In a basic hydraulic circuit, the force exerted by a cylinder is dependent upon the cylinder bore size and the pump pressure. (There is no force generated unless there is resistance to the movement of the piston).  With 1000 psi pump pressure exerted against a 12 square inch piston area (approximately 4" diameter), a force of 12,000 pounds is developed by the cylinder.  The speed at which the piston will move is dependant upon the flow rate (gpm) from the pump and the cylinder area.  Hence, if pump delivery is 1 gallon per minute (231 cu. in./min.) the cylinder piston will move at a rate of 20 in./min (231 cu.in. / 12 cu.in./min.)

The simplest hydraulic circuit consists of a reservoir, pump, relief valve, 3-way directional control valve, single acting cylinder, connectors and lines (as shown in Figure 2).  This system is used where the cylinder piston is returned by mechanical force.  With the control valve in neutral, pump flow passes through the valve and back to the reservoir.  With the valve shifted, oil is directed to the piston side of the cylinder, causing the piston to move, extending the rod.  If the valve is returned to neutral, the oil is trapped in the cylinder, holding it in a fixed position, while pump flow is returned to the reservoir.  Shifting the valve in the opposite direction permits the oil to pass through the valve back to the reservoir.  The relief valve limits the system pressure to a pre-set amount.  Relief valves are commonly incorporated into the directional control valve.

A hydraulic system using s double acting cylinder and a 4-way valve differs from the single acting cylinder system, (as shown in Figure 3) in that the cylinder can exert force in both directions.  With the control valve in neutral, flow is returned to the reservoir.  When shifted in one direction, oil is directed to the piston side of the cylinder, causing the cylinder to extend.  Oil from the rod side passes through the valve back to the reservoir.  If the valve is shifted to neutral, oil in the cylinder is trapped, holding it in a fixed position.  When the valve is shifted, in the opposite position, oil is directed to the rod side of the cylinder, causing the rod to retract.  Oil from the piston side passes through the valve back to the reservoir.  Cylinder extend force is the result of the pressure (psi) times the piston area (minus any force resulting from the pressure acting against the rod side of the piston).  Retract force is a result of the pressure (psi) times the area difference between the rod and the piston (minus any force resulting from pressure acting against the piston side of the cylinder).

Rotary hydraulic motor circuits are basically the same as cylinder circuits.  Systems may be uni-directional or bi-directional (as shown in Figure 4).  The amount of rotary force (torque) available from the motor is a function of pressure (psi) and motor size.  All of the systems described above are open center systems due to the oil flowing through the control valve back to the reservoir.  Most systems are this type.  Closed center systems use control valves with the inlet port blocked and variable displacement pumps.  With the Control valve in neutral, the pump is "de-stroked" to zero flow.

Figure 1

Figure 2

Figure 3

Figure 4

The above system shows a front end loader powered by a PTO driven pump. A 2-spool directional control valve with built-in relief controls the lift and bucket cylinders of the loader.  A return line filter is used to prevent contamination.

The diagram shows a winch powered by a hydraulic motor.  The directional control valve with built-in relief features optional flow control to control winch coil speed.  The hydraulic pump and motor must be matched to the torque requirements of the winch.

Logsplitter

A directional control valve with optional pressure kick-out feature controls a double acting cylinder.  A pressure gauge is recommended to help spot potential system problems.  The tank should be at least one and a half times the pump gpm output and the oil level must remain above the pump intake at all times.

Helpful Nomographs

Cylinder Speed

Pump Delivery = Cylinder Area  x  Piston Velocity

Cylinder Force

Force = Cylinder Area  x  Line Pressure

Pump & Motor Horsepower

HP  =  PSI x GPM x 0.000583

Pump Drive  =  theoretical HP / Efficiency

Motor Output  =  theoretical HP x Efficiency

Pipe Flow Capacity

Area  =  GPM x 0.3208 / Velocity (ft/sec.)

*  Recommendations are for oils having a maximum viscosity of 315 SSU at 100 deg F operating at temperatures between 65 deg F and 155 deg F.

Hydraulic Motor Torque

Torque  =  Displacement x Pressure / 24 π

Hydraulic Motor Speed

Flow Rate  =  Speed x Displacement / 231

ACCUMULATOR: A container which stores fluid under pressure. Used as an energy source or to absorb hydraulic shock. Common types are piston, bladder and diaphragm.

BLEEDER (BLEED VALVE): A device for removal of pressurized fluid. Used to bleed air from system.

CAVITATION: A gaseous condition within a liquid stream caused when pressure is reduced to the vapor pressure. To be avoided due to destructive effects on pumps and motors.

CIRCUIT; PILOT: Used to control a main circuit or component.

CIRCUIT; REGENERATIVE: Used to increase cylinder speed by directing rod end discharge to the piston side of the cylinder. Can be incorporated into directional control valve as fourth position.

CYLINDER: A device which converts hydraulic energy into linear mechanical motion and force.

CYLINDER; DOUBLE ACTING: A cylinder which can apply force and motion in either direction.

CYLINDER; SINGLE ACTING: A cylinder which can apply force in one direction only.

CYLINDER; DEPTH CONTROL: A mechanical or hydraulic device, adjustable, for limiting cylinder stroke.

CYLINDER; REPHASING: A cylinder design which permits the use of two or more cylinders in series, automatically synchronizing cylinder position at the end of each stroke.

DETENT: A spring device which maintains the spool of a directional control valve in position.

DETENT RELEASE: A mechanical, hydraulic or electrical device for releasing the detent.

FILTER: A device incorporated into a hydraulic system to remove contaminants from the oil.

FITTING: A device for connecting hose or pipe to hydraulic components.

FLOAT SPOOL (POSIT10N): A spool valve design which connects all ports to the tank (return) port, usually in a detented fourth position, allowing a cylinder or motor to "float".

FLOW RATE: The volume of fluid passing through the system or component in gal. per min. (or 11m)

FLUID POWER SYSTEM: The transmission and control of power through the use of fluid pressure.

MOTOR: A device which converts hydraulic energy into rotary motion, either fixed or variable.

PORT: The internal or external terminus of a passage. The point where the fitting is attached.

PRESSURE: The force per unit area, expressed in pounds per square inch (psi), bars, or atmospheres.

PRESSURE; BACK: The pressure encountered on the downstream or return side of a component.

PRESSURE; CRACKING: The pressure at which a pressure operated valve begins to pass fluid.

PRESSURE; MAXIMUM RATED: The maximum pressure at which a component should be operated on a continuous basis, usually the relief valve setting at maximum flow rate.

PUMP: A device which converts mechanical energy into hydraulic. energy, either fixed or variable.

RESERVOIR: A container which stores the liquid in a fluid power system.

SEAL: A device which prevents or controls the escape or passage of hydraulic fluid.

VALVE: A device which controls fluid flow rate, direction, or pressure.

VALVE; DIRECTIONAL CONTROL: A device for directing or preventing the oil flow in a system.

OPEN CENTER (TANDEM) TYPE: Has the inlet port connected to the outlet (tank) port in neutral.

CLOSED CENTER TYPE: Has the inlet port blocked from the outlet and work ports in neutral.

TWO-WAY: A 2 port valve with inlet and outlet ports.

THREE-WAY: A 3 port valve (in, out ~r,d work) normally used with a single acting cylinder or uni-directional motor.

FOUR-WAY: A 4 port valve (in, out and 2 work) used with double acting cylinders, bi-directional motors.

TWO, THREE, FOUR POSITION: The number of positions in which a valve can be positioned.

SERIES TYPE: A multiple spool valve in which the return oil from the first spool is directed to the inlet of the second spool (and from the second to the third, etc.). This type valve permits simultaneous operation of two or more functions with the same oil flow. However, the total pressure requirements of all functions are accumulative.

PARALLEL TYPE: A multiple spool valve in which the inlet oil is connected to all spools simultaneously. If more than one spool is actuated, the function requiring the lowest pressure will operate first.

SERIES-PARALLEL TYPE: A multiple spool valve which has all spools connected to the open center passage in neutral. However, when actuated, the upstream valve takes full priority. The return oil is directed to downstream spools as in a series type valve.

POWER BEYOND (HIGH PRESSURE CARRYOVER): A sleeve attachment which permits the oil flow from one valve (when in neutral) to be used by another valve downstream. Hence, a 3-spool valve cou1d be connected to a 2-spool valve to create a 5-spool valve, The first valve takes priority and must have a separate outlet port to return oil  from an activator back to the reservoir.

LOAD CHECK (LIFT CHECK): A device which prevents a load from dropping when a valve is shifted, until ample pressure and flow is available to hold or move the load