CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of application Serial No. 07/417,497, filed on October 5, 1989.

BACKGROUND OF THE INVENTION Field of the Invention: The invention herein relates to capstans for tape drives. More particularly it relates to capstans for use with externally driven tape cartridges.

Description of Related Art: For several years magnetic tape cartridges have been commonly used for backup of computer, data processing and word processing files. Such a tape cartridge is designed in a box-like form and contains two tape reels, one a tape supply reel and the other a take-up reel. The device is designed to operate at very high linear tape speed, so that a large quantity of data can be transferred very quickly to the storage tape. The various types of cartridges available all differ slightly in various details. They are, however, all of generally similar configuration since they are intended to be used interchangeably in the commercial tape drives or players. A typical commercial cartridge structure is described in U.S. Patent No. 3,692,255, and will be discussed in more detail below. In this and other similar cartridges, the tape reels are driven by a belt which is in turn driven by a capstan. The capstan itself contacts a drive wheel through an openinq in the cartridge case, which dri"»e wheel is part of the tape drive device. When the cartridge is inserted into the tape drive the capstan and the drive wheel are automatically

engaged. The capstan has a vertical axial hole therethrough and is rotatably mounted on a vertical spindle mounted within the cartridge case. Experience has shown that at the high rotational speeds involved the capstans are subject to significant wear. Consequently, in the past, it has been desired to form the capstans from resistant plastics. One which has commonly been used is the family of acetal resins and resins with similar physical properties. Use of such resins has provided for reasonably long service lives for the capstans before there is sufficient wear that replacement-is required. Principal wear surfaces are the inner surface of the spindle hole, the bottom of the capstan where it rubs against the inside surface of the bottom plate of the cartridge and the major outer diameter where it contacts the motor capstan. It has also been common to add small amounts of friction reducing polymers, principally polytetrafluoroethylene ("PTFE") . Initially capstans of these types of compositions were found to be quite suitable. However, as tape cartridge capacities were increased and the tapes were necessarily run at higher speeds to allow rapid transfer of data, it was found that the high rotational speeds caused static electric charges to build up on the capstan because of friction with the spindle, cartridge base plate and/or drive belt. Such static charge buildup is objectionable because there is a tendency for large accumulated amounts of static electricity to discharge suddenly, causing electrical noise which can adversely affect the operation of the playback transducer. In the past the industry has tried to deal with this in two different ways. First, there have been attempts to use conductive fillers in the acetal resin in order to make the capstan conductive so that the static charge will be dissipated, generally through the metallic spindle or other

ground. For instance, powdered carbon has been added to the acetal resin. However, the carbon powder filled acetal resin capstans showed very poor wear characteristics as indicated in U.S. Patent No. 4,607,808. Thus, while the carbon powder filled capstan did serve to dissipate static charge, the wear characteristics were sufficiently poor that the carbon powder filled capstans were not practical. Second, there have been attempts to manufacture multi- part capstans, such as the one shown in the aforesaid U.S. Patent No. 4,607,808, in which one part is wear resistant and the other is conductive. The capstan of that patent is formed of a wear resistant polymer sleeve which is on the inside of the capstan and fits around and in contact with the spindle. The sleeve material is fluorocarbon-filled acetal resin. The main body of the capstan, which surrounds the sleeve, is of carbon powder filled resin. The patent discloses success with this two-part capstan although the patentees acknowledge that it is not understood why the capstan should work to dissipate the static charge, since a significant portion of it (the sleeve) is non-conductive. However, even though there may be the ability to discharge the static charge and to avoid undue spindle wear, such multipart capstans are difficult to fabricate (as acknowledged in the patent) and still suffer from integrity problems. Consequently, there is still a significant need in the industry for a capstan in a unitary form which is made of a material which will provide both acceptable service life with minimal wear and reliable and safe dissipation of static electricity.

SUMMARY OF THE INVENTION In its broadest form, the invention herein comprises a drive capstan for a belt driven tape cartridge which comprises

a generally unitary cylindrical body having means for contacting and being driven by an external driving means and having an axially aligned mounting hole through the center thereof for being rotatably mounted on a spindle, the body being formed entirely of a composition comprising a resin matrix having dimensional stability in rotation and having dispersed therethrough 1-45 percent by weight of a carbon component and 1-45 percent by weight of a fluorocarbon component. (All percentages herein are by weight unless otherwise noted. ) The carbon component may be the form of carbon fibers, the fibers having a fiber length range of 0.1-10 mm, or in the form of granulated carbon ("carbon powder") , in which the particle sizes are in the range of 2- 100 μm, or as a mixture of the two forms. - The fluorocarbon component may also be the form of fibers, the fibers having a fiber length range of 0.1-20 mm, or in granulated form ("fluorocarbon powder") , in which the particle sizes are also in the range of 2-100 μm, or as a mixture of the two forms. In various preferred embodiments, the resin may be an acetal or nylon resin or the equivalent and the fluorocarbon component may be a fluorocarbon such as polytetra- fluoroethylene ("PTFE") , fluorinated ethylene propylene ("FEP") or one of the polymers commonly referred to as "perfluoroalkoxy polymers" ("PFA") . In another preferred embodiment the means to contact and be driven by the external drive means comprises a circumfer- ential flange extending radially outward from the top of the body.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a plan view of a typical tape cartridge embodying the capstan of this invention. In the view shown the top cover of the cartridge is partially cut away to allow

illustration of the interior mechanism. Figure 2 is a plan view of one embodiment of the capstan of the present invention. Figure 3 is a cross sectional view of the capstan of Figure 2 taken on the line 3-3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before turning to the description of the capstan of this invention, it is instructive to briefly describe a typical cartridge into which the capstan is mounted. The typical cartridge is shown in Figure 1 and is of the type generally shown in the aforesaid U.S. Patent No. 3,692,255. The cartridge, designated 2, has a generally rectangular base 4 and top 6 separated by perimeter wall 8, all of which combine to form the hollow interior 9 of the cartridge. Within the interior 9 are tape reels 10 and 12. Since the tape can be driven and recorded on in either direction, either reel 10 or reel 12 can serve as the feed or take-up reel respectively. Conventional tape drives are designed, however, to drive the cartridge in the direction such that reel 10 is the feed reel and reel 12 is the take-up reel. The tape 14 is unwound from reel 10 and fed past recording head 16 to take-up reel 12 in the forward direction. The path is determined by the number and location of various guide posts and rollers such as 18 and 20 whose functions are not critical to the present invention and whose placement and number will vary from cartridge to cartridge. Further description will be found in the aforementioned patent. Access by head 16 to tape 14 is by opening of door 22 which is commonly designed to occur automatically as the cartridge is inserted into the tape drive. The reels 10 and 12 are driven by continuous belt 24 which is wound around pulleys 26 and 28- The belt is under

tension and passes in contact with the surfaces of tape 14 along peripheral segments of reels 10 and 12 as shown generally at 30 and 32. The belt is led around and in contact with capstan 34 as shown at 36. Rotation of capstan 34 thus drives belt 24 which in turn drives the reels 10 and 12 and feeds tape 14 past head 16 for recording or playback. Capstan 34 is itself driven by drive wheel 38 which, when the cartridge is inserted into the tape drive, is in peripheral contact with capstan 34 through opening 42 in the peripheral wall 8 of the cartridge. Drive wheel 38 is in turn connected to a flange 40 which -is connected to the driving mechanism (not shown) of the tape drive. The capstan 34 is wholly within the interior 9 of cartridge 2 as shown in Figure 1 so that the drive wheel 38 projects into the cartridge 2. Capstan 34 rotates on spindle 46 which is normally a vertical metal pin extending upwardly from base 4. It may or may not reach all the way to the underside of top 6, but will be at least tall enough that it will provide for stable rotation of capstan 34. The spindle 46 fits into central axial hole 48 in capstan 34 and has a slight clearance fit such that capstan 34 can rotate freely with no significant wobble. As described above, it is rotation about spindle 46 that has proved to be a problem for capstans of the prior art. One piece capstans which have not been electrically conducting have had good service life and wear resistance at the inner wear surface of 50 of hole 48 while the one piece conductive capstans have showed excessive wear on that surface, leading the practice of using two piece sleeved capstans of the type shown in the aforementioned Patent No. 4,607,808. The unitary capstan of the present invention with its critical composition is uniquely able to combine electrical conductivity for dissipation of static charge with long service life and

minimal rotational wear. The structure of a preferred embodiment of the capstan of the present invention is shown in Figures 2 and 3. In order to reduce mass and thus minimize the system's inertial moment at the high rotational accelerations involved, the preferred design of Figures 2 and 3 has a central core 52 through which is bored or molded hole 48. Horizontal ribs 54 connect the inner core 52 with the outer belt contact shell 56 which, in a further preferred form, expands radially outwardly at the top to form flange 44. Belt 24 is in contact with shell 56 at the central segment thereof as- shown in Figure 3. This central segment 58 preferably has a slight outward peripheral bulge as shown in Figure 3 to serve as a belt guide. It will be recognized that other configurations of the capstan are possible. For instance, the ribs 54 can be joined horizontally to form a solid web and/or they can be thickened vertically, even to the point of completely filling the openings 60 and 62 (in which structure the ribs 54 effectively do not exist as separate entities and the capstan is essentially formed as a monolithic block) . Similarly, either or both of the outer peripheral shell 56 or core 52 can have walls of different thicknesses, and, as with the ribs 54, can extend horizontally to the point of merger, again forming a monolithic block structure. The practical minimum and maximum limits of such dimensions, and whether a solid body is satisfactory, will be readily apparent to those skilled in the art and will be based on factors such as strength and integrity at high rotational speeds and acceptable weight. The bottom 64 of central core 52 rotates on the inner surface of base 4 of cartridge 2 in a sliding contact. This provides an additional wear point which in the past has been an area of potential capstan failure. In the present design it will be noted that the lower portion of peripheral shell 56

is elevated slightly above the bottom 64 of central core 52 so that the area of sliding ^' contact is minimized while yet providing appropriate stability of the capstan 34 during high speed rotation. Also present in the bottom surface 64 is circumferential oil groove 61 into which a lubricant can be placed to provide lubrication for the rotating capstan. A like groove 61' can be in the top surface 63 if the top 63 of core 52 bears against a boss (not shown) projecting downwardly from the inside of the cartridge case top 6. The material of which the capstan of this invention is made is critical to the success of the invention. The matrix component of the composition will be a resin which is dimensionally stable in rotation and which will accept fillers. These resins will be characterized as being hard, rigid, strong, tough and resilient, as well as being dimensionally stable under exposure to flexing and creep when subjected to high centrifugal forces. It has been found that acetal and nylon resins are both satisfactory for purposes of this invention, although other resins having such characteristic properties substantially equivalent to acetals or nylons are also contemplated for use herein. Most preferred are the acetals, because of their ease of molding. The acetal and nylon resins are well-known and widely described. The acetals can basically be considered to be polyoxy ethylene thermoplastic polymers obtained by polymerization of formaldehyde. Typical acetal resins and their method of manufacture are shown in Noller, Chemistry of Organic Compounds (1965), p. 244 and in Brady, et al. , Materials Handbook (11th Edn. ; 1979) , pp. 7-8. The nylons are polyamide resins formed from the polymerization of a dibasic acid and a diamine. Typical nylon resins and their method of manufacture are shown in Brady, et al., pp. 533-535. Various suitable acetal and nylon resins are commercially available-

Also critical to the compositions used in the capstan of this invention is the discovery of the unexpected synergy of the other two required components, carbon and a fluorocarbon or fluorocarbon ether. As noted, it has been known in the past, as described in the aforesaid U.S. Patent No. 4,607,808, to incorporate carbon alone into an acetal resin used for capstan manufacture. However, in the past such carbon additions have proved to provide inferior and unsuccessful capstans, in that they tend to wear rapidly and disintegrate easily under the high speeds and high centrifugal forces involved. Critical to the -present invention is the recognition that the combination of carbon and fluorocarbon into the resin not only provides excellent charge dissipation properties and electrical conductivity, but also provides a strong coherent mass which can be molded or otherwise formed into the capstan of the present invention as a unitary body and which will provide excellent self-integrity and resistance to wear and disintegration in service. Different forms of carbon and fluorocarbon components may be used. If used in the fibrous form, suitable carbon fibers will have fiber lengths in the range of 0.1-10 mm, preferably 0.5-2 mm, and more preferably 0.8-1.6 mm. The carbon fibers will have fiber diameters in the range of 2-40 μm, preferably 5-20 μm, and more preferably 7-14 μm. If used in granular form (also referred to a particulate or powdered form) , the carbon will have particle diameters in the range of 2-100 μm, preferably 10-50 μm, and more preferably 15-35 μm. The carbon component will be present as l%-45%, preferably 10%-20%, and more preferably about 15%, of the composition. Mixtures of the two forms may also be used. The fluorocarbon may be any of several generally equivalent fluorocarbon resins. The preferred resin is polytetrafluoroethylene ("PTFE") , since it is widely available

and has appropriate starting and running coefficients of friction. Other materials which are suitable include fluorinated ethylene propylene ("FEP") or the material commonly referred to as "perfluoroalkoxy polymer" ("PFA") , or any others with substantially equivalent starting and running coefficients of friction. The fluorocarbon polymers (including PTFE, FEP and PFA) are described in Seymour, Engineering Polymer Sourcebook, Ch. 15 (1990) . If used in the fibrous form, the suitable fluorocarbon fibers will have fiber lengths in the range of 0.1-20 mm, preferably 0.2-5 mm, and more preferably about 0.5 mm. The fluorocarbon fibers will have fiber deniers in the range of 2-200, preferably about 5- 7. If used in granular form the fluorocarbon material will have particle diameters in the range of 2-100 μm, preferably 10-50 μm, and more preferably 15-35 μ . Mixtures of the two forms may also be used. The fluorocarbon component will be present as l%-45%, preferably 2%-10%, and more preferably about 5%, of the composition. The fibers used in the present invention are often referred to as "milled" fibers, but any commercial fibers may be used. The powders and fibers are dispersed generally randomly and homogeneously throughout the matrix resin. The fibers, powders and resins may be purchased separately and blended and molded by the capstan manufacturer. However, it may also be possible to purchase pre ixed quantities of certain fibers and/or powders and resin from resin manufacturers. A typical mixed composition is a material referred to as "Antistat Acetal RTP 899X 53689" from the RTP Company of inona, Minnesota. Other materials commonly used in the manufacture of acetal and nylon resin products may also be present, including colorants. The type and quantity of such additive materials will be well known to those skilled in the art and the exact

nature of such additives is not critical to the present invention, as long as such additives do not adversely affect the integrity, electrical conductivity, and service life of the present capstan. The capstan of the present invention may be manufactured by any common resin molding or forming procedure. The embodiment shown in Figures 2 and 3 is most common formed by molding. It will be evident that there are numerous embodiments of this invention which, while not expressly described above, are clearly within the scope and spirit of the invention. The above description is therefore to be considered exemplary only, and the scope of the invention is to be limited solely by the appended claims. I claim: