Unit Seven: Pumps and Compressors

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Unit Seven Pumps and Compressors
Unit Seven Pumps and Compressors
Pumps and compressors are primary sources of flow
in fluid power systems. Maximum system
horsepower is controlled by the size of these
components along with system flow.
The three basic types of pumps are the Gear,
Piston, and Vane designs.
The following hydraulic formula illustrates a
relationship between Horsepower, Pressure, and
Flow.
Hydraulic Horsepower GPM x PSI x .000583
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HYDRAULIC PUMPS AND POSITIVE DISPLACEMENT
HYDRAULIC PUMPS AND POSITIVE DISPLACEMENT
In its most basic sense, positive displacement
means what you take in you put out. In other
words, for each revolution of a hydraulic pump of
this type, a specific quantity of fluid is
produced relating to the displacement of the pump.
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The Rule of 1500
The rule of 1500 is a engineering reference to a
predictable relationship between Horsepower,
Flow, and Pressure. This is used when sizing an
electric motor for a particular hydraulic system.
In any hydraulic system operating at a pressure
of 1500psi, every gallon of flow produced by the
pump will require at least one horsepower to
drive it.
In pneumatic systems there is a similar
relationship.
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Pumps and Compressors
Before we go any further it should be pointed out
that no matter what design of pump or compressor
is being discussed they all produce flow the same
way.
Pumps and compressors produce flow by creating a
pressure differential. An example would be a
person drinking water through a straw.
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Pumps and Compressors
Hydraulic Pump Symbol
Pneumatic Compressor Symbol
Its important to note that the above symbols do
not indicate a specific design type, just
function. As you can see, the only difference is
the triangle.
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Vane Pumps
In the above illustration, only the internal
parts are shown. Normally, one port would be
connected to the increasing volume side and
another port would be connected to the
decreasing volume side. The outer piece does
not move. The center piece rotates and is off
center. The dark lines are the vanes and they
move in and out.
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Vane Pumps
To understand pump operation, first imagine that
the area in green is attached to a port and is
under low pressure. Fluid, as influenced by
atmospheric pressure, rushes in to fill the voids
as the assembly rotates.
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Vane Pumps
As the fluid passes from left to right it becomes
trapped between the rotating group and the
outlet port. Fluids always take the path of
least resistance as does electricity so out to
the system it goes. All pumps operate in this
manner regardless of design or configuration.
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Balanced Vane Design
In normal operation most pumps are loaded to
one side because of pressure at the outlet port.
This has an effect on bearing life. A balanced
vane pump has its ports located in four distinct
locations around its shaft to offset this effect
and extend service life.
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Cartridge Assembly
A lot of vane pump manufacturers have
incorporated the rotating group into a removable
assembly that can be replaced independently of
the housing.
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Double Pump
Schematic symbol for a double pump
Although vane pumps are sometimes put together in
pairs to form a double pump, any design could
be made a double pump. All this means is that
you have two pumps driven by one motor which may
have their flows put together or separated.
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Variable Volume Vane Pump
Variable volume means that the amount of oil
which is displaced by a pump each revolution can
change whereas in otherfixed displacement
models it cannot. What controls the amount of
oil displaced by fixed displacement pumps?
Speed and Displacement
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Variable Volume Vane Pump Operation
The key to understanding this illustration is
knowing that displacement depends on the amount
of offset that exists between the rotor and cam
ring. The more the offset the more the
displacement and the less the offset the less the
displacement. If the rotor becomes centered,
there is NO displacement. If the rotor travels
from one side to the next, flow reverses ports.
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Volumetric Output of a Pump
Theoretical Pump Flow Speed x Displacement

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What this means is that other than an internal
mechanical mechanism that changes flow rate the
only two thins that control flow are the physical
size of the pump and how fast you run it.
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Pressure Compensated Variable Volume Vane Pump
Operation
As pressure builds in the system it is felt
everywhere including the pump. The cam ring will
push away from the pressure direction toward the
path of least resistance which is the spring.
When the pressure of the system is equal to the
tension of the spring, the rotor will be in the
center of the cam ring and flow will stop while
pressure is maintained.
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Pressure Compensated Variable Volume Vane Case
Drain
All pumps experience internal leakage but it is
worst in the models illustrated here. To
alleviate this pressure, and thus prevent the
front seal from blowing out, a case drain is
provided.
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Gear Pumps
In a gear pump, an increasing volume is generated
as teeth un-mesh or move away from each other.
The fluid drawn in is forced around the teeth,
not through the middle. As the teeth move toward
each other, fluid is forced from the outlet port.
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Piston Pumps
There are two major categories of pumps Axial
and Radial.
Axial(swash plate)piston pump
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Piston Pumps
Radial piston pump
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Piston Pumps
Piston pumps operate under the same controlling
principles as all other pumps. With this design,
a piston moves back and forth in a barrel. As
the piston moves back, a larger volume is created
that provides a vacuum. As the piston moves
forward, the volume is decreased and fluid is
forced out. Axial piston pumps have pistons that
move in parallel to the drive shaft axis. Radial
piston pumps have pistons that move at 90 degrees
to the drive shaft axis. Either type can be made
variable volume by adjusting the amount of stroke
the piston travels in the cylinder bore.
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Pressure Compensated Axial Piston Pump
Low pressure-full stroke condition
Compensator fires at pressure setting- no flow
In the axial pump above, a pressure build up
causes the compensator rod to push against the
swash plate which in turn decreases the amount of
flow to zero when the tension of the spring has
been reached.
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Overcenter Axial Piston Pumps
As in the vane pump, reverse flow in the piston
pump is accomplished by moving the rotating group
beyond a center point. In the piston pump the
swash plate is the member that moves to or 0
degrees to achieve this feature. These types of
pumps are often found in hydrostatic
transmissions.
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Compressors
Compressors operate by drawing in air at lower
than atmospheric conditions and then trapping,
and compressing it. Once compressed, the air is
allowed to escape to the path of least
resistance, usually the receiver tank. All
compressors operate under the same principles as
pumps but the fluid is a gas.
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Compressors
Compressors fall into one of two main categories
Dynamic and Displacement.
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Dynamic Compressors
Dynamic compressors are not positive
displacement. They move air by adding kinetic
energy to it or in other words they throw the
air. Examples of dynamic compressors would
include a leaf blower, hair dryer, and common
fan. Dynamic compressors are known for low
pressures but high volumes of air. A jet engine
is another example of a dynamic compressor.
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Displacement Compressors
Standard displacement compressors can be single
stage where the air is compressed once or
multi-stage where the air is compressed two or
more times to achieve higher efficiency. In
operation, air is drawn in as the piston moves
down. When the piston moves up air is compressed
and then released to the receiver tank.
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Multi-stage Compressors
In multi-stage compressors, air is compressed
twice in order to get it to the receiver tank at
a higher pressure but lower temperature. Single
stage compression is less efficient because so
much heat is given up in the receiver which
translate into lost pressure.