Hydraulic system for controlling a work implement

Abstract

A hydraulic system includes a cylinder assembly with a rod, a pressurized fluid source, a fluid tank, and a control valve. The control valve is fluidly connected to the cylinder assembly. The hydraulic system also includes a line relief, first makeup valve, and second makeup valve fluidly connected to a head end of the cylinder assembly. The second makeup valve is configured to provide fluid flow to the head end of the cylinder assembly when the rod is urged toward an extended position and the control valve is in a neutral position.

What is claimed is: 
     
         1 . A hydraulic system, comprising:
 a hydraulic cylinder assembly including a cylinder, a rod, a head end including a head pressure, and a rod end including a rod pressure;   a pressurized fluid source;   a fluid tank;   a metering control valve including a rod extension position, a rod retraction position, and a neutral position, the control valve being fluidly connected to the head end, the rod end, the fluid source, and the fluid tank, the metering control valve including a valve opening configured to direct fluid from the pressurized fluid source when the metering control valve is in the rod extension position, the valve opening having an opening area less than about 3% of a maximum opening area of the valve opening over about the first 50% of full spool displacement;   a line relief and first makeup valve fluidly connected to the head end of the cylinder assembly; and   a second makeup valve fluidly connected to the head end of the cylinder assembly, the second makeup valve being configured to provide fluid flow to the head end of the cylinder assembly when the rod is urged toward an extended position at a time when the metering control valve is in the neutral position.   
     
     
         2 . The hydraulic system of  claim 1 , wherein the metering control valve includes a rod extension port selectively fluidly connected to a pilot fluid source, and a rod retraction port selectively fluidly connected to the pilot fluid source. 
     
     
         3 . The hydraulic system of  claim 2 , wherein the metering control valve moves into the rod extension position when the rod extension port is connected to the pilot fluid source, connecting the pressurized fluid source to the head end of the hydraulic cylinder assembly. 
     
     
         4 . The hydraulic system of  claim 2 , wherein the metering control valve moves into the rod retraction position when the rod retraction port is connected to the pilot fluid source, connecting the pressurized fluid source to the rod end of the hydraulic cylinder assembly. 
     
     
         5 . The hydraulic system of  claim 1 , further including:
 a load control valve fluidly connected between the metering control valve and the head end of the cylinder assembly.   
     
     
         6 . The hydraulic system of  claim 1 , wherein the second makeup valve is a spring-biased and normally closed check valve. 
     
     
         7 . The hydraulic system of  claim 1 , further including a combination valve, the combination valve including a line relief, the first makeup valve, a spring-biased and normally-closed pressure relief valve; and a spring-biased and normally closed check valve. 
     
     
         8 . The hydraulic system of  claim 1 , further including a second hydraulic cylinder assembly including a cylinder, a rod, a head end including a head pressure, and a rod end including a rod pressure, the metering control valve and the line relief and first makeup valve being fluidly connected to the head end and the rod end of the second hydraulic cylinder assembly, and the second makeup valve being fluidly connected to the head end of the second hydraulic cylinder assembly, the second makeup valve being configured to provide fluid flow to the head end of the second hydraulic cylinder assembly when the rod of the second hydraulic cylinder assembly is urged toward an extended position at a time when the metering control valve is in the neutral position. 
     
     
         9 . A machine comprising:
 a power source;   a work implement; and   a hydraulic system including
 a hydraulic cylinder assembly including a cylinder, a rod, a head end including a head pressure, and a rod end including a rod pressure, 
 a fluid source, 
 a fluid tank, 
 a metering control valve including a rod extension position, a rod retraction position, and a neutral position, the control valve being fluidly connected to the head end, the rod end, the fluid source, and the fluid tank, the metering control valve including a valve opening configured to direct fluid from the pressurized fluid source when the metering control valve is in the rod extension position, the valve opening having an opening area less than about 3% of a maximum opening area of the valve opening over about the first 50% of full spool displacement, 
 a line relief and first makeup valve fluidly connected to the head end of the cylinder assembly, and 
 a second makeup valve fluidly connected to the head end of the cylinder assembly, the second makeup valve being configured to provide fluid flow to the head end of the cylinder assembly when the rod is urged toward an extended position at a time when the metering control valve is in the neutral position. 
   
     
     
         10 . The machine of  claim 9 , wherein the metering control valve includes a rod extension port selectively fluidly connected to a pilot fluid source, and a rod retraction port selectively fluidly connected to the pilot fluid source. 
     
     
         11 . The machine of  claim 10 , wherein the metering control valve moves into the rod extension position when the rod extension port is connected to the pilot fluid source, connecting the pressurized fluid source to the head end of the hydraulic cylinder assembly. 
     
     
         12 . The machine of  claim 10 , wherein the metering control valve moves into the rod retraction position when the rod retraction port is connected to the pilot fluid source, connecting the pressurized fluid source to the rod end of the hydraulic cylinder assembly. 
     
     
         13 . The machine of  claim 9 , further including:
 a load control valve fluidly connected between the metering control valve and the head end of the cylinder assembly.   
     
     
         14 . The machine of  claim 9 , wherein the second makeup valve is a spring-biased and normally closed check valve. 
     
     
         15 . The machine of  claim 9 , further including a combination valve, the combination valve including a line relief, the first makeup valve, a spring-biased and normally-closed pressure relief valve, and a spring-biased and normally closed check valve. 
     
     
         16 . The machine of  claim 9 , further including a second hydraulic cylinder assembly including a cylinder, a rod, a head end including a head pressure, and a rod end including a rod pressure, the metering control valve and the line relief and first makeup valve being fluidly connected to the head end and the rod end of the second hydraulic cylinder assembly, and the second makeup valve being fluidly connected to the head end of the second hydraulic cylinder assembly, the second makeup valve being configured to provide fluid flow to the head end of the second hydraulic cylinder assembly when the rod of the second hydraulic cylinder assembly is urged toward an extended position at a time when the metering control valve is in the neutral position. 
     
     
         17 . The machine of  claim 9 , wherein the work implement includes a bucket selectively operable to open and close, the machine further including:
 a stick selectively operable for extending and retracting the bucket; and   a boom selective operable for raising and lowering the bucket, the hydraulic cylinder assembly controlling operation of the boom.   
     
     
         18 . The machine of  claim 17 , wherein at times when the bucket is operably closed and the stick is operably retracted, interaction of the bucket with a surface causes extension of the boom while the metering control valve is in the neutral position. 
     
     
         19 . The machine of  claim 18 , wherein the second makeup valve provides fluid flow to the head end of the cylinder assembly while the metering control valve is in a neutral position. 
     
     
         20 . A method for operating a machine that includes a bucket, a stick, and a boom, the method comprising:
 selectively operating a metering control valve in a neutral position to maintain the boom in desired position;   selectively operating a stick and a bucket of the machine to close the bucket and retract the stick while the metering control valve is in the neutral position;   at times when said selective operation of the stick and bucket causes extension of the boom while the metering control valve is in the neutral position, providing fluid flow to the head end of the cylinder assembly via a line relief and first makeup valve fluidly connected to the head end of the cylinder assembly and a second makeup valve fluidly connected to the head end of the cylinder assembly; and   selectively operating the metering control valve in a rod extension position to raise the boom to a desired position, the metering control valve including a valve opening configured to direct fluid from the pressurized fluid source when the metering control valve is in the rod extension position, the valve opening having an opening area less than about 3% of a maximum opening area of the valve opening over about the first 50% of full spool displacement.