GB1586634A - Hydraulic servo control device for adjusting the transmission ratio of a hydrostatic transmission - Google Patents

Description

(54) HYDRAULIC SERVO CONTROL DEVICE FOR ADJUSTING THE TRANSMISSION RATIO OF A HYDROSTATIC TRANSMISSION (71) We, ZAHNRADFABRIK FRIEDRICH SHAFEN AKTIENGESELLSCHAFT, of Friedrichshafen-on-the-Bodensee, Federal Republic of Germany, a Joint-Stock Company organised under the laws of the Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a device for reversing a hydrostatic drive of the kind in which an adjustable pump, which can be changed over to two directions of delivery, has a hydraulically.

operated control element biassed to adjust said pump to a zero delivery position and operable in two opposite directions by means of a 3/2way reversing valve and has a servo control pressure which is adjustable by means of a tapping throttle valve of variable outlet cross-section which is connected to a pressure pipe extending between a control pump driven at the input rotational speed of the transmission and the 3/2-way reversing valve in order to be able at will to lower the control pressure which is dependent on the input rotational speed of the transmission, for braking purposes.

In known drives of the foregoing kind (German Patent 1,555,480) controlled braking of the output shaft of the drive is permitted by the fact that either the input rotational speed of the drive is reduced by closing the throttle (engine braking) or that an outlet throttle valve is operated by means of a brake pedal and lowers the control pressure for the hydraulic pump control element. By virtue of this reduction of pressure the pump control element moves towards zero because of its bias. If in such a drive the reversing valve effecting the reversing of the direction of rotation is operated during a braking process of this kind, it is quite impossible for the driver to vary the control speed by operating the outlet throttle, since the adjustment of the pump, at least into the neutral position, is effected from outside solely through the action of the adjusting and restoring forces of the pump control element.

The operation of returning the adjustable hydraulic control pump to its neutral position in this case usually means full braking because of the restoring forces applied. This means that for a laden fork-lift truck travelling in the forward direction, for example, there is a risk that the load will slip off the fork, the goods comprising the load will be damaged, and the operator may possibly be at risk. It has been attempted to achieve a partial improvement by damping the adjustment time, but in that case other disadvantages must be accepted.

If the driver effects the reversal only after reaching the reversal point, there is the additional disadvantage that the driver will be temporarily overburdened by the combinationdf switching or operating manipulations requiring to be carried out simultaneously, for example in the case of shunting movements such as are required for loading work. It is precisely in such circumstances that the reversing operation requires increased attention because of the drivers inability to intervene in the process.

The problem which the invention aims at solving is the improvement of the reversing operation so that after the switch command has been given for the reversing of the direction of travel of the vehicle the driver will be able himself to control the progress of the reversing operation.

Accordingly, the present invention consists in a hydraulic servo control device for adjusting the transmission ratio of a hydrostatic transmission, in which an adjustable pump, which can be changed over to two directions of delivery, has a hydraulically operated control element biassed to adjust said pump to a zero delivery position of said pump and operable in two opposite directions by means of a 3/2-way reversing valve and has a servo control pressure, which is adjustable by means of a tapping throttle valve of variable outlet cross-section which is connected to a pressure pipe extending between a control pump driven at the input rotational speed of the transmission and the 372-way reversing valve in order to be able at will to lower the control pressure which is dependent on the input rotational speed of the transmission, for braking purposes, characterised in that a direction of rotation or travel selector means is provided the operation of which causes a biassing force, which acts in the direction of change over, to act on the 3/2-way reversing valve, a locking means being provided which prevents movement until the hydraulic control element of the control pump has reached a pre-determined position as the result of an arbitrary reduction of adjusting pressure in the adjusting pressure pipe.

It is already known (German Published Patent Specifications 19 15 107 and 21 29 830) for a reversing valve for the main working circuit of a hydrostatic drive to be connected to the pump adjustment in such a manner that the change-over is made only when the pump is in the zero position, but this relates to hydraulic pumps which deliver in one direction only and therefore require an additional directional valve in the main circuit of the hydrostatic transmission. In this case it is not possible for the driver himself to control the progress of the reversing operation in all its phases.

Furthermore, it is known in transmissions in general for a gear change to be initiated by operation of a preselector switch, the changeover being triggered by a locking element only when a reversing valve is operated (German Published Patent Specification 14 26 450).

In contrast thereto the present invention provides the advantage of enabling the driver himself to control and adjust the deceleration to rest and the acceleration of the drive following the change-over while at rest, not only in respect of timing but also in respect of magnitude, so that he can thus adapt deceleration and acceleration in the optimum manner to operating conditions, the load to be transported or moved, and the environment. By virtue of preselection of the change of direction of travel he is thus relieved of an additional change-over operation requiring to be performed at the reversing point itself.

The arrangement of the invention is also found advantageous for working machines, winches, and rotating and slewing drives.

Advantageous further developments of the invention are indicated in the sub-claims.

In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically and by way of example two embodiments thereof and in which: Figure 1 shows an adjusting device with a preselector device and mechanical locking means, and Figure 2 shows analogously the same adjusting device with only hydraulic components.

The embodiment of Figure 1 shows a hydrostatic drive 1 comprising an adjustable hydraulic pump 2 and a hydraulic motor 3. The drive is effected by a driving motor, preferably an internal combustion engine, via a drive shaft 4.

A feed pump 5 and a control pump 6 are also connected to the shaft 4. The adjustment of the hydraulic pump 2 is effected by an adjusting device 7 which is connected via a connecting element 10 to position indicator 11. By way of ducts 12 and 13 the adjusting device 7 is operatively connected to a three-position two-way reversing valve 8, which has a preselector device comprising the parts 14, 15, and 16 and a mechanical locking system comprising the parts 17, 18, 19, and 20. By means of a duct 9 the reversing valve 8 is connected to the delivery side of the control pump 6 and to an adjustable throttle 21 by-passing the pump 6. The throttle 21 is connected by means of a linkage 22 to the brake pedal 23.

The preselection operation for the mechanical locking system will now be described with reference to Figure 1. In Figure 2, in which a hydraulic locking system is shown, parts serving equivalent functions are given reference numerals increased by 100.

The hydrostatic drive 1 is adjusted, i.e.

reversed, by the adjusting device 7 which is actuated by the reversing valve 8. The desired control position of the reversing valve 8 is preselected for a desired direction of travel by means of the adjusting device 14, 15, 16.

The position indicator 11 of the adjusting pump 2 releases the catch 17 of the locking system only when the pump is in the neutral position (as shown), so that the reversing valve 8 and consequently, by way of the hydraulic adjusting device 7, the hydrostatic drive 1 can be reversed only in that position.

By means of the control throttle 21 the control pump 6 produces a pressure which is dependent on rotational speed and which is applied to the adjusting device 7. The control throttle 21 is opened by means of the brake pedal 23 and the control pressure thus reduced. The hydraulic pump 2 is set back and the vehicle braked hydrostatically. With the aid of the brake pedal, or its speed of adjustment, the chronological fall of control pressure and consequently the deceleration of the vehicle can therefore be adjusted.

When the brake pedal 23 is released the control pressure is built up again and the vehicle re-accelerated.

WHAT WE CLAIM IS: 1. A hydraulic servo control device for adjusting the transmission ratio of a hydrostatic transmission, in which an adjustable pump, which can be changed over to two directions of delivery, has a hydraulically operated control element biassed to adjust said pump to a zero delivery position of said pump and operable in two opposite directions by means of a 3/2-way reversing valve and has a servo control pressure, which is adjustable by means of a tapping throttle valve of variable outlet cross-section which is connected to a pressure pipe extending between a con trol pump driven at the input rotational speed of the transmission and the 3/2-way reversing valve in order to be able at will to lower the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. in that a direction of rotation or travel selector means is provided the operation of which causes a biassing force, which acts in the direction of change over, to act on the 3/2-way reversing valve, a locking means being provided which prevents movement until the hydraulic control element of the control pump has reached a pre-determined position as the result of an arbitrary reduction of adjusting pressure in the adjusting pressure pipe. It is already known (German Published Patent Specifications 19 15 107 and 21 29 830) for a reversing valve for the main working circuit of a hydrostatic drive to be connected to the pump adjustment in such a manner that the change-over is made only when the pump is in the zero position, but this relates to hydraulic pumps which deliver in one direction only and therefore require an additional directional valve in the main circuit of the hydrostatic transmission. In this case it is not possible for the driver himself to control the progress of the reversing operation in all its phases. Furthermore, it is known in transmissions in general for a gear change to be initiated by operation of a preselector switch, the changeover being triggered by a locking element only when a reversing valve is operated (German Published Patent Specification 14 26 450). In contrast thereto the present invention provides the advantage of enabling the driver himself to control and adjust the deceleration to rest and the acceleration of the drive following the change-over while at rest, not only in respect of timing but also in respect of magnitude, so that he can thus adapt deceleration and acceleration in the optimum manner to operating conditions, the load to be transported or moved, and the environment. By virtue of preselection of the change of direction of travel he is thus relieved of an additional change-over operation requiring to be performed at the reversing point itself. The arrangement of the invention is also found advantageous for working machines, winches, and rotating and slewing drives. Advantageous further developments of the invention are indicated in the sub-claims. In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically and by way of example two embodiments thereof and in which: Figure 1 shows an adjusting device with a preselector device and mechanical locking means, and Figure 2 shows analogously the same adjusting device with only hydraulic components. The embodiment of Figure 1 shows a hydrostatic drive 1 comprising an adjustable hydraulic pump 2 and a hydraulic motor 3. The drive is effected by a driving motor, preferably an internal combustion engine, via a drive shaft 4. A feed pump 5 and a control pump 6 are also connected to the shaft 4. The adjustment of the hydraulic pump 2 is effected by an adjusting device 7 which is connected via a connecting element 10 to position indicator 11. By way of ducts 12 and 13 the adjusting device 7 is operatively connected to a three-position two-way reversing valve 8, which has a preselector device comprising the parts 14, 15, and 16 and a mechanical locking system comprising the parts 17, 18, 19, and 20. By means of a duct 9 the reversing valve 8 is connected to the delivery side of the control pump 6 and to an adjustable throttle 21 by-passing the pump 6. The throttle 21 is connected by means of a linkage 22 to the brake pedal 23. The preselection operation for the mechanical locking system will now be described with reference to Figure 1. In Figure 2, in which a hydraulic locking system is shown, parts serving equivalent functions are given reference numerals increased by 100. The hydrostatic drive 1 is adjusted, i.e. reversed, by the adjusting device 7 which is actuated by the reversing valve 8. The desired control position of the reversing valve 8 is preselected for a desired direction of travel by means of the adjusting device 14, 15, 16. The position indicator 11 of the adjusting pump 2 releases the catch 17 of the locking system only when the pump is in the neutral position (as shown), so that the reversing valve 8 and consequently, by way of the hydraulic adjusting device 7, the hydrostatic drive 1 can be reversed only in that position. By means of the control throttle 21 the control pump 6 produces a pressure which is dependent on rotational speed and which is applied to the adjusting device 7. The control throttle 21 is opened by means of the brake pedal 23 and the control pressure thus reduced. The hydraulic pump 2 is set back and the vehicle braked hydrostatically. With the aid of the brake pedal, or its speed of adjustment, the chronological fall of control pressure and consequently the deceleration of the vehicle can therefore be adjusted. When the brake pedal 23 is released the control pressure is built up again and the vehicle re-accelerated. WHAT WE CLAIM IS:

1. A hydraulic servo control device for adjusting the transmission ratio of a hydrostatic transmission, in which an adjustable pump, which can be changed over to two directions of delivery, has a hydraulically operated control element biassed to adjust said pump to a zero delivery position of said pump and operable in two opposite directions by means of a 3/2-way reversing valve and has a servo control pressure, which is adjustable by means of a tapping throttle valve of variable outlet cross-section which is connected to a pressure pipe extending between a con trol pump driven at the input rotational speed of the transmission and the 3/2-way reversing valve in order to be able at will to lower the

control pressure which is dependent on the input rotational speed of the transmission, for braking purposes, characterised in that a direction of rotation or travel selector means is provided the operation of which causes a biassing force, which acts in the direction of change over to act on the 3/2-way reversing valve, a locking means being provided which prevents movement until the hydraulic control element of the adjustable pump has reached a pre-determined position as the result of an arbitrary reduction of adjusting pressure in the adjusting pressure pipe.

2. A servo control device according to Claim 1, wherein the locking means and the direction of rotation or travel selector means are constructed as mechanical means for the preselection of the control position of the reversing valve, and are connected to the latter in such a manner that after locking of the reversing valve has been effected and the direction of rotation or travel selector means has subsequently been operated the reversing valve is placed under spring bias, and after the adjustable pump has reached its neutral position the locking means is released.

3. A servo control device according to Claim 1, wherein the reversing valve is reversed hydraulically, the reversing valve being connected to a control valve constituting the selector means by way of a further control valve constituting the locking means for preselection of the control position of the reversing valve, and the operation of said further control valve being effected in dependence on the adjustment of the hydraulic adjustable pump, in such a manner that when the neutral position of the adjustable pump is reached the hydraulic locking of the reversing valve is cancelled.

4. A hydraulic servo control device for adjusting the transmission ratio of a hydrostatic transmission, substantially as herein described with reference to and as shown in Figure 1 or 2 of the accompanying drawings.