referring to the drawings in detail and initially to fig1 there is disclosed apparatus embodying the invention for feeding paperboard blanks such as corrugated blanks or sheets , generally designated b , one - by - one to the nip of feed rolls 18 suitably mounted in a main frame 6 which forms part of box - making machinery including other mechanisms for printing , cutting , slotting , folding or gluing and the like which are not shown . the boards b are , of course , flat and are stacked one on top of the other in a suitable hopper with the lowermost board resting on a feed surface 36a which extends horizontally in the plane between the feed rolls 18 . the forwardmost edges of the boards are positioned by one or more gate members 32 ( one shown ) and by a trailing edge support generally designated 46 having a lower ledge on which the rearmost edges rest . the gates 32 and the trailing edge support 46 are adjustable in the lengthwise direction of the machine to accommodate boards of different lengths . additionally , they are adjustable in vertical planes so as to precisely set the distance between the lowermost edge of the gate 32 and the feed surface 36 . additionally , vertical adjustability of the trailing edge support 46 is employed to compensate for warpage occurring in any of the boards . the feed surface 36a is formed by the upper surface of a vacuum box 36 which encloses a vacuum chamber supplied with a vacuum through a duct 40 connected to any suitable source of vacuum and which communicates with the vacuum chamber through apertures 37 formed in the bottom wall of the vacuum box 36 as best shown in fig3 . the boards b are fed one - by - one across the feed surface 36 and into the nip of the feed rolls 18 by means of a plurality of driven feed wheels generally designated 51 mounted for rotation in the vacuum chamber of the vacuum box 36 on shafts 50 which are journaled for rotation in opposite end frame members 34 ( see fig3 ). in the specific embodiment shown , there are three rows of feed wheels 51 extending transversely of the longitudinal or feeding direction of the machine with each row containing ten feed wheels and with the feed wheels in adjacent rows being aligned with each other in the longitudinal or feeding direction of the machine . the feed wheel shafts 50 run parallel to each other and are equally spaced from each other , and the feed wheels in each of the rows are uniformly spaced from each other . the feed wheel shafts 50 are geared to rotate in opposite directions to drive the feed wheels in forward or reverse directions for a purpose to be described below . as seen in fig2 feed wheels 51 are provided with a high friction outer cylindrical surface 51a formed from any suitable material such as rubber or urethane so that the coefficient of friction between the feed wheel surfaces and the boards b will be in the range of 0 . 7 to 1 . 0 . the friction surface 51a may be provided on aluminum hubs 51b or any other suitable construction may be employed . in order to raise and lower the boards b off and on to the feed wheels 51 for purposes which will be explained below , a grate is provided to extend between the feed wheels throughout the extent of the feed wheels . referring to fig1 and 2 , the grate includes a plurality of vertical ribs 56 formed by plate - like structures of any suitable material , such as steel , which extend parallel to each other between the feed wheels 51 and the longitudinal or feeding direction of the machine . extending between and fixed to the ribs 56 are a plurality of rod members 55 which are recessed slightly downwardly from the upper surface of the ribs 56 as best shown in fig2 and 3 . the grate 55 , 56 is mounted and actuated up and down in the vertical direction at the desired time by means of preferably two elongated shafts 52 and 52a which extend transversely of the machine through the vacuum box 36 below the level of the feed wheels and which are journaled at their opposite ends in the frame members 34 at opposite sides of the machine . shafts 52 and 52a each contain mounting lugs 58 fixed thereto and having pins 57 receivable in apertures 53 fixed to depending leg portions of certain grate ribs 56 as best shown in fig2 . by structure to be described below , shafts 52 and 52a are rotatable in small amounts in opposite directions in order to raise or lower the grate 55 , 56 . when the grate 55 , 56 is in its lowered position shown in fig1 the upper surface of the grate is below the upper surfaces 51a of the feed wheels which are thus able to contact the underside of the board b to drive it by friction . when the grate is in its raised position , its upper surface is above the upper surfaces 51a of the feed wheels whereby the board b is out of contact with the feed wheels which occurs after the board b is engaged and driven by the nip rolls 18 . the vacuum box 36 with the feed wheels , grate and grate mounting and actuating means , and the side frame members 34 may be provided as a unit into existing machines or , of course , new machines . referring to fig1 this is effected by mounting the frame members 34 onto cross - frame members 8 of the existing machine through means of struts 35 fixed to and between frame members 34 and cross - frame members 8 . cross - frame members 8 are fixed in the main frame 6 which is shown in fig1 . referring to fig4 feed wheels 51 are driven in opposite rotative directions about the axis of their shafts 50 through mechanism including a plurality of wheel gears 70 fixed to shafts 50 on one of the ends thereof located outwardly of frame 34 ( see fig3 ). two of the wheel gears 70 are in mesh with a much larger gear 74 to be driven thereby while the remaining wheel gear 70 is driven through means of an idler gear 72 in mesh with two of the wheel gears 70 as shown in the right - hand side of fig4 . as shown in fig3 rocker gear 74 is mounted at its center 85 on a pivot shaft 86 which is journaled in frame 34 . in accordance with the present invention rocker gear 74 is rotatably oscillated about its axis 85 by means of a rocker shaft 10 and a linkage operatively interconnecting rocker shaft 10 and rocker gear 74 . in the specific embodiment shown in fig2 and 4 , a mechanism is employed which is the equivalent of a four - bar linkage , the mechanism including a rocker arm 81 fixed to and projecting from a sleeve 81a which , in turn , is mounted about rocker shaft 10 and fixed thereto by means of a key 82 as best shown in fig4 . the mechanism further includes a connecting link 80 pivotally connected at one end at pivot 83 to the rocker arm 81 and pivotally mounted at its opposite end by pivot 84 to the rocker gear 74 at a location offset from the axis 85 of the rocker gear 74 as best shown fig4 . this action of the feed wheels 51 and their actuation through the linkage and the rocker shaft is to be contrasted with conventional feed wheels that are driven in one rotative direction by a geneva or indexing drive . it will be seen that upon rotation of rocker shaft 10 in one direction , the feed wheels 50 will be rotated in one direction , and upon rotation of the rocker shaft 10 in the opposite direction , feed wheels 50 will be rotated in the opposite direction . the forward rotational direction of the feed wheels 51 is of course , utilized to drive the boards b to the nip rolls 18 when the grate is in its lower position . in accordance with the invention , the reverse or rearward rotational direction of the feed wheels 51 is utilized to urge the board b away from the gate 32 to relieve pressure against the gate 32 and prevent jamming . this latter action occurs at the end of the reverse rotation phase of the feed wheels when the grate is lowered and just prior to the forward rotational phase of the feed wheels 51 which serves to feed the board b to the nip rolls 18 . referring now to fig2 rocker shaft 10 is driven by means of a crank gear 16 which in turn is driven by a gear 20 which , in turn , is driven from the main drive of the associated box - making machinery ( not shown ). gear 20 is fixed to the shaft of one of the feed rolls 18 while being in mesh with another feed roll gear 20a which is fixed to the shaft of the other feed roll 18 , whereby feed rolls 18 are driven in counterdirections for feeding the boards b into box - making machinery . crank gear 16 drives rocker shaft 10 in oscillation by means of a mechanism which amounts to a four - bar linkage including a first link generally designated 12 which may be termed a &# 34 ; feed lever &# 34 ; having a passage therein receiving rocker shaft 10 and being fixed thereto through means of a key 1 . the mechanism further includes a connecting link 14 having one end pivotally connected by a pivot 3 and having its opposite end pivotally connected by a pivot 22 through the crank gear 16 at a location offset from the axis of rotation 24 of the crank gear 16 . it will therefore be seen that rotation of the crank gear 16 by feed roll gear 20 will cause rocker shaft 10 to oscillate through means of the linkage mechanism including links 12 and 14 . referring to fig7 the rocker shaft 10 may be driven by a quick return slider crank instead of the linkage 12 , 14 described above . as shown in fig7 this mechanism includes a slider 12a keyed to rocker shaft 10 and having an elongated slot 12b receiving a block 13 pivoted to crank gear 16 to slide in slot 12b . block 13 is , of course , mounted to crank gear 16 at a location offset from the center 24 of crank gear 16 . it will be seen that rotation of crank gear 16 will function to oscillate rocker shaft 10 by virtue of the action of the slide 12a . referring now to fig2 and 5 , shafts 52 and 52a are rotatably oscillated in small increments in order to raise and lower the grate 55 , 56 and such action is achieved through a cam 90 and cam follower lever 100 , 94 which is driven from a gear 26 ( see fig2 ) which , in turn , is driven from a gear 23 that is fixed concentrically to the inner surface of crank gear 16 so as to be rotatable therewith . cam 90 is fixed to a shaft 30 which is journaled in the frame member 34 and which is fixed to gear 26 at the center axis thereof so as to be driven thereby . referring to fig5 cam 90 has a circumferential surface portion 92 which may be termed &# 34 ; the high - dwell surface &# 34 ; and a shorter circumferential surface portion 91 which may be termed &# 34 ; the low - dwell surface &# 34 ; extending between the high - dwell surface 90 with transitional surface portions extending therebetween for the extent of the angle α . the cam follower lever includes a dog leg portion 100 having a hub receiving the end of shaft 52 to which it is suitably keyed for movement therewith . on one side of dog lever 100 is a cam follower or roller 94 which engages on the surface of cam 90 . the other shaft 52a is operatively connected to be oscillated in small increments in unison with shaft 52 through means of a link 104 , whose opposite ends are pivotally connected by pivots 104a to the dog leg lever 100 and to a follower lever 106 fixed to shaft 52a by means of a hub received about shaft 52 and keyed thereto in any suitable manner . it will be seen that when cam follower 94 engages the high - dwell surface 90 , the grate will be at rest in an elevated position and when the cam follower 94 engages the low - dwell surface 91 , the grate will be at rest in a depressed or lowered position . additionally , when the cam follower surface 94 engages the transition surface enclosed by the angle α , that is , between the high - dwell and low - dwell surfaces 90 and 91 , the cam follower will be lowered or raised depending on the particular transition surface engaged which , in turn , will slightly rotate the dog leg lever 100 and , in turn , the associated shaft 52 to actuate the grate 55 and 56 . motion of the dog leg lever 100 is transmitted to the adjacent shaft 52a through means of the link 104 and follower lever 106 to rotate shaft 52a in unison with shaft 52 . in the latter regard , dog lever 100 , link 104 and follower lever 106 act as a parallelogram linkage with the pivots of the parallelogram linkage lying at 52 , 104a and 52a . referring now to fig5 cam follower 94 is urged onto the surface of cam 90 by a spring mechanism including a coil compression spring 110 having one end bearing against a block 112 fixed to the frame 34 and having an opposite end bearing against a nut and washer assembly 111 received about a rod 114 which extends through the axis of the coil spring 110 and through the block 112 after which it is pivotally connected by pivot 116 to the dog leg lever 100 as shown in fig5 . it will be seen that spring 110 urges the rod 114 in the direction to impose a clockwise ( as viewed in fig5 ) bias on dog leg lever 100 which , of course , urges cam follower roller 94 into engagement with the surface of cam 90 . in order to interrupt the feeding of the boards b , it is necessary to position the grate 56 spaced above the feed wheels 51 , that is , when the cam follower 94 is engaged on the high - dwell surface 92 of the cam 90 . for this purpose , a latch mechanism in accordance with another aspect of the invention is provided . as shown in fig5 a in the preferred embodiment , the latch mechanism includes an aperture 126 formed in the lower leg portion of dog leg lever 100 for receiving a pin 122 that is mounted for reciprocal movement in a housing 120 fixed in the frame 34 . housing 120 has an elongated passage receiving a rod 124 of an air cylinder 125 with the pin 122 being fixed to the extremity of the rod 124 . when the cam follower is positioned on the high - dwell surface 92 of cam 90 , the aperture 126 in dog leg lever 100 will be aligned with pin 122 whereupon if it is desired to interrupt the sheet feeding by virtue of the grate 56 being positioned above the feed wheels 51 , air cylinder 124 is energized to extend pin 122 into the aperture 126 . when received in the aperture 126 , the latch pin 122 holds the cam follower 94 at the level of the high dwell surface of the cam 90 which thus maintains the grate in the raised position regardless of the rotation of cam 90 . when in its raised position , the grate prevents contact of the boards with the feed wheels 51 and thus feeding of the boards is interrupted . only a small air cylinder 124 is needed to actuate pin 122 and therefore such action occurs quickly and efficiently to interrupt sheet feeding at precisely the desired point . when sheet feeding is to be resumed , air cylinder 125 is actuated to retract pin 122 from aperture 126 in the dog leg lever 100 , thereby freeing the latter for operation as described above . the latter must be effected when the cam follower is positioned above the high - dwell surface . fig6 shows two graphs which illustrate the relationship of the velocity of the feed wheels 51 to the position of the grate 55 , 56 throughout a full cycle of operation . the upper graph shows the relationship of rocker shaft ( 10 ) velocity versus crank gear ( 16 ) angle while the lower graph shows grate level versus the crank gear angle . the solid lines in the graphs refer to the embodiment of fig1 to 5 utilizing a four - bar linkage to drive the rocker shaft 10 while the dotted lines refer to the embodiment which utilizes a quick return slider crank shown in fig7 to drive the rocker shaft 10 . the contact period shown in the graphs refers to the phase when a board b is contacting the feed wheels 51 . note , however , that the smaller portion of the contact period ( during negative velocity ) occurs when the feed members 51 are rotating in a reverse direction which is utilized , as indicated above , to relieve the pressure of the board b against the gate 32 . that is to say that the positive velocity values depicted on the graph refer to rotation of the feed wheels 51 in the forward or board - feeding direction , while the negative velocity values refer to the rotation of the feed wheels in the reverse direction . additionally , the positive values for the grate level refer to when the grate is above the surface of the feed wheels 51 while the negative values refer to when the grate is below the surface of the feed wheels . it will therefore be seen from fig6 that when the feed members are increasing velocity during the contact period , the grate 55 , 56 is in its lowered position and therefore , the board is being conveyed to the nip rolls 18 . as the feed wheels 51 approach their peak velocity , the board begins to enter the nip rolls 18 and the grate begins to rise . the board enters the nip rolls near the point at which the feed wheels 51 reach a peak velocity which is matched to the constant velocity of the nip rolls . after the board is engaged in the nip rolls , the grate rises above the surface of the feed wheels 51 after which the grate reaches its uppermost level as the feed wheels decelerate and the board is being conveyed by the nip rolls 18 off the surface of the grate . the grate remains in its elevated position when the velocity of the feed wheels 51 changes from positive to negative ( when the direction of the feed wheels 51 is reversed ). however , at the end of this reversing phase of the feed wheels as they approach zero velocity , the grate begins to lower until the next board to be fed engages the feed wheels 51 at a very low negative velocity at which time the reversing feed wheels function to urge the board away from the gate 32 to relieve pressure until the feed wheels come to zero velocity and then rotate in the forward direction to drive the board toward the nip rolls 18 . the above cycle is then repeated . the board is displaced the distance from gate 32 to the nip rolls 18 to arrive at the same velocity as the nip rolls by virtue of the proper selection of the four bar linkage and gear ratio shown in fig4 as described above . the grate is lowered and raised at the appropriate time as described above and this timing is governed by the length of the low - dwell surface 91 and the extent of transition angle α which are selected accordingly . at any time during the relatively long phase when the grate is elevated , if it is desired to interrupt the cycle of operation , the air cylinder 125 ( fig5 and 5a ) need only be actuated to extend latch pin 122 into aperture 126 of cam follower lever 100 which will thus hold the grate in the elevated position . when it is desired to resume operation , the air cylinder is actuated to retract latch pin 122 from the cam follower lever 100 during the same relatively long phase when the grate is elevated . it will be seen that among its several advantages described above , the feeding mechanism of the present invention not only employs a conventional rocker shaft to drive the feed wheels in two opposite directions , but also it may be easily adjusted to various new or existing box - making machine drives to provide the desired timing , sequencing and torques by substituting different size linkages between the rocker shaft 10 and the rocker gear 74 or between the crank gear 16 and the rocker shaft 10 . this is a clear advantage over the indexing or geneva drive systems heretofore employed with rotary feeders which systems are more complex and require the substitution of different gears or gear arrangements in order to adapt to different drive systems . additionally , it will be appreciated that the rocker shaft drive utilized by the present invention will provide sufficient torque capacity to enable larger surfaces to be utilized on the feed wheels 51 to increase speed and accuracy of feeding . it should be pointed out that although the preferred embodiment of the invention described and illustrated above utilizes feed members in the form of rotatable wheels or rolls , the present invention may alternatively utilize other types of feed members such as , for example , endless belts that move in horizontal paths below the boards to be fed . therefore , it should be understood that the term &# 34 ; rotary feeder &# 34 ; as used herein is not limited to the specific rotatable feed members 51 that will perform in accordance with the teachings of the present invention .