7193 _TFTD OF THE INYFNTI~M This invention relates to a method and apparatus for feedingflexible sheet material sequentially from a stack onto a movable conveyor platform. BACKGROUND OF THE INVENTION - Devices have been developed for feeding sheet material, such as greeting cards, at high speed sequentially from a stack. An example machine for this purpose is described in my United States Patent No. 3,908,983 issued September 30, 1975 entitled Card Feeder. However, such devices are suitable only for relatively thick sheet material since the sheets in a stack of thinner material tend to stick together which makes the feeding of single sheets difficu]t and also the pressure in the nip required for thinner sheets tends to cause the movement of the conveyor platfor~ to smear the ink on the sheets. Postal machine devices are known which will separate letters from a stack but such machines would not be practical for thin sheet material since the friction deterrent means would peel and crease the sheets being fed through the device. An example of such a postal machine ls di~closed in United States Patent No. 1,955~066 issued April 17, 1954 to G. Hiller. It is an ob~ect of the present invention to provide an improved method and d~vice for feeding thin sheet material sequentially from a stack. SUMMARY OF THE INVENTION Essentially the invention consists of a method of feeding flexible material sequentially from a stack thereof, comprising the steps llZ7193 of; holding the stack of flexible sheets above a forwardly moving conveyor platform, the stack being held in a downwardly sloping position in the forward direction of travel of the platform and spaced - therefrom, the lowermost sheets of the stack being fanned forwardly; retarding the forward movement of those sheets adjacent the lowermost sheet of the stack by friction means; allowing the single lowermost single sheet of the stack to move forwardly on the plat.~orm between the platform and the friction means, and distorting said lowermost sheet arcuately from its normal plane by interaction between the friction means and the platform to effect at least partial lateral separation between said lowermost sheet and the adjàcent next lowermost sheet, the lowermost sheet being supported upwardly beneath the friction means. In another aspect the invention consists of a device for feeding flexible sheet material sequentially from a stack thereof, camprising: a conveyor providing a planar, forwardly movable platform means to hold the stack of flexible sheets above the platform in a downwardly sloping position in the forward direction of travel of the platform and spaced therefr~m, said holding means including means whereby the lowermost sheets of the stack are fanned forwardly; friction means positioned above the platform forwardly of the stack to retard the forward movement of these sheets ad~acent the lowe~ost sheet of the stack, the friction means being spaced from the platform to allow passage of a single sheet therethrough and pro~ecting below the upper planar surface is laterally distorted arcuately to effect at least partial lateral separation of said lowermost sheet from the ad~acent next lowermost sheet; means to support the platform ad~acent the friction means; and means to move the platform forwardly whereby said lowermost sheet is advanced towards said friction means, the conveyor comprising at least two laterally spaced endless bands and llZ7193 the friction means is positioned between the bands, an auxiliary endless band being positioned between said two laterally spaced bands and beneath the friction means~ the upper surface of the auxiliary band being lower than the planar upper surface of the platform formed by said two laterally spaced bands, the friction means being spaced from the auxiliary band to allow passage of a single sheet therethrough. BRIEF DESCRIPTION OF THE DRAWINGS Example embodiments of the invention are shown in the accompanying drawings in which: ` Figure 1 is a perspective view of a sheet material feeding device; - 2a - . 9~ ~ Figure 2 is a view in cross-section taken along line ~-2 of Figure 1; arA Figure 3 is a view taken along line 3-3 of Figure 2. Figure 4 is a perspective view of an alternate embodiment of a sheet material feeding device; Figure 5 is a side view in elevation of the guide, the conveyor platform, and the friction means of the device of Figure 1, taken adjacent the centre auxiliary band, with a stack of sheets being fed therethrough; Figure 6 is a view taken along line 3-3 of Figure 5; and Figure 7 is a cross-section taken along line 4-4 of Figure 3. DETAILED DESCRIPTION The example embodiment shown in Figures 1 to 3 of the drawings compr~ses a pair of journalled rollers 10, suitably ~. driven and mounted on a frame (not shown), which carry a plurality of hori~.ontally disposed bands 12 adjustable in known manner for yaw and for speed, the bands being laterally spaced to provide a gap 13. Bands 12 form a conveyor platform 18 movable in the direction of arrows 14 and are ].ocated between a pair of uprlght, spaced guide flanges 15 fixed on parallel, spaced guide rails 16 which are adjustably mounted on the frame. A curved gui~ebar 20, mounted on a crossbar 22tis located above bands 12 and in front of guide flanges 15 ~with respect to the direction of travel of bands 12). Crossbar 22 is fixed on a pair of lateral walls 24 mounted on the frame of the device. The upper portion of guide bar 20 forms an arm ,: 27~3 26 sloping downwardl~ and rearwardly and the guide bar then curves forwardly in an arc 27 to form a horizontally ~isposed forwardly directed tongue 28 l~ing astride 13 above bands 12 to define a passage 29. Arc 27 of guide bar 20 has a centrally disposed longitudinal slot 30 extending into arm 26 and into tongue 28. Friction means, in the form of a wheel 32, oriented in a vertical plane lying in the direction of movement of bands 12 and keyed on a shaft 34, has a rim 36 of hard rubber, or other material such as a tungsten carbide coating on steel, of a high coefficient of friction and is located in the lower portion of slot 30 of guide bar 20 adjacent bands 12, As seen more particularly in Figure 3 of the drawings, wheel 32 is so located that the periphery of rim 36 extends downwardly slightly beyond the lower surface of tongue 28 but does not extend laterall~ beyond the rearward surf'ace of arm 26 or 27, i.e.only the lower portion of wheel 32 is exposed below guide bar 20. Also the bottom of friction wheel 32 lies in gap 13 between bands 12 and slightly below the plane of the upper surface of the bands, as seen in Figure 3 of the drawings, but not enough to crease the sheet material being fed by the device. Shaft 34 is journalled in a pair of bearings 36 mounted one on each of frame walls 24. Each bearing 36 is adjusbable vertically along a pair of rails 38 by screws 40 mounted in brackets 42. Rails 38 and brackets 42 are fixed on side walls 24 and shaft 34 extends through a slot 44 in each side wall. A sprocket 46 is keyed on shaft 34 adjacent one bearing 36 and is engaged b~ a chain 48 which is driven through a reducing gear (not shown) from the drive of roller 10 . 1127~ 93 Guide bar 20 may be adjustable, for example in ~he manner shown in Figures 5 and 6 of my United States Patent No. 3,908,983 mentioned above. Similarly chain 48 may be tensioned in the manner shown in Figure 1 of that patent and sprocket 46 may be driven as shown in Figure lA of my above-mentioned patent. A retainer bar 50, spaced behind guide bar 20, is mounted on a further crossbar 52 which is in turn mounted in known manner on lateral frame walls 24 and ad~ustable in the direction of movement of bands 12, for example as shown in Figure 4 of my above-mentioned United States patent No. 3,908,983. Retainer bar 50 is contoured in a manner similar to arm 26 of guide bar 20 but terminates in a lower lip 54 spaced above bands 12, as seen in Figures 1 and 2 of the drawings. A roller 56, freely rotatable in the fixed frame of the device, supports bands 12 directly below wheel 32, as seen in Figure 2 of the drawings. In the operation of the example embodiment shown in Figures 1 to 3 of the drawings a stack 60 of cards is placed between retainer bar 50 and guide bar 20. Stack 60 may be centered and tilted and guide bar 20 may be moved horizontally, as described in my U.S. Patent No. 3,908,983. The position of wheel 32 in slot 30 of guide bar 20 ls ad~ustable by screws 40 which al~ ad~ust the width of gap 38 between the wheel and bands 12. Because ofthe curvature of lip 54 of retainer bar 50 and arc 27 of guide bar 20, the weight of stack 60 fans the lower cards of the stack forwardly. When these adjustments have been made the operation of the device is initiated by moving bands 12 in the direction of arrows 14. ~s the bottom or first card 61 in stack 60 is moved downwardly by the weight of the stack, the leading edge - 1127'193 of that card touches bands 12 and is pulled forward by the continuous movement of the bands in the direction of arrows 14 -a~ TI ~1 F-i~u~ 2 an~ 3 . Tne 1 o~r~ ~vveme~t ~ ~u~ium card 61 allows the trailing edge of that card to clear lip 54 of retainer bar 50 and drop onto bands 12. As bottom card 61 passes forwardly between friction wheel 32 and bands 12 it is distorted arcuately from its normal plane, as seen in Figure 3 of the drawings, because the bottom of the friction wheel lies below the plane of the upper surface of the card. ~his effects at least partial separation between bottom card 61 and the next lowermost card 62 and prevents those two cards from sticking together and prematurely pulling card 62 from stack 60. As the leading edge of bottom card 61 passes forwardly between friction wheel 32 and bands 12 the leading edges of the cards immediately above it in stack 60, i.e. second card 62 and third card 63, are forced forwardly by the weight of the stack and the curvature of bars 20 and 50. At this point the leading edge of card 62 comes into contact with friction wheel 32 which pro~ects slightly below arc 27 of guide bar 20. Friction wheel 32 is geared to rotate at an extremely slow speed, say 1:3000 in relat~n to the speed of bands 12, and the wheel may be rotated in either direction. Bottom card 61 continues to be carried forward by bands 12, affected only by the friction of the leading edge of card 62 which keeps it clear of friction wheel 32, and is carried forward by the bands through passage 29 between the bands and tongue 28 of guide bar 20, to be delivered by the bands to a subsequent station. When card 61 has passed friction wheel 32 and bands 12 the leading edge of the next card 62 drops onto bands 12 and is carried forward, causing the trailing edge of card 62 to clear lip 54 of retainer bar 50, whereupon card 62 passes friction wheel and bands 12 and through passage 29 in the same manner. -- 6 -- ~.27193 In the example embodiment shown in Figures 4 to 7 of the drawings an endless auxiliary band 70 is located between a pair of endless primary bands 72 which form a conveyor pLlatrorm 74. Primary bands 72 pass around a pair of front pulleys 76 and a pair of rear pulleys 78, both pairs of pulleys being preferably of the same diameter. Front pulleys 76 are fixed on a front drive shaft 80 journally mounted in side frame 24 and suitably driven while rear pulleys 78 are fixed on a rear driven shaft 82 also journally mounted in side frame 24. Rear shaft 82 is driven from front shaft 80 through a further belt and pulley arrangement 84 located on each side of pulleys 76 and 78 (see Figure 4). Auxiliary band 70 passes around a further front pulley 86 freely ~- rotatable on front drive shaft 80 and around a further rear pulley 88 freely rotatable on shaft 82. Rear pulley 88 is of the same diameter as rear pulleys 78 of primary bands 72 while f`ront pulley 86 of auxiliary band 70 is of a diameter slightly less than the diameters of front pulleys 76 of primary bands 72. ~ In the embodiment of Figures 4 to 7 certain other elements are shown in modifi.ed form but performing the same functions as those in the previous embodiment of Figures l to 3 thus spaced guide flanges 15 of a different configuration are ad~ustably mounted slidably on a crossbar 90 which also slidably carries retainer bar 50 mounted on an adjustable bracket 92. A pair of curved guide bars 94, mounted on a fixed crossbar 96, are located above platform 74 and laterally of primary bands 72. As in the previous embodiment friction wheel 32 is keyed on a journelled shaft 34 which is suspended from crossbar 96 by adjustable mounts 98 and driven by means not shown. Conventional exit belts lO0 are located in front of friction wheel 32 and ~; platform 74. Driven shaft 82 -is adjustable by mounts 102 to vary the tension on bands 70 and 72. In the operation of the embodiment of Figures 4 to 7 ~ 7193 friction wheel 32 operates in the same manner as in the previous embodiment and guide bars 94 perform the same function as guide bar 20 by fanning the lowermost cards of stack 60 forwardly while the friction wheel allows the lowermost card 61 to move forward and restrains the cards immediately above it. As before, bottom card 61 is distorted arcuately from its normal plane (see Figure 6) which effects at least partial separation between the bottom card and the next lowermost card in the stack. The invention provides an improved continuous feed- ing device for high speed operation. It is particularlyadvantageous in feeding thin sheets such as bank notes where adjacent notes tend to stick together. The importance of feeding such notes at high speed in accurately counted batches will be appreciated. Also, the invention inhibits smearing of ink on the sheet material because there is less pressure applied between the bands and the friction means. In this respect the embodiment of Figures 4 to 7 affords a device in which auxiliary band 70 provides more frictional contact area beneath lowermost card 61 and assists in its separation from the next lowermost card in stack 60. The device of Figures 4 to 7 is useful for thinner cards in the range of .0035" - .005". Paper, acetate or other sheet material may be fed by the method and device of this invention as long as it is flexible. There must be lateral clearance between the friction means and the bands of the conveyor platform to produce an arcuate or wavy contour in the sheet being fed between them which breaks the hold between that sheet and the next succeeding one in the stack. The lowermost surface of the friction means is usually located below the upper surface of the platform depending upon the gauge or thickness of the sheet material being fed through t he device but in any event below the upper planar surface of the sheet material as it lies on the platform. Of course the number of bands or friction means may be increased as 1127193 long as there are at least two bands and an interdisposed f~ri~t~n me~ r at lea~ o fricti^~ m~.~a,~g al.d an inte- ~disposed band. The friction means is preferably a friction wheel as described. Friction wheel 32 may rotate continuously or it may be governed to effect a pulsed rotation. A friction wheel approximately three-quarters of an inch in diameter has been found satisfactory for feeding most sheet material. The purpose of a friction means is to retard the advancement of second card 62 while the purpose of rotation of the example embodiment is to prevent non-uniform wear of the friction surface which would adversely affect the efficiency of the feeding operation.