Expansible chamber devices – Displacement control of plural cylinders arranged in... – Parallel cylinders
Reexamination Certificate
2001-07-09
2003-04-29
Look, Edward K. (Department: 3745)
Expansible chamber devices
Displacement control of plural cylinders arranged in...
Parallel cylinders
C091S506000
Reexamination Certificate
active
06553891
ABSTRACT:
FIELD OF INVENTION
The invention relates to a hydrostatic variable displacement pump of swash plate construction in which the servosystem is integrated in a servosystem housing designed as a cover; the cover closes off a housing opening of the variable displacement pump, a servoarm projecting through said opening for connection to the servosystem.
BACKGROUND OF THE INVENTION
In the known hydrostatic variable displacement pumps of swash plate construction which operate with a closed circuit, the displacement pistons are guided in cylinders of a cylinder block and rotate about the shaft of the variable displacement pump. During the rotation, the displacement pistons are supported on the swash plate by means of sliding blocks, each displacement piston executing a complete stroke with each 360° rotation. The swash plate has a planar running surface on which the sliding blocks slide, the displacement pistons being connected in an articulated manner to the sliding blocks.
The swash plate is usually referred to as a rocker device or adjustable-angle plate, to be precise depending on whether they are mounted in cylinder shells on rollers or can be pivoted about bearing journals. The swash plate is pivoted by adjustment of the servosystem such that the angle position of its running surface is changed in relation to the stroke direction of the displacement pistons. With the change in the angle position of the running surface of the swash plate, the stroke of the displacement pistons, and thus the volume stream produced by the pump, is changed. The force necessary for changing the angle position of the adjustable-angle plate is usually produced hydraulically by virtue of pressure being produced on one, two or possibly even more servopistons which act on the swash plate.
The axis of action of the servopistons is located outside the axis of rotation of the rocker device or adjustable-angle plate, with the result that a lever arm is thus produced. The servopiston or servopistons is/are connected directly or indirectly to the swash plate such that the force displacing the servopistons produces a pivoting torque of the swash plate via the lever arm.
Hydrostatic variable displacement pumps also require spring forces which guide the pivoting angle of the pump back to 0°, i.e. into the neutral position thereof, if the servo-adjustment means of the pump is not activated.
In pivot-through pumps, the swash plate can be pivot in opposite directions from the center position determined by the spring forces, with the result that two delivery directions are produced. For both delivery directions, there is in each case a volume stream which begins from zero and increases to a specific maximum value. As a result, the center position is also referred to as the zero position. It is often the case that the springs which determine the zero position are installed such that they act as compression springs both in the case of positive pivoting angles and in the case of negative pivoting angles.
Variable displacement pumps in which the springs are accommodated outside the servopiston and servocylinder and are connected to the actual servopiston of the servosystem via corresponding lever systems are known. This means that the springs, rather than acting directly on the moveable servopistons, act on the servopistons via force-deflecting means. (See FIG.
5
).
In most of the known variable displacement pumps of the type described above, the springs are installed directly in the servocylinder pressure chamber formed by the servopiston and servocylinder. The springs are thus located in the chamber in which the servopressure for adjusting the angle position of the swash plate also acts. In this case, the springs do indeed act directly on the servopiston, i.e. the spring force is transmitted directly from the spring to the servopiston. However, the dimensioning of the springs is limited by the size of the servocylinder pressure chamber. This means that the spring force cannot be adapted to different force conditions and sizes of the servosystem independently of the size of the servocylinder pressure chamber.
In order that the springs always operate as compression springs regardless of the pivoting direction of the servosystem, they are prestressed between two spring plates. In such devices, a rod is located between the spring plates with a low as possible amount of axial displacement play. The two spring plates can be moved toward one another both in the servopiston and on the rod, but they cannot move apart from one another beyond the distance between the two spring plates. The rod is connected to the housing of the variable displacement pump such that it cannot be displaced axially. In this case, the rod has to be adjusted such that the springs, prestressed to the length between the two spring plates, position the swash plate such that the stroke of the displacement pistons becomes zero (see FIG.
4
).
A disadvantage of this configuration is that the spring-force requirement, which determines the geometrical dimensions of the springs, also has an influence on the amount of space required in the servopiston and/or in the servocylinder. This produces an undesirable relationship between the necessary spring force and the necessary servopiston diameter with a corresponding servopiston stroke, which relationship restricts the flexibility of design relatively pronounced extent and cannot be broken up as desired by means of construction. This means that a large spring force also always requires corresponding large servopiston and servocylinder, and a large servocylinder pressure chamber. High spring forces for smaller servopistons and servocylinders are barely possible with such known systems. A further disadvantage of this configuration is that the overall space necessary for the spring produces a dead volume in the servocylinder pressure chamber. In particular in the case of large springs, the dead volume is often greater than the displacement volume of a servopiston stroke. As a result, the servocylinder pressure chamber is not emptied to the full extent during the stroke of the servopiston. If, for example, air is located in this chamber, then air extraction must additionally be ensured by corresponding design measures.
Also known are variable displacement pumps (see FIG.
3
), in which the servopiston is arranged in the interior of the tank chamber of the variable displacement pump. There is provided a pivot-back piston against which the servocylinder operates and wherein the spring is arranged on the outer circumference of the piston (see FIG.
3
). Double-acting servopistons with inner springs are also known, wherein the servopiston virtually always is arranged at right angles to the pump axis. The application of force for the lever arm in relation to the swash plates should be located as far as possible, as should the center line of the springs, on the center longitudinal axis of the servopiston, in order that the hydraulically mechanical forces on the servopiston do not try to press the servopiston onto the wall of the servocylinder and thus increase friction and wear. This is only expedient in practice, however, when all the springs are located on one side of this application of force (see, in particular, FIG.
4
).
In another prior art system the hydrostatic variable displacement pump has a cylinder block in which displacement pistons are guided and circulate with the cylinder block. The displacement pistons are supported on a swash plate, which can have its angle position pivoted in relation to the stroke direction of the displacement pistons, with the result that during the 360° rotation of the cylinder block, in which the displacement pistons execute a complete stroke, the stroke of said pistons can be adjusted.
The above servosystem has a piston device with at least two servopiston surfaces subjected to the action of pressure, the servopiston surfaces either being assigned to a single servopiston or each belonging to a separate servopiston. The spring device is arranged outside the servocylinder pressure cham
Fiebing Carsten
Thoms Reinhardt
Leslie Michael
Look Edward K.
Sauer-Danfoss Inc.
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