Power plants – Pressure fluid source and motor – Condition responsive control of pump or motor displacement
Patent
1993-03-26
1994-06-07
Look, Edward K.
Power plants
Pressure fluid source and motor
Condition responsive control of pump or motor displacement
60423, F16D 3102
Patent
active
053178718
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a circuit capable of varying a pump discharge volume in a closed center-load sensing system and, more particularly, to an improvement in a circuit capable of varying a pump discharge volume, which circuit is suitable for use with construction machines such as power shovels.
Description of the Related Art
In one embodiment of a conventional construction machine such as a power shovel, as shown in FIG. 6, two pumps 53 and 54, driven via a power take-out apparatus (hereinafter referred to as a PTO) 52 disposed on an engine 51, are connected by separate pipes 61 and 62, respectively, to two change-over valves 59 and 60 (hereinafter referred to as "two pumps/two valves") for changing an oil pressure applied to a boom actuator 55, an arm actuator 57, and a turning actuator 58, for operating a construction machine.
In an alternative system, as shown in FIG. 7, oil discharged from two pumps 71 and 72 flows through pipes 74 and 75 into a common pipe to change-over valve unit 73 (hereinafter referred to as "two pumps/one valve") having two stacked type change-over valves connected in parallel to actuators 57 and 58 of various types. A closed center-load system is used in the change-over valve unit 73 in the above case, and a load sensing system 80, for varying the respective discharge volumes in accordance with the valve opening irrespective of a load pressure P, is used with the two pumps 71 and 72. The discharge volume of the single pump 72 is changed by shutting off a flow control valve 81.
However, since the two pumps 53 and 54 of the "two pumps/two valves" shown in FIG. 6 are driven by using the PTO 52, and are connected to two change-over valves 59 and 60, placed in different places, by means of two pipes, the construction is complex so that a wide space is needed and the cost thereof is high. The "two pumps/one valve" shown in FIG. 7 has the same drawback as the above construction because it has two pumps, and also has a performance problem described below, with reference to FIG. 8.
1 When the spool stroke of the change-over valve is in an S range, i.e., in a throttling range (stroke and flow-rate characteristics), even if just the one pump 72 is used instead of the two pumps 71 and 72, the fine control curve (A) becomes constant with respect to stroke and cannot be made variable. Or, when only the pump 72 is used, since a pump capable of discharging a large amount must be used, the fine control curve (A) is also constant.
2 Even if the flow control valve 81 is changed and only the single pump 72 is discharged, the problem of item 1 is not solved though the maximum flow-rate is reduced from (B) to (C).
In addition, to simplify the construction and reduce the space required for the apparatus, "one pump/one valve" may be used. However, when the flow-rate required for a maximum discharge amount of the pump is low, as in the turning of a power shovel, even if the rotation of the pump is decreased by an engine, the maximum speed, i.e., the turning speed (D) of the actuator, does not change, as shown in FIG. 9, causing a problem in that the actuator does not turn as fast as the operator thinks it will.
Accordingly, the inventor of the present invention has proposed in Japanese Patent Laid-Open No. 2-261902 that the discharge volume of a pump be changed by varying the discharge pressure of a pump used to change a change-over valve, the load pressure of an actuator, and the energization force of a spring, or that the discharge volume of a pump be changed in accordance with the rotational speed of an engine. As a result, since the discharge volume of a pump changes in accordance with the rotational speed of an engine and since the operation speed of a construction machine is in accordance with the setting of the rotational speed of the engine, the actuator does not turn as fast as the operator thinks it will.
In recent years, there has been a demand for construction machines such as hydraulic shovels that can be operated easily
REFERENCES:
patent: 3543508 (1970-12-01), Schwab
patent: 3754400 (1973-08-01), Parquet
patent: 3826090 (1974-07-01), Bahl
patent: 4810171 (1989-03-01), Krebs
patent: 5146746 (1992-09-01), Nagai et al.
patent: 5249421 (1993-10-01), Lunzman
Imai Hiroshi
Ito Daijiro
Kabushiki Kaisha Komatsu Seisakusho
Look Edward K.
Ryznic John
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