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Hydraulic pumps
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Hydraulic pumps
Different types of hydraulic pumps
Hydraulic pumps are manufactured depending on different functional and hydraulic system requirements, such as operating medium, required range of pressure, type of drive, etc. A large range of design principles and configurations exists behind hydraulic pumps. Consequently, not every pump can fully meet all sets of requirements to an optimum degree. Three different types of hydraulic pumps exist:

Hydraulic Gear Pumps

External gear pump
Internal gear pump
Gear ring pump
Screw spindle pump

Hydraulic Vane Pumps

Single chamber vane pump
Double chamber vane pump

Hydraulic Piston Pumps

Axial piston pump
Radial piston pump

Hydraulic Gear Pumps
A gear pump is used in many hydraulic systems. It has few moving parts, works smoothly, and operates very well at pressures up to 250 bar. The displacement chambers are formed between the housing of the pump and the rotating gear wheel (or gear wheels, depending on model).

External Gear Pump
External gear pumps are used in industrial and mobile (e.g. log splitters, lifts) hydraulic applications. Typical applications are lubrication pumps in machine tools, fluid power transfer units and oil pumps in engines. These pumps have some unique features:

Low weight
Relatively high working pressures
Wide range of speeds
Wide temperature and viscosity range (i.e. flexibility)
Low cost

In an external gear pump, only one of the gear wheels is connected to the drive. The other gear wheel rotates in the opposite direction so that the teeth of the rotating gear wheels interlock. With use of a bearing block, the gear wheels are positioned in such a way that they interlock with the minimum clearance. Volume is created between the gear tooth profiles, housing walls and surfaces of the bearing blocks.

Hydraulic Vane Pumps
The vane pump finds its use in die casting and injection moulding machines in industry, as well as in land and road construction machinery.

Hydraulic vane pumps operate with much lower flow pulsation, i.e. constant flow. As such, vane pumps produce less noise while maintaining a relatively high speed.

Key features of the vane pump:

Low flow pulsation
Very low noise levels
Wide range of speeds
Wide viscosity range

The operating pressure of vane pumps does not normally exceed 175 bar. However, in specially designed vane pumps the operating pressure may go over 200 bar and up to 300 bar. Hydraulic vane pumps are available as single chamber vane pumps or double chamber vane pumps.

Both types use the same parts, i.e. they comprise a rotor and vanes. The vanes may be radially moved within the rotor, and the centrifugal force of the rotor pushes the vanes out to touch the housing. The difference between the two types is in the shape of the stroke ring that limits the stroke movement of the vanes.

Typical parameters are:

Displacement volume: 6 to 640 cc
Maximum pressure: up to 200 bar
Speed range: 500 to 3,000 rpm

Single Chamber Vane Pump
In a single chamber vane pump, the stroke movement of the vanes is limited by a ring with a circular internal track. The position of this so-called stroke ring is off-centre with respect to the rotor, resulting in change of volume in the displacement chambers. The displacement chambers are created by the rotor, two vanes, the internal surface of the ring and the control discs on one side.

In a single chamber vane pump, the system pressure is only on one side of the rotor. This causes a significant load on the bearings. To reduce this load, the forces acting on the rotor must be in balance. This is the reason why double chamber vane pumps were designed, as mentioned below.

Double Chamber Vane Pump
For double chamber vane pumps, the process of filling the chambers (suction) and emptying is in principle the same as for single chamber vane pumps. In this case, however, the stroke ring (i.e. stator) has a double eccentric internal surface. The rotor can be placed in the axis of the stator because of these surfaces, which differentiates them from single chamber vane pumps.

This set up causes each vane to carry out two strokes per rotation of the shaft. All radial loads on the rotor are now neutralized (two pressure ports on each opposite side). The end result is that two pumps have been built together as one. Due to the twin cam forms of the stator, two displacement processes occur per revolution.

Hydraulic Piston Pumps
Hydraulic piston pumps can handle large flows at high hydraulic system pressures. Typical applications are mobile and construction equipment, marine auxiliary power, metal forming and stamping, machine tools and oil field equipment.

In these pumps, the pistons accurately slide back and forth inside the cylinders that are part of the hydraulic pump. The sealing properties of the pistons are excellent.

Key features of hydraulic piston pumps are:

Compact size
High power density
Optimum efficiency and reliability
High speed and torque
High operating pressures

Hydraulic piston pumps operate at very high volumetric efficiency levels due to low fluid leakage. The plungers may consist of valves at the suction and pressure ports or with input and output channels. Piston pumps with valves at the ports are better suited to operate at higher system pressures due to better sealing characteristics.

Axial Piston Pump
The design of an axial piston unit is based on two important principles. First, the design of the axial piston pump may be based on the swash plate principle or bent axis design. Secondly, hydraulic system parameters have to be taken into account: whether the usage is to take place in an open or closed loop circuit is of great importance.

In closed loop circuits, the return line (i.e. the suction line of the pump) is under pressure. This must be incorporated in the design of axial piston units used in closed loop systems. It is also imperative to have a variable displacement volume hydraulic pump in operation in these systems. In fixed displacement volume configuration, the axial piston unit can be used both as pump and as motor.

In bent axis design, the displacement volume is dependent on the swivel angle: the pistons move within the cylinder bores when the shaft rotates. In swash plate design, the rotating pistons are supported by a swash plate; the angle of the swash plate determines the piston stroke.

Typical parameters are:

Displacement vol.: 5 to 1,000 cc
Maximum pressure: up to 450 bar
Speed range: 1,500 to 11,000 rpm

Radial Piston Pumps
Radial piston pumps are used in applications that involve high pressures (operating pressures above 400 bar and up to 700 bar), such as presses, machines for processing plastic and machine tools that clamp hydraulics. Radial piston pumps are the only pumps capable of working satisfactorily at such high pressures, even under continuous operation.

Radial piston pumps are available in two different configurations. With eccentric cylinder block, the piston rotates within the rigid external ring. Eccentricity determines the stroke of the pistons. Or, with an eccentric shaft, the rotating eccentric shaft causes radially-oscillating piston movements to be produced. Most models have an odd number of pistons to reduce the flow pulsation.

Typical parameters are:

Displacement volume: 0.5 to 100 cc
Maximum pressure: up to 700 bar (size dependent)
Range of speeds: 1,000 to 3,000 rpm (size dependent)



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