Lecture Five
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
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.
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 1500 psi, every
GPM of flow produced by the pump will require at least one
horsepower to drive it. This relationship is linear – in other
words, at a pressure of 750 psi, each GPM of flow would require
½ horsepower. At 3000 psi, each GPM would need 2 horsepower,
and so on.
In pneumatic systems there is a similar relationship. However,
since the relationship is not linear (because air compresses), the
math required to calculate it is beyond the scope of this course.
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, all pumps
produce flow the same way.
Pumps and compressors produce flow by creating a “pressure
differential.” Fluids always flow from a higher pressure to a lower
pressure. By using the increasing pump volume at the inlet, the
pump (or compressor) creates a region of decreasing inlet pressure
(vacuum). This causes the higher reservoir pressure (atmospheric) to
push the fluid to the lower pressure inlet area. An example would be
a person drinking water through a straw. One sucks, creating a
lower pressure (vacuum) inside the mouth, and the higher
(atmospheric) pressure on the liquid surface pushes it up the straw.
The fluid is trapped inside the pump, and it is carried to the outlet
where the volume is decreased and pressure increased. Fluid will
take the path of least resistance, which is out the device outlet.
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. Remember, pumps and compressors
only produce FLOW. Pressure is created by resistance to that
flow. If there is no resistance, flow moves at atmospheric
pressure, and there is no (gage) pressure.
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 (called the “cam ring”) does not move.
The center piece (the “rotor”) rotates and is off center. The dark
lines are the “vanes”. They move in and out because of centrifugal
force to maintain a seal against the cam ring.
Vane Pumps
To understand pump operation, first imagine that the area in
green is attached to an inlet port and is under low pressure
(vacuum). Fluid, as influenced by the atmospheric pressure in the
reservoir, rushes in to fill the voids as the assembly rotates and
the volume increases causing a pressure decrease (Boyle’s Law).
Vane Pumps
As the fluid passes from left to right it becomes “trapped” between
the rotating group and the pump housing. It is carried around to the
outlet port. There, the volume containing the fluid decreases, causing
the pressure to increase. 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
The previous slides showed normal vane pump. In normal operation
vane and gear pumps are “loaded” to one side because of pressure at
the outlet port. This “loading” has an adverse effect on bearing life.
A balanced vane pump has its ports located in four distinct locations
around its shaft (2 inlets and 2 outlets, each 180 ° apart) to balance
the forces on the shaft. The result is extended bearing service life.
Cartridge Assembly
A lot of manufacturers of fixed displacement vane pumps have
incorporated the rotating group into a removable assembly that
can be replaced independently of the housing, so piping can
remain connected. This assembly, called a “cartridge”, consists of
the rotor, vanes, cam ring and port plates. It minimizes down time
to rebuild a pump, and reduces piping leaks in the system.
Cartridge assemblies are rarely used in unbalanced vane pumps.
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.
Typically, one of the pumps is several times larger than the
other, so the larger pump will produce much more flow. An
application for this type of pump will be discussed in Unit 11.
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 “total amount” of oil
(flow - GPM) produced by a fixed displacement pump?
Speed and Displacement
Variable Volume Vane Pump Operation
The key to understanding this illustration is knowing that amount of
displacement depends on the 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. Flow STOPS, but outlet
pressure is maintained. If the rotor travels past the cam ring
centerline from one side to the other, flow reverses ports.
Volumetric Output of a Pump
Theoretical Pump Flow = Speed x Displacement
231
What this means is that if there is no internal mechanical
mechanism that changes the displacement volume
causing flow rate change, the only two things that control
flow from a pump 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 outlet. As outlet pressure rises, 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, Then
flow will stop while pressure in the system is maintained as
determined by the spring adjustment.
Pressure Compensated
Variable Volume Vane Case
Drain
All pumps experience internal leakage but it is worst in the
models illustrated here. To alleviate the internal pressure
caused by this leakage, a case drain is provided. This
relieves the internal pressure and prevents the front seal on
the pump shaft from blowing out,
Gear Pumps
In a gear pump, an increasing volume (therefore, decreasing
pressure) is generated as teeth un-mesh or move away from each
other. The fluid drawn into the pump inlet from the reservoir is
forced around the teeth toward the pump outlet, not through the
middle. As the teeth move toward each other, the volume decreases,
and the increasing pressure forces the fluid out the outlet port.
Piston Pumps
There are two major categories of pumps: Axial and
Radial.
Axial(swash plate)piston pump
Piston Pumps
Illustrated above are examples of Radial piston pumps. Their pistons meet the drive
shaft axis at 90 degrees . They are more compact yielding space savings and can be ran
at a wide speed range. As motors, they are very popular in cabling systems.
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 (pressure increases) 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. Axial piston pumps are, by far, the most common.
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 stroke of the pistons. This reduces amount of flow to
zero when the pressure completely balances the tension of the
spring, in the same manner as the variable volume pressure
compensated vane pump. The symbols for the pressure
compensated vane and piston pumps are the same.
Over-center (reversible) 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. The symbol above is could indicate any the
piston and vane pump with reverse flow capability. The
description of the component on the system schematic would
tell the technician the details of the pump used.
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 into the receiver tank. Compressors create flow
using the same differential pressure principles as pumps do, but
the fluid is a gas. Pump and Compressor symbols are identical,
except for the flow arrow. In both cases, the arrow points “out” of
the device, indicating flow is produced. For pumps, the arrow is
solid – for compressors, transparent. For a quick determination if
a schematic is hydraulic or pneumatic, find the flow producer
symbol. Solid is hydraulic – transparent is pneumatic.
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 primarily used
for low pressures but high volumes of air. Dynamic compressors
(fans) are very common at low pressures. When typical pneumatic
system pressures are required, their high cost limits applications to
oil free air requirements, such as in the food and pharmaceutical
industries. 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. The
hot compressed air is cooled after the first stage in an “intercooler”
to reduce the air temperature entering the next stage. Since the
entering air temperature is lower, the outlet air temperature is also
lower. Single stage compression is less efficient because more heat
has to be given up in the receiver which translates into lost pressure.
Screw Compressors
Dry and Oil flooded
Screw Compressors
Oil Separation
Review
1.
Explain the purpose of the pump.
2.
List three different types of pumps.
3.
Assuming fixed displacement, what two things determine the flow rate of a pump?
4.
What is the relationship between flow and input horsepower?
5.
What is the rule of 1500?
6.
Name the major internal components of a vane pump assembly.
7.
What type of vane pump is balanced?
8.
What is pressure compensation?
9.
What is the purpose of a case drain in a hydraulic pump?
10.
What is the difference between a axial piston pump and a radial piston pump?
11.
What compressor type is the most common?
12.
What is multi-staging?
13.
Why is multi-staging more efficient?
14.
What is a dynamic compressor?
15.
What is the most common use for dynamic compressors?