Limit switches: Figure 18-10 shows an outline drawing and symbols for a limit switch. While some electrical components are shown to indicate function and location, no wiring appears on fluid power circuit diagrams.
Shock absorbers: The cross-sectional view in Figure 18-11 shows an oil-filled shock absorber. (The figure also includes a proposed symbol.) When cushioned cylinders or other decelerating devices are not satisfactory or desirable, shock absorbers are one viable alternative. Shock absorbers are available in sizes from 3/8 in. or less up to models that can stop a loaded overhead crane traveling at full speed in two feet or less. Some are adjustable, some are self- adjusting. Some use metering orifices (as the figure shows), others use tapered metering cones.
Because they may absorb a lot of energy in a short period, most have the ability to transfer fluid from the last stroke to a reservoir for cooling. The same reservoir replenishes the shock absorber with cooled fluid for the next stroke.
The model depicted in Figure 18-11 uses a spring-returned piston with an integral check valve that travels through a bore with several metering orifices in it. As the piston moves through its bore, there are fewer holes for fluid to pass through. Thus, resistance to movement increases throughout the stroke. As the piston strokes, it smoothly decelerates the load at a controlled rate until it stops. Fluid forced out of the bore during the deceleration stroke is sent into an oil chamber that is partially filled with a closed-cell foam accumulator. This accumulator makes it possible for the oil chamber to accept the extra fluid and then force it back to the bore on the return stroke. An oil-fill port allows replenishment of any lost fluid.
Most shock absorber manufacturers offer formulas in their catalogs and/or computer programs to size their products for specific applications.
Hose-break valves: In pneumatic systems, there usually is more air available than is required, so if a hose ruptures or is disconnected suddenly, air will flow profusely and the loose end of the hose can whip about dangerously. A hose-break valve set for a flow greater than working flow will close automatically when flow tries to increase above its capacity. Air hose-break valves never shut off completely so when the line is reconnected, the small bleed bypass fills the repaired section and the hose-break valve opens for use.
The drawing and symbol in Figure 18-12 represent a typical hose-break valve. Air flows into the right-hand port at a rate up to a certain cfm setting. The distance between the shut-off poppet and its seat determines the maximum flow rate before the shut-off poppet closes and stops flow. Reverse flow is never blocked, but is restricted to cause a pressure drop. When pressure at the right-hand port drops or when pressure at the left-hand port rises, the shut-off poppet opens to pass flow.