1. INTRODUCTION
A basic hydraulic circuit consists of a power supply, pump, reservoir, relief valve and a control
valve. Basic hydraulic power units can have specific control valves and activators to properly
control hydraulic devices. Examples, Single or Double Acting Hydraulic Cylinders, Hydraulic
Motors or to send fluid and pressure to a remote location. Custom designing a hydraulic circuit
is to specifically build the complete circuit to satisfy all the requirements of the power unit.
CIRCUIT COMPONENT
Types of Control Valves:
A) Mechanically - Operated, Spring Loaded or Detent Features.
B) Solenoid - Operated, 120- Volt AC, 12/24 - Volt DC.
C) Open - Center Valves
D) Closed - Center Valve
Types of Hydraulic Pumps:
A) Gear Pumps, Single & Double Stage.
B) Pressure Compensated, Variable Displacement Piston Pumps.
C) Vane Pumps
D) Tandem/Multiple Circuit Pumps.
Flow Control Valves:
A) Adjustable Pressure Compensated Flow Control Valves.
B) Needle Valves (Flow Control when Reverse Flow Check is not needed).
C) Flow Control Valve (Restricts in one direction, free-flow opposite directions).
Hydraulic Selector Valves:
A) 3 - way, 2 - position (single-circuit).
B) 3 - way, 2 - position (double-circuit).
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2. Flow Dividers:
A) Rotary Flow Dividers - Provides proportional division of a single pump output.
B) Proportional Flow Divider - Split one inlet flow into two equal outlet flows.
Flow Valves:
A) Inline Check Valve.
B) Pilot Operated Check Valves (Single + Double).
C) Lock Valves - Lock Cylinder Position in Control Valves Neutral Position.
Relief Valves:
A) Differential Poppet Style
B) Pilot-Operated Style
Heat Exchangers:
A) Oil Cooler without Fan.
B) Oil Cooler with Fan. AC/DC
C) Oil Cooler, Shell type, Water-cooled.
Oil Filters:
A) Return Line Type
B) Suction Supply
C) Pressure Type
Control Methods:
A) Mechanical.
B) Solenoid.
C) Foot Pedal(s).
D) Hand Held Pendants.
E) Wireless.
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3. Interaction of Components
The diagram show the sequences in a basic hydraulic circuit in simplified form - the actuation
and spring return of the final control element (4/2-way valve), the advance and return of the
drive component (double acting cylinder) and the opening and closing of the pressure relief
valve.
HYDRAULIC BASIC SYSTEM
Structure of a Hydraulic System
This simplified block diagram shows the division of hydraulic systems into a signal control
section and a hydraulic power section. This signal control section is used to activate the valves
in the power control section.
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4. Hydraulic Power Section
The diagram of the hydraulic power section is complemented in this case by a circuit diagram to
allow correlation of the various function groups; the power supply section contains the hydraulic
pump and drive motor and the components for the preparation of the hydraulic fluid. The energy
control section consists of the various valves used to provide control and regulate the flow rate,
pressure and direction of the hydraulic fluid. This drive section consists of cylinders or hydraulic
motors, depending on the application in question.
PRESSURE
Hydraulic pressure is generated when a flowing fluid meets resistance which is generally related
to the load that is being moved.
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5. A force is applied via the lever to produce system pressure (p = F/A or F = p x A).
If more force is applied, the system pressure rises until the load moves, if the load remains
constant the pressure will increase no further. The load can therefore be moved if the necessary
pressure is generated. The speed at which the load moves will be dependent upon the volume
of fluid which is fed to the load cylinder. For example, as the mold is opening or closing, the
pressure generated in the system represents the resistance of the toggle lever to movement.
Adding to that resistance would be the weight (i.e. mass) of the mold and toggle lever and also
the friction between the toggle lever bushings and the tie bars. When the two mold halves touch
and the toggle begins to straighten out, the increasing pressure
represents that which is required to stretch the tie bars in the generation of a particular clamp
force. Similarly when injecting material into the mold the pressure generated in the injection
system represents the resistance of the injection ram to movement. Adding to that resistance
would be the mass of the injection ram and screw, the friction between all moving components
and the resistance of the plastic melt as it is forced quickly into the mold cavity.
Pressure Control
In order to safeguard the system, pressure relief valves are installed. The valves serve to limit
the amount of pressure that can develop in the hydraulic system since the various hydraulic
components are expensive and they are subject to pressure limitations before failure occurs.
One characteristic of fluid flow that is important to note here is that flow occurs always in the
path of least resistance. Pressure would continue to rise in the circuit consistent with the load
being
moved. The pressure relief valve is always set to allow flow to travel through the relief valve well
before pressure rises above safe levels and causes damage to the system and its components.
In other words, the path of least resistance is employed here to safeguard the system after the
other movements have taken place.
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6. Pressure Override
An extremely important concept to understand about pressure relief valves is their pressure
override characteristics. Pressure override is the difference between the pressure at which the
relief valve just starts to crack open and the pressure at the full open position. For direct acting
pressure relief valves this pressure differential can be as high as 30% and proportional pressure
relief valves range from
10% - 20%.
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7. Pressure Intensification
Another important concept to keep in mind is that of pressure intensification. This law of
hydraulics is often forgotten when troubleshooting hydraulic circuits.
For example, if two pistons of different size are connected by a rod, the pressure existing on the
smaller area will always be greater. This principle also applies to the cap side and the rod side
of a normal double acting piston.
If P1 = 1,000 psi and A1 = 10 square inches, then F1 = 10,000 pounds of force.
If F1 = 10,000 pounds of force and if A2 = 5 square inches, then P2 = 2,000 psi.
Hydrostatic pressure
A. Hydrostatic pressure is the pressure created above a certain level within a liquid as a
result of the weight of the liquid mass. Hydrostatic pressure is not dependent on the
shape of the vessel concerned but only on the height and density of the column of liquid.
B. Hydrostatic pressure can generally be ignored for the purpose of studying hydraulics
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8. Pressure propagation
A. If a force F acts on an area A of an enclosed liquid, a pressure p is produced
which acts throughout the liquid (Pascal's Law).
B. Hydrostatic pressure has been ignored here. The term pressure propagation is
also used to mean the pulse velocity in liquids (approx. 1000 m/s).
Power transmission
A. If a force F_1 is applied to an area A_1 of a liquid, a pressure p results. If, as in this
case, the pressure acts on a larger surface A_2, then a larger counter-force F_2 must be
maintained. If A_2 is three times as large as A1, then F_2 will also be three times as
large as F_1.
B. Hydraulic power transmission is comparable to the mechanical law of levers.
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9. Pressure transfer
A. The fluid pressure p_1 exerts a force F_1 on the surface A_1 which is transferred via the
piston rod to the small piston. The force F_1 thus acts on the surface A_2 and produces
the fluid pressure p2 . Since the piston area A_2 is smaller than the piston area A_1, the
pressure p_2 must be larger than the pressure p_1.
B. The pressure-transfer (pressure-intensification) effect is put to practical use in
pneumatic/hydraulic pressure intensifiers and also in purely hydraulic systems when
extremely high pressures are required which a pump cannot deliver.
Pressure transfer (2)
A. A pressure-transfer effect also occurs in conventional double acting cylinders with single
piston rod.
B. This effect also causes problems in hydraulics. If, for example, an exhaust flow control is
fitted to a differential cylinder for the advance stroke, a pressure- intensification effect
results in the piston-rod chamber.
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10. FLOW CONTROL
Hydrodynamics
As well as understanding the concept of speed in hydraulics, it is also important to have some
insight into flow characteristics. For example, the drawing below shows that when oil is flowing
through different diameter pipes an equal volume flows in an equal unit of time. If that is true
and if the shaded quantity Q1 equals
the shaded quantity Q2, then velocity V2 must be greater than velocity V1.
As the diameter of the pipe decreases, the flow rate will increase. Specifically, if the pipe diameter
decreases by one half in the direction of oil flow, the cross sectional area will decrease by four times,
and visa versa. Oil flow velocity through different pipe sizes can be calculated using the formula:
The same gallons per minute will have to travel 4 times faster through the smaller pipe.
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11. Types of flow
A. A distinction is made between laminar flow and turbulent flow. In the case of laminar
flow, the hydraulic fluid moves through the pipe in ordered cylindrical layers. If the flow
velocity of the hydraulic fluid rises above a critical speed, the fluid particles at the center
of the pipe break away to the side, and turbulence results.
B. Turbulent flow should be avoided in hydraulic circuits by ensuring they are adequate
sized.
One goal in the initial design of hydraulic power transmission systems is to encourage laminar
flow as much as possible since an increase in turbulence will increase flow resistance and
hydraulic losses as well.
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12. Turbulent flow is wasteful in most hydraulic applications, it is desirable to have turbulence in the
oil flow as it travels through the heat exchanger for cooling purposes. If turbulence exists as the
oil flows through the heat exchanger, more of the oil molecules come into contact with the heat
exchanger cooling tubes and more efficient cooling is the result.
DIRECTIONAL CONTROL
The drawing below shows a piston being extended, held stationary and then retracted, simply
by changing the position of a directional valve. Even though the drawing is simple in nature, it
still demonstrates the principle involved in directional control. In addition to simple directional
control valves, we also employ proportional directional control valves on some machines to
control the clamp opening and closing function.
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13. REFERENCE
http://www.capetronics.com/basics_of_hydrailics.htm
http://www.fostermfgcorp.com/page/circuits/circuits.htm
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