referring to fig1 reference numeral 10 generally designates an automatic reset plow unit according to the present invention . the plow unit 10 is mounted to a horizontal beam 11 by means of an upright shaft 12 . the shaft 12 is journaled in upper and lower blocks 13 , 14 of a yoke designated 15 . the yoke 15 is thus free to pivot about a vertical axis extending along the axis of the shaft 12 . in the illustrated embodiment , the plow unit 10 may be one of a number of similar units secured to the beam 11 in accordance with the teachings of the kinzenbaw u . s . pat . no . 3 , 817 , 333 , issued june 18 , 1974 for &# 34 ; plow system with plurality of plow units and means for adjusting spacing between units in a continuous manner .&# 34 ; the coulter may be provided in front of the plow unit 10 and attached to the yoke 15 , if desired . secured to the rear of the yoke 15 is a linkage assembly generally designated by reference numeral 20 . a plow bottom 21 is mounted to the linkage assembly 20 by means of a shank 22 . in fig1 the plow bottom is seen in its normal plowing position , just prior to striking an object designated 24 . the soil being plowed is not shown for clarity in fig1 - 3 , but may be seen at s in fig4 . the linkage assembly 20 includes an upper link 25 , a generally upright rear link 26 , and a lower line 27 . the forward ends of the upper link 25 and lower link 27 are pivotally connected to a member 28 which is rigidly secured to the yoke 15 . thus , the linkage assembly includes four pivot connections : upper forward and rear connections designated 30 and 31 respectively , and lower forward and rear connections 32 , 33 . the upper link 25 includes a depending portion 35 which is pivotally connected at 36 to a cam member 37 . the central portion of the fixed link 28 is broken away to show a seat designated 38 on the cam 37 . a roller 39 is mounted to the fixed link 28 ( which may comprise two separate side plates spaced by the roller 39 , although only one is shown in the drawing ). the cam member 37 may be formed of steel plate material and it is provided with a cam surface including a first portion designated 40 adjacent the seat 38 for engaging the roller 39 in the normal , plowing position , and it also includes a turning point or threshold 40a ( fig4 ) and a second portion 41 ( see fig2 and 3 ), the function of which will be described presently . the cam surface 40 , 41 may be reinforced to extend beyond the width of the cam 37 . the upper link 25 is also formed of two similarly shaped plate members which are arranged on either side of the cam 37 so as to straddle it . the plate members are connected together by tubes forming journals for the three pivot positions indicated at 30 , 31 and 36 . referring now to fig2 the rear end of the cam member 37 is pivotally connected at 43 to the upper end of a spring unit generally designated 45 , the lower end of which is pivotally connected at 42 ( see fig2 ) to the rear link 26 . still referring to fig2 the cam member 37 is cut away at 44 to allow clearance for the pivotal connection at 30 for all positions of the unit , and the upper link 25 is bent at 47 to allow clearance for the upper pivotal connection 43 of the spring unit 45 . the spring unit 45 includes four compressed coil springs ( two on either side of the cam member 37 ). only one pair of the spring is seen in the drawing , but the other pair is similar . the springs are mounted coaxially with threaded rods 50 provided with inner guide sleeve 51 . each pair of springs , shown diagrammatically by the dashed lines 52 , is held between upper and lower end castings 54 , 55 ( fig3 ). the upper end castings are connected to a pivot shaft 43 , and the lower end castings are connected to a pivot shaft 42 by means of bolts threaded into the castings and engaging one of the shafts . the upper end casting is held down by nuts on the threaded rods 50 to establish a preload on the springs . the upper pivot shaft 43 is journaled in the cam member 37 and the lower pivot shaft 42 is journaled in the link 26 . thus , a pair of springs is easily replaced if damaged or if a larger spring constant is desired . as the castings 54 , 55 are brought together , the springs are further compressed , thereby increasing the force urging the plow bottom 21 downwardly . referring now to fig3 the shank 22 is attached to an extension 26a of the rear link 26 by a bolt 60 ; and the plow bottom is attached directly to the shank by welding and to the extension 26a by a bolt 61 . the bolt 61 is dimensioned to shear first , at a predetermined thrust on the plow bottom to avoid permanent damage to the unit . it will be observed that whereas the upper forward and rear pivot positions 30 , 31 are generally horizontally aligned , the rear lower pivot position 33 in the linkage assembly 20 is lower than the corresponding forward pivot position 32 . in other words , the length of the upper link 25 is smaller than that of the lower link 27 . thus , when the two links 25 , 27 are raised in unison about the forward pivot positions 30 , 32 respectively , this relationship causes the plow bottom 21 to initially move rearwardly at a rate greater than it moves upwardly . this is illustrated in fig2 where the horizontal displacement of the plow tip from the normal plowing position is ( fig1 ) is indicated by x and the corresponding vertical displacement of the plow tip is indicated by y . thus , as the plow bottom 21 strikes the object 24 , the upper link 25 and lower link 27 begin to rotate clockwise about their respective pivot locations 30 , 32 . because the link 27 is longer and inclined , the top of the rear link 26 is relatively constrained , whereas the bottom is moved outwardly , causing the tip of the plow to move more rearwardly than upwardly during this initial period , and thereby allowing greater time for the system to react to the presence of the obstructions . at the same time , as mentioned above , it is not desirable that this action take place every time heavy or compacted soil is encountered . the present invention advantageously provides that an initial force be overcome prior to achieving the reset action , and this is provided by having the roller 39 engage the seat 38 in the cam member 37 and by the preload on the springs unit 45 . referring to fig2 it will be observed that the spring unit 45 exerts a force on the cam member 37 which tends to keep the seat 38 in engagement with the roller 39 . that is , the cam member 37 is pivotally connected at 36 to the extension 35 of the top link 25 and the spring unit 45 exerts a clockwise force about this point . as the plow bottom 21 begins to raise , the lower part of the spring unit 45 ( the pivotal connection 42 ) is raised , whereas the cam surface 40 rides on top of the roller 37 and urges the cam member 37 counterclockwise about pivot 36 , thereby further compressing the springs of the spring unit 41 and increasing the force urging the plow bottom to a plowing position . referring now to fig3 after the threshold 40a of the cam surface passes the roller 39 , the roller engages and travels along the cam surface 41 . this surface is designed to increase the compression of the springs , but at a lesser rate , until the roller engages the uppermost portion , designated 41a in fig4 which , it will be observed , is curved forwardly so as to provide a stop or limit position for the roller 39 , beyond which the unit cannot move . in summary , the cam surface includes a first portion ( surface 40 ) which acts initially to compress the spring unit at a higher rate for a given movement of the plow bottom , and a second portion 41 which continues to increase the compression of the springs as the plow bottom is raised , but at a lesser rate . these two operating portions are separated by a threshold 40a which partially defines the seat 38 . the seat 38 together with the preload of the springs acts to hold the plow bottom at a fixed plowing depth for a wide range of soil conditions , but the plow will quickly and readily raise when a larger obstruction is encountered and reset when it is passed . persons skilled in the art will appreciate that the amount of compression as well as the rate of compression depends upon the shape of the two cam surfaces , and these may be changed , as indicated above , to account for varying soil conditions in different locales . referring now to fig1 - 4 as showing the sequence for reset , it will be recalled that the horizontal displacement from the normal plowing positions is indicated by x and the vertical displacement by y . the locus of the tip of the plow during reset in clearing the object 24 is indicated by the curved line designated 60 in fig4 and in this position , the spring unit 41 is under maximum compression . the force urging the plow bottom 21 downwardly to the resetting position is determined not only by the spring compressive force but also by the distance between the axis of shaft 36 ( which is the pivot location for the cam 37 ) and a line through the axes of shafts 42 and 43 ( sometimes called the line of application of spring force ). see the distance d in fig3 and 4 . in other words , the reset force is a moment coupled about the pin connection 36 , and even though the compressive spring force increases through the positions of fig1 through fig4 the actual reset force decreases because the distance d decreases more than the spring force increases . the resetting action , of course , is the reverse sequence , and it is considered an important advantage of the present invention that the plow bottom 21 is inclined downwardly as it re - enters the soil , which may be indicated by the line s in fig4 . hence , as soon as the plow bottom 21 clears the object 24 , it is urged downwardly and with a downward inclination and maximum force to re - enter the soil . further , the downward inclination of the plow bottom will cause it to be rapidly and fully seated assisted by the pull of the draft vehicle until the seat 38 again engages the roller 39 , as seen in fig1 . thereafter , the mechanism holds the plow bottom at uniform plowing depth , as determined by the operator . having thus described in detail a preferred embodiment of the invention , persons skilled in the art will be able to modify certain of the structure which has been illustrated and to substitute equivalent elements for those disclosed while continuing to practice the principle of the invention ; and it is , therefore , intended that all such modifications and substitutions be covered as they are embraced within the spirit and scope of the appended claims .
a linkage assembly supports a moldboard plow . a cam member having a seat engaging a roller is pivotally connected to the linkage assembly . a spring assembly is pivotally connected between the cam member and the linkage assembly to urge the plow in a normal plowing position . when the plow bottom encounters an object the cam seat disengages the roller and the plow rises quickly . after the obstruction is passed , the plow resets . during reset , the roller rides on a cam surface which controls the force of the spring urging the plow to reset . the locus of the plow tip when it initially strikes an object is more rearward than upward to give the unit better ability to respond at higher ground speeds . the tip of the plow is inclined downwardly during reset to facilitate rapid re - entry to proper plowing depth .