floating cup principle
TRANSCRIPT
1. Introduction to Pumps: One of the important element to considered as the heart of hydraulic
system is the power generating element. Power generating elements
are those which imparts power to the fluid using mechanical energy
or in other words a device which converts mechanical energy into
hydraulic energy is called Hydraulic pump.
Hydraulic energy is a source of hydraulic power. It imparts hydraulic
energy to the oil. Fig shows the pump as a source of hydraulic
energy.
The mechanical energy delivered to the pump via a prime mover such
as an electric motor. Due to mechanical action, the pump creates a
partial vacuum at its inlet. This permits atmospheric pressure to
force the fluid through the inlet line and into the pump. The pump
then pushes the fluid into the hydraulic system.
Pressure in the system develops from resistance to the flow
determined by the force needed to move the load (i.e., cylinder or
1
fluid motor). A pump rated for 35 000 kPa (5000 psi), for example, is
capable of operating at that pressure.
2. Pumping theory:
A pump operates on the principle whereby a partial vacuum is created at pump inlet due to the internal operation of the pump. This allows atmospheric pressure to push the fluid out of the oil tank (reservoir) and into the pump intake. The then mechanically pushes the fluid out of the discharge line.
This type of operation can be visualized by referring to the simple piston pump of fig. Note that this pump contains two ball check valve, which are described as follows:
Check valve1 is connected to the pump inlet line and allows fluid to enter the pump only at this location.
Check valve2 is connected to the pump discharge line and allows the fluid to leave the pump only at this location.
As the piston is pulled to the left, a partial vacuum is created in pump cavity 3, because the close tolerance between the piston and cylinder (or the use of piston ring seal) prevents air inside cavity 4 from traveling into cavity 3.this flow of air, if allowed to occur, would destroy the vacuum. This vacuum holds the ball of check valve 2 against its seat and allows atmospheric pressure to push fluid from the reservoir into the pump via check valve1. this inlet flow occurs because the force of the fluid pushes the ball of the check valve1 off its seat.When the piston is pushed to the right, the fluid movement closes inlet valve 1 and opens outlet valve 2.the quantity of the fluid
2
displaced by the piston, is forcibly ejected out the discharge line leading to the hydraulic system.
3. Classification of pumps:There are two broad Classifications of pumps as identified by the fluid
power industry.
1. Hydro-dynamic or Non positive displacement pump (NPD):
Pumps wherein of the fluid in motion is used to displace and
transfer the fluid are called non positive displacement pumps. These
types of used for low pressure and high volume applications. Their
application is limited in the field of fluid power. They are primarily
used for transfer of fluid from one point to another.
Centrifugal and axial flow pumps are examples of this type.
2. Hydro-static or Positive displacement pump:
This type is universally used for fluid power systems. As the name
implies, a positive displacement pump ejects a fixed amount of fluid
into the hydraulic system per revolution of the pump shaft rotation.
Such a pump is capable of overcoming the pressure resulting from
the mechanical loads of the system as well as the resistance to flow
due to friction.
There are three types of positive displacement pumps: Gear, Vane
and Piston pumps.
Gear pumps:
a. External gear pumps
b. Internal gear pumps
c. Lobe pumps
d. Screw pumps
3
Vane pumps:
a. Unbalanced vane pump (fixed or variable displacement)
b. Balanced vane pumps (fixed displacement only)
Piston pumps:
a. Axial design
b. Radial design
Axial piston pumps:
In the axial piston pump, the pistons stroke in the same
direction on a cylinder block’s center line (axially). Axial piston
pumps may be an in-line or angle design. In capacity, piston
pumps range from low to very high. Pressures are as high as
5,000 psi, and drive speeds are medium to high. Efficiency is
high, and pumps generally have excellent durability. Petroleum
oil fluids are usually required. Pulsations in delivery are small
and of medium frequency. The pumps are quiet in operation but
may have a growl or whine, depending on condition. Except for
in-line pumps, which are compact in size, piston pumps are
heavy and bulky.
(1) In-Line Pump:
In an in-line piston pump (diagram A), a drive shaft and
cylinder block are on the same centerline. Reciprocation of
the pistons is caused by a swash plate that the pistons run
against as a cylinder block rotates. A drive shaft turns a
cylinder block, which carries the pistons around a shaft. The
piston shoes slide against a swash plate and are held against
it by a shoe plate. A swash plate's angle causes
4
the cylinders to reciprocate in their bores. At the point where a
piston begins to retract, an opening in the end of a bore slides over
an inlet slot in a valve plate, and oil is drawn into a bore through
somewhat less than half a revolution. There is a solid area in a valve
plate as a piston becomes fully retracted. As a piston begins to
extend, an opening in a cylinder barrel moves over an outlet slot, and
oil is forced out a pressure port.
Pump displacement depends on the bore and stroke of a piston and
the number of pistons. A swash plate's angle (Figure 3-19, diagram B)
determines the stroke, which can vary by changing the angle. In a
fixed angle's unit, a swash plate is stationary in the housing. In a
variable unit's, it is mounted on a yoke, which can turn on pintles.
Different controls can be attached to the pintles to vary pump
delivery from zero to the maximum. With certain controls, the
5
direction of flow can be reversed by swinging a yoke past center. In
the center position, a swash plate is perpendicular to the cylinders,
and there is no piston reciprocation; no oil is pumped.
Bent-Axis Axial Piston Pump:
In an angle- or a bent-axis-type piston pump, the piston rods are
attached by ball joints to a drive shaft's flange. A universal link keys
a cylinder block to a shaft so that they rotate together but at an
offset angle. A cylinder barrel turns against a slotted valve plate to
which the ports connect. Pumping action is the same as an in-line
pump. The angle of offset determines a pump's displacement, just as
the swash plate's angle determines an in-line pump's displacement.
In fixed-delivery pumps, the angle is constant. In variable models, a
yoke mounted on pintles swings a cylinder block to vary
displacement. Flow direction can be reversed with appropriate
controls.
6
4. Problems associated with axial with Axial piston pump:
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, maybe, 3000 rpm. The pistons are usually less than half an
inch (13 mm) in diameter with similar stroke lengths. Keeping the
wall to piston seal tight means that very small clearances are
involved and that material 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 (a sliding contact) 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 (250 °F) 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
7
the reservoir or to the inlet side of the pump again. Hydraulic fluid
that has been used is always cooled before recalculating through the
pump. It is also filtered by micrometer-sized filters before reuse too.
Despite the problems indicated above this type of pump can contain
most of the necessary circuit controls integrally (the swash-plate
angle control) to regulate flow and pressure, be very reliable and
allow the rest of the hydraulic system to be very simple and
inexpensive.
5. Floating Cup:
8
The Floating Cup principle is a new axial piston principle for hydrostatic
pumps, motors and transformers. It can be manufactured utilizing low cost
production technologies. Through drive of multiple units is possible. The
sound output is low, due to a balanced design and low pressure and flow
pulses. Torque efficiency is unequalled, also at very low speed (more than
95% at 0.1 rpm and 350 bar). The overall efficiency lies above current axial
piston pumps. 'Floating Cup' refers to the cylinders of the principle. Each
piston gets its own cup-like cylinder. These cups are free floating on a
barrel plate.
On average the cups and the barrel rotate at the same rotational
speed. A closer look at the kinematics of the floating cup principle
however reveals that the cups make a small movement on the barrel
plate. The size of this cup trajectory is strongly dependent on the tilt
angle between the barrel and the rotor. Furthermore, the nonuniformity
of the joint between the barrel and the rotor shaft can create an angular
difference between the cup and the barrel position. This article will
focus on the combined effect of the barrel tilt angle and the
nonuniformity on the cup movement.
6. Construction
9
The core of the Floating Cup principle is the shaft on which the rotor plate is fixed. The pistons are locked onto the rotor: there is no movable joint between the pistons and the rotor. The pistons are double faced. Unlike conventional axial pistons machines the pistons are not running in a collective cylinder block or barrel. Instead each piston has its own cup-like cylinder. The cylinders are supported by means of a barrel, one on each side of the rotor.To create a positive displacement the barrel plates have to be maneuvered in an angular position. This makes the cylinders move up and down over the pistons.
10
11
12
a. Key elements:
Characteristic are two elements in the design: The pistons and the
cups. The pistons are fixed in a rotor thereby avoiding the expensive
piston joint applied in bent axis units as well as the slippers that are
used in in-line pumps and motors.
The cup like cylinders are (hydrostatically balanced) floating on
the barrel plates. Like in all piston machines they seal off the
displacement volume. The piston seals directly to the cup, without
piston rings, thereby minimizing the friction.
Where as in the conventional axial piston pump the piston rods are
connected to the drive shaft flange by ball-and-socket joints. The
pistons are forced in and out of their bores as the distance between
the drive shaft flange and the cylinder block changes. A universal
joint connects the block to the drive shaft to provide and positive
drive.
In in-line piston pumps, the pistons are connected to a shoe plate
which bears against an angled swash plate. As the cylinder rotates
the piston reciprocate because the piston shoe follows the angled
swash plate.
13
b. Universal joint:
The barrels are directly driven by the shaft. Therefore a kind of
homokinetic joint is introduced on both sides. The driving torque for
the barrels is limited to some friction and inertia forces. There is no
hydraulic power supplied to or taken by the barrels. The conversion
from hydraulic power to mechanical power (or vice versa) occurs
directly in the cylinders.
The relative movement between the cylinder cups and the barrel
plate is small, much smaller than for instance between a slipper and
the swash plate in case of an in-line pump. This is important for
wear reduction and friction losses.
14
c. Double configuration
The Floating Cup principle has been built with double faced pistons to
create a mirrored design. An important advantage of this
construction is the complete balancing of hydraulic forces in axial
direction. This enables the use of small, simple bearings.
When built completely symmetrical, the displacement generated on
each side of a piston pair is completely in phase and the whole unit
will behave as a 12-piston machine. However, in order to reduce flow
pulsations, pressure pulsations and noise, it is more attractive to
have 24 displacement volumes. This is realized by simply changing
the orientation of the two port plates around the central shaft of the
unit. Although this slightly affects the complete balancing of the
hydraulic forces in axial direction, the torque on the shaft is still very
small compared with conventional axial displacement units and also
in conventional type it is very tough task to obtain a complete
balancing of hydraulic forces. Though the conventional is small and
compact they are noisy in operation.
Normally the axial piston pumps are most expensive and provide the
highest level of performance. They can be operated at high speeds
(up to 5000 rpm) to provide a high horsepower to weight ratio. They
produce essentially a nonpulsating flow and can operate at the
highest pressure levels. Due to very close fitting pistons, they
efficiencies compare to that of gear type or vane type. Since no side
load occurs to the pistons, the life expectancy is at least several
years. However, because of their complex design, piston pumps
cannot normally be repaired in the field.
7. Characteristics 15
Characteristic for the floating cup technology are the low friction of
the principle leading to a high efficiency and low starting torque. The
pulsations are low, especially due to the high number of pistons.
Noise emissions are low due to low pulsations and the balanced
construction. Furthermore the floating cup can be produced at low
costs. The power density of the slipper type, bent axis and Floating
Cup machine are comparable.
Measurements by the Institute fur Fluid-technische Antriebe und
Steuerungen of the University of Aachen (IFAS) on the latest
prototypes prove the high efficiency and low torque loss of the
Floating Cup concept.
16
a. Low friction
In the Floating cup design the hydrostatic forces do not cause any
friction between the cup and piston, contrary to the situation in the
slipper type and bent axis machines. Consequently the torque loss of
the FC machine is very small and not dependent on the operating
pressure.
Because of the increased piston number the torque variation is very
small and together with the small torque loss this guarantees an
excellent start up behavior,
As the hydrostatic forces on each piston and subsequently also on
the rotor are balanced small. This means low bearing friction, low
noise and low cost
Measurements on the latest prototypes by the Fluid Power Institute
(IFAS) of the Technical University of Aachen proves the high
efficiency and low torque loss of the Floating Cup concept.
17
b. Low pulsations:
The advantage of having a phase shift is of course that the number of
displacement volumes is effectively doubled and, consequently, the
pulsations are out of phase. This leads to a very smooth flow output.
This again has benefits in terms of reduction of wear of the hydraulic
system and the decrease of leakage, for example in fittings and hose
connections.
c. Low noise:
Doubling the number of pistons by introducing the phase shift of the
port plates has positive effects on lowering the noise levels. Reducing
pressure pulsations directly affects the sound output. Pressure
pulsations generated by the pump in the system will be reduced as
well. This results in a reduction of fluid borne noise.
Also the sources of mechanical sound are strongly reduced. Most
important in this aspect is the almost complete balancing of hydraulic
forces. The forces on the bearings decrease, reducing the transfer of
pulses and vibrations to the housing. This results in lower noise
levels.
d. Low mass:18
Comparison of the weight of the FC pump with conventional slipper
type and bent axis type pumps shows interesting differences.
Especially in larger displacement volumes, the Floating Cup pump has
a much higher power to weight ratio. The larger the displacement
volume of the pump or motor gets, the lower the mass of the FC
principle will be in comparison with bent axis or slipper type
machines. The weight of an average 125 cc Floating Cup pump is 28%
less than a 128 cc state of the art bent axis. Compared with a slipper
type pump the weight of the FC is 62% less.
e. Low cost:
19
At first sight it may seem that the FC concept will result in a cost
increase, given the high number of parts. However, quite the
opposite is true. The cups can be manufactured using low cost metal
forming techniques like deep drawing. Many of the other parts of the
design can be manufactured with the same or similar non-swarf
technologies like forging and fine blanking. Their precision and
surface quality is excellent for hydraulic parts. In the automotive
world these production methods are already widely used, including
for hydraulic components like hydraulic valve lash adjusters.
The FC concept also offers cost advantages because of reduced
tolerances. The introduction of the floating cups breaks the chain of
tolerances, which is hindering the possibilities for cost reduction of
conventional axial machines. An expensive barrel is replaced by low
cost parts. Compared with current bent axis pumps and motors, the
costs of the bearings are strongly reduced. Finally regarding costs for
use of multiple units, through drive of two or more units (piggy
backing) can be realized easily.
8. Measurements:
20
Efficiency, low speed (0.1 rpm) and pulsations have been measured
on a 28cc Floating Cup pump with 24 pistons. To make a comparison
of the test data possible, a bent-axis pump and a slipper type pump
have been tested under the same conditions. Measurements were
conducted in accordance with ISO 4409, by the Technical Universities
of Aachen and Eindhoven. As the Floating Cup pump is still under
development, further improvements are expected.
a. Efficiency measurements:
Efficiency of the Floating Cup pump has been measured in a field of
pressures ranging from 50 to 350 bar (50 bar intervals) and speeds
from 500 to 3000 rpm (500 rpm intervals). In the 4 figures on the
21
left, hydraulic and hydro mechanical efficiencies are combined into
overall efficiency. Measurements of the Floating Cup pump have
been compared to a bent axis, as well as slipper type pump.
All measurements were performed at an oil temperature of 40º C with
HLP46 oil. The measurements were performed in accordance with
ISO4409
b. Pulsations:
IFAS has performed comparing measurements for pressure pulsations
in the output line. A Floating Cup pump and a bent axis pump have
been measured. Shown below are the individual pressure pulsations
of these both pumps during one revolution.
c. Low speed measurements:
In order to learn more about the prospects of the Floating Cup
Technology for hydraulic motors, measurements were performed on
the Floating Cup Pump running as a motor at very low speed (0.1
22
rpm). The results were compared with a conventional bent-axis pump
with the same displacement volume (28.2 cc) also running as a
motor.
The three diagrams present the torque losses, the hydromechanical
efficiency and the flow pulsations of the Floating cup machine.
The torque losses and torque variations during a revolution are
extremely low and almost independent from the input pressure. The
low leakage and minimal torque losses result in a high overall
efficiency and excellent motor behaviors.
9. Variable floating cup pump:
The floating cup principle can be made variable by changing the port
plate angles on which the barrel plates are rotating.
The small swash angle allows a compact construction of a variable
23
floating cup pump with a power density above conventional variable
piston pumps. The control mechanism uses control pistons and
cylinders similar to the ones used in the rotation group, minimizing
the costs for the control mechanism. Any conventional control
(pressure, load sensing etceteras) may be connected to the two front
actuators.
10. Application:
The floating cup principle was developed for pumps, hydraulic motors
and transformers.
Several floating cup pumps have been built and tested by both the
24
industry and technical universities. A 70cc hydraulic transformer has
been built and tested in the framework of the EU IBIS program in
which a Mecalac excavator has been fitted with FC transformer
technology.
FC for mobile application:
Through drive
High efficiency
Low noise
Low cost
FC for industrial application:
Low pulsation
Low noise
High efficiency
Through drive
FC for motor application
Excellent startup behavior
Low noise
Compact
Low cost
11. CONCLUSION:
The Floating Cup principle offers many benefits over conventional hydraulic axial
displacement machines. It allows for high efficiency, low noise levels and low
starting torques at a competing price level. Constant displacement Floating Cup
pumps have been build and tested thoroughly and the first Floating Cup pump with
variable displacement has been presented already. It differs from conventional open
circuit pumps in the fact that oil is fed through the swash plates, which vibrate
25
constantly during operation. To secure the high efficiency, the sealing interface
between barrel and swash plate must remain tight under all nominal working
conditions. Here, a method for dynamical analysis is being presented that enables
for the dedicated design of the Floating Cup swash system.
A first prototype based on the new floating cup principle has been designed, built
and tested. The new pump features a high number of pistons arranged in a double
ring, back-to-back configuration. Each piston has a ball shaped end, which is sealing
directly on the cylinder wall.
Experiments have proven the viability of the new concept. The floating cup principle
has demonstrated to be stable in a wide range of pressures and rotational speeds.
Furthermore, in a series of tests conducted by the IFAS of the University of Aachen,
the efficiency of the floating cup pump was measured. It has been proven that the
floating cup pump has a high efficiency in a wide range of operating conditions, with
a maximum efficiency of around 97%. In addition, the hydro-mechanical losses are
very low at the operating condition of low speeds in combination with high loads.
This makes the floating cup principle also very attractive for application in
hydrostatic motors.
Further research needs to be done especially regarding pulsations, noise and costs.
It is expected that the floating cup pump will decrease the pressure pulsations in
the output line by a factor of 4 to 5. Moreover a reduction of fluid borne and
structure borne noise is expected. Finally, contrary to current axial piston machines,
the new pump design can be produced by utilizing modern, low cost production
techniques like extrusion and deep drawing.
BIBLOGRABHY:
Fluid Power-Anthony Esposito Hydraulics & Pneumatics – H.D.Ramachandra www.google.com
26
www.yahoo.com www.howstuffworks.com
27