Design of hydraulic circuits

Dr. S. & S.S. GHANDHY GOVERNMENT ENGINEERING COLLEGE, SURAT
MECHANICAL ENGINEERING DEPARTMENT
“ACTIVE LEARNING ASSIGNMENT”
for partial fulfillment of term work in
Oil Hydraulics and Pneumatics (2171912)
by
ENR. NO. NAME
150233119001 BHANANI AMAN C.
150233119002 BHAVSAR DHVANIL H.
150233119014 PATEL SNEH H.
150233119018 SIDDHPURA HARDIK K.

Design of Hydraulic Circuits
Contents
■ Basic hydraulic circuits
■ Industrial hydraulic circuits
■ Power losses in flow control circuits

Basic hydraulic circuits
■ A basic hydraulic circuit consists of power supply, pump, reservoir, relief valve and
control valve.
■ Basic hydraulic power units can have specific control valves and activators to properly
control hydraulic devices. Example: 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.

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.

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.

Simple Hydraulic Circuit
A simple open centre hydraulic circuit The equivalent circuit schematic

Interaction of components
■ The animations 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.

Circuit diagram: Hydraulic power
unit
■ The illustration shows the detailed circuit
symbol for a hydraulic power unit.
■ Since this is an combination unit, a dot/dash
line is placed around the symbols
representing the individual units.

Pressure relief valve (1)
■ In this design incorporating a poppet valve,
a seal is pressed against the inlet port P by
a pressure spring when the valve is in its
normal position.
■ In this situation, for example, an unloaded
piston rod is executing an advance stroke
and the entire pump delivery is flowing to
the cylinder.

Pressure relief valve (2)
■ As soon as the force exerted by the inlet
pressure at A exceeds the opposing spring
force, the valve begins to open.
■ In this situation, for example, the piston rod is
fully advanced; the entire pump delivery is
flowing at the pre-set system pressure to the
tank.

PRV used to limit system pressure
■ This illustration shows a pressure relief valve within a basic hydraulic circuit (used to
control a double acting cylinder).

Circuit diagram: Brake valve
■ This circuit incorporates not only a brake
valve on the piston-rod side but also a non-
return valve on the inlet side via which oil
can be taken in from a reservoir during the
vacuum phase following the closure of the
directional control valve.
■ The following animation shows the events
which occur in the two working lines.

Industrial Hydraulic Circuits
■ Typical hydraulic circuits for control of industrial machinery are described from here.
Graphical hydraulic circuit diagrams incorporating component symbols are used to
explain the operation of the circuits.

Unloading System for Energy Saving
■ An “unloading” system is used to divert pump flow to a tank during part of the
operational cycle to reduce power demand. This is done to avoid wasting power idle
periods. For example, it is often desirable to combine the delivery of two pumps to
achieve higher flow rates for higher speed while a cylinder is advancing at low
pressure. However, there may be considerable portions of the cycle, such as when the
cylinder is moving a heavy load, when the high speed is no longer required, or cannot
be sustained by the prime mover. Therefore, one of the two pumps is to be unloaded
resulting in a reduction of speed and consequently, power. The components of this
system are: A, B: Hydraulic pumps, C, E: Pilot operated Spring loaded Relief valves, D:
Check valve

Mode 1: Both Pumps Loaded
■ In Figure, when both pumps are delivering, oil from
the pump A passes through the unloading valve C and
the check valve D to combine with the pump B
output.This continues so long as system pressure is
lower than the setting of the unloading valve
Mode 2: One pump unloaded
■ In Fig., when system pressure exceeds the setting of
the unloading valve C, it makes pump A to discharge
to the tank at little pressure. Although the system
pressure, supplied by pump B, is high, the check valve
prevents flow from B through the unloading valve.
Thus only pump B now drives the load at its own
delivery rate.Thus the load motion becomes slower
but the power demand on the motor M also reduces.
If the system pressure goes higher, say because load
motion stops, pump B discharges when its relief valve
settings would be exceeded. C.

Reciprocating Cylinder with Automatic
Venting at End of Cycle
■ A reciprocating cylinder drive is a very common hydraulic system. In systems where it
is not necessary to hold pressure at the end of a cycle, it is desirable to unload the
pump by automatically venting the relief valve, to save energy. Figures below show
such a system. The system components are : A : Reservoir with Filter, B : Hydraulic
pump, C, E : Check valve, D : Pilot operated relief valve, F : Two-position electro-
hydraulic pilot operated Four-way Directional valve, G : Cam operated pilot valve, H :
Double acting Single rod Cylinder, I : Limit Switch.

Extension Stroke
■ Consider the beginning of the machine cycle when the
solenoid of the spring offset directional valve F is
energized. Pump output is connected to the cap end of
the cylinder. The vent line drawn from the directional
valve output connected to the cap end of the cylinder is
blocked at the cam-operated pilot valve G. Thus, vent
port of the relief valve D is blocked, and the cylinder
moves under full pump pressure applied to the cap end.
Retraction Stroke
■ At the extreme end of the extension stroke, the limit
switch is made on by the cylinder rod to break the
solenoid circuit for the directional valve F. The
directional valve now shifts to its right position and the
pump gets connected to the rod end of the cylinder
which now retracts. Note that the relief valve vent
connection is still blocked.
extension

AutomaticVenting at End of Retraction Stroke
■ At the extreme end of the retraction stroke, the cam on the
cylinder is operated by the rod to shift valve G. The relief
valve vent port is thus connected, through E and G, to the
line from the cap end of the cylinder, and to tank through
the F and the inline check valve C. This vents the relief valve
D and unloads the pump.
Push Button Start of Cycle
■ If another cycle of reciprocating motion is desired, a start
button connected to the solenoid circuit is depressed to
energize the solenoid, and, in turn, the directional valve
shifts to direct pump output into the cap end of the
cylinder. This causes the check valve in the vent line to
close. Pressure again builds up and the cylinder starts
extending. This releases the cam, which, under spring
action, shifts and the vent port of E is again blocked at G.
Thus the cycle repeats.
retraction

Push Button Start of CycleAutomaticVenting at End of Retraction Stroke

Regenerative Reciprocating Circuit
■ Conventional reciprocating circuits use a four-way directional valve connected directly
to a cylinder. In a regenerative reciprocating circuit, oil from the rod end of the cylinder
is directed into the cap end to increase speed, without requiring to increase pump
flow. Such a circuit is shown below in Figures below.The circuit components are :A :
Hydraulic Pump, B : Relief valve, C : Four-way two position solenoid operated valve, D :
Double-acting Single-rodCylinder.The operation of the regenerative circuit is shown
in Figures below.

Regenerative Advance
■ In Figure, the “B” port on the directional valve C, which conventionally connects to the
cylinder, is plugged and the rod end of the cylinder is connected directly to the pressure
line. With the valve shifted to the left most position, the “P” port is connect to the cap end
of the cylinder. If the ratio of cap end area to rod end annular area in the cylinder is 2:1, the
pressure being the same at both end, the force at the cap end is double that at the rod
end. There is therefore a net force on the cylinder to move the load. Similarly, at any speed
of the cylinder, the flow into the cap end would be double that of the rod end. However, in
this connection, the flow out of the rod end joins pump delivery to increase the cylinder
speed. Thus only half of the flow into the cap end is actually supplied by the pump.
However, the pressure during advance will be double the pressure required for a
conventional arrangement for the same force requirement. This is because the same
pressure in the rod end, effective over half the cap end area, opposes the cylinder’s
advance.

■ In the reverse condition shown in Figure, flow from the pump directly enters the rod
end of the cylinder through two parallel paths, one through the directional valve and
the other directly. Exhaust flow from the cap end returns to the tank conventionally
through the directional valve. Note that, in contrast to the conventional case, the force
on the cylinder as well as the pump flow remains unchanged during extension and
retraction. Thus, the speed of the piston during both advancement and retraction
remain same.

Sequencing Circuits
■ In many applications, it is necessary to perform operations in a definite order.
Following is one of several such circuits.The components of the system are as follows.
A : Reservoir and Filter ; B : Hydraulic Pump ; C : ; Relief valve : D ; F1, F2, G : Relief valve
with integral check valve ; H, J : Cylinders ; I : CheckValve
■ The sequence of operation realized by the circuit shown in Figures is:
Step A – Extend Cylinder H
Step B – Extend Cylinder J while holding pressure on Cylinder H
Step C – Retract Cylinder J
Step D – Retract Cylinder H

Step A
■ Pressing a pushbutton would start the
cycle and shift the directional valve E to
the position shown in Fig. At first the
fluid flows through the integral check
valve in G into the cap end of H and
returns freely through the check valve in
F2. The pump pressure is low during this
period, only to the extent of pushing the
load on H.

Step B
■ Once H reaches its rod end, the pressure
builds up and now the flow develops
through F1 into the cap end of J and out
through the rod end to go back directly
to tank through F2, E and C. Note that a
pressure equal to the setting of the valve
F1 is maintained on H. When J is fully
extended, pressure increases further and
is limited by the setting of D, providing
overload protection to B.

Step C
■ Similarly, when the other solenoid of E
is energized, the directional valve shifts
to the other position, as shown in Fig.
Now, pump delivery is directed through
D, E and F2, into the rod end of J. As
before, the flow out of the cap end of J
flows to tank through F1, E and C. Step C
is illustrated in Fig.

Step D
■ On completion of Step C, the pressure
increases again, and the flow is
directed through F2 to the rod end of H
and out through the cap end to flow
into the tank through the valve G at its
pressure setting and then freely to tank
through F1, E and C. Note that F2
maintains a pressure equal to the
setting of H at the rod end of J during
the retraction of H. Note further that,
while H is retracting, a back pressure is
provided to it by G, to prevent rapid
falling of the load during lowering,
under gravity.

Power losses in flow control circuits.
■ Energy losses in the current hydraulic systems, ranging between 30% and 50%, can no
longer be accepted and therefore relevant scientific research carried out in the last 20
years has analysed the main causes, vulnerable places in the installations and ways to
reduce them.
■ In fact, energy losses are determined, among others, by the friction of the fluid layers
between them and with the pipes through which they pass and by the pressure drops
on the equipment's, at bends and diameter changes.
■ Finally all these cumulated hydraulic pressure losses turn into heat, and thus to the
energy loss is also added the destructive action of the high temperature and the
obligation to introduce additional cooling equipment in the system.

Power losses in components and
systems
■ In this section will be taken into consideration losses in pumps, distribution and control
systems, pipes and hydraulic motors.
■ Losses in pumps are determined by internal losses and mechanical friction, and the
total efficiency, which represents the energy efficiency, will be determined as the
product of volumetric efficiency and mechanical efficiency.
■ An increase in the technological level of pumps manufacture, together with improved
materials and increased tribological performances, made that the volumetric
efficiency determined primarily by side clearances, as well as the mechanical efficiency
determined by friction, both have values over 90%, so that in the end the total
efficiency will also be over 90%.

■ Losses in the distribution and control section are local losses determined by either
construction of the equipment or the working methodology of the system.
■ If losses on every component can be treated as local losses and reduced by improving the
forms of flow, within fairly narrow limits, technological losses recorded on flow control
valves and regulators can be minimized through a proper design of the whole system and
especially through the use of adjustable pumps with high level of automation.
■ Upgrades in this area of a hydraulic system could lead to the greatest reductions in energy
losses with current equipment's and technologies. In fact, the most important thing is to
devise a system by which the discharges to the tank through the safety valve to be
minimized.
■ Losses on pipelines and auxiliary components are generally quite high and are comprised
of linear losses and losses on auxiliary equipment's such as filters, accumulators and
coolers.
■ Generally, losses on auxiliary components can be treated as local losses with relatively
small values, with rather small possibilities of reduction, as some of these components
don’t permanently intervene into operation (accumulators), and others can be bypassed.

■ The big problem are the linear losses in the pipelines, which generally have high values and
on which is working much and generally efficient. Designers choose the shortest routes,
reduce them to the minimum, avoiding the forming of local areas of turbulence.
■ Energy losses in hydraulic motors are quite important, even though not essential. Losses in
rotary motors are similar to energy losses in pumps because also in this case the one that
counts is the tribological element and less the technological element, through which are
produced at normal prices side clearances that can reduce internal flow losses. Hydraulic
cylinders, with their component materials and the structure may reduce losses, but can not
remove them.
■ In any case, in the cylinders used today in hydraulic systems, we find the friction between
the rod and rod cap seal, between the piston and cylinder body and in the couplings by
which the cylinder is attached to the mechanical equipment. Much important and more
dangerous are the problems caused by a poor grip on the machine, because high radial
forces are introduced which induce high friction and therefore high power losses.
■ Switching-type digital hydraulics represents a solution of great interest which provides close
proximity between the available flow rate and the required flow rate in each phase of work
and also greatly reduces the number of hydraulic equipment's for distribution and control.
■ Another great advantage is the reduced number of pipelines and hence linear losses.
Otherwise the problem of the pumps and motors is similar toType C systems.

Meter in flow-control circuits at rest.
■ There are three types of flow control circuits from which to
choose. They are: meter-in, meter-out, and bleed-off (or
bypass). Air and hydraulic systems use meter-in and meter-
out circuits, while only hydraulic circuits use bleed off types.
Each control has certain advantages in particular situations.
■ meter-in flow-control circuit for a cylinder. Notice that a
bypass check valve forces fluid through an adjustable orifice
just before it enters the actuator.
■ extending hydraulic cylinder and indicates the pressures
and flows in various parts of the circuit. With a meter-in
circuit, fluid enters the actuator at a controlled rate. If the
actuator has a resistive load, movement will be smooth and
steady with a hydraulic circuit. This is because oil is almost
non compressible.

Meter-in flow-control circuit with
cylinder extending.
■ In pneumatic systems, cylinder movement may be jerky
because air is compressible.
■ As air flows into a cylinder, as depicted in Figure,
pressure increases slowly until it generates the
breakaway force needed to start the load moving.
Because the subsequent force needed to keep the load
moving is always less than the breakaway force, the air
in the cylinder actually expands.
■ The expanding air increases the cylinder speed, causing
it to lunge forward.
■ The piston moves faster than the incoming air can fill
the cylinder, pressure drops to less than it takes to keep
the cylinder moving and it stops.
■ Then pressure starts to build again to overcome
breakaway force and the process repeats. This lunging
movement can continue to the end of the stroke. A
meter-out circuit is the best control to avoid air-cylinder
lunging.

Meter-in flow-control circuit for overrunning
load with cylinder extending
■ If the actuator has an overrunning load, a meter-in flow
control will not work.
■ When the directional valve shifts, the vertical load on the
cylinder rod makes it extend.
■ Because fluid cannot enter the cylinder ’s cap end fast
enough, a vacuum void forms there. The cylinder then free
falls, regardless of the setting of the meter-in flow
adjustment.
■ The pump will continue to supply metered fluid to the cap
end of the cylinder and will eventually fill the vacuum void.
After the vacuum void fills, the cylinder can produce full
force.

Design of hydraulic circuits

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