Hydraulic Systems: Control Valves and Schematics
Control Valves: Ways vs. Positions
Number of blocks indicates the number of positions.
Number of ports indicates the number of ways. For example, A, B, P, and T indicate four ways.
Two-position valves: oil can move in one of two ways.
Examples
Two position, two way
Two position, three way: pressure, outlet/work port, and return to tank.
Three position, four way: three positions (blocks), four ports (A, B, P, T).
A and B: work ports.
Spool shifted left, spool centered, or spool shifted right indicates 3 positions.
Positioning is done through the blocks
Ways of oil travel is done through the porting.
Valve Centers
Open Center: In the center position, oil flows back to the tank.
Closed Center: In the center position, oil flow is blocked (dead).
Schematic Components
Actuators: push button, lever, or pedal (symbols used in schematics).
Pilot Operated Valves: Indicated by color fill.
Electrical Actuation: controlled by a solenoid.
Prime Movers: Internal combustion engine or electric motor.
Hydraulic Filters: Symbol is typically a diamond or a square.
Hydraulic Cooler, right, or the heater: Note the the difference between, like, for example, a hydraulic cooler, right, or the heater on your unit.
Steering Accumulators: Store hydraulic pressure to allow steering if the prime mover fails.
Basic Schematic Elements
Reservoir: Vented (open) or pressurized.
Inlet Strainer: prevents large particles from entering the pump.
Prime Mover: internal combustion engine connected to a pump.
Relief Valve: protects the system from overpressure.
Directional Control Valve: e.g., three positions, four ways, closed center.
Cylinders: Actuators for movement (e.g., boom, bucket).
Filter: Removes contaminants with a bypass valve.
Safety with Hydraulics
Depressurize systems before working on them.
Allow the system to cool down.
Wear safety glasses and avoid contact with hot oil.
High-pressure fluid can cause severe injection injuries.
Accumulators store energy and must be bled down.