PUMPS....................... ................
INTRODUCTION ON PUMP
A Pump is an essential for any hydraulic system, as it acts as the input devices, with takes the power from the prime mover and expends it to the actuator to do some useful work. The function of a pump is similar to the function of the heart of a human body. Heart sucks the blood from the artery and sends it to the veins. Hydraulic pump sucks oil from lower energy level and delivers it to higher energy level. The pump creates the flow, but pressure comes on the delivery line due to resistance applied on the pump.
All pumps have their individual pressure capabilities. Under these pressures, pumps can deliver enough amount of flow required by the system at optimum energy consumption. Sometime it is called as an energy converter, as it converts mechanical energy into the pressure energy. In case of pump failure, the entire system collapses.
In general pumps are two types:
‐a) Non
‐positive displacement type or hydrodynamic pumps.b) Positive displacement type or hydrostatic pumps.
NON
‐POSITIVE DISPLACEMENT PUMPNon
‐positive displacement pumps are not normally used in the hydraulic system, as pressure increases, flow rate of the pump decreases, even though pump rpm remains constant. Therefore, the speed of an actuator never remains constant at variable pressure. Centrifugal pump is a common pump under this category. This type of pump has no positive seal between suction and delivery, therefore, under pressure some part of oil flows back from delivery to suction and decreases the net flow rate of the pump.POSITIVE DISPLACEMENT PUMP
Positive displacement pumps are used in hydraulic system. In this type of pump, the flow rate remains constant at any pressure, if rpm of the pump remains constant. There is a positive seal between suction and delivery in this type of the pump, therefore, oil sucked by the pump flows to delivery side only without returning to tank. In this way it maintains a constant flow rate at any pressure. The constant flow rate keeps the speed of the actuator constant. Practically, at higher pressure, some oil leaks internally, but, the effect of that leakage is negligible in comparison to the flow rate of centrifugal pump.
Below are different types of positive displacement pumps:
(i) Gear pump
‐
External gear pump‐
Internal gear pump(ii) Vane pump
‐
Unbalanced type‐
Balanced type‐
Intra‐van type or double vane type(iii) Piston pump
‐
Axial piston pump‐
In‐ line axial piston pump‐
Bent axis axial piston pump‐
Radial piston pump(i) GEAR PUMP
Gear pump is coming under the category of positive displacement pump. It is a very common type of pump used in hydraulic system. It is robust, less dirt sensitive, less noisy, simple in construction and capable to take higher pressure up to 210 bars in modified condition. Otherwise, it is very much suitable up to a pressure of 80 bars to protect the life of the pump.
In external gear pump, two spur gears are running in meshed condition inside a casing. One gear is driving the other gear. Gears are also surrounded by two pressure or side plates. In the middle of the casing suction and delivery ports are made. The suction and delivery ports of the pump are determined by the diameter as suction side has the bigger diameter than the delivery side. Theoretically, a gear pump can be stalled keeping any direction of rotation, but, practically it should not be. Manufacturer always provides correct direction of rotation mark and the suction and delivery ports. If the pump is rotated in wrong direction, the pump will fail very soon. Normally, suction port is made of bigger diameter than the delivery port to avoid the cavitation. In case of wrong direction of rotation, the bearing drain line oil will be connected with the delivery side and high thrust of the oil will create severe wearing and damage the pressure plate. Also, the shaft seal will damage. As the gear teeth unmesh, it increases the volume and creates the partial vacuum in the suction side. The atmospheric pressure pushes the oil to the suction side to make up the vacuum pressure and thus, suction is crated. The oil which is sucked by the pump is now entrapped between the gear teeth and the casing and is carried over by the gear teeth to the delivery side. In delivery side, the gear teeth are meshing to each other, thus it expels the oil from the gear teeth chambers and send it to the system at any pressure. It is called delivery. Because the low clearances between the mating parts, it requires no priming. Negative suction is possible, but positive suction is better. Also, to avoid unnecessary cavitation, the suction side is given low pressure drop by designing suitable suction pipe diameter, less bend, less suction head, et. Normally a suction lift is equal to 2.5” oh Hg is recommended. The flow rate of a pump is generally depends upon the rpm of the pump, as all other parameters are fixed those are not liable to change. Thus, gear pump is called a fixed displacement pump. Clearances between the parts of a gear pump are very important. At higher pressure, these small clearances increase the leakage of the oil and hence reduce the volumetric efficiency of the pump.
There are following type of clearances:
a) Radial or tip clearance: it is the clearance between the tip of the gear teeth and casing. It should be minimum, otherwise, leakage will increase.
b) Axial clearance: it is the gap between the face of the gear and pressure plate. It should be kept minimum as possible.
c) Bearing clearance: it is also very important; otherwise it will affect the radial clearance and sometimes allow rubbing the tip of the teeth with the casing.
d) Back lash: minimum backlash should be given between the gears.
A gear pump always behaves like unbalance type by pressure, as high pressure acting on delivery side cannot be compensated with the low oil pressure acting on suction side. This unbalanced force tends to push the shaft in suction side, creates ovality on bearing and casing. In high pressure pump, such type of effect is very high. To reduce the effect, a small slot is cut along the periphery of gears in the pressure plates, which extends the pressure zone and changes the direction of acting of resultant forces. For a general gear pump, the maximum pressure can go up to 80 bars. To take thrust on the bearing, gear pump always has the bush bearing. Also shaft diameters are made larger to reduce intensity on the bearing.
A gear pump is very much suitable for low rpm and contaminated surroundings. But, it creates too much noise because of mechanical contact of gear teeth and the oil friction. Its direction cannot be changed as per the desire and spares are also not very much available. A worn out gear pump cannot be replaced except replacing the whole pump with the new unit.
(ii) VANE PUMP
Vane pump is a positive displacement pump. It is used in hydraulic system because of its high efficiency, low noise level and long life.
This pump has a somewhat limited pressure capability because of its unbalanced hydraulic loading. Its displacement, however, can be varied or even reduced to zero by moving the ring towards the centre line of the rotor.
Balanced vane pumps operate in the same manner like unbalanced vane pump. The difference is only in the inner contour of the ring, which is an ellipse rather than a circle. These configurations forms two sets of pumping chambers on opposite sides of the rotor, but are interconnected through the passage within the housing. Forces caused by the pressure build up on one side are canceled out by equal and opposite forces on the rotor. The displacement of a vane pump cannot be adjusted.
Interchangeable ring with different contours and widths are available, making it possible to quickly modify a pump to increase or decrease its delivery.
The direction of rotation can be reversed easily by reversing the position of the cam ring. By doing so, major diameter of the cam ring will be reversed by 90 degree and hence reverse the flow direction.
The intra-vane design provides a means of controlling the outward thrust of the vane against the ring and maintains the tip loads within reasonable limits. In the intra-vane cartridge, full system pressure is applied only the area between the vane and the thrust. This area is small and thrust is correspondingly light. During vane travel through pressure areas, full system pressure is applied against the bottom area of the outer vane. The valving of the pressure is applied against the bottom area of the vane is through holes drilled in the rotor. This selective application of pressure maintains the vane in substantially constant radial hydraulic balance in all positions. Vane tip wear is compensated for, automatically. As the vane wears, pressure moves the vane further out in the rotor slot holding the vane against the cam ring.
The flex side plates for both inlet and outlet are symmetrical. Pressure is fed behind each side plate into kidney shape cavities which are sealed by special seal packs. The two flex side plates and their associated kidney shaped cavities function in the following manner. As pressure builds up in the outlet, pressure also builds up in the cavities. The pressure in the cavities holds the flex side plates in hydrostatic balance against the rotor and provides optimum running clearances for minimum internal leakage and minimum friction.
The flex side plates also provide passages for feeding under vane pressure to the space between the vane and the inset. The bronze faces of the flex side plates ride next to the rotor and provide excellent wear and cold start characteristics.
The inlet and outlet support plates hold the flex side plates in the position and contain passage which allows fluid to pass from the inlet to the pumping cartridge and from the cartridge to the outlet port.
(iii) PISTON PUMP
All piston pumps operate on the principle that a piston reciprocating in a bore will draw in fluid as it is retracted and expel it on the forward stroke.
Piston pumps are highly efficient units available in the wide range of capacities from very small to high. Most are capable of operating in the medium of high pressure range (1500 PSI-3000 PSI) with others going much higher.
Being variable and reversible they lend them-selves very well to large press applications and hydrostatic drives.
Because of their closely fitted parts and finely machined surfaces, cleanliness and good quality fluids are vital to long service life.
There are two basic designs of the piston pump: radial and axial. Both are available as fixed and variable displacement types. A radial pump has the pistons arranged radially in a cylinder block while in the axial piston pump; the pistons are parallel to each other and to the axis of the cylinder block. The axial piston pump is further divided into two types; in line axial and bent axis.
Ø Radial piston pump:-
In a radial piston pump the cylinder block rotates on a stationary pintle and inside a circular reaction ring or rotor. As the block rotates centrifugal force, charging pressure or some form of mechanical action causes the pistons to follow the inner surface to the ring which is offset from the centre line of the cylinder block.
As the piston reciprocate in their bores, porting in the pintle permits them to take in fluid as they move outward and discharge it as they move in.
The size and number of the pistons and, of course the length of their stroke determines pump displacement. In some models the displacement can be varied by moving the reaction ring to increase or decrease piston travel.
Ø In-line piston:-
In in-line axial piston pump, the cylinder block and drive shaft are on the same centre line and the pistons reciprocate parallel to the drive shaft. The simplest type of axial piston pump is swash plate type.
The cylinder block in this pump is turned by the drive shaft. Pistons fitted in bores in the cylinders are connected through piston shoes and a retainer ring, so that the shoes bear against an angled swash plate. As the block turns, the piston shoes follow the swash plate, causing the pistons to reciprocate. The ports are arranged in the valve plate so that the pistons pass the inlet as they are being pulled out and pass the outlet as they are forced back.
In these pumps, the displacement is also determined by the size and number of piston as well as their stroke length, the letter being a function of the swash plate angle.
In variable displacement models of the in-line pump, the swash plate is installed in a movable yoke “pivoting” the yoke on pintles changes the swash plate angle to increase or decrease the piston stroke, the yoke can be positioned manually, with a servo control, with a compensator control, or by any of several other means. The maximum angle of the unit is limited to seventeen and half degree by construction.
Ø Bent axis axial piston pump:-
In a bent axis axial piston pump, the cylinder blocks turns with the drive shaft, but at an offset angle. The piston rods are attached to the drive shaft flange by ball joints, and are forced in and out of their bores as the distance between the drive shaft flanges and cylinder block to the drive shaft to maintain alignment ad assure that they turn together. The link does not transmit forces except to accelerate and decelerate the cylinder block and to overcome resistance of the block revolving in the oil filled housing.
The displacement of this pump varies with the offset angle, the maximum angle being 30 degrees, the minimum zero.
Fixed displacement models are usually available with 23 degree or 30 degree angles. In the variable displacement construction a yoke with an external control is used to change the angle. With some controls, the yoke can be moved over centre to reverse the direction of flow from the pump.
Various models are used to control displacement of variable displacement bent-axis pumps. Typical controls are the hand wheel, pressure compensated, and servo.
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