FIELD OF THE INVENTION 
     This invention relates to feed mechanisms and particularly to a tractor feed mechanism used in printers for the incremental feeding of a continuous paper print medium commonly referred to as a web or print forms. 
     BACKGROUND OF THE INVENTION 
     A typical paper feed mechanism has an endless, flexible and relatively inextensible sprocket driven belt. The belt has a row of uniformly spaced feed pins extending perpendicularly relative to the outer surface of the belt. The belt has drive teeth around its inside surface and is entrained around to form trace regions between a pair of spaced drive members, such as sprockets, with grooves that mesh with the drive teeth. The paper has one or more rows of uniformly spaced perforations and is driven by the feed belt whose pins are aligned with the row of perforations and have the same spacing so as to enter the perforations near one end of the trace and withdraw from perforations at the other end of the trace. The belt and the length of the trace regions are designed so that several pins simultaneously engage the perforations and drive the paper within the perforations. The sprocket and drive belt assembly are supported between a pair of frame members into which the sprockets are journaled. In some tractors, the drive mechanism comprises a single sprocket and a shoe around which the belt is wound, the shoe being essentially non-rotatable. One problem with the shoe structure is that it is stationary and thereby imposes increased frictional loads and wear on the belt as it slides over the shoe. 
     The belt is supported in the trace regions between the sprockets by a support structure attached to or made part of one or both of the side frames. The support structure has a horizontal support surface which engages and supports the underside of the flexible belt over at least that portion of the trace distance where several of the feed pins will be within the perforations and in full drive engagement with the paper. The support surface may be a continuous curve or may have a flat portion between upward and downward linear ramps. The gear teeth ride on the support surface. It is common to provide the horizontal support surface with a longitudinal guide slot or channel in which either the gear teeth on the underside of the belt or both gear teeth and belt can enter and be guided by the side walls of the channel. The purpose of the channel is to limit the lateral movement of the belt in the trace region in order that belt and hence the feed pins are maintained in alignment with the perforations in the paper during entry, withdrawal and full engagement with the paper. A problem with previous tractor designs made of plastic was that friction between the guide structure and the belt caused excess loading and wear of the belt thereby causing misalignment of the pins with the perforations of the paper. 
     High speed printers use as many as four tractors supported on parallel guide and drive shafts of a carriage mechanism. The tractors are movable transversely on the guide shafts to obtain proper alignment of the feed pins with the perforations along the edges of the paper. The adjustment mechanism may comprise lead screws or cables connected to the tractors in a manner whereby all or some of the tractors can be moved laterally along the shafts. In the past, tractor designs were such that many different parts were needed for the different tractors. 
     Originally tractors were made largely from metal parts to assure long wear and precision required for the feed pins to be aligned with and to enter and withdraw from the perforations without damage to the paper. Such tractors tended to be costly and required a large number of different parts especially to provide tractors for feeding along both edges of the forms. Later tractors were made with parts made almost entirely with plastic materials which could be fabricated by casting or molding processes. The belt was also made entirely of plastic materials molded entirely as a single endless belt or alternatively from a thin strip of plastic or metal with plastic drive elements molded thereto. Examples of such tractors using various plastic materials are shown in U.S. Pat. Nos. 3,825,162; 3,930,601; 4,226,353; 4,457,463 and 4,614,287. 
     Some of the problems associated with plastic tractors have been lack of adequate structural strength, dimensional instability and rapid wear which affect proper alignment of the belt with the paper and the pins with the perforations. Another problem has been the build up of electrical charge which makes the paper cling to the forms path and resist refolding. 
     It has been common practice to apply a tensioning force to the belt to maintain a taut pin belt during assembly and operation. Such use has been primarily in tractors where the belt is wrapped around one sprocket and a guide shoe. The guide shoe is mounted between two frame plates so as to be movable therebetween relative to the drive sprocket which is journaled to the frame plates. A compression spring applies pressure to the movable shoe within the space between the sides of the frame members. It is common to lock the shoe in place after the belt has been stretched taut by the spring. As a result of wear through use, the belt eventually develops some slack thereby causing misalignment of the pins and loss of paper control. The shoe is then unlocked so as to allow movement of the shoe relative to the sprocket to thereby restore the necessary amount of tautness to the belt. A problem with prior belt tensioning structures is the tendency of the tensioner to become jammed against the frame members so that when released it will not move solely under the force of the spring to restore the belt to its initial tautness. One reason for this is the lack of adequate space for the tensioner within the tractor. Another is paper dust lodging within the spaces occupied by the tensioner. Yet another is that the materials used tend to change shape or size thereby causing adherence to the frame elements. Some kind of manual operation is then required to unfreeze the shoe from the frame members. Examples of tractor belt tensioning devices are shown in U.S. Pat. Nos. 3,930,601; 4,226,353; 4,453,660; 4,457,463; 4,462,531; 4,638,935 and 4,723,697 and in articles published in the IBM Technical Disclosure Bulletin, Vol. 16 No. 1, Jun. 1973, p. 309; and Vol. 22 No. 7, Dec. 1979, pp. 2636-2637. 
     SUMMARY OF THE INVENTION 
     It is the object of this invention to provide a tractor feed mechanisms made mostly of plastic parts which overcomes or minimizes all of the above mentioned problems. 
     Basically the invention achieves this by providing an improved two part frame structure for supporting and guiding the feed belt and for supporting the drive mechanism that moves the belt. The belt has a tension member with gear teeth and feed pins projecting from opposite surfaces of the tension member. The belt is entrained around a pair of spaced sprockets one of which is journaled to the frame members and the other of which is journaled to a carrier movably supported by the frame members. The frame members are structured and assembled to form a slot for the belt with paper guide surfaces on either side of the belt. The tension member of the belt has borders between the edges of the gear teeth and the edges of the tension member. Friction is minimized by supporting the belt solely along its borders in the trace regions between the sprockets, by the sprockets where the belt wraps around them and by engaging only the edges of the gear teeth to provide lateral guidance. Ledges having a horizontal support surface and a vertical end wall are formed on the tractor frame members for that purpose. The borders of the belt slide on the horizontal surfaces of the ledges in the trace regions of the belt. The horizontal surfaces are made wider than the borders of the belt thereby preventing the edges of the belt from making contact with the frame members. A portion of the horizontal ledge surfaces between the sprockets is coplanar with the paper guide surfaces. The belt makes no engagement with the ledges where the belt wraps around the sprockets. Vertical support to the belt during wrap around comes exclusively from the sprockets. Lateral guidance for the belt both in the trace regions and during wraparound comes solely from the vertical end walls of the ledges engaging the edges of the gear teeth in the trace regions between the sprockets. The vertical end walls of the ledges form a channel in which the gear teeth travel. The spacing between the end walls is greater than the width of the gear teeth so that only one edge of the gear teeth slides against an end wall. As a result of the fact that there is no sliding contact of the bottom of the gear teeth or the edges of the belt for vertical and lateral guidance, the loading on the belt is more uniform and damage to the belt edges and wear are greatly reduced. 
     The movable sprocket carrier frame is supported between the frame members within an enclosure formed by guide plates extending from the frame members and a cross wall connected between the guide plates. Alignment of the carrier frame and the sprocket is maintained by guide ribs formed on the guide plates which ribs engage the edges of the carrier frame. Friction is reduced by spacing and dimensioning the guide surfaces of the ribs where they engage surfaces of the carrier frame. 
     The carrier frame has a central web with an elongate opening that aligns with corresponding elongate openings in the frame members of the tractor to form a passageway for the guide shaft on which the tractor is mounted. A guide tube through the elongated openings surrounds the guide shaft. Retention means in the form of a plate extending from the periphery of the tube is held between the frame members so that the guide tube and tractor move together along the guide shaft. The retention plate and the carrier frame are structured to allow the tube to move lengthwise relative to the carrier frame, which is clamped between the frame members, so as to accommodate tolerance variations in spacing between the drive and guide shafts on which the tractor feed mechanism is supported. 
     In accordance with this invention a tractor feed mechanism in which various the parts, including the frame members, the sprockets and the carrier frame are made from polycarbonate having carbon fiber fillers. Such material provides structural strength and is also beneficial in preventing the accumulation of electrostatic charge on the paper which prevents easy refolding of the paper. Friction is further reduced by adding polytetraflouroethylene to the carbon fiber filled polycarbonate. Nylon filled with carbon fiber and polytetraflouroethylene is used for the sprockets. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a plan view of a schematic of a forms feeding system for a printer using four tractors for feeding paper; 
     FIG. 2 is a three dimensional view of a tractor assembly for using the invention; 
     FIG. 3 is an exploded three dimensional view of the tractor assembly of FIG. 2; 
     FIG. 4 is a section view of the tractor assembly 
     of FIG. 2 taken along line 4--4; 
     FIG. 5 is an enlarged fragment of the section view of FIG. 4; 
     FIG. 6 is an enlarged view of a portion of one end of the assembly of FIG. 2 showing details of the paper separation structure of the guide frames; 
     FIG. 7 is a plan view showing details of the interior of one of the frame members shown in FIG. 3; 
     FIG. 8 is an exploded three dimensional view of the belt tensioning elements of the tractor of FIG. 2; 
     FIG. 9 is a three dimensional view showing the assembly of the tensioning elements of FIG. 10; 
     FIG. 10 is an exploded three dimensional view of another embodiment of the elements for tensioning the pin belt of the tractor of FIG. 2; 
     FIG. 11 is an enlarged section view of the tractor assembly of FIG. 2 taken along section lines 11--11; 
     FIG. 12 is an exploded three dimensional view of carrier frame element of FIGS. 8 and 9 showing details of the carrier and the guide tube portions of the paper feed carriage mechanism of FIG. 1; 
     FIG. 13 is a three dimensional view showing the interior of the second frame member of the tractor assembly of FIGS. 2 and 3; 
     FIG. 14 is a fragment of a section view of the tractor in FIG. 2 taken along lines 14--14 in FIG. 2; 
     FIG. 15 is another fragment of a section view taken along lines 15--15 in FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a front view of a forms carriage in a printer. Four forms tractors 10-13 increment the print forms 14 (shown by broken lines) by engaging perforations known as feed holes (not shown) at both edges of the forms. As is well known, the feed holes are uniformly spaced to be engaged by similarly spaced feed pins which are part of a feed belt of each of the tractors 10-13 to be described more completely hereinafter. 
     Tractors 10-13 are driven by a motor 15 which is operated under feedback control from signals produced by and emitter 16 and transducer 17. Motor motion is transmitted to the feed belts of the tractors 10-13 through a timing belt 18 that engages toothed pulleys 19 and 20 on each end of two splined drive shafts 21 and 22 that drive the feed belts of the tractors. Pulleys 19 and 20 are clamped to the drive shafts 21 and 22. Drive shafts 21 and 22 are supported at each end by bearings (not shown) in side plates 23 and 24. Guide shafts 25 and 26, also supported by side plates 23 and 24, as will be seen hereinafter, prevent the tractors 10-13 from rotating about the drive shafts 21 and 22. Tractors 10-13 are slidable on drive shafts 21 and 22 and guide shafts 25 and 26. 
     A cable system comprising cables 27 and 28 and adjustment knob 29 moves tractors 10-13 along drive shafts 21 and 22 and guide shafts 25 and 26. The cable system is designed to move the right pair of tractors 11 and 13 with respect to the left pair 10 and 12 to accommodate forms of various widths or to move all four tractors together to align the forms with the print columns of the printer. Further details of the cable system may be obtained by reference to an article published in the IBM Technical Disclosure Bulletin, Vol. 29 No. 12, May 1987 at pages 5518 and 5519. 
     FIG. 2 shows a tractor 30 which can be assembled as either the lower right or upper left tractors 10 or 13 in the printer carriage of FIG. 1. As seen in FIGS. 2 and 3, tractor 30 which is mounted on drive shaft 21 and guide shaft 25 has a body or frame 31 comprising a pair of elongate frame members 32 and 33 clamped together by screws 34 and nut plate 35. Located between frame members 32 and 33 is feed belt 36 entrained around spaced sprockets 37 and 38. Sprocket 37 is the drive sprocket and is driven by drive shaft 21. Sprocket 38 is the idler sprocket. The span of belt 36 between the sprockets is sometimes referred to as the trace region. As is conventional, belt 36 has an upper or forward trace region on the top side and a lower or return trace region on the bottom side of tractor 30. 
     As seen in FIG. 3, feed belt 36 has feed pins 36b and gear teeth 36c, sometimes called lugs, projecting respectively from the top and bottom sides of a thin endless flexible tension member or web such as band 36a. Preferably band 36a is made of stainless steel or plastic such as polyimide or polyester. Feed pins 36b and gear teeth 36c are integral elements and are attached by molding through perforations (not shown) formed in the center of the band 36a. Feed pins 36b may be tapered to facilitate insertion and removal from the feed holes in paper 14. Gear teeth 36c are shaped to mesh within notches in the sprockets 37 and 38. In a preferred form in which the invention is practiced, gear teeth 36c are wedge shaped. The width of gear teeth 36c is less than the width of band 36a  so that band 36a has overhang regions between its edges and the edges of gear teeth 36c whereby belt 36 can be provided with vertical support in the trace regions thereof. 
     Frame members 32 and 33 have guide surfaces 32a and 33a respectively on either side of belt 36. Guide surfaces 32a and 33a are convex and extend the entire length of tractor 30. Guide surfaces 32a and 33a each have the same convex contour, namely, a slightly curved central region between linear sloped ends. The curved central regions extend roughly the length of the central portion of the trace regions of belt 36. The linear sloped ends of guide surfaces 32a and 33a extend from the point of juncture with the central section to the ends of the frame members 32 and 33 respectively. Paper 14 is retained on feed pins 36b and in engagement with the top of band 36a over at least a portion of the trace region of belt 36 and on the guide surfaces 32a and 33a throughout their length by door 39 which is concave and has ridges 39a on its inner side. Ridges 39a, which coact with guide surfaces 32a and 33a to form a paper gap, have a concave contour which essentially parallels the guide surfaces 32a and 33a. Frame members 32 and 33 have identical convex guide surfaces 32b and 33b on either side of the return portion of belt 36. Door 39 is rotatably hinged by pins 40 either to flange elements 41 or alternately with flanges 42 on frame member 33. A flat spring 43 supported as a simple beam by either of the platforms 44 or 45 operates to maintain door 39 closed when paper 14 is being fed or in at least one open position when paper 14 is being inserted or removed from tractor 30 as described in copending application Ser. No. 150,348 filed Jan. 29, 1988. 
     As shown in FIGS. 3 and 4, drive sprocket 37 comprises wheel 37a and hub 37b. Wheel 37a has V-shaped notches 37c which mesh with the wedge shaped gear teeth 36c on belt 36. Hub 37b has a splined passageway 37d for engagement with drive shaft 21. Idler sprocket 38 has wheel 38a and hub 38b. V-notches 38c in wheel 38a mesh with gear teeth 36c of belt 36. Hub 38b may be solid or may have a cylindrical passageway 38d for receiving a pin support shaft 38e in the alternate embodiment shown in FIG. 10. Both sprockets are preferably molded from plastic. Both wheels 37a and 38a are the same width and are narrower than gear teeth 36c. 
     Hub 37b of sprocket 37 is journaled in cylindrical bearings 32c and 33c formed respectively in the planar sidewalls 32d and 33d of frame members 32 and 33. Near the other ends, sidewalls 32d and 33d have aligned fan-shaped clearance openings 32e and 33e. Between their ends, sidewalls 32d and 33d have aligned elongate openings 32f and 33f which provide a passageway for guide shaft 25. Frame members 32 and 33 are plastic molded parts in which the guide surfaces, the bearings as well as other openings plus other features are formed in one operation. 
     For the purpose of applying tension to belt 36, idler sprocket 38 is rotatably supported on a carrier 46 which is slidably supported between frame members 32 and 33 for movement relative to drive sprocket 37. As seen in FIG. 8, carrier 46 comprises a rectangular frame 46a. Laterally spaced sprocket support arms 46b and 46c project from one end of carrier frame 46a. On the ends of support arms 46b and 46c are C-bearings 46d and 46e in which hub 38b of sprocket 38 can be snap fitted so as to be journaled therein. In the embodiment of FIGS. 9 and 10, support arms 46b and 46c have rectangular slots 46f and 46g which receive the flattened ends 38f and 38g of pin support shaft 38e so as to hold it stationary. In this arrangement, pin shaft 38e is stationary and sprocket 38 rotates thereon. This arrangement reduces the susceptibility of the sprocket 38 to becoming clogged and wearing due to paper dust compared to the C-bearing arrangement. 
     Carrier frame 46a has end plate 46h which is mutually perpendicular with side plates 46j and 46k buttressed by a center plate or web 46l. Tensioning force for moving frame 46 is supplied by compressed coil springs 48 acting on end plate 46h. Retention pins 46m projecting from end plate 46h maintain the coil springs in position. End plate 46h and side plates 46j and 46k are made slightly wider than the spacing between the sidewalls 32d and 33d of the assembled frame members 32 and 33 so that carrier 46 can be clamped by the frame members. A guide hole 46n through web 46l forms a passageway for guide tube 47 through which guide shaft 25 passes and along which guide tube 47 with tractor 30 is slidable. As seen in the figures, guide hole 46n is elongate with straight sides and circular ends and having the same dimensions as and is aligned with elongate openings 32f and 33f in sidewalls 32 d and 33d of the frame members 32 and 33. The distance between the parallel sides of guide hole 46n is only slightly larger than the diameter of guide tube 47 thereby enabling carrier 46 with idler sprocket 38 to move easily but not too loosely as a unit relative to tube 47 and its concentrically housed guide shaft 25. This allows tube 47 to move relative to the tractor 30 to compensate for tolerance variations in the separation between guide shaft 25 and drive shaft 21 without disturbing the alignment of tractor 30 relative to the direction of feeding of paper 14. 
     Attached to tube 47 is retention plate 47a. Plate 47a is relatively thin but substantially rigid and is affixed to the perimeter of sleeve 47. When assembled to tractor 30, retention plate 47a is located between carrier frame 46a and either sidewall 32d of frame member 32 or sidewall 33d of frame member 33. Retention plate 47a is formed with lateral extensions 47b and 47c and longitudinal extensions 47d and 47e. These extensions reside in guide notches 46p, 46q and 46r in end plate 46h and side plates 46k and 46j end plate 46h of carrier frame 46a respectively. Notches 46p, 46q and 46r are wide and deep enough so that retention plate 47a is permitted movement lengthwise relative to carrier 46 when tube 47 is caused to be moved lengthwise in elongate openings 32f, 33f, and 46n as a result of tolerance variations in the spacing between drive shaft 21 and guide shaft 25. Retention plate 47a serves the further function of maintaining the tractor 30 aligned with the direction of paper feeding during this adjustment of the sleeve 47. Tube 47 and retention plate 47a are also molded plastic parts. 
     Projecting from the inner surfaces of sidewalls 32d and 33d are ledges. The ledges are defined by horizontal surface 32g and vertical end wall 32h on frame member 32 and by horizontal surface 33g and vertical end wall 33h on frame member 33. The ledges are identical, oval shaped and encompass the bearing and clearance openings in the respective sidewalls. At their ends, the ledges function as inner dust barriers for protecting sprockets 37 and 38. Between the sprockets, the ledges provide vertical support and lateral guidance to the trace regions of belt 36. In the trace regions, horizontal surfaces 32g and 33g are coplanar with the central regions of the guide surfaces 32a and 33a of frame member 32 and 33 respectively. On either side of the central regions, the horizontal surfaces 32g and 33g form curved ramps which drop below the linear sloped ends of guide surfaces 32a and 33a. In the vicinity of the sprockets, horizontal surfaces 32g and 33g drop below and continue below the periphery of sprocket wheels 37a and 38a for the entire distance where belt 36 wraps around the sprocket wheels. In like manner the horizontal ledge surfaces are also coplanar with the central regions of guide surfaces 32b and 33b and form curved ramps below the linear sloped ends thereof on the opposite sides. 
     The horizontal surfaces 32g and 33g provide vertical support to the underside of belt 36 only along the borders which overhang the edges of gear teeth 36c and the edges of band 36a. The vertical end walls 32h and 33h of the ledges engage the edges of one side of the gear teeth 36c and in this way provide lateral guidance to the entire length of belt 36. While the horizontal surfaces 32g and 33g drop below the periphery of the sprockets, the drop is less than the height of the gear teeth 36c of belt 36 thereby assuring that lateral guidance of belt 36 continues as belt 36 leaves one trace region, wraps around the sprockets and returns to the second trace region at the proper entrance angle. The width of horizontal surfaces 32g and 33g is greater than the width of the overhanging borders of band 36a so that the edges of gear teeth 36c will always encounter vertical end walls 32h or 33h of the respective ledges and thereby prevent the edges of band 36a from engaging any part of sidewalls 32k or 33k along the ramps and in the vicinity of the sprockets. Of course the edges of band 36a make no engagement along the central region of the guide surfaces 32a, 33a, 32b and 33b where horizontal surfaces 32g and 33g are coplanar therewith. 
     In accordance with this invention, the distance between end walls 32h and 33h is greater than the transverse width of gear teeth 36c. Thus, lateral guidance to belt 36 is achieved by gear teeth 36c making contact with either vertical end wall 32h or 33h but not both at the same time. This is shown most clearly in FIGS. 11, 14 and 15 where one edge of gear tooth 36c is shown in contact with end wall 32h and there is a clearance between the opposite edge of gear tooth 36c and end wall 33h. This arrangement can be produced when the tractors on one side are moved along shafts 21 and 25 by cable adjustment while the other tractors are held stationary. Thus by preventing the edges of of belt 36 from engaging any part of the frame members 32 and 33, and limiting lateral guidance of the belt by engagement of the gear teeth with either of the end walls 32h and 33h but not both, by supporting belt 36 by ledges engaging only the overhangs of the belt and limiting such support to the trace regions of belt 36, the amount of frictional contact between the belt 36 and frame members 32 and 33 is greatly reduced. This in turn reduces friction loading and the wear of belt 36 compared to previous structures thereby allowing smaller drive motors to be used and extending the life of the belt and the frame members. 
     Projecting from the inside and in the central part of sidewall 32a are a pair of spaced guide plates 32i and 32j connected by vertical cross wall 32l. Cross wall 32l with guide plates 32i and 32j form an enclosure in which carrier frame 46a is housed and is movable to apply tension to belt 36. Tension force is provided by coil springs 48 being held in a compressed state between cross wall 32l and end plate 46h of carrier frame 46a. Upper ribs 32m depending from guide plate 32i engage side plate 46k of carrier frame 46a. Lower ribs 32n rising from guide plate 32j engage side plate 46d of carrier frame 46a. Ribs 32m and 32n provide lengthwise guidance and alignment to carrier frame 46a with minimum amount of friction thereby enabling an accurate determination of the force of coil springs 48 on belt 36 and permitting a relatively weak spring force to effectively move carrier 46 in spite of any friction produced by engagement of frame 46a with sidewalls 32d and 33d. 
     The projecting ends of guide plates 32i, 32j and cross wall 32l abut against sidewall 33d of frame member 33 and when assembled with screws 34 tightened into nut plate 35, frame 45a is clamped in to maintain tension in belt 36. Carrier arms 46b and 46c are aligned with and extend laterally so as to be partially within clearance openings 32e and 33e. Sprocket 38, therefore, is free to rotate either in C-bearings 46d and 46d or on pin support 38e. 
     Guide plates 32i and 32j have oppositely facing outer surfaces 32p and 32q. Surfaces 32p and 32q are convex and are contoured to be parallel with the central guide sections of the guide surfaces 32a, 33a, 32b and 33b of frame members 32 and 33 respectively. The ends of guide plates 32i and 32j which extend slightly beyond the central guide regions into the linear sloped ends regions of guide surfaces 32a and 33a are beveled to provide clearance to the bottoms of gear teeth 36c. Vertical end walls 32h and 33h of frame members 32 and 33 form a channel for gear teeth 36c of belt 36 with surfaces 32p and 32q of guide plates 32i and 32j. However, the height of end walls 32h and 33h is greater than the height of the gear teeth 36c so that gear teeth 36c are always clear of engagement with surfaces 32p and 32q. Thus friction loading of belt 36 is confined to the continuous surface of band 36a. By confining the vertical support of belt 36 to the overhang portions of band 36a, the loading of belt 36 is constant and is not affected by the intermittent contacts that would be produced by the ends of gear teeth 36c coming into contact with surfaces 32p and 32q. 
     In accordance with this invention, frames 32, 33, carrier frame 46, sleeve 47 and door 39 are molded from polycarbonate containing at least 15 percent carbon fibers to provide stiffness, electrical conductivity and greater heat conductivity than glass plastics with glass fibers. 15 percent carbon fiber by weight is required to dissipate an electrical charge. Another benefit of the carbon fibers is that they are not abrasive to steel as are glass fibers. This prevents the pressure between steel band 36a and walls 32g and 33g from abrading steel band 36a. Band 36a is less than 0.002 inches thick to provide allowable bending stresses and has no allowance for wear. Frames 32, 33, and carrier 46 also contain 15 percent PTFE for lubrication of both the belt support surfaces 32g, 32h, 33g and 33h; bearing surfaces 32c, 33c, 46d and 46e and shaft 38e. 
     Sprockets 37 and 38 are molded from nylon 6/10 containing at least 15 percent carbon fibers and 15 percent PTFE. Tests have shown that the combination of nylon and polycarbonate produces little wear of either member. Nylon 6/10 was selected because it is among the least hydroscopic of the nylons. As in the polycarbonate, the 15 percent carbon fibers provides electrical and thermal conductivity and the 15 percent PTFE provides optimum lubrication. Because the PTFE is a dry lubricant, it will not hold paper dust the way liquid lubricants, such as oil, do so that bearing wear due to abrasion by paper dust is eliminated. 
     While the novel features of the present invention have been shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art, that the foregoing and other changes can be made in the form and details without departing from the spirit and scope of the invention.
A tractor mechanism has an endless pin belt entrained around spaced drive and idler sprockets mounted between a pair of frame members. The idler sprocket is supported by a carrier movable longitudinally by springs to apply tension to the belt. The edges of the belt overhang gear teeth of the belt and are prevented from contact with the frame members by the gear teeth engaging an endless vertical wall on one of the frame members. The frame members and sprocket carrier are molded polycarbonate with carbon fiber and solid lubricant fillers to reduce electrostatic charge buildup and wear of the belt. The frame members are bolted together to clamp the sprocket carrier in place to maintain the belt tension applied by the springs. The sprocket carrier and frame members have aligned elongate openings for receiving a tractor guide shaft with a concentric plastic tube, the latter being attached to a retainer plate held loosely between the sprocket carrier and a frame member.