Positive Displacement Pump
Positive Displacement Pump
These notes include descriptions of the following pump types
Below are identified various types of positive displacement pumps which are used in hydtrostatic drive systems
with basic notes identifying operating information.
External Gear Pump
The gear pump is used for fluid transfer and power transfer and process duties. The gear
pump is widely used in the agricultural and mobile vehicle industry for hydraulic
transmission systems.
The pump can be used for transferring a wide variety
of fluids depending on the materials of construction.
The pump includes two gears one gear is driven by the prime mover. The pump
bearings are generally provided with internal bearings and packed glands or mechanical
seals. The most popular gear types are straight spur. These can be
noisy and subject to vibration if they are not manufactured to high standards.
Helical gears can be used to minimise vibration but high side loads result.
The used of double helical gears to eliminate side loads results in expensive costly units.
These pumps are reliable low cost units which can be run for long periods if operated correctly. They have good high pressure
operating characteristics. Close tolerances are required between the internal components for the pump
to operate effectively
The gear pump has moderate efficiency..
These pumps can transfer fluids at reasonable flow rates at developed heads of up to 200 bar. For pressures
above 50 bar the pumps have to be specially designed with hydraulic balancing. These pumps have moderate self priming capabilities
Approx Efficiencies of a typical gear pump /motor
Internal Gear Pump
The internal gear pump has similar characteristics to the external gear pump. The pump has improved suction
and delivery characteristics and is smoother in operation. The pump is based on an external gear located within,
and meshing with, a larger internal gear. A crescent vane is included to seperate the inlet volume from the discharge
volume between the two gears.
The internal gear pump is however more complicated and expensive to manufacture and maintain than the external gear pump.
The internal gear pump can develop liquid pressures of 200 barg.
Note: Gerotor pumps very similar to internal gear pumps without the vane . The operation of tyhe gerotor pump is similar to that of an internal gear pump. The inner gear rotor (gerotor element) is power driven and draws the outer gear rotor around as they mesh together. This forms the inlet and outlet discharge pumping chambers between the rotor lobes. The tips of the inner and the outer lobes make contact to seal the pumping chambers from each other. The inner gear has one tooth less than the outer gear, and the volumetric displacement is determined by the space formed by the extra tooth in the outer gear.
Lobe Pump
This pump is based on two parallel rotors located within a shaped case.
The rotors include a number of lobes these are arranged such that as the rotors are
rotated they contain spaces which increase and reduce in volume.
Fluid enters these spaces through the inlet connection and is trapped as the rotors
rotate. The fluid is compressed and forced out of the discharge connection
as the rotor continues to rotate. This pump is effectively a development of
the external gear pump.
The rotors are synchronised by external timing gears
and therefore the internal contact between the lobes is a sealing contact and not a
driving contact. The rotors need not actually contact.
Various
shapes of rotor are used, the tri-lobe rotor is probable the most popular. The
lower the number of lobes the better the pump is for handling viscous and solids laden
fluids. The rotor can be made from a wide selection of materials from exotic
steel to synthetic rubber-with steel internal support. When soft rotors are
used this type of pump can achieve high levels of volumetic efficiency.
This type of pump includes for relatively low internal fluid velocities with low level of shear. The resulting flow
includes some level of pulsation. The pump can run dry, subject to the design of the bearings and the pump is
self priming especially if the rotors are wetted. As the pump has clean internal surface with few crevices the pump can be
used for hygiene related applications.
The pump can transfer fluid at flowrates up to 500 m3/hr (200 dia pump) and can deliver total heads
of 20 bar.
Sliding Vane Pump
The vane pump includes a ring mounted inside a cylindrical case The ring
includes a number of radial slots in which are located sliding vanes. The ring
is mounted eccentric to the case and the vanes are designed to press against the inside
wall of the case. The vanes are forced against the wall by hydraulic pressure or
spring force or due the the centrifugal force resulting as the ring is rotated.
The prime mover is use to rotate the ring and liquid flow into compartments between the
vanes and the case inner circumference. As the ring rotates the liquid is
trapped in the compartment and is then compressed and forced out through the discharge
connection.
The older designs of vane pump are based on an eccentric ring as described above . These are not hydraulically
balanced and are thus limited in the hydraulic presssure which can be developed. More modern designs include
for an elliptical inner ring which results in two pressure cycle per revolution. These pumps can develop much
higher pressures at high rotational speeds.
The vanes outer edges are subject to continuous wear and the vanes need to be replaced
after periods of continuous use. Modern pumps are design for convenient
maintenance by having the internal components design as cartidges.
The hydraulic circuit based on these pumps should include a relief valve.
This design of pump include a large number of mechanical parts related to its duty.
In transfer duties these pumps can develop high suction heads. They are smooth
operating and have higher efficiency compared to gear pumps.
They can handle suspended non-abrasive solids. Certain designs can
tolerate significant vane wear. (carbon vanes)
This type of pump when pumping hydraulic oil can develop head of 200 barg.
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Approx Efficiencies of a typical vane pump /motor
Rotary Piston Pumps
This general type of pump includes a number of variations some of which are described below.
- Radial Piston Pump
- Swashplate Piston Pump
- Wobble Plate Pump
- Bent Axis Piston Pump
The pumps are extensively used for power transfer applications in the off shore ,
machine tool industry, power transmission , agricultural, aerospace and construction industries,...etc.etc.
All of these pumps work on a similar principle.
The pump includes a block with a number of symmetrically arranged cylindrical
pistons around a common centre line. The pistons are caused reciprocate in and
out under the action of a separate fixed or rotating plate (axial piston pump) or an eccentric bearing ring (radial piston pump)
or some other mechanical feature. Each piston is interfaced with the inlet and
outlet port via a special valve arrangement such that as it moves out of its cyclinder it draws
fluid in and as it moves back it pushes the fluid out. The pumps are
engineered to allow rotational speeds from less the 1 RPM to over 25,000RPM.
Radial Piston Pumps.
Radial Piston pumps include a rotating cylinder containing equally spaced radial pistons
arranged radial around the cylinder centre line. A springs pushes the pistons
against the inner surface of an encircling stationay ring mounted eccentric to the cylinder.
The pistons draw in fluid during half a revolution and drive fluid out
during the other half. The greater the ring eccentricity the longer the pistons stroke
and the more fluid they transfer.
Swashplate Pumps.
Swashplate pumps have a rotating cylinder containing parallel pistons arranged radially around the cylinder centre line.
A spring pushes the pistons against a stationary swash plate located at one end of the cylinder ,
which sits at an angle to the cylinder. The pistons draw in fluid during half a revolution and drive fluid out
during the other half. The greater the swashplate angle relative to the cylinder centre line the further the
longer the pistons stroke and the more fluid they transfer.
Wobble Plate Pumps.
This pump includes a stationary piston block containing a number parallel pistons arranged radially
around the block centre(at least five). The end of each piston is forced against a rotating
wobble plate by springs. The wobble plate is shaped with varying thickness around its centre line and thus as it rotates
it causes the pistons to reciprocate at a fixed stroke.
The pistons draw in fluid from the cavity during half a revolution and drive fluid out
at the rear of the pump during the other half. The fluid flow is controlled
using non-return valves for each piston.
These pumps can generate pressures of up to 700 bar.
Bent Axis Pump.
Bent axis piston pumps have a rotating cylinder containing parallel pistons arranged radially
around the cylinder centre line. The cylinder is driven by an shaft which is arranged at an
angle to the cylinder axis. the shaft includes a flange with a mechanical connection to each
piston. As the shaft rotates the pistons are made to reciprocate over a stroke
based on the relative angle of the shaft and cylinder.
Approx Efficiencies of a typical piston pump /motor
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