floating cup principle

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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. 1

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Page 1: Floating Cup Principle

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

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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

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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

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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

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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

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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.

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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

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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:

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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

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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.

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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.

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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.

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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

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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.

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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.

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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.

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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:

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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:

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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

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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

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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

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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

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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

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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

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www.yahoo.com www.howstuffworks.com

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