Hydraulic drive system for construction machine

Power plants – Pressure fluid source and motor – Condition responsive control of or by input to input pump...

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60434, 60452, 417 34, F16D 3102

Patent

active

051559960

DESCRIPTION:

BRIEF SUMMARY
1. Technical Field
The present invention relates to a hydraulic drive system for construction machines such as represented by hydraulic excavators or loaders, and more particularly to a hydraulic drive system for a construction machine which is equipped with pump control means capable of controlling the delivery rate of a hydraulic pump driven by a prime mover dependent on the input amount of a control lever.
2. Background Art
In a hydraulic drive system for construction machines such as hydraulic excavators or cranes, pump control means is employed which controls the delivery rate of a hydraulic pump driven by a prime mover dependent on the input amount of a control lever. The pump control means is operated under a load sensing control system or a negative control system. The load sensing control is, as described in DE-A1-3422165 (corresponding to JP, A, 60-11760 or U.S. Pat. No. 4,617,854) by way of example, to detect a maximum load pressure among a plurality of hydraulic actuators and control the delivery rate of the hydraulic pump such that a delivery pressure of the hydraulic pump becomes higher a fixed value than the maximum load pressure. This permits to increase or decrease the delivery rate of the hydraulic pump dependent on respective load pressures of the hydraulic actuators for saving on energy and an improvement in operability.
The negative control is to detect a pilot pressure signal concerned with the input amount of the control lever for controlling the delivery rate of the hydraulic pump dependent on the pilot pressure signal. This control system also permits saving on energy and an improvement in operability.
However, the conventional hydraulic drive system equipped with either control system mentioned above has accompanied problems below.
The delivery rate of a variable displacement hydraulic pump is determined by the product of a displacement volume, i.e., the tilting amount of a swash plate for the swash plate type, and a revolution speed. The larger the tilting amount, the greater will be the delivery rate. The tilting amount of the swash plate has a maximum value determined by the pump structure, and the delivery rate of the hydraulic pump becomes maximum at the maximum tilting amount of the swash plate for a certain setting revolution speed of a prime mover. Furthermore, the prime mover is reduced in its revolution speed upon input torque of the hydraulic pump exceeding output torque of the prime mover, and may stall in the worst case. To avoid this, therefore, the hydraulic pump is generally subjected to input torque limiting control. The input torque limiting control limits a maximum value of the delivery rate so that the input torque of the hydraulic pump will not exceed the output torque of the prime mover.
Thus, the hydraulic pump has a limitation in the delivery rate, i.e., the available maximum delivery rate. Accordingly, when the hydraulic pump reaches the available maximum delivery rate during operation of (one or more) actuators, the pump delivery rate can no longer be increased beyond the available maximum delivery rate and hence undergoes saturation. Upon the pump delivery rate getting into saturation, the actuator can no longer be driven at a speed commanded by the control lever. In the load sensing control, particularly, because the delivery pressure of the hydraulic pump is reduced so as not to ensure a fixed differential pressure with respect to the maximum load pressure, a larger part of a hydraulic fluid delivered from the hydraulic pump is caused to flow into the actuator on the lower pressure side, which disables smooth combined operation.
One conceivable method of solving the above problem is to set the hydraulic pump so as to produce the higher capability, i.e., the larger capacity. But, this creates an increase in the cost of parts and is disadvantageous in economy. As an alternative, the foregoing problem could be alleviated if the prime mover for driving the hydraulic pump is set to a maximum revolution speed at all times. However, setting the prime mover to

REFERENCES:
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patent: 3987624 (1976-10-01), Cooke et al.
patent: 4118149 (1978-10-01), Hagberg
patent: 4330238 (1982-05-01), Hoffman
patent: 4534707 (1985-08-01), Mitchell
patent: 4617854 (1986-10-01), Kropp
patent: 4635440 (1987-01-01), Kropp
patent: 4742676 (1988-05-01), Kropp et al.
patent: 4884402 (1989-12-01), Strenzke et al.
Patent Abstracts of Japan, vol. 10, No. 155; Jun. 4, 1986, (JP-A-61-11 429).

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