Conventional drive oil hydraulic circuitry of an oil hydraulic motor, for example, as shown in FIG. 7, is known.
In other words, more specifically, an operation valve 2 connects a delivery path 1a of an oil hydraulic pump 1 to a first main circuit 3 and a second main circuit 4, or the operation valve 2 disconnects the delivery path 1a from the first and second main circuits 3, 4. The first and second main circuits 3, 4 are respectively connected to a first port 6.sub.1 and a second port 6.sub.2 of an oil hydraulic motor 5. A counterbalance valve 7 is arranged between the first and second main circuits 3, 4. When the operation valve 2 is moved to a neutral position "N", because of non-return valves 8 placed in the first and second main circuits 3, 4, the side of the oil hydraulic motor 5 is disconnected from the counterbalance valve 7. Thus, the oil hydraulic motor 5 is not caused to rotate by an external force. When the operation valve 2 is moved to either a first position "I" or a second position "II", the counterbalance valve 7 is switched over to either the first position "I" or the second position "II", this switching being caused by highly pressurized oil in either the first main circuit 3 or the second main circuit 4.
The counterbalance valve 7 for use in such oil hydraulic circuitry is switched over to the first and second positions by highly pressurized oil in the first and second main circuits 3, 4. The counterbalance valve 7 returns to the neutral position "N" when the highly pressurized oil runs out.
When the oil hydraulic motor 5 is not driven by the first main circuit 3 or the second main circuit 4, it can be caused to rotate by an external load, thereby acting as a pump.
For this reason, if the counterbalance valve 7 is in the neutral position "N" when the oil hydraulic motor 5 is first rotated by an external load while the operation valve 2 is in the neutral position, either the first main circuit 3 or the second main circuit 4 becomes a high pressure circuit, thereby causing a great shock.
To moderate such a shock during cessation, the speed for the counterbalance valve 7 to return to the neutral position "N" from the first position and the second position is rendered slow. The counterbalance valve 7 throttles the pressurized oil in the first and second positions, thus causing it to flow to the tank 9.
For example, throttles 11, 11 are arranged in circuits 10, 10 which connect the first and second main circuits 3, 4 to the counterbalance valve 7. The speed for the counterbalance valve 7 to return to the neutral position "N" from the first and second positions is made slow by increasing the throttle amounts of these throttles 11, 11. In such an arrangement, however, the time required for the counterbalance 7 to return to the neutral position "N" is delayed. This causes cavitation and the oil hydraulic motor to stop for long periods.
An object of the present invention is therefore to provide a counterbalance valve in which an oil hydraulic motor can slow down without shock, while cavitation is prevented, thus coming to rest in a short period time.