An axial piston pump is a positive displacement pump that has a number of
pistons in a circular array within a cylinder block. It can be used as a
stand-alone pump, a hydraulic motor or an automotive air conditioning
compressor. Description
An axial piston pump has a number of pistons arranged in a circular array
within a housing which is commonly referred to as a cylinder block, rotor
or barrel. This cylinder block is driven to rotate about its axis of symmetry by
an integral shaft that is, more or less, aligned with the pumping pistons.
Mating surfaces. One end of the cylinder block is convex and wears against a
mating surface on a stationary valve plate. The inlet and outlet fluid of the
pump pass through different parts of the sliding interface between the cylinder
block and valve plate. The valve plate has two semi-circular ports that allow
inlet of the operating fluid and exhaust of the outlet fluid respectively.
Protruding pistons. The pumping pistons protrude from the opposite end of the
cylinder block. There are numerous configurations used for the exposed ends
of the pistons but in all cases they bear against a cam. In variable
displacement units, the cam is movable and commonly referred to as a swash
plate, yoke or hanger. For conceptual purposes, the cam can be represented by
a plane, the orientation of which, in combination with shaft rotation,
provides the cam action that leads to piston reciprocation and thus pumping.
The angle between a vector normal to the cam plane and the cylinder block axis of
rotation, called the cam angle, is one variable that determines the
displacement of the pump or the amount of fluid pumped per shaft revolution.
Variable displacement units have the ability to vary the cam angle during
operation whereas fixed displacement units do not.
Reciprocating pistons. As the cylinder block rotates, the exposed ends of the
pistons are constrained to follow the surface of the cam plane. Since the cam
plane is at an angle to the axis of rotation, the pistons must reciprocate
axially as they precess about the cylinder block axis. The axial motion of
the pistons is sinusoidal. During the rising portion of the piston's
reciprocation cycle, the piston moves toward the valve plate. Also, during
this time, the fluid trapped between the buried end of the piston and the valve
plate is vented to the pump's discharge port through one of the valve plate's
semi-circular ports - the discharge port. As the piston moves toward the
valve plate, fluid is pushed or displaced through the discharge port of
the valve plate. Effect of precession. When the piston is
at the top of the reciprocation cycle, the connection between the trapped fluid
chamber and the pump's discharge port is closed. Shortly thereafter, that same
chamber becomes open to the pump's inlet port. As the piston continues to precess
about the cylinder block axis, it moves away from the valve plate thereby
increasing the volume of the trapped chamber. As this occurs, fluid enters
the chamber from the pump's inlet to fill the void. This process continues
until the piston reaches the bottom of the reciprocation cylinder - commonly
referred to as bottom-dead-center or BDC. At BDC, the connection between the
pumping chamber and inlet port is closed. Shortly thereafter, the chamber
becomes open to the discharge port again and the pumping cycle starts over.
Variable displacement. In a variable displacement unit, if the vector normal
to the cam plane is set parallel to the axis of rotation, there is no movement
of the pistons in their cylinders. Thus there is no output. Movement of the
swash plate controls pump output from zero to maximum.
Pressure. In a typical pressure-compensated pump, the swash
plate angle is adjusted through the action of a valve which uses pressure
feedback so that the instantaneous pump output flow is exactly enough to
maintain a designated pressure. If the load flow increases, pressure will
momentarily decrease but the pressure-compensation valve will sense
the decrease and then increase the swash plate angle to increase pump output flow
so that the pdesired pressure is restored. In reality most systems use
pressure as a control for this type of pump. The operating pressure reaches,
say, 200 bar and the swash plate is driven towards zero angle and with the
inherent leaks in the system allows the pump to stabilise at the delivery volume
that maintains the set pressure. As demand increases the swash plate is
moved to a greater angle, piston stroke increases and the volume of fluid
increases; if the demand slackens the pressure will rise, and the pumped
volume diminishes as the pressure rises. At maximum system pressure the output is
once again almost zero. If the fluid demand increases beyond the capacity of
the pump to deliver, the system pressure will drop to near zero. The swash plate
angle will remain at the maximum allowed, and the pistons will operate at
full stroke. This continues until system flow-demand eases and the pump's
capacity is greater than demand. As the pressure rises the swash-plate angle
modulates to try to not exceed the maximum pressure while meeting the flow
demand. Design difficulties
Designers have a number of problems to overcome in designing axial piston
pumps. One is managing to be able to manufacture a pump with the fine
tolerances necessary for efficient operation. The mating faces between the
rotary piston-cylinder assembly and the stationary pump body have to be almost a
perfect seal while the rotary part turns at perhaps 3000 rpm. The pistons are
usually less than half an inch in diameter with similar stroke lengths.
Keeping the wall to piston seal tight means that very small clearances are
involved and that materials have to be closely matched for similar coefficient
of expansion. The pistons have to be drawn outwards in
their cylinder by some means. On small pumps this can be done by means of a
spring inside the cylinder that forces the piston up the cylinder. Inlet fluid
pressure can also be arranged so that the fluid pushes the pistons up the
cylinder. Often a vane pump is located on the same drive shaft to provide this
pressure and it also allows the pump assembly to draw fluid against some
suction head from the reservoir, which is not an attribute of the unaided axial
piston pump. Another method of drawing pistons up the
cylinder is to attach the cylinder heads to the surface of the swash plate. In
that way the piston stroke is totally mechanical. However, the designer's
problem of lubricating the swash plate face is made even more difficult.
Internal lubrication of the pump is achieved by use of the operating
fluid—normally called hydraulic fluid. Most hydraulic systems have a maximum
operating temperature, limited by the fluid, of about 120 °C so that using
that fluid as a lubricant brings its own problems. In this type of pump the
leakage from the face between the cylinder housing and the body block is
used to cool and lubricate the exterior of the rotating parts. The leakage is
then carried off to the reservoir or to the inlet side of the pump again.
Hydraulic fluid that has been used is always cooled and passed through
micrometre-sized filters before recirculating through the pump.
Uses Despite the problems indicated above
this type of pump can contain most of the necessary circuit controls
integrally to regulate flow and pressure, be very reliable and allow the
rest of the hydraulic system to be very simple and inexpensive.
Axial piston pumps are used to power the hydraulic systems of jet aircraft, being
gear-driven off of the turbine engine's main shaft, The system used on the F-14
used a 9-piston pump that produced a standard system operating pressure of
3000 psi and a maximum flow of 84 gallons per minute.
They are also used in some pressure washers. For example Kärcher has several
models powered by axial piston pumps with three pistons.
Axial reciprocating motors are also used to power many machines. They operate on
the same principle as described above, except that the circulating fluid is
provided under considerable pressure and the piston housing is made to rotate and
provide shaft power to another machine. A common use of an axial reciprocating
motor is to power small earthmoving plant such as skid loader machines.
Another use is to drive the screws of torpedoes.
See also Swashplate engine
References External links
Figure 8 is an axial piston pump www.rotarypower.com, Manufacturer of
Axial Piston Pumps Tecnapol, Axial Piston Pumps
repair/rebuild