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
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 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 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
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 blocks 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.
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.
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 fluidnormally 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.
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 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