GB2101720A

GB2101720A - Control circuit for hydrostatic transmission of construction vehicle

Info

Publication number: GB2101720A
Application number: GB08216207A
Authority: GB
Inventors: Makoto Mihara
Original assignee: Hitachi Construction Machinery Co Ltd
Current assignee: Hitachi Construction Machinery Co Ltd
Priority date: 1981-06-10
Filing date: 1982-06-03
Publication date: 1983-01-19
Also published as: NL8202325A; DE3221543A1; JPS57205637A

Abstract

When an hydraulic motor 1 is adjusted from a low speed mode to a high speed mode by pilot pressure 9, the maximum pressure of a drive circuit for the hydraulic motor is changed. As shown, the pilot pressure 9 also acts on a valve 13 so that a relief valve 5 becomes effective instead of a relief valve 14 set at a lower value. The control circuit prevents insufficiency of the running torque in high speed running on flat road and excessiveness of the running torque in low speed running on soft road. <IMAGE>

Description

SPECIFICATION Control circuit for travelling hydraulic motor a) Field ofthe invention: The present invention relates to a control circuit for travelling hydraulic motor used for construction machines such as a hydraulic crawler crane, a hydraulic shovel or the like, especially to a control circuit for travelling hydraulic motors capable of two speed controls for high and low speed runnings by using a hydraulic motor.

d) Description of prior art: In the control circuitfortravelling hydraulic motor, the maximum pressure value is set by a relief valve, so that the maximum torque at high rate becomes half ofthe maximum torque at low rate in inversely proportional relationship.

In generai, in the case of a soft-road running or a running in a load-hanging state, a comparatively large torque for running is needed, so that a low speed running is practical. Especially in the case of the running in a load-hanging state, the low speed running is preferable to prevent the hanged load from swing and vibration. On the other hand, in the case of a flat-road running without hanging load, running resistance is comparatively small, so that a high speed running is preferable to reduce the time required to reach the destination.

Then, as metioned above the running torque is inversely proportional to the running speed, therefore if the relief pressure of the relief valve is set to obtain the maximum running torque required for the low speed running in the soft road, the running torque required for the high speed running in the the flat road is undesirably insufficient, in turn if the relief pressure is set to produce the maximum run ningtorque required for the high speed running in the flat road, the running torque required for the low speed running in the soft road is excessively increased thereby to undesirably demand large strength and durabilityforthe hydraulic motor and deceleration device of the travel device.

The main object of the present invention is to provide a control circuit for travelling hydraulic motors in which the maximum running torque in high and low speed runnings can be set regardless of the speed ratio, though the maximum pressure of the drive circuit for the hydraulic motor is decided by the relief valve thereof.

Another object of the present invention isto provide a control circuit for travelling hydraulic motors with high reliability and practically although the maximum running torque can be set regardless of the speed ratio.

Further objects of the present invention will appear more fully from the following descriptions and drawings.

In the accompanying drawings: Fig. lisa diagram of a first embodiment of the control circuit for travelling hydralic motor of the present invention; Fig. 2 is a diagram of a second embodiment cfthe present invention; Fig. 3 is a diagram showing a modification of the embodiment shown in Fig. 2; and Fig. 4 is a diagram of a third embodiment of the present invention.

Referring now to Figs. 1 to 4 there are shown some embodiments of the present invention showing a control circuit for travelling hydraulic motor used for a crawler crane and the like, in which running speed can be controlled in high and low rates. In Fig. 1 numeral 1 designates a hydraulic motor in which the injection amount to be injected to the motor 1 can be changed in two-stepwise state for high and low speed runnings by applying pilot-liquid pressure to a control cylinder 1 a. Numeral 2 designates a brake valve disposed in the drive circuit for the hydraulic motor 1, the brake valve being composed of a shock relief valve 2a for absorbing the shock pressure applied from a load side and a double check valve 2b for stopping the rotation of the hydraulic motor 1 and preventing over-running thereof.Numeral 3 designates a direction control valve for controlling the rotational direction ofthe hydraulic motor, by switching the flowing direction of the delivery pressure liquid of a hydraulic pump 4. Numeral 5 designates a main relief valve disposed at the delivery side of the hydraulic pump 4, and bythe main relief valve 5 the maximum pressure of the drive circuit of the hydraulic motor 1 is set. Numeral 7 designates an auxiliary hydraulic pump for supplying the pressured liquid to the control cylinder la of the hydraulic motor 1 through a pilot control circuit 9. At the delivery side of the auxiliary hydraulic pump 7, a relief valve 8 for setting the circuit pressure of the pilot control circuit 9.Numeral 10 designates a switch valve 10 disposed at the pilot control circuit 9 for connecting or disconnecting the pilot control circuit 9 to a liquid tank 6, and by operating the switch valve 10 the injection amount to be injected to the hydraulic motor 1 is changed in two-stepwise state by actuating the control cylinder 1 a, and as a result, the low speed mode of the hydraulic motor 1 as shown in Fig. 1 is changed to a high speed mode of the hydraulic motor 1.

A set pressure switching circuit is connected to the drive circuit. There is provided a shuttle valve 11 for selecting hydraulic pressure of the circuit of the high pressure side in the drive circuit disposed between the hydraulic motor 1 and the direction control valve 3, and a branch conduit 12 branched from the shuttle valve 11 is connected to the liquid, or oil, tank 6. On the branch conduit 12 there are provided with branch conduit 12 to a disconnecting position by means of liquid pressure produced by the pilot control circuit 9, and a relief valve 14 having a lower set value of pressure than that of the drive circuit of the hydraulic motor 1.Hence, the pressure of the shock relief valve 2a for setting the pressure for the drive circuit of the hydraulic motor 1, or the main relief valve 5 is set to the pressure P1 corresponding to that maximum running torque requiredforthe high speed running. Further the pressure of the relief valve 14 disposed on the branch conduit 12 may be set to the pressure P2 corresponding to the max imum running torque required for the low speed running. As stated above the interrelationship there between is P1 > P2.

In this embodiment of the present invention, a hydraulic Icck mechanism may be provided in a neutral position within the direction control valve 3 thereby to omit the brake valve 2.

According to the control circuit for travelling hydraulic motor thus constructed, in the low speed running the switch valve 10 is to set to the position as shown in Fig. 1 thereby to connect the liquid tank 6 to the pilot control circuit 9 and to change over the hydraulic motor 1 to a low speed mode. In this mode, if the direction control valve 3 in neutral position as shown in Fig. 1 is changed over to the position A or B, the delivery from the hydraulic pump 4 flows into the hydraulic motor 1 through the drive circuit thereby to rotate the hydraulic motor 1 in forward or backward direction with low speed. At this time, the pressured liquid at the high pressure side of the drive circuit flows through the branch conduit 12 and the shuttle valve 11 and acts to the relief valve 14 thereby to open the relief valve 14.Accordingly the maximum pressure used for the hydraulic motor 1 becomes to the pressure P2 set by the relief valve 14.

In turn, in high speed running the switch value 10 is changed over to the position A as shown in Fig. 1 then the delivery from the auxiliary hydraulic pump 7 flows into the pilot control circuit the hydraulic motor 1 is changed over to the high speed mode, and the pilot switching valve 13 is changed overto the position A by the pressure of the pilot control circuit 9 thereby to disconnect the branch conduit 12.

In this mode, if the direction control valve 3 is changed over from the position A to the position B the high speed running is effected. Atthistime, although the liquid at the high pressure side of the drive circuit flows into the branch conduit 12 through the shuttle valve 11, the branch conduit 12 is discon nected by the pilot switching valve 13, so that the relief valve having lower set value out of the shock relief valve 2a orthe main relief valve 5 is opened by the liquid pressure of the drive circuit.Accordingly, the maximum pressure used for the hydraulic motor 11 atthe high speed mode becomes to the pressure P1 corresponding to the maximum running torque required for the high speed running, and on the other hand the maximum pressure used for the hyd raulic motor 1 atthe low speed mode becomes to the pressure P2 corresponding othe maximum running torque required for the low speed running. Namely, the maximum running torque required for high and low speed runnings can be set regardless of the speed ratio.

Referring now to Fig. 2, a second embodiment of the present invention is shown, in which the drive circuit located at the upstream side of the hydraulic pump4 is connected to a branch conduit 22 con nected to the oil tank 6. A set pressure switching circuit is connected to the drive circuit. On the branch conduit 22 there are provided the pilot switching valve 13 and the relief valve 14.

According to the embodiment thus constructed the shuttle valve 11 as shown in Fig. 1 may be omit ted. The operation of the second embodiment of Fig.

2 is the same as the first embodiment of Fig. 1.

Referring to Fig. 3 a modification of the second embodiment in Fig. 2 is shown, in which a relief valve 5' is corresponding to the main relief valve 5 of the embodiments mentioned above and the pres sure ofthe reliefvalve S' is set lowerthan that ofthe shock relief valve 2a, so that the relief valve 5' serves also as the reliefvalve 14.

According to the modification thus constructed, the maximum pressure used for the drive circuit at the high speed running becomes to the pressure P, set by the shock relief valve 2a, and the maximum pressure at the low speed running becomes to the pressure P2 set by the relief valve 5'. Further by a mere provision of the pilot switching valve 13 at the upstream side of the relief valve 5', the branch conduit 12 and the relief valve 14 as shown in Fig. 2 can be omitted. However, it is necessary for the modification of Fig. 3 to connect the port P to the portT at the neutral position of the direction control valve 3'.

In connection with this, it may be practical without any hindrance that the port P is connected to the port Tatthe neutral position of the direction control valve 3 in the embodiments of Figs. 1 and 2.

Referring now to Fig. 4 a third embodiment ofthe present invention is shown, in which the set pressure switching circuit is provided within the brake valve 2', and there are provided a pair of branch conduits 32 and 42 connected to the drive circuit in the brake valve 2', and the pilot switching valve 13 and the relief valve disposed on the branch conduits 32 and 42 respectively.

According to the third embodiment thus constructed, the shock relief valve 12a and the relief valve 14 having the set pressure different from that of the relief 12a can be compactly disposed within the brake valve 2' and no piping work becomes needed.

In the third embodiment, it is explained that the high speed mode is selected when the pilot oil pressure acts to the control cylinder 1a of the hydraulic motor 1, however in a case of the hydraulic motor of the type selecting the low speed mode when the pilot oil pressure acts to the control cylinder 1 a, the switching position of the pilot switching valve 13 may be reversed. Further to switching valve 13 is also explained as a pilot switching valve changed over by oil pressure produced at the pilot control circuit, it may be arranged to be responded to the switching operation of the switch valve 10. Further a variable displacement pump may be used as the hydraulic pump, since the delivery pressure is similary set by the relief valve.

As stated above, according to the present invention, the set pressure switching circuit including a simple hydraulic apparatus is additionally provided with the control circuit for travelling hydraulic motors controlled in the high and low speed modes by pilot signals, whereby the maximum running tor que in the high and low runnings can ze set regard less of the speed ratio. As a result, insufficiency of the running torque in high speed running of flat road and execessiveness of the running torque in low speed running of soft road, which are unsolvable by the conventional technique, are prevented. Accord inglythe strength and durability for the travel apparatus may be reduced, in comparison with the conventional control circuit for travelling hydraulic motors, thereby to make the travel apparatus light and inexpensive. Further the set pressure switching circuit is composed of usual switch valves and relief valves, so that it is effective in reliability and practice.

Claims

1. A control circuit for travelling hydraulic motors comprising a hydraulic motorswitchable in high and low speed modes in response to control signals, a direction control valve for controlling the rotational direction of said hydraulic motor, a hydraulic pump, and a drive circuit for supplying the pressured liquid to said hydraulic motor from said hydraulic pump, said drive circuit being provided between said hydraulic motor and said hydraulic pump, said drive circuit providing a set pressure switching circuit having a first relief valve for setting the maximum pressure of said drive circuit, a second relief valve for setting the pressure different from the maximum pressure of said first relief valve, and a switch valve for being changed over in response to said control signals, whereby the maximum running torque in high and low speed runnings is set regardless of the speed ratio.

2. A control circuit for travelling hydraulic motors comprising a hydraulic motor switchable in high and low speed modes in response to control signals, a brake valve disposed on a drive circuit, a direction control valve for controlling the rotational direction of said hydraulic motor, a hydraulic pump, and a drive circuit for supplying the pressured liquid to said hydraulic motor from said hydraulic pump, the maximum pressure of said drive circuit being set by shock relief valve disposed in said brake valve for absorbing a shock load pressure or by a main relief valve disposed at the delivery side of said hydraulic pump, and a set pressure switching circuit being disposed in said drive circuit providing between said hydraulic motor and said hydraulic pump, and including a relief valve having the set pressure lower than said maximum pressure and a switch valve for being changed over in response to said control signals, whereby said maximum pressure of said drive circuit in the high speed running is set to be higher than that in the low speed running.

3. Acontrol circuit for travelling hydraulic motors according to Claim 1 or 2, wherein said set pressure switching circuit is connected to said drive circuit between said hydraulic motor and said direction control valve, through a shuttle valve.

4. A control circuit for travelling hydraulic motors according to Claim 1 or 2, wherein said set pressure switching circuit is connected to the circuit at the delivery side of the hydraulic pump.

5. A control circuit for travelling hydraulic motors according to Claim 2, wherein said set pressure switching circuit is disposed within said brake valve.

6. A control circuit for travelling hydraulic motors comprising a hydraulic motor switchable in high and low speed modes in response to control signals, a direction control valve for controlling the rotational direction of said hydraulic motor, a hyd raulic pump, a drive circuit for supplying the pressured liquid to said hydraulic motor from said hyd raulic pump, and a brake valve disposed within said drive circuit of the hydraulic motor, the maximum pressure of said drive circuit being set by a shock relief valve disposed in said brake valve for absorbing a shock load pressure, a set pressure switching circuit being disposed at the delivery side circuit of said hydraulic pump and including a relief valve having a set pressure lower than that of said shock relief valve and a switch valve for being changed over in response to said control signals, whereby the maximum pressure of said drive circuit in high speed running is set higher than that in low speed running.

7. A control circuit for travelling hydraulic motors substantially as hereinbefore described with reference to and as illustrated in Fig. 1; or in Fig. 2; or in Figs. 2 and 3; or in Fig. 4.