Transcript Document

Unit Seven
Unit Seven: Pumps and
Pumps andCompressors
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
HYDRAULIC PUMPS AND POSITIVE
HYDRAULIC
PUMPS AND POSITIVE
DISPLACEMENT
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Variable Volume Vane Pump
“Variable volume” means that the “amount” of oil which is
displaced by a pump each revolution can change whereas in
other”fixed” displacement models it cannot. What controls the
“amount” of oil displaced by fixed displacement pumps?
Speed and Displacement
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.
Volumetric Output of a Pump
Theoretical Pump Flow = Speed x Displacement
231
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.
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.
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.
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.
Piston Pumps
There are two major categories of pumps: Axial and
Radial.
Axial(swash plate)piston pump
Piston Pumps
Radial piston pump
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.
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.
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.
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.
Compressors
Compressors fall into one of two main categories; Dynamic and
Displacement.
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.
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.
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.