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 fluid
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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: •
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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 displaced by 2
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 Centrifugal and axial flow pumps flow pumps are examples of this type.
2. Hydro-static or Positive displacement 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 p ositive displacement pumps: Gear, Vane and Piston pumps.
Gear pumps: a. Externa Externall gear pumps pumps b. Interna Internall gear gear pumps pumps c. Lo Lobe be pump pumps s d. Scre Screw w pump pumps s
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Vane pumps: a. Unbalanced Unbalanced vane vane pump (fixed (fixed or variable variable displacemen displacement) t) b. Balanced Balanced vane pumps pumps (fixed displacem displacement ent only) only)
Piston pumps: a. Axia Axiall desi design gn b. Radi Radial al desi design gn
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.
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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 5
controls can be attached to the pintles to vary pump delivery from zero to the maximum. With certain controls, the 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 fixeddelivery 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 stat ionary pump body have to be almost a perfect seal while the rotary part turns at, maybe, 3000 rpm. 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 ver y small clearances are involved and that material have to be closely matched for similar coefficient of expansion. 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, 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. 7
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 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.
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5. Floating Cup:
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 t o 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.
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6. Construction 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. rot or. 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. 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 t o reduce flow pulsations, pulsations, pressure pulsations and noise, 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. perf ormance. 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.
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7. Characteristics 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 b e produced at low costs. 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
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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 st rongly 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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d. Low mass: Comparison of the weight of the t he 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.
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e. Low cost: 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.
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8. Measurements: Efficiency, Efficiency, low speed (0.1 rpm) and pulsations have been measured on a 28cc Floating Cup pump with wit h 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:
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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 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 pulsat ions 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: 22
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 rpm). The results were compared with a conventional bent-axis pump with wit h 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: 23
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 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: 24
The floating cup principle was developed for pumps, hydraulic motors and transformers. Several floating cup pumps have been built and tested by both the 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
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High efficiency
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Low noise
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Low cost
FC for industrial application: •
Low pulsation
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Low noise
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High efficiency
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Through drive
FC for motor application •
Excellent startup behavior
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Low noise
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Compact
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Low cost
11. CONCLUSION: 25
The Floating Cup principle offers many benefits over conventional hydraulic axial displacement machines. It allows for high efficiency, low n oise 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 I t differs from conventional open circuit pumps in the fact that oil is fed through the swash plates, which vibrate 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 e fficiency 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.
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BIBLOGRABHY:
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Fluid Power-Antho P ower-Anthony ny Esposito Hydraulics & Pneumatics – H.D.Ramachandra H.D.Ramachandra www.google.com www.yahoo.com www.howstuffworks.com
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