Fixed displacement pump
It is a hydraulic pump, where the displacement remains fixed per cycle, thus delivering a constant flow.
Variable displacement pump
This is a pump, the displacement of which is variable. Vane pumps and piston pumps are available as variable displacement pumps. Adjustment may be either direct or pilot actuated. If adjustment is possible via zero into the negative range, so that the feed direction is reversed, then a over center pump is obtained.Variable displacement pumps only supply as much flow as required. Therefore their power loss remains small.
Axial piston pump
The displacement pistons (in most cases 7 to 11) are arranged parallel to the drive shaft and are operated via a bent axis (up and down movement is dependent on the swivel angle) or aswashplate (up and down movement is dependent on the inclination of the swashplate). A third version of axial piston pump is the wobble plate axial piston pump.
A- Bent axis design
The bent axis mechanism is a displacement machine, the displacement pistons of which are arranged at an angle to the drive shaft Axial piston units – swashplate design, pump function.
B- The swashplate Design
The swashplate mechanism is a positive displacement machine, the displacement pistons of which are arranged axially to the drive shaft. The reaction force of the pistons is carried by the swashplate.
As the drive shaft rotates, the cylinder (with its pistons) is turned. The pistons follow an up and down movement, which is determined by the inclination of the swashplate. Fluid is fed to the pump via the low pressure side (inlet) and then delivered via the pistons on the high pessure side (outlet) into the system.
The control plate is a plate shaped or spherical functional element in axial piston units. It separates the pressure side from the suction side by means of its control openings.
1-Drive shaft 2 -Swashplate (swivel angle)
3-Cylinder 4-Through drive
5-Control kidneys 6-Control plate
7-Piston 8-Slipper pad
Radial piston pump
Radial piston pumps are used in applications involving high pressures (above 400 bar).
Drive shaft (1) is eccentric to pump elements (2). The pump elements comprise piston (3), cylinder sleeve (4), pivot (5), compression spring (6), suction valve (7) and pressure control valve (8). The pivot is screwed into housing (9). The piston is positioned with the so-called slipper pad on the excentre. The compression spring causes the slipper pad to always lie on the excentre when the eccentric shaft rotates and the cylinder sleeve to be supported by the pivot.
There are two types of vane pump in common use: single and double stroke.
With both designs the displacement chamber is formed between the circular-shaped stator, the rotor and the vanes. The vanes may be moved radially inside the rotor.
What differs is the form of the ring which limits the stroke movement of the vanes.
.Vane pump, single stroke
Positioning device for vane pump
The position of the stator ring may be influenced at three positioning devices in a variable displacement vanepump:
• adjustment screw for the stroke volume (1),
• height adjustment screw (2) and
• setting screw for max. operating pressure (3).
Pressure control device for a pump
The pressure controller determines the maximum operating pressure.
When the pre-set pressure level is reached flow is set to zero to prevent the pressure in the system from rising further.
The pressure controller comprises control piston (1), housing (2), spring (3) and adjustment device (4). The control piston is fitted with one longitudinal and two transverse bores, permitting hydraulic fluid to flow through the control piston. Connection to the tank in the position set is closed by means of a bridge. As long as the force Fp resulting from the pressure is lower than the opposite force of spring FF the pump will remain in this state.
External gear pump
External gear pumps are widely used, especially in the mobile hydraulics sector. As the gears rotate, the hydraulic fluid is carried via the external spaces between the teeth from the suction side to the pressure side. These pumps are of simple design, cost-effective and robust and operate within a wide speed range (500 to 6000 r.p.m.) at relatively high pressures (up to 300 bar).
Ring gear pump
This pump operates in accordance with the planetary principle. The rotor has one gear less than the internally geared stator. As a result of the way the internal and external gears mesh, a good seal is obtained without an additional sickle seal. The pump operates quietly up to pressures of 160 bar.
Screw pumps are similar to internal gear pumps in that their main characteristic is that they possess an extremely low operating noise level. Screw pumps contain 2 or 3 worm gears within a housing. The worm gear connected to the drive has a clockwise thread and transmits the rotary movement to further worm gears, which each have anti-clockwise threads. An enclosed chamber is formed between the threads of the worm gears. This chamber moves from the suction port to the pressure port of the pump without change in volume. This produces a constant,uniform and smooth flow and hence operation tends to be very quiet.