1. Field of the Invention
The present invention relates to a working machine such as an excavator which is operated using a battery as a drive source.
2. Description of the Prior Art
Usually, in a conventional working machine such as a hydraulic excavator, a hydraulic pump is actuated by utilizing the power of an engine mounted for traveling and a hydraulic oil discharged from the hydraulic pump is fed to various hydraulic actuators such as a rotating motor, a boom cylinder and an arm cylinder to actuate various portions. However, in such a machine utilizing the engine power, there is a fear that noises and exhaust gases generated from the engine may exert a bad influence on the working site environment. For this reason it is the present situation that strict restrictions are placed on operations in cities and towns and in tunnels, further, on night operations.
In an effort to eliminate such bad influence there has recently been proposed a working machine wherein an electric motor is rotated by utilizing the electric power of a battery mounted on the working machine and a hydraulic pump is actuated by the electric motor (see, for example, Japanese Patent Laid Open No. 9-144061).
The battery mounted on the above working machine is lower in energy density (energy capable of being stored per unit mass) than fossil fuels such as gasoline and gas oil which are engine fuels. Thus, battery-powered working machine is disadvantageous in that the continuous working time is short as compared with working machines which utilize the engine power. As an example of means for increasing the continuous working time there is mentioned a means wherein energy is recovered when a negative work such as boom lowering or rotation braking work is performed and it is regenerated in the battery (that is, the attainment of a high efficiency is intended by energy regeneration). However, a large number of valves, including electromagnetic proportion valves, are mounted on a hydraulic excavator or the like and there occurs a large pressure loss or relief loss in those valves, so that it is very difficult to effect an efficient recovery of energy to the battery through the medium of such a hydraulic circuit.
If the operation of the boom and the rotation are conducted directly with use of an electric motor which is driven by a battery, without using such a hydraulic circuit, it will be possible to effect an efficient recovery of energy. However, in comparison with the hydraulic actuator, such an electric motor is smaller in the force and torque which it can generate per unit weight; besides, a reduction mechanism is required. Thus, the use of such a battery-powered electric motor is disadvantageous in that moving portions becomes more complicated in structure than in the use of a hydraulic actuator. The weight of moving portions also increases. Therefore, if the electric motor in question is provided at the front end of a boom, not only the moment of inertia of the boom and that of the entire swing structure will become larger, but also the weight balance of the entire working machine will become unstable.