In this type, the axial reciprocating motion of the pistons is obtained by a swash plate that is either fixed or variable in its degree of angle. As the piston barrel assembly rotates, the pistons rotate around the shaft, with the piston shoes in contact with and sliding along the swash plate surface. Since there is no reciprocating motion when the swash plate is in vertical position, no displacement occurs. As there is an increase in the swash plate angle, the pistons move in and out of the barrel as they follow the angle of the swash plate surface. The pistons move out of the cylinder barrel during one half of the cycle of rotation thereby generating an increasing volume, while during the other half of the rotating cycle, the pistons move into the cylinder barrel generating a decreasing volume. This reciprocating motion results in the drawing in and pumping out of the fluid. Pump capacity can easily be controlled by altering the swash plate angle, larger the angle, greater being the pump capacity. The swash plate angle can easily be controlled remotely with the help of a separate hydraulic cylinder. A cross-sectional view of this pump is shown in Figure 3.16.
The cylinder block and the drive shaft in this pump are located on the same centerline. The pistons are connected through shoes and a shoe plate that bears against the swash plate. As the cylinder rotates, the pistons reciprocate due to the piston shoes following the angled surface of the swash plate. This operation of drawing in and drawing out of the fluid is illustrated in the Figure 3.17.
The outlet and the inlet ports are located in the valve plate so that the pistons pass the inlet as they are being pulled out and pass the outlet as they are being forced back in.
These types of pumps can also be designed to have a variable displacement capability. In such a design, the swash plate is mounted in a movable yoke. The swash plate angle can be changed by pivoting the yoke on pintles.
The positioning of the yoke can be accomplished by manual operation, servo control or a compressor control and the maximum swash plate angle is usually limited to 17.5° (Figure 3.18).
As the name implies, these pumps discharge a fixed quantity of oil per revolution of the pump shaft. In other words, they produce flow proportional to their displacement and rotor speed. A majority of the pumps used in fluid power applications belong to this category. These pumps are capable of overcoming the pressure that results from the mechanical loads on the system as well as the resistance to flow due to friction. Thus the pump output flow is constant and not dependent on system pressure. Another advantage associated with these pumps is that the high-pressure and low-pressure areas are separated and hence the fluid cannot leak back and return to the low-pressure source. These features make the positive displacement pump most suited and universally accepted for hydraulic systems.
The advantages of positive displacement pumps over non-positive displacement pumps are:
• Capability to generate high pressures
• High volumetric efficiency
• Small and compact with high power to weight ratio
• Relatively smaller changes in efficiency throughout the pressure range
• Wider operating range i.e. the capability to operate over a wide pressure and speed range.
As discussed earlier, it is important to understand that pumps do not produce pressure; they only produce fluid flow. The resistance to this flow as developed in a hydraulic system is what determines the pressure. If a positive displacement pump has its discharge port open to the atmosphere, then there will be fluid flow, but no discharge pressure above that of atmospheric pressure, because there is no resistance to flow.
If the discharge port is partially blocked, then the pressure will rise due to the resistance to flow. In a scenario where the discharge port of the pump is completely blocked, theoretically an infinite resistance to flow is possible. This will result in a rapid rise in pressure which will result in breakage of the weakest component in the circuit. This is exactly the reason why positive displacement pumps are provided with safety controls, which help prevent the rise in pressure beyond a certain value.
They are also known as hydro-dynamic pumps. In these pumps the pressure produced, is proportional to the rotor speed. In other words, the fluid is displaced and transferred using the inertia of the fluid in motion. These pumps are incapable of withstanding high pressures and are generally used for low-pressure and high-volume flow applications.
Normally their maximum pressure capacity is limited to 20-30 kgf/cm3. They are primarily used for transporting fluids from one location to the other and find little use in the hydraulic or fluid power industry.
Because of fewer numbers of moving parts, non-positive displacement pumps cost less and operate with little maintenance. They make use of Newton’s first law of motion to move the fluid against the system resistance. Although these pumps provide a smooth and continuous flow, their flow output is reduced as the system resistance (resistance to flow) is increased. In fact it is possible to completely block the outlet to stop all flow even while the pump is running at the designed speed. Thus the pump flow rate depends not only on the rotational speed (rpm) at which it is driven but also on the resistance of the external system. As the resistance of the external system increases, some of the fluid will slip back, causing a reduction in the discharge flow rate. When the resistance of the external system becomes very large, the pump will produce no flow and thus its volumetric efficiency becomes zero. Examples of these pumps are the centrifugal and axial (propeller) pumps.
In a centrifugal pump, a simple sketch of which is illustrated in Figure 3.2, rotational inertia is imparted to the fluid. Centrifugal pumps are not self-priming and must be positioned below the fluid level.
Principle of operation
The fluid from the inlet port enters at the center of the impeller. The rotating impeller imparts centrifugal force to the fluid and causes it to move radially outward. This results in the fluid being forced through the outlet discharge port of the housing. The tips of the impeller blades merely move through the fluid while the rotational speed maintains the fluid pressure corresponding to the centrifugal force established.
Centrifugal pumps are generally used in pumping stations, for delivering water to homes and factories. The advantages of non-positive displacement pumps are:
• Low initial cost and minimum maintenance
• Simplicity of operation and high reliability
• Capable of handling any type of fluid, for example sludge and slurries.
Since the impeller imparts kinetic energy to the fluid, centrifugal pumps are also known as hydrokinetic power generators.
The sole purpose of a pump in a hydraulic system is to provide flow. A pump, which is the heart of a hydraulic system, converts mechanical energy, which is primarily rotational power from an electric motor or engine, into hydraulic energy. While mechanical rotational power is the product of torque and speed, hydraulic power is pressure times flow. The pump can be designed in such a way that either flow or pressure is fixed, while the other parameter is allowed to swing with the load. In other words, by fixing the pump flow, the pressure goes up as the load restriction is increased. Conversely, the flow goes down with an increase in load restriction when the pump delivers fixed pressure.
The pumping action is the same for every pump. Due to mechanical action, the pump creates a partial vacuum at the inlet. This causes the atmospheric pressure to force the fluid into the inlet of the pump. The pump then pushes the fluid into the hydraulic system (Figure 3.1).
The pump contains two check valves. Check valve 1 is connected to the pump inlet and allows fluid to enter the pump only through it. Check valve 2 is connected to the pump discharge and allows fluid to exit only through it.
When the piston is pulled to the left, a partial vacuum is created in the pump cavity 3. This vacuum holds the check valve 2 against its seat and allows atmospheric pressure to push the fluid inside the cylinder through the check valve 1. When the piston is pushed to the right, the fluid movement closes check valve 1 and opens outlet valve 2. The quantity of fluid displaced by the piston is forcibly ejected from the cylinder. The volume of the fluid displaced by the piston during the discharge stroke is called the displacement volume of the pump.