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HYDRAULICS – PRINCIPLES OF OPERATION
The target group for this chapter is mechanics who have no prior
knowledge of hydraulic machines. An understanding of the basic
principles of hydraulics will help resolve many of the problems
and issues encountered during troubleshooting.
Hydraulic system
The term "hydraulic" refers to the trans-
fer of power using some form of fluid.
The fluid that is used in this context is
oil, which apart from serving as the
medium for transferring power, also
lubricates and cools the system's com-
ponents. Hydraulic power transfer nor-
mally operates in a sealed system, that is
to say the fluid medium that is used is
returned to a container from where it is
reused. A hydraulic system consists of a
power source, a pump of some sort and
a recipient that performs some kind of work, for example a
hydraulic motor. Often, transmission links of some kind are also
required, for example hoses or pipes. A comparison with a
mechanical system helps clarify this set-up, for example with a
motor or engine, a chain and a wheel.
PRESSURE AND FLOW
Two terms are fundamental when it comes to hydraulics, namely
pressure and flow. Proper understanding of how these work and
interact will help solve most problems and answer most questions
that may arise.
Pressure
In order to describe pressure,
we use the example of a static
hydraulic system. This exam-
ple also shows how hydraulics
can be used for motion and
power gearing. The figure here
shows a sealed container con-
taining water. The container
has two movable pistons, one
with an area of 10 cm
2
and the
other just 1 cm
2
. We place a
weight of 1 kg on the smaller
piston and 10 kg on the large
piston. The following will apply:
Weight balance. Since the weights are proportional to the
areas on which they are placed (1 kg/cm
balance, and the pistons will not move.
Movement gearing. The piston areas in contact with the fluid
have a ratio of 1 to 10. If we press down the small piston 10 mm
into the fluid, the large piston will rise 1 mm. The same fluid
volume will now have exchanged places between the two pistons,
and we have used hydraulics to create movements of different
magnitudes.
Power gearing. Gearing of movement also promotes gearing
of power – that which is lost in terms of movement is gained in
terms of power. The load of the small piston gives a lifting power
on the large piston that is 10 times greater.
These basic physical properties are used for functions in
hydraulic and pneumatic systems and in such applications, very
complex functions can often be carried out using surprisingly
simple mechanical designs.
Hydraulic motor
Pump
10
1
2
cm
2
cm
2
), the system remains in
Dimensions for pressure
Pressure must always be measured
over a given area in order for it
to be meaningful as a dimen-
sion, usually per square centi -
metre. Pressure was previously
often measured as kg/cm
2
(actually kp/cm
2
), a method that
is still widely used because it is so easy to grasp since we have a
clearer understanding of weight as a dimension. Since engineers
and physicists prefer the dimension of Newton for weight and
power (10 N corresponds to 1 kp), the "bar" is often nowadays
used as the unit for expressing pressure, for instance in hydraulic
systems (1 bar corresponds to 1 kp/cm
Newton/cm
2
gives a pressure of 1 bar. Note that each square
centimetre-sized surface unit of the container we de scribed earlier
is sub jected to a pressure of 1 bar. We can thus measure pressure
anywhere we like in the container.
Flow
Flow in a hydraulic system is the fluid's speed of movement,
often measured in litres per minute (l/min). In more or less static
systems, such as a hydraulic jack, flow is unimportant. However,
if we want to drive various devices hydraulically at set speeds,
such as cranes, vehicles, power cutters and so on, flow is a vital
parameter.
The hydraulic unit pumps a predetermined volume at a given
rotational speed or pump stroke. If the pump's speed is altered,
then flow too is changed. If we know the pump's capacity, we can
calculate the movement at the other (receiving) end. If we know
the hydraulic
piston's area, we can calculate the rotational speed, and if we
know the hydraulic motor's flow-through volume per revolution,
then we can calculate the speed.
Measurement of pressure and flow
In order to be able to check the function of hydraulic cutting
machines, you must be able to measure pressure and flow. Of
course, to do this you need a hydraulic unit with a capacity equi-
valent to that required for the cutting machine in question. In
the case of Husqvarna hydraulic cutters, the hydraulic unit must
provide a flow of 40 l/min (10 gpm) and a pressure of up to 150
bar (2000 psi), at which point an overpressure valve in the unit
must trip.
As shown in the diagram below, the measurement equipment
must be connected to the pressure side, which is represented by
the upper half.
PRESSURE
0
Power Cutter
Measurement equipment
Measurement equipment for hydraulic applications is available in
a number of configurations from specialist retailers. It is relatively
easy to build a suitable measurement device, the procedure for
which is described on page 31.
10 Newton
1 bar
10 Newton
2
). A power of 10
FLOW
Bar
L/min
140
40
0
Hydraulic unit
15
2
1 cm
27
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