referring to fig1 and 2 , a tractor shown generally at 10 has attached to its rear body 11 an upper implement hitch pivot 12 and a lower implement hitch pivot 13 . the upper implement hitch pivot 12 and the lower implement hitch pivot 13 are attached to the rear body of the tractor 11 through an upper linkage assembly comprising two links 12a , 12b and a lower linkage assembly comprising two lower links 13a , 13b respectively and pivot joints 14 , 15 , 16 , 17 , 18 and 19 . the two links 12a , and 12b lie in a plane abc and the lower links 13a , 13b in a plane def . the planes abc , def are inclined as shown to the ground plane xy and intersect each other at d in the line t 1 t 11 which is the &# 34 ; track grip line &# 34 ;, that is to say that line on which the tractor when pulling an implement such as a plough or disc harrow is found to concentrate its grip with the ground . this line may often contain the center of gravity of the working assembly of tractor and implement but whether it does or not , the lower links 13a and 13b should ideally converge towards d in t 1 t 11 . it will also be seen that the center line of the upper linkage assembly , that is the line bisecting the angle between the two upper links 12a and 12b converges with the lower links towards the point d . it will be understood that the geometry of fig1 and 2 is an ideal arrangement and that in practice the intersection line of the planes may fall slightly outside of the line t 1 t 11 but this must be kept to a minimum by the careful design inter alia of the height of the upper implement hitch pivot a above the ground plane xy ; the height of the lower implement hitch pivot joints 14 and 15 above the said ground plane and the positioning of the connecting points of the linkage assemblies , viz . 12a , 12b ; 13a , 13b , to the tractor body 11 . indeed , the importance of these heights are noted in british standard specifications 1841 , 1968 &# 34 ; attachment of mounted implements to agricultural wheeled tractors &# 34 ;; and 4621 , 1970 &# 34 ; implement headstock for tractor / implement connection by three - point linkage or automatic coupler &# 34 ;. it is to be appreciated that the intersection of the planes as described above ensures that there is no effective change of trim or weight distribution of the tractor and its attachments from its static state . indeed it provides an arrangement equivalent to the tractor having a draw bar mounted at ground level , yet the ground clearance under the tractor and its attachments is not reduced from an appreciable working clearance , a feature not hitherto achieved in practice . a preferred form of the upper linkage assembly is shown in fig4 a and b . in fig4 a the linkage assembly 12 comprises two links 12a and 12b which contain hydraulic rams 210 and 211 respectively . the piston rods 212 and 213 of the rams are pivoted to a common pin 214 and the cylinders 215 and 216 of the rams are pivoted to the rear of the tractor 10 at the pivots 18 and 19 . the cylinders 215 and 216 are provided with hydraulic connections 217 , 218 , 219 and 220 which interconnect the cavities 221 , 222 , 223 and 224 of the cylinders . hydraulic locking valves 225 and 226 are provided in the respective connections between the cavities 221 , 222 and 223 , 224 . a double - acting valve chest 227 supplied with hydraulic fluid under pressure from a hydraulic pump ( not shown ) is connected as shown to the pairs of cavities 221 , 222 and 223 , 224 . it will be seen that by setting the valve 227 to deliver hydraulic fluid into the cavities 221 , 222 and to return fluid from the cavities 223 , 224 to the tank at the pump , the two links 12a and 12b are shortened . if the valve 227 is set to deliver hydraulic fluid to the cavities 223 and 224 and to return fluid from the cavities 221 and 222 to the tank the two links 12a and 12b are lengthened . if the valve 227 is set to a neutral position , hydraulic fluid can flow between the cavities 221 and 222 and between the cavities 223 and 224 . thus the apex at a is free to move laterally in the direction of the arrows x - y . if the valve 227 is set to a neutral position and the locking valves 225 and 226 are closed no fluid can flow in the system and the two links 12a and 12b become rigid . if the valve 227 is set to allow the flow of fluid between the cavities on opposite sides of the pistons the links 12a and 12b become free to float in the fore and aft direction by being lengthened and shortened , as required . the links 13a and 13b can be lifted and lowered by hydraulic rams 24 , 25 ( fig3 and 6 ). in a preferred embodiment i make the two lifting and lowering rams 24 , 25 for the links 13a , 13b to have cylinders and pistons with identical swept volumes in all four chambers and the two cylinders are so connected that they provide , from the driving seat of the tractor , the following manipulations for the links 13a , 13b of the implement being drawn : a : float in unison of links 13a , 13b out of the plane def , b : lift of links 13a , 13b out of the plane def away from the ground plane xy , d : lowering by free drop or under power of links 13a , 13b out of the plane def and toward the ground plane xy , e : l controlled tilt of 13a , 13b out of plane def to left ( e ) these manipulations may be achieved by operating the hydraulic circuit as shown in fig3 where the hydraulic cylinders 24 and 25 are linked to the back plate 11 of the tractor at points 20 , 22 and to the links 13a , 13b at 21 and 23 respectively . the cylinders 24 and 25 are shown diagrammatically in fig3 a , 3b , 3c , 3d , 3e and 3f with pistons 24a , 25a and ports i , ii , iii , iv . the pump line to the system is denoted by the letter p and the line to tank by the letter t with appropriate arrows for signifying the direction of the fluid flow . in this manner the manipulations a , b , c , d , e and f referred to above may be obtained from the hydraulic connections shown in fig3 a , 3b , 3c , 3d , 3e and 3f respectively . finally , as a special refinement to the linkage of fig1 and 2 , a depth sensing device or draught control for the implement such as a plough being drawn by the tractor is provided . this depth sensing device is shown in fig4 b . when the implement being drawn digs into the ground it tends to turn about the lower linkage assembly ( 13a , 13b of fig1 and 2 ) and put the upper linkage assembly ( 12a , 12b , fig1 and 2 ) into compression with a consequent rise in the pressure of the oil in the chambers 223 , 224 ( fig4 b ). if now spring 228 is set to a given pressure then , when the pressure in 223 and 224 rises above it the piston 229 in cylinder 230 moves to actuate lever 231 which opens the oil supply to the rams 24 , 25 , fig3 to lift the lower linkage assembly 13a , 13b and thereby suitably adjust the depth of action of the implement . when the pressure in 223 , 224 falls the spring 228 returns the piston 229 to its normal position and the oil supply to rams 24 , 25 is cut off and the depth of cut of the drawn implement maintained . referring to fig5 this shows a mounting which has been devised whereby a tractor linkage can be mounted on the rear of a tractor in such a way that stress on the rear gearbox can be greatly reduced by transfer of stress to the main longitudinal chassis members of the tractor , a standard pivoted draw bar can be retained when the linkage is attached , and power take - off can be provided to an implement coupled to the tractor by the linkage . the mounting comprises a yoke 104 mounted at the rearward end of the rear gearbox 105 of a tractor . the gearbox 105 has a frame 106 bolted to its rear face , the frame having two side plates which support a bottom plate 107 having an arcuate rear edge as shown . this rear edge supports the free end of a draw bar 108 and the curvature of this edge allows the draw bar to pivot as indicated by the arrows 109 . the dependent legs of the yoke 104 are welded to the side plates of the frame 106 and the lower edge of the crossbar of the yoke 104 abuts against the rear face of the gearbox 105 . the overlap between the gearbox and the crossbar of the yoke to provide this abutment is about half an inch . the two extremities of the crossbar of the yoke are connected by two tie rods 110 and 111 respectively to the main longitudinal chassis members partly shown at 112 and 113 . as shown the tie rods terminate in plates 114 and 115 which are bolted to the chassis members 112 and 113 . a linkage according to the invention is mounted on the yoke 104 at points such as 116 , 117 , 118 and 119 . because there are two divergent parts to the upper linkage assembly there is no obstruction for a power take - off shaft such as the shaft and universal joint 120 . the two upper links are normally in tension and these tensile forces are transmitted through the rods 110 and 111 to the main chassis members 112 and 113 thus relieving the gearbox studs of the considerable stress which is normally applied to them . referring now to fig6 this shows an arrangement which has been devised to enable one man to hitch a three - point linkage to an implement . the arrangement comprises a frame including a main tubular member 121 of square cross - section . by means of two struts 121 and 122 welded at their lower ends to the tubular member 121 a head 123 is carried above the member 121 . the tubular member 121 is supported by the links 13a and 13b ( shown in broken lines ) which are pivoted to members 124 and 125 affixed to the member 121 . two lower hitch - hooks 126 and 127 are slidably mounted on the tubular member 121 and can be clamped thereto by clamps 128 and 129 . by these means the lower hitch - pins such as the pins 130 and 131 of any standard implement can be engaged by the lower hitch - hooks 126 and 127 . the head 123 carries a ball - and - socket universal joint ( not visible in fig6 ) to which the two upper links 12a and 12b are attached . thus by appropriate operation of the hydraulics in the links 12a and 12b the head can be caused to move forwards and backwards , and left and right . the head 123 also carries two upper hitch - hooks 132 and 133 which can be brought into engagement with an upper hitch - pin such as the pin 134 of an implement . when the upper hooks engage the upper hitch - pin a clamp member 135 can be forced down upon the hooks 132 and 133 whereby all four hitch - hooks are firmly held in engagement with the hitch - pins of the implement . for this purpose a rod 136 is screw - threaded through the clamp member 135 and is provided with a handle 137 enabling the driver to operate the clamp . thus to hitch the linkage to an implement the driver of the tractor so operates the hydraulics as to lower the member 121 to a low level and to pull the head 123 back towards the tractor . he then reverses the tractor towards the implement until the lower hitch - hooks are underneath the lower hitch - pins of the implement . he then so operates the hydraulic rams 24 and 25 as to bring the lower hitch - hooks into engagement with the lower hitch - pins . the driver then so operates the rams in the links 12a and 12b as to move the head 123 forwardly and click the hooks 132 and 133 over the upper hitch - pin 134 . finally the driver winds down the clamp member 135 by means of the handle 137 until the implement hitch - pins are firmly locked into the upper and lower hooks . the height of the hooks 132 and 133 can be adjusted by using appropriate pairs of the apertures 138 . thus , it will be appreciated that the hitch can be used to couple the linkage to implements of all standard categories without adding to or modifying the implements . in a modification ( not shown ) of the arrangement of fig6 the upper hitch - hooks are arranged to be substantially longer than shown . the hooks are urged downwardly by a spiral spring coaxial with the spindle of the hooks . with this modification the rams in the links 12a and 12b are initially urged forwardly to bring the upper hitch - hooks above the upper hitch - pin 134 . the rams 24 and 25 ( fig3 ) are then operated to bring the upper hitch - hooks into engagement with the upper hitch - pin 134 and then to rotate the upper hitch - hooks against the spring until the lower hitch - hooks can be moved under the lower hitch - pins 130 and 131 . by means of the rams 24 and 25 the lower hitch - hooks can then be brought into engagement with the lower hitch - pins and can be locked to them by any suitable lock such as the automatic spring - loaded locks used on crane hooks .
a three - point linkage for coupling farm implements to a tractor has two lower links pivoted at their forward ends to a tractor and lie in a plane which from the rear to the front ends of the links is downwardly inclined and the links converge towards a point at or near ground level , and an upper link whose center line converges with the lower links towards the said point . in lieu of a single upper link , a pair of links which diverge from one another as they extend toward the tractor are preferred .