HYDRAULIC DRIVE SYSTEM FOR CONSTRUCTION MACHINE

• US 20120031088A1
• Filed: 09/14/2010
• Published: 02/09/2012
• Est. Priority Date: 11/18/2009
• Status: Abandoned Application

• Current Assignee: Hitachi Construction Machinery Company Limited (Hitachi, Ltd.)
• Original Assignee: Hitachi Construction Machinery Company Limited (Hitachi, Ltd.)
• Inventors: Takebayashi, Yoshifumi, Tsuruga, Yasutaka, Takahashi, Kiwamu, Mori, Kazushige
• Application Number:
  US13/127,333
• Publication Number:
  US 20120031088A1
• Time in Patent Office: Days
• US Class Current: 60/452
• CPC Class Codes: E02F 9/2225 using pressure-compensating valves
• CPC Class Codes: E02F 9/2228 including an electronic controller
• CPC Class Codes: F15B 2211/3116 the pump port being open in the centre position, e.g. so-called open centre
• CPC Class Codes: F15B 2211/355 Pilot pressure control
• CPC Class Codes: F15B 2211/50554 the pressure control means controlling a pressure downstream of the pressure control means, e.g. pressure reducing valve
• CPC Class Codes: F15B 2211/575 Pilot pressure control
• CPC Class Codes: F15B 2211/6355 having valve means

Abstract

A hydraulic drive system for a construction machine allows automatic changes of the operational characteristics of a boom directional control valve by judging whether or not a hydraulic boom cylinder needs driving pressure at the time of a boom lowering operation.

A solenoid switch valve is controlled such that a pilot oil passage with a pressure reducing valve is selected to set the limit of a boom-lowering spool stroke of a boom directional control valve to a middle position L1 when the rod-side pressure of a hydraulic boom cylinder detected by a pressure sensor is less than a threshold value and such that a pilot oil passage is selected to set the limit of the boom-lowering spool stroke of the boom directional control valve to a maximum stroke position L2 when the rod-side pressure is equal to or greater than the threshold value.

Claims

1. A hydraulic drive system for a construction machine, the system comprising:

a hydraulic pump (28);

a hydraulic boom cylinder (20) for actuating a boom (17);

an operating device (30) for controlling the operation of the boom (17); and

a boom directional control valve (31) for controlling the flow of pressurized oil routed from the hydraulic pump (28) to the hydraulic boom cylinder (20) in response to the operation of the operating device (30), the boom directional control valve (31) being an open center valve, the system having characteristics that allow the orifice area of a center bypass oil passage of the boom directional control valve (31) to become larger than the orifice area of a meter-in oil passage of the boom directional control valve (31) when a spool of the boom directional control valve (31) is in the middle position of a boom-lowering spool stroke and that allow the orifice area of the center bypass oil passage to become smaller than the orifice area of the meter-in oil passage or allow the center bypass oil passage to completely close when the spool is in the maximum stroke position of the boom-lowering spool stroke,

wherein the system further comprises;

stroke limit varying means (38a, 38b, 39, 40;

38a, 38b, 39, 43;

45, 46;

45, 47) for selecting either the middle position or the maximum stroke position as the limit of a boom-lowering spool stroke of the boom directional control valve (31);

pressure judging means (41, 42;

44) for detecting or receiving an oil-feeding-side pressure of the hydraulic boom cylinder (20) upon lowering the boom (17) and for judging whether or not the oil-feeding-side pressure is equal to or greater than a predetermined threshold value; and

control means (42;

44) for controlling the stroke limit varying means (40;

43;

46;

47) such that the limit of the boom-lowering spool stroke of the boom directional control valve (31) is set to the middle position when the oil-feeding-side pressure of the hydraulic boom cylinder (20) upon lowering the boom (17) is less than the threshold value and such that the limit of the boom-lowering spool stroke of the boom directional control valve (31) is set to the maximum stroke position when the oil-feeding-side pressure of the hydraulic boom cylinder (20) upon lowering the boom (17) is equal to or greater than the threshold value.

2. The hydraulic drive system of claim 1,

wherein the stroke limit varying means includes:

a first pilot oil passage (38a) for outputting a spool-control pilot pressure generated based on a boom-lowering operation by the operating device (30) to a pressure receiver of the boom directional control valve (31) without any change to the spool-control pilot pressure;

a second pilot oil passage (38b) for reducing, with the use of a pressure-reducing valve (39), a spool-control pilot pressure generated based on a boom-lowering operation by the operating device (30) and then outputting the reduced pressure to the pressure receiver of the boom directional control valve (31); and

pilot-oil-passage selecting means (40;

43) for selecting either the first pilot oil passage (38a) or the second pilot oil passage (38b) and

wherein the control means (42;

44) controls the pilot-oil-passage selecting means (40;

43) such that the second pilot oil passage (38b) is selected when the oil-feeding-side pressure of the hydraulic boom cylinder (20) upon lowering the boom (17) is less than the threshold value and such that the first pilot oil passage (38a) is selected when the oil-feeding-side pressure of the hydraulic boom cylinder (20) upon lowering the boom (17) is equal to or greater than the threshold value.

3. The hydraulic drive system of claim 1,

wherein the stroke limit varying means includes:

a pilot oil passage (45) for outputting a spool-control pilot pressure generated based on a boom-lowering operation by the operating device (30) to a pressure receiver of the boom directional control valve (31); and

a variable pressure-reducing valve (46;

47), located on the pilot oil passage (45), for limiting the maximum value of the spool-control pilot pressure in a variable manner and

wherein the control means (42;

44) controls a limit value set for the variable pressure-reducing valve (46;

47) such that the limit value becomes a predetermined first limit value when the oil-feeding-side pressure of the hydraulic boom cylinder (20) upon lowering the boom (17) is less than the threshold value and such that the limit value becomes a predetermined second limit value larger than the first limit value when the oil-feeding-side pressure of the hydraulic boom cylinder (20) upon lowering the boom (17) is equal to or greater than the threshold value.

4. A hydraulic drive system for a construction machine, the system comprising:

a hydraulic pump (28);

a hydraulic boom cylinder (20) for actuating a boom (17);

an operating device (30) for controlling the operation of the boom (17); and

a first boom directional control valve (31) for controlling the flow of pressurized oil routed from the hydraulic pump (28) to the hydraulic boom cylinder (20) in response to the operation of the operating device (30), the first boom directional control valve (31) being an open center valve, the system having characteristics that allow the orifice area of a center bypass oil passage of the first boom directional control valve (31) to become larger than the orifice area of a meter-in oil passage of the first boom directional control valve (31) when a spool of the first boom directional control valve (31) is in the middle position of a boom-lowering spool stroke and that allow the orifice area of the center bypass oil passage to become smaller than the orifice area of the meter-in oil passage or allow the center bypass oil passage to completely close when the spool is in the maximum stroke position of the boom-lowering spool stroke,

wherein the system further comprises;

a second boom directional control valve (48), the second boom directional control valve (48) being an open center valve, the orifice area of a center bypass oil passage of the second boom directional control valve (48) being larger than the orifice area of a meter-in oil passage of the second boom directional control valve (48) when a spool of the second boom directional control valve (48) is in the middle position and the maximum stroke position of a boom-lowering spool stroke;

directional-control-valve selecting means (51a, 51b, 52) for selecting either the first boom directional control valve (31) or the second boom directional control valve (48) and actuating the selected boom directional control valve in response to the operation of the operating device (30);

pressure judging means (41, 42) for detecting or receiving an oil-feeding-side pressure of the hydraulic boom cylinder (20) upon lowering the boom (17) and for judging whether or not the oil-feeding-side pressure is equal to or greater than a predetermined threshold value; and

control means (42) for controlling the directional-control-valve selecting means (52) such that the second boom directional control valve (48) is selected when the oil-feeding-side pressure of the hydraulic boom cylinder (20) upon lowering the boom (17) is less than the threshold value and such that the first boom directional control valve (31) is selected when the oil-feeding-side pressure of the hydraulic boom cylinder (20) upon lowering the boom (17) is equal to or greater than the threshold value.

5. The hydraulic drive system of claim 4,

wherein the directional-control-valve selecting means includes:

a first pilot oil passage (51a) for outputting a spool-control pilot pressure generated based on a boom-lowering operation by the operating device (30) to a pressure receiver of the first boom directional control valve (31);

a second pilot oil passage (51b) for outputting a spool-control pilot pressure generated based on a boom-lowering operation by the operating device (30) to a pressure receiver of the second boom directional control valve (48); and

pilot-oil-passage selecting means (52) for selecting either the first pilot oil passage (51a) or the second pilot oil passage (51b) and

wherein the control means (42) controls the pilot-oil-passage selecting means (52) such that the second pilot oil passage (51b) is selected when the oil-feeding-side pressure of the hydraulic boom cylinder (20) upon lowering the boom (17) is less than the threshold value and such that the first pilot oil passage (51a) is selected when the oil-feeding-side pressure of the hydraulic boom cylinder (20) upon lowering the boom (17) is equal to or greater than the threshold value.