1. Field of the Invention
The present invention relates to a fine operation mode changeover device for a hydraulic excavator which is intended to enable performance of accurate operation by simply changing over the operation to a fine operation mode when fine control of operation of a working machine such as a hydraulic excavator is provisionally required, for example, in leveling of a ground surface and in position adjustment on a dump vessel, and easy cancellation of the fine operation mode of the hydraulic excavator for returning to normal operation, thereby improving operability and work efficiency of the machine.
2. Description of the Related Art
When temporary fine control of the operation is required in such work as ground leveling by a hydraulic excavator or position adjustment on a dump vessel, such fine operation can be carried out with a far smaller quantity of fuel than in typical excavating work. Therefore, an engine revolution rate at a specified torque T.sub.0 is controlled to N.sub.1, N.sub.2 or N.sub.3 (rev/min) and a required quantity of fuel, that is, V.N.sub.1, V.N.sub.2 or V.N.sub.3 (cc/min), while maintaining a capacity V (cc/rev) of a hydraulic pump to be driven by the engine at a fixed level, is controlled by reducing a fuel injection quantity as shown in an engine torque graph in FIG. 8, thereby reducing fuel consumption of the engine. As is well known, an absorption torque T of the hydraulic pump is denoted as T=kP.times.V wherein k is a proportional constant and P is a load pressure. If the capacity V (cc/rev) of the hydraulic pump is fixed, the load pressure P.sub.0 of the hydraulic pump, having the absorption torque T.sub.0 illustrated in FIG. 8, is proportional to the absorption torque T.sub.0. An oil quantity of a hydraulic pump is reduced by a method which reduces the capacity V of the hydraulic pump by fixing a fuel injection to the engine as shown in FIG. 9 (the rotation rate of the engine is approximately fixed) and changing over the operation of the working machine to the fine operation mode. However, in the case of a method for reducing the oil quantity of the hydraulic pump by decreasing the rotation rate of the engine from the engine rotation note N.sub.1, in a state where the capacity V of the hydraulic pump is kept fixed as shown in FIG. 8, the matching points A2, A3 for the lower engine rotation rates N.sub.2, N.sub.3, respectively, with the absorption torque T.sub.0 corresponding to a specified load are further away from the center of the equivalent fuel consumption efficiency curve FC of the engine (hereinafter referred to as the equivalent fuel consumption curve with 100% at the center) than the matching point A.sub.1 for the initial engine rotation N.sub.1, and therefore this method is disadvantageous in that the fuel consumption of the engine lowers accordingly and the operator will suffer from a great deal of fatigue in frequently repeated operations for adjusting the rotation rate of the engine during the work by the hydraulic excavator. In the method as shown in FIG. 9, assuming that k is a proportional constant and P is a load pressure, the absorption torque T of the pump is denoted as T=kP.times.V as described above, and therefore, when the maximum torque is required, reduction of the value V with respect to the maximum load pressure P set by the relief valve will result in reduction of absorption torque of the hydraulic pump from T.sub.S to T.sub.1. Accordingly, this method is also disadvantageous in that the matching point with the hydraulic pump is shifted from A.sub.S to A.sub.1, which is further away from the center of the equivalent fuel consumption curve FC of the engine. Therefore, the fuel consumption efficiency of the engine deteriorates due to the location of the matching point A.sub.1 being further from the center of the equivalent fuel consumption curve FC than is the location of the matching point A.sub.S. The operating valves should be controlled in a small range where the operating strokes of operating valves are small, and the operability is deteriorated since only an insufficient capacity of the hydraulic pump can be obtained from the reduced absorption torque T.sub.1 of the hydraulic pump because the load sensing control is not effected. As shown in FIG. 8, frequent changeover operations of the fine operation mode and the ordinary operation mode will bring about a considerable degree of fatigue to the operator.