referring now in more detail to the drawing , there is shown a sheet feed machine for feeding sheets to a printer 10 such as a duplicator model no . 350 sold by the a . b . dick company . the sheet feed machine is seen to include a pair of angle iron guide plates 12 and a pair of upright guide pins 14 located upon mutually parallel inclines . the guide pins are mounted by wing nuts 16 to a pair of rails 18 which are in turn mounted to the inside of parallel side frame members 20 . a main feed cylinder 22 is rotatably mounted between frames 20 beneath the guide plates 12 . an idler roller 24 is rotatably mounted between the rails above and to one side of the main feed roller 22 . an apex roller 25 is rotatably mounted to the side frames above a plane which passes between the axes of rollers 22 and 24 . the location of the apex roller is adjustable atop rails 18 along plane 27 shown in fig2 for the purpose hereinafter described . an endless belt 26 is seen to be looped over the three rollers 22 , 24 and 25 . a stack of envelopes 28 is shown set atop that portion of the belt located between rollers 22 and 25 with a portion overhanging that portion of the belt located between rollers 24 and 25 . a lower edge of the stack is supported upon and between the guide plates 12 between the guide pins 14 . a retainer plate 30 is mounted to a bar 32 that extends between the two side frame members 20 with a lower lip portion resting upon a bar rest 34 just behind the guide plates 12 and just above belt 26 . the retainer plate is biased downwardly by a compression spring 35 held atop a retainer plate upper - flange by a knurled screw 36 . the just described mechanism serves to feed envelopes successively from the bottom of stack 28 into a stream of envelopes with adjacent envelopes overlapped . this may be best visualized in fig2 by following the travel of belt 26 as it passes from idler 24 in the direction of arrow 37 towards and then over the apex roller 25 . initial movement of the belt urges the bottom - most envelope to be stripped from the stack downwardly beneath the retainer plate lower lip until the trailing edge of the envelope distal the guides 12 has passed over the apex roller 25 . as this trailing edge passes over the apex roller 25 the next portion of the belt 26 itself to pass over the apex roller 25 will follow and thus fail to engage the bottom - most envelope but instead engage the envelope immediately thereabove . at this point there will be seen that an overlap gap has been created which approximates the distance between the apex roller 25 and the edge of the stack distal the guides 12 . thus , slight relocations of the apex roller can effect charges in the overlap gap thereby altering the speed of operations . continued movement of the belt successively causes a stripping of envelopes from the bottom of the stack in a mutually overlapped configuration . in this manner several layers of envelopes will simultaneously be caused to pass beneath the retainer plate 30 . that the retainer plate 30 is spring biased in a downward direction provides an adjustment means by manual rotation of the knurled screw 36 in finely adjusting the just described operation . after the sheets have left the sheet input station and passed beneath the retainer plate lower lip 32 , they are routed around under the main feed cylinder 22 at the sheet inversion station . this is accomplished by the simultaneous actions of conveyor belt 26 as it moves about the main feed cylinder and another conveyor belt 40 which is routed over a drive roller 42 , an idler 44 and a tension adjust roller 45 . as a portion of the main feed cylinder 22 lies in a plane passing tangentially over rollers 42 and 44 , belt 40 is seen to curve inwardly as it passes over the main feed cylinder . the velocity of the drive roller 42 is set to cause the conveyor belt 40 to move at the same speed as the conveyor belt 26 does over the main feed cylinder 22 . it thus is seen that with a stream of envelopes carried atop belt 26 they will be sandwiched between belts 26 and 40 as they are conveyed down and around the main feed cylinder 22 . in this manner it is seen that the stream of envelopes is inverted . after the stream of sheets has been inverted it is guided onto a horizontal feeder tray 50 over which another endless conveyor belt 52 is driven as shown in fig3 . this conveyor belt 52 is routed over a drive shaft 54 , a front idler 55 and a pair of tension rollers 56 and 57 . a u - shaped leg 58 is pivotably mounted to parallel frame support 60 to each side of the feeder tray which leg is sized to be placed atop the printer elevator . an electric motor 66 is mounted between another u - shaped leg 64 having its output drive shaft coupled with a drive belt 68 that drives shaft 54 . unshown endless chains in turn couple the main drive shaft with the other drive cylinder 22 and roller 42 for driving belts 26 and 40 while the main drive shaft itself drives the conveyor belt 52 . the u - shaped leg 64 is mounted atop a stanchion 70 which is telescopically received within another stanchion 72 that extends uprightly from a platform 74 supported upon a floor by a set of casters 75 . this telescoping arrangement of the stanchion enables the height of the sheet feed machine to be adjusted . this height adjustable feature , coupled with the pivotably u - shaped front leg 58 and strap 62 , enable the sheet feed machine to be readily connected with numerous sheet processing machines such as printers in a minimum of time and with a minimum of effort and training . the sheet feed machine also includes a mechanism for holding the stream of sheets down upon the top of the conveyor belt 52 with adjacent sheets in the stream maintained in mutual relative position . this hold - down mechanism includes a beam 71 pivotably mounted by pivot pin 76 above the conveyor belt 52 . a collar 73 is slideably mounted by set screw 79 to the beam which collar supports a trolley arm 80 to which a trolley 82 is pivoted . the trolley in turn supports a pair of mutually spaced rollers 84 . another collar 85 is slideably mounted by a screw 86 to the beam . a pair of legs 78 is pivotably mounted to this collar 85 with each leg supporting a roller 90 . a tension spring 92 couples the two legs 78 together . finally , a rear wheel 94 is pivotably mounted to the beam closely adjacent to the pivot pin 76 . with this hold - down arrangement the members in a stream of sheets passing atop belt 52 and tray 50 are held in firm engagement atop the belt 52 with relative movement between the members in the stream inhibited . it will also be seen that the collars 73 and 85 enable the spacing of the various rollers of the hold - down mechanism to be altered to accommodate variations in sheet sizes and overlap spacings . the rear most roller 94 is purely optional , being preferred only for very small envelopes . finally , the sheet feed machine is seen to include a microswitch 96 to which an actuating arm 97 is mounted . the microswitch 96 serves to terminate , through unshown electrical circuitry , the operation of the electric motor 66 in driving the various conveyor belts 26 , 40 and 52 when a stack 98 of sheets has been accumulated beneath a suction pickup arm 99 of the printer or other sheet processing machine . once this actuating arm 97 has fallen from the removal of sheets from the stack faster than they are being resupplied , motor 66 is reactivated to feed additional sheets into the pickup station . the sheets are accumulated here by the provisions of stop 100 . it should be understood that the just described embodiments merely illustrates principles of the invention in preferred forms . many modifications , additions and deletions may , of course , be made thereto without departure from the spirit and scope of the invention as set forth in the following claims .