: The air-exhaust mufflers in Figure 18-2 reduce the noise level of air-operated equipment. They are made in several different configurations out of many types of material, but the end result of all of them is the smooth discharge of air.
The sintered-bronze elements on the left are similar to filters made of the same material. They separate the flowing air into numerous paths to lessen or eliminate the loud crash of air as it leaves an actuator. The sintered-bronze element in the center has a protective metal covering and an adjustable poppet valve to control flow. It works as an inexpensive meter-out flow control when used with a 5-way directional control valve. Because a 5-way valve has two exhaust ports, these speed-control mufflers can regulate speed independently in both directions of travel. The muffler on the right is similar to those used on internal combustion engines. It may be made of plastic or aluminum. It is bulky, but causes less restriction on fast-moving actuators.
Accessory Items for pneumatics and hydraulics
The components described in the rest of this chapter are common to hydraulics or pneumatics. The main difference between them is the materials used to make them. Many pneumatic components can be made of plastic or aluminum to resist corrosion and keep cost down. These materials work well at low pressure. Most hydraulic components see high to very high pressure and need to be much more robust. Cast iron and steel are common materials for hydraulic parts due to their strength and the absence of corrosion. Aluminum is also preferred by some because of its light weight.
Pressure gauges: The gauges shown in Figure 18-3 come in a variety of shapes, sizes, and designs. The most common is the round model that has a moving needle to designate system pressure. The round gauge on the left and the plunger gauge measure psig, not atmospheric pressure. Because atmospheric pressure is in and around an actuator, it doesn't help or hinder performance, so it is not important when determining the amount of work being done.
The gauge marked PSIA reads atmospheric pressure instead of zero and can be used to check vacuum as well as pressure. Some of these gauges set on zero and read psi clockwise and vacuum (in inches of mercury) counter-clockwise.
Other designs include battery-operated digital-readout units. These gauges are accurate and very fast reading.