| 摘要 |
<p>When a solenoid proportional valve (91A) of a direction control valve (8A) is left open due to failure and an operator makes a reverse lever operation for operating an operation lever (4A) in an x2 direction, a low level signal is outputted from an operation position sensor (30A2) and a high level signal is outputted from operation position sensors (30A1, 30B1, 30B2). Since the output of an AND circuit (6b2) becomes a low level signal and the output of an AND circuit (6b1) becomes a high level signal, a change-over signal is outputted from an amplification circuit (6h) to a solenoid change-over valve (121), so that the solenoid change-over valve (121) is switched to a right side position in the figure to allow a pilot line (51) to keep communicating with a tank (97). The change-over signal is not outputted from the amplification circuit (6h) to a solenoid change-over valve (122), and the solenoid change-over valve (122) is kept at a left side position in the figure. Accordingly, the pilot line (52) keeps communicating with a pilot pump (96). At this time, a driving signal from a metering calculation portion (6a) is inputted to a solenoid proportional valve (92A) of a direction control valve and the solenoid proportional valve (92A) is excited, so that a secondary pilot pressure is given from the solenoid proportional valve (92A) to a pilot operation portion (22A) of the direction control valve (8A). Because a primary pilot pressure to the solenoid proportional valve (91A) is reduced by the solenoid change-over valve (121) to the tank pressure, the direction control valve (8A), that has existed at the left side position in the figure, is switched to the left and can be easily returned to a neutral position. <IMAGE></p> |
