Tractor with hydraulic draft control
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a hitch system of the type which is responsive to draft loads to adjust the working depth of an implement connected thereto to maintain a preselected draft force and more particularly relates to draft control using a pilot-operated hydraulic valve which increases the sensitivity of the direction control mechanism to deviations in the draft load. 
     Prior art hitch systems have generally provided for draft sensing connected directly to the control mechanism as disclosed in the U.S. Pat. No. 3,731,745 granted to J. A. Koch on May 8, 1973 and the U.S. Pat. No. 2,940,530 granted to W. H. DuShane on June 14, 1960. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide hydraulic draft load control incorporating a pilot-operated valve for maintaining a desired draft load. The valve is highly sensitive to deviations in the draft load and corrects the position of the draft links accordingly. When the draft load increases, a draft sensor signals the pilot-operated valve which amplifies the signal to cause the draft links to be raised. Likewise, when the draft load decreases, the sensor signals the pilot-operated valve which amplifies the signal to cause the draft links to be lowered. 
     The above and additional objects and advantages of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a section view of the rear portion of a tractor embodying a draft sensing hitch system used with the present invention; and 
     FIG. 2 is a schematic of the draft load control of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown the rear portion of a tractor 10 including a chassis 12 and wheels 14 (only one shown) on an axle 16. Attached to the chassis 12 for vertical movement about a pivotal connection 20 and extending rearwardly therefrom is a rearwardly extending draft link 18. The draft link 18 has integral therewith a sensor cylinder or sensor 48. 
     A horizontal transverse rockshaft 24 is journaled for rotation in the chassis 12 and a crank arm 26 is secured to and extends rearwardly from the rockshaft 24. Innerconnecting the crank arm 26 and draft link 18 is a lift link 28. An extensible and retractable hydraulic lift cylinder 30 (shown in FIG. 2) is located in the chassis 12 and is connected to the rockshaft 24 for rocking the latter to cause the draft link 18 to be moved between a variable lowered working position and a raised transport position. 
     The sensor 48 includes a rod and piston assembly 50. The piston has a shaft 49 secured to it for selectively activating a ball check valve 51 which is connected by a supply line 52 to a fluid pump 54. The shaft 49 and the valve 51 cooperate to compensate for fluid leakage around the piston which is drained through a loss line 56 to a reservoir 88. The rod end chamber of the sensor 48 is connected by a sensor line 58 to a pilot-operated load control valve 60. 
     The pilot-operated valve 60 includes a valve housing 62 surrounding a valve body 64 having a longitudinally extending bore 65 with a main piston 68 positioned therein. The main piston 68 is slidably mounted in the bore 65 and in cooperation with the valve body 64 defines a main chamber 70. 
     A pilot piston 74 is slidably mounted in another longitudinally extending bore 66 and sleeved over a portion of the main piston 68 for relative sliding therebetween. The main piston 68 contains a groove 77 cooperating with the pilot piston 74 to form a sub-chamber 78 which is connected by a fluid passage 80 with the main chamber 70. The sub-chamber 78 is selectively connected by port means 76 and a supply line 82 with the fluid pump 54. The pilot piston 74 further cooperates with the valve body 64 to form a sensor connected chamber 86 which is connected by the sensor line 58 to the sensor 48. 
     In order to afford bi-directional movement of the main piston 68 as will be hereinafter described, the main chamber 70 is vented to the reservoir 88 by a restricted vent 90. First and second passages 92 and 96 are provided to prevent hydraulic lock between the main and pilot pistons 68 and 74. 
     To prevent leakage where the pilot piston 74 exits from the valve body 64, a pilot piston seal 98 is provided. Likewise, to prevent leakage where the main piston 68 exits from the valve body 64, a main piston seal 100 is provided. 
     A control mechanism, as is commonly known in the art and of the type shown in the U.S. Pat. No. 2,940,530 granted to W. H. DuShane, is illustrated at 110. The mechanism 110 includes a load control arm 112 pivoted at a load control arm pivot 114 and connected to the main piston 68 by a pin 116. The load control arm 112 moves a link 113. The link 113 in turn is connected to an intermediate link 115. The intermediate link 115 is connected to a valve link 118 which is connected to operate a three-position, three-way, direction control valve 122. 
     The direction control valve 122 has a first port in the first side thereof to which the fluid pump 54 is connected and a second port in the first side thereof to which the reservoir 88 is connected. The port in the second side of the direction control valve 122 is connected by a lift cylinder line 124 with the lift cylinder 30 for adjusting the position of the draft link 18. A positioning spring 123 biases the direction control valve 122 towards the connection of the lift cylinder line 124 with the reservoir 88. 
     A draft control lever 126 for manipulation by an operator to obtain a desired draft load is pivoted at a control lever pivot 128 and pivotally supports the intermediate link 115 which is pivotally connected to the valve link 118 through a pin 117. The draft control lever 126 is further operatively associated with a loading lever 132 which is pivoted at loading lever pivot 134 and abuts a pin 130 on the draft control lever 126 at one extremity and a bushing pin 136 sliding in a slot 137 in the valve housing 62 at the other. The bushing pin 136 is inserted in a guide bushing 138 which guides and selectively compresses a valve spring 140. The valve spring 140 biases the pilot piston 74 towards the main piston 68 and operates as will hereinafter be described. 
     With an earth-working implement (not shown) connected to the draft link 18 and being pulled through the soil by the tractor 10, the operator moves the draft control lever 126 to a position representative of the new desired draft load. This positioning places the control machanism 110 and the load control valve 60 in positions unique to the new desired draft load. Further, a preselected preload is imposed on the valve spring 140. 
     Since the earth-working implement is connected to the draft link 18, it imposes a draft load on the sensor 48. The draft load acting on the rod and piston assembly 50 provides a fluid pressure in the sensor connected chamber 86 which positions the pilot piston 74 such that the port means 76 is in restricted fluid communication with the subchamber 78. The restricted fluid communication allows merely enough fluid from the pump 54 to reach the main chamber 70 as required to make up the loss of fluid through the vent 90. 
     When the actual draft load increases in excess of the desired draft load, referring now to FIG. 2, the pressure in the sensor connected chamber 86 increases and forces the pilot piston 74 in the load control valve 60 to compress the preloaded valve spring 140 until the subchamber 78 is in full fluid communication with the port means 76. Pressurized fluid from the fluid pump 54 passes through the fluid passage 80 into the main chamber 70 to urge the main piston 68 to the left. 
     Movement of the main piston 68 acting through the draft control mechanism 110 linkages positions the direction control valve 122 to connect the fluid pump 54 with the lift cylinder 30 to cause the rockshaft 24 to rotate clockwise and decrease the implement draft until the desired draft load is sensed by the sensor 48. 
     With the reduction in pressure in the sensor connected chamber 86, the valve spring 140 slides the pilot piston out of full fluid communication with the port means 76. As the fluid in the main chamber 70 is lost through the vent 90, the main piston 68 is urged to the right by the positioning spring 123 on the direction control valve 122 acting through the draft control mechanism 110 linkages. 
     At the desired draft load, the restricted fluid communication between the subchamber 78 and the port means 76 is reestablished allowing the direction control valve 122 to assume the ports blocked position. 
     When the draft load decreases below the desired draft load, the draft load control operates substantially in the reverse manner as described for draft load increases. The pressure in the sensor 48 decreases and causes the restricted fluid communication to cease. As the fluid is lost through vent 90, the main piston 68 moves to reestablish restricted fluid communication and positions the direction control valve 122 to connect the lift cylinder 30 to drain into the reservoir 88. The rockshaft 24 rotates counter clockwise and the implement is lowered to increase draft until the desired draft load is reached. At the desired draft load, the pressure from the sensor 48 permits the restricted fluid communication to be reestablished and the direction control valve 122 to be returned to the ports blocked position. 
     To decrease the desired draft load, the draft control lever 126 is moved clockwise to a new position which in turn positions the direction control valve 122 to fluidly connect the fluid pump 54 to the lift cylinder 30. As the draft decreases, decreased pressure in the sensor 48 causes the restricted fluid communication in the load control valve 60 to cease. Movement of the main piston 68 to the right to reestablish fluid communication allows the control mechanism 110 linkages to move under urging by the positioning spring 123 to a new unique configuration so as to reposition the direction control valve 122 into its ports blocked position. 
     To increase the draft load, the draft control lever 126 is moved counterclockwise to a new position which in turn positions the direction control valve 122 to fluidly connect the lift cylinder 30 to the reservoir 88. As the draft increases, increased pressure in the sensor 48 causes full fluid communication in the load control valve 60. Movement of the main piston 68 to the left to reestablish restricted fluid communication forces the control mechanism 110 linkages to move into a further unique configuration so as to reposition the direction control valve into its ports blocked position. 
     As the draft control lever 126 is moved counterclockwise to call for an increase in the draft load, the loading lever 132 will pivot about the loading lever pivot 134 so as to urge the guide bushing 138 to cause increased compression of the valve spring 140. The valve spring 140 is a constant rate spring, thus equal fluctuations in draft load at different draft loads cause equal changes in the lift cylinder 30 position and the load control valve 60 is equally sensitive for small or large draft loads. Further, the ability to change the preload of the valve spring 140 to counterbalance the fluid pressure in the sensor responsive chamber 86 reduces the necessary movement of the pilot piston 74 and thus allows for a shorter pilot piston 74 than would otherwise be required. 
     While the invention has been described in conjunction with a specific embodiment, it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the aforegoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.