GB2203198A

GB2203198A - Adjustable hydrostatic piston machine

Info

Publication number: GB2203198A
Application number: GB08804812A
Authority: GB
Inventors: Josef Riedhammer
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 1987-04-02
Filing date: 1988-03-01
Publication date: 1988-10-12
Anticipated expiration: 2008-03-01
Also published as: FR2613433A1; GB8804812D0; DE3711041A1; FR2613433B1; GB2203198B; JPS63255573A; DE3711041C2

Description

1 2203198 ADJUSTABLE HYDROSTATIC PISTON MACHINE This invention concerns an

adjustable hydrostatic piston machine.

Adjustable hydrostatic piston machines (for example, axial piston machines) are known in which the tilting axis of the regulating part lies outside the neutral point of the piston forces and in which the regulating part is connected to at least one regulating piston which is displaceable against the force of a spring in a regulating cylinder subject to the action of an arbitrarily selectable control pressure. If the neutral point of the piston forces lay precisely on the tilting axis of an axial piston machine, there would be no restoring forces dependent on the pressure acting on the pistons at the regulating part. However, given the inevitable manufacturing tolerances, it is.not possible to ensure that the neutral point is located exactly on the tilting axis. In order to avoid having the neutral point located on one side of the tilting axis in some of the machines of a batch and on the other side of the tilting axis in the other machines of this batch, the tilting axis is deliberately so located that the neutral point lies on a pre-selected side If 6 of the tilting axis and thus produces restoring forces acting in a direction known in advance.

In axial piston machines, the tilting axis is preferably disposed in such a manner that the restoring forces act in the direction of the zero stroke position so as to produce a restoring action dependent on the delivery pressure. In the cas-e of an axial piston machine in which the regulating part is connected to a regulating piston which is displaceable in a regulating cylinder which can be acted upon by control pressure and is braced against the force of a spring, the control pist.on is always supposed to be displaced through a particular distance against the spring at a particular control pressure so that the regulating part adopts a particular position at a particular given control pressure. This clear correlation no longer exists when the regulating part is additionally subject to the action of restoring forces dependent on the delivery pressure and this has the result that the position of the regulating part is dependent not only on the magnitude of the set control pressure but also on the magnitude of the delivery pressure and thus of the restoring forces. If the forces act in the direction of the zero stroke position, this can lead to a desired X X power limiting or power regulating effect. However, in many cases it is desired that the setting of the regulating part of the hydrostatic transmission be adjustable precisely through the control pressure, i.e. irrespective of any other variable. This desire often applies, in particular, in the area of small angular deflections, namely, in the area of low rotational output speeds in the case of the pump of a transmission. In the case of a vehicle propulsion system, for example, (and in particular the system of a forklift truck). it must be possible to select low road spe'eds very precisely.

It is already known for the restoring forces to be partially offset by pistons acted upon by the delivery pressure in order to keep the operating forces (which are to be applied at a hand lever or a pedal at any time) small in spite of the high restoring forces (see German Auslegeschrift 11 15 140); in the case of this known device, the only object was to keep the operating forces small over the entire operating range. Purther, it is already known for the force of a piston acted upon by the delivery pressure to be made to act on the piston acted upon by the control pressure contingent on the setting of the piston machine (see German 4 - 1 Offenlegungsschrift 34 34 588); in the case of this known device, this is done in order to guarantee a return on reaching a critical pressure even when adjusted for a large swept volume. Again, it is already known for a piston acted upon by the delivery pressure to additionally be made to act controlled according'to the deflection of the pump (see German Offenlegungsschrift 34 34 588).

In the case of hydrostatic transmissions it is also known for the total range of adjustment to be spl it into two sub-ranges one of which is influenced differently from the other (see German Offenlegungsschrift 25 22 719); in this case the object had been to achieve a different regulating function at high road speeds to that at low road speeds.

The principal object of the present invention is to create an adjustable hydrostatic piston machine which will provide a refinement through which the setting of the regulating part of the machine is only determined by the control pressure within a predetermined control pressure range (and this is preferably the range neighbouring the zero stroke position or the range extending as far as the zero stroke position) while the intrinsic restoring forces of the pump are used to limit the power or to 0 1 - 5 limit the torque over the rest of the range of adjustment, in particular at fairly high rotational output speeds.

Accordingly, the present invention consists in an adjustable hydrostatic (and preferably axial) piston machine in which the tilting axis of the regulating part les outside the neutral point of the piston forces and in which said regulating part is connected to at least one regulating piston which is displaceable against the force of a spring in a regulating cylinder subject to the action of an arbitrarily selectable control pressure, wherein there.is provided an auxiliary compensating piston which is adapted to be acted upon by the delivery pressure and is of such dimensions that the force produced by the delivery pressure acting on the auxiliary compensating piston is the same as the intrinsic restoring force produced by the delivery pressure, the auxiliary compensating piston being controlled so that the force produced at this auxiliary compensating piston counteracts the intrinsic restoring force, and the movement of the auxiliary compensating piston is limited by a movement- limiting stop.

Preferably, in-a machine as described in the preceding paragraph, the arrangement is such that - 6 the movement of said auxiliary compensating piston starts from the zero stroke position.

In a machine as described in either of the two preceding paragraphs, the tilting axis may be disposed offset relative to the neutral point of the piston forces so that the restoring force acts in the direction of the zero stroke position.

In a machine as described in any one of the three preceding paragraphs, a power control piston is preferably included which is acted upon by the delivery pressure and which is connected to the regulating part and which exerts a force on it acting in the direction of the zero stroke position when subjected to the action of the pressure, said power control piston being provided with an additional stop against which the power control piston bears while the regulating part is short of a pre-determined critical position.

In a machine as described in the preceding paragraph, the pre-determined critical position from which the power control piston bears against the associated stop may be the same as the position of the regulating part from which the movement limiting stop of the auxiliary compensating piston is effective.

The spring force and the intrinsic restoring 1 - 7 force can act in the same direction so that the auxiliary piston (compensating piston) relieves the spring by the amount of the intrinsic restoring force. In this case, the annular space around the piston rod of the auxiliary piston is subject to the action of the delivery pressure. In order to obtain the simpler design in which the delivery pressure acts on the side of the auxiliary piston remote from the piston rod, the auxiliary piston can be arranged as a ram on the other side of the tilting axis like the spring. As the distance between the tilting axis of the regulating part and the neutral point is an arbitrary design choice and therefore the magnitude of the intrinsic restoring force is known, the diameter of the auxiliary piston can be calculated and selected so that the force produced by the delivery pressure acting on the auxiliary piston is equal to the intrinsic restoring force, bearing in mind that the intrinsic restoring force is dependent on the angular deflection of the regulating part and contingent. on the actual precise distance between the neutral point and the tilting axis. However, these magnitudes can be taken into account accordingly at the design stage.

The embodiment of adjustable hydrostatic 1.5 piston machine according to the present invention (which embodiment is about to be described with reference to the drawing and which is given by way of example) provides proportional adjustment with overlaid power control without the expense involved in a pilot control system and without the auxiliary equipment for power control and thus represents a very inexpensive design.

Referring now to the accompanying diagrammatic drawing, the middle part of the drawing shows a view of the control base, the left-hand part of the drawing shows the regulating units on the left-hand side of the axial piston machine swung out through 900 and the right-hand side of the drawing shows the regulating units on the right-hand side of the axial piston machine also swung out through 900.

Two kidney-shaped control elements 2 and 3 can be seen in a control surface 1 of the control base. A rocker 4 is mounted so that it can tilt about the tilting axis 15. As the rocker 4 can be tilted out of the position parallel with the control surface 1 in both directions, regulating units are provided for both tilting directions and all of these are fixed to a control base assembly 7 on which the control surface 1 is also formed. The power control cylinder 8 is fixed in the control base assembly 7 1 t - 9 and the power control piston 9 is displaceable in said cylinder 8. An arrangement of nested concentric springs 10 is disposed inside the cylinder 8 and inside the piston 9.

The rocker 4 on the left-hand side of the drawing is identical to the rocker on the right-hand side, and in both cases are shown in their deflected positions.

A so-called compensating cylinder 11 is also fixed in the control base assemblY 7 and a piston 12 which is braced against an arrangement of nested concentric springs 13 is displaceable in this cylinder.

The arrangement on the other side is the same with a compensating cylinder 14 in which a piston 16 is displaceable, and a power control cylinder 17 in which a piston 18 is displaceable, connected with the control base assembly 7. Arrangements of nested springs 23, 24 are disposed inside the cylinder 14/ piston 16 and cylinder"17/Piston, respectively.

An auxiliary compensating piston 19 can be seen on the left7hand side of the drawing and the auxiliary compensating piston 20 on the right-hand side thereof. A power control piston 21 can be seen on the left-hand side of the drawing and a power control piston 22 on the right-hand side thereof.

The manner of operation is as follows:- At the start of the deflection from the zero stroke position, the piston 20 and the control pressure act on the piston surfaces 16 and 9 in the direction of further deflection. This force is opposed by the intrinsic restoring force dependent on high pressure, and the forces of the springs 13 and 24. The end face in front of the piston 20 is connected to the kidney-shaped control element 3. Like the intrinsic restoring force, the force produced by the delivery pressure acting on the piston 20 is also contingent on the magnitude of the delivery pressure which acts in the kidneyshaped element 3. For this the diameter of the piston 20 is selected so that the intrinsic restoring force is roughly offset in the range of adjustment from zero degrees to a predetermined angle (60, for example) with the result that in this range the angular deflection of the rocker 4 is only contingent on the magnitude of the control pressure.

When the rocker 4 reaches said predetermined critical design angle, the compensating piston 20 or 19 comes to rest against a stop 5 or 28 with the result that the auxiliary compensating p iston 19 or 20 cannot exert any more force on the deflecting p piston 9 or 16 if the rocker 4 is deflected further. Thus, in the range of adjustment extending beyond the predetermined critical design angle, the position of the rocker 4 is dependent on both the control pressure and the intrinsic restoring force. Since the intrinsic restoring force provided by the present pump design is not sufficient to guarantee the restoring force necessary for control for constant torque, the power control piston 22 is also provided; this produces an additional restoring force which is dependent on the delivery pressure acting on this piston 22. In the fine control-range determined by the design and extending from zero to 60 for example, this power control piston 22 cannot exert any force as. the movement of this power control piston 22 is limited by a stop 6.

In the event of deceleration by means of the hydrostatic pump, the pressure sides are reversed in the illustrated arrangement, i.e. the higher pressure is now to be found in the kidney-shaped element 2. The rocker 4 is now subject to the action of a deflecting force which is dependent on pressure. Now the auxiliary compensating piston 19 acts in the direction of the zero stroke position and assists the actions"of the springs 13 and 24. As the deflecting force of the pump is largely offset If by the auxiliary compensating piston 19 or 20, the pump position is only dependent on the control pressure. The power control piston subject to the action of the pressure cannot become effective as it is fixed by the stop 27.

Claims

The division of the regulating movement into a portion in which the control pressure alone is effective and another portion in which there is a clear power regulating characteristic is reinforced by the second stop 27 or 6, as mentioned in Claims 4 and 5. CLAIMS:

1. An adjustable hydrostatic and preferably axial piston machine in which the tilting axis of the regulating part lies outside the neutral point of the piston forces and in which said regulating part is connected to at least one regulating piston which is displaceable against the force of a spring in a regulating cylinder subject to the action of an arbitrarily selectable control pressure, wherein there is provided an auxiliary compensating piston which is adapted to be acted upon by the delivery pressure and is of such dimensions that the force produced by the delivery pressure acting on the auxiliary compensating piston is the same as the intrinsic restoring force produced by the delivery pressure, the auxiliary compensating piston being controlled so that the force produced at this auxiliary compensating piston counteracts the intrinsic restoring force, and the movement of the auxiliary compensating piston is limited by a movement-limiting stop.

2. A machine as claimed in Claim 1, wherein the arrangement is such that the movement of said auxiliary compensating piston starts from the zero stroke position.

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3. A machine as claimed in Claim 1 or Claim 2, wherein the tilting axis is disposed offset relative to the neutral point of the piston forces so that the restoring force acts in the direction of the zero stroke position.

4. A machine as claimed in any one of the preceding Claims and further including a power control piston which is acted upon by the delivery pressure, is connected to the regulating part and exerts a force on it acting in the direction of the zero atroke position when subjected to the action of the pressure, said power control piston being provided with an additional stop against which the power control piston bears while the regulating part is short of a pre-deterined critical position.

5.. A machine as claimed in Claim 4, wherein the pre-determined critical position from which the power control piston bears against the associated stop is the same as the position of the regulating part from which the movement limiting stop of the auxiliary compensating piston is effective.

6. An adjustable hydrostatic and preferably axial, piston machine constructed, arranged and adapted to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying diagrammatic drawing.

1 1 1 - 15

7. Any features of novelty, taken singly or in combination, of the embodiments of the invention hereinbefore described with reference to the accompanying diagrammatic drawing.

Published 1988 at The Patent Offtce, State House, 66171 High Holborn, London WC1R 4TP. Further copies may be obtained from The Patent OftIce, Sales Branch, St Mary Cray, Orpington, Kent BR5 3BD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con- 1187.