Patent Publication Number: US-7715166-B2

Title: Method and reciprocating apparatus for permanent magnet erasure of magnetic storage media

Description:
TECHNICAL FIELD 
   This invention relates to magnetic storage media erasure and the mechanisms that may be applied to enhance such erasure. 
   BACKGROUND 
   Prior degaussers typically require manual manipulation of magnetic storage media, such as magnetic storage tape, hard disk drives, and the like, by a user in between passes of the media through a magnetic field to achieve uniform multidirectional magnetic field exposure for optimum erasure performance. The raw magnetic strength applied to certain magnetic storage media such as hard disk drives can overcome the lack of multidirectional exposure in the plane of the disk along the directions of the circular tracks recorded on the disk. Meanwhile, hard disk drive technology has advanced to the era of perpendicular recording on the disk with increasing coercivity ratings requiring a higher applied magnetic field strength for erasure. 
   Perpendicular recording and the possibility of further coercivity increases in magnetic disk drives are creating a demand for perpendicular magnetic field strength and an approximately equivalent horizontal magnetic field strength to be applied in degaussers. Typically, the disks are only partially constrained in the drive by the frictional force of parked heads, and strong erase fields acting on the spindle rotors might overcome that force, leading to less than certain demagnetization results. 
   Certain prior attempts to erase hard disk drives included apparatuses that apply a degaussing magnetic field almost directly to the disk while rotating the disk within the drive. The relationship, however, between external features of various hard disk drive brands and models and their internal components like spindle motors and head motors are not universally obvious. Therefore, in general application it is desirable to treat the entire volume of each hard disk drive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above needs are at least partially met through provision of the method and apparatus for permanent magnet erasure of magnetic storage media described in the following detailed description particularly when studied in conjunction with the drawings wherein: 
       FIG. 1  is an elevational view perpendicular to a magnetic media path through one embodiment of a Halbach-like array permanent magnet degausser in accordance with various embodiments; 
       FIG. 2  is an elevational view parallel to a magnetic media path through one embodiment of a permanent magnet degausser in accordance with various embodiments; 
       FIG. 3  is a perspective view of a permanent magnet degausser in accordance with various embodiments; 
       FIG. 4  is a perspective view of a permanent magnet degausser in accordance with various embodiments; 
       FIG. 5  is a perspective view of a permanent magnet degausser in accordance with various embodiments; 
       FIG. 6  is a perspective view of a permanent magnet degausser in accordance with various embodiments; 
       FIG. 7  is a perspective view of the permanent magnet degausser of  FIG. 6  with additional magnetic elements surrounding the magnetic media conveyance path in accordance with various embodiments; 
       FIG. 8  is a perspective view of a permanent magnet degausser with a cleated conveyor belt in accordance with various embodiments; 
       FIG. 9  is a side view of a media miss feed in a permanent magnet degausser in accordance with various embodiments; 
       FIG. 10  is a block diagram including a wiring diagram of a protective sensor connected to a direction selector and a motor controller of a permanent magnet degausser in accordance with various embodiments; 
       FIG. 11  is a perspective view of a permanent magnet degausser with a reciprocal media conveyor in accordance with various embodiments; 
       FIG. 12  is an elevational view of the permanent magnet degausser of  FIG. 11  demonstrating various cross-sectional views; 
       FIG. 13  is a view of the permanent magnet degausser of  FIG. 11  along the cross sectional view I of  FIG. 12 ; 
       FIG. 14  is a view of the permanent magnet degausser of  FIG. 11  along the cross sectional view II of  FIG. 12 ; 
       FIG. 15  is a partial view of the permanent magnet degausser of  FIG. 11  along the cross sectional view III of  FIG. 12 ; 
       FIG. 16  is a partial view of the permanent magnet degausser of  FIG. 11  along the cross sectional view IV of  FIG. 12 ; and 
       FIG. 17  is a partial view of the permanent magnet degausser of  FIG. 11  along the cross sectional view V of  FIG. 12 . 
   

   Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the arts will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Generally speaking, pursuant to these various embodiments, a permanent magnet degausser includes at least one magnetic field generator comprising magnetic elements arranged near a media conveyance path and a conveyor for transporting magnetic media through a magnetic media conveyance path. A passive belt or protector plate may be provided to assist the passage of the magnetic media through the applied magnetic field. The conveyor may be a continuous motion conveyor belt including cleats for holding the magnetic media or a reciprocal media conveyor including magnetic storage media bin. The magnetic field generator may include permanent magnets of varying intrinsic coercivities and/or remanences. 
   Various embodiments of these degaussers allow for high volume and high quality demagnetization of magnetic storage media such as hard disk drives, magnetic tape, and other magnetic media devices. Additionally, when using permanent magnet magnetic field generators, the degaussers can operate without expensive and complicated electronics for generating the magnetic fields. Permanent magnet arrangements also typically allow for high field strength and uniformity in a variety of field directions to provide improved erasure of hard disk drives. 
   Production of the Magnetic Field 
   With reference to the figures, and in particular to  FIG. 1 , permanent magnet arrays  5 , also called magnetic assemblages  5 , comprising Halbach-like arrays having ¾ of one full Halbach period face each other across a gap  2 . This illustrates an example of a magnetic field generator&#39;s having at least two magnetic assemblages  5  disposed on opposite sides of the magnetic media conveyance path  46  thereby defining a gap  2  for the path  46 . A more thorough discussion of the application of Halbach-like arrays in the field of permanent degaussing as developed by one of the current inventors is discussed in U.S. patent application Ser. No. 10/897,882, titled “Permanent Magnet Bulk Degausser” and filed Jul. 23, 2004 (“the &#39;882 application”), which is incorporated herein in its entirety. 
   As depicted in  FIG. 1 , Halbach arrays of classic linear form consist of touching or closely spaced permanent magnets, generally depicted as squares in cross-section, including inner permanent magnet members  4  and outer permanent magnet members  6 . Ferromagnetic plates  8  can line the permanent magnet array surfaces opposite the gap  2  for structural support, to aid in assembly, for enhancement of magnetic performance, or to provide a magnetic circuit element. Cold rolled steel plate, for example, provides excellent and economical mechanical and magnetic properties for the plates  8 . 
   The directions of magnetizations depicted by the solid arrows of  FIG. 1  represent an example vertical generator, a group of permanent magnets creating a generally vertical magnetic field in the gap  2  relative to the permanent magnet arrays. This embodiment includes permanent magnet elements  4  with the same vertical magnetic field directions  10  above and below the gap  2  in cooperation with the outward horizontal magnetic field directions  12  provided by permanent magnet elements  6  above the gap  2  and inward horizontal magnetic field directions  14  provided by permanent magnet elements  6  below the gap  2 . Such an arrangement generates a generally upwardly vertical magnetic field of peak strength toward the center of the gap  2 . 
   The directions of magnetizations depicted by the outlined arrows of  FIG. 1  represent an example horizontal generator, a group of permanent magnets creating a generally horizontal magnetic field in the gap  2  relative to the permanent magnet arrays. This embodiment includes permanent magnet elements or segments  6  with opposite vertical magnetic field directions  20  facing each other above and below the gap  2  in cooperation with the same horizontal magnetic field directions  22  provided by the permanent magnets  4  between them. Such an arrangement generates a generally right pointing horizontal magnetic field of peak strength toward the center of gap  2 . 
     FIG. 2  illustrates the permanent magnet degausser of  FIG. 1  showing the sides of outer permanent magnets or segments  6  and plates  8 . The horizontal plates  8  may extend outward to or beyond the vertical structural members  30 . The horizontal plates  8  preferably have a thickness sufficient to prevent or minimize magnetic saturation in the vertical generator, and roughly equivalent thicknesses for both plates  8  is preferable for the magnetic properties of a horizontal generator. 
   In a preferred embodiment, the permanent magnet degausser includes at least two plates  30  extending between the at least two magnetic assemblages  5  to surround the magnetic media  42  on four sides as it passes through the gap  2 . In a preferred vertical generator embodiment, the vertical plates  30  are steel to return vertical flux between the magnetic assemblages and thick enough to prevent saturation. The vertical plates  30  need not touch both horizontal plates  8  as rod shaped gussets  32 , which may or may not touch both horizontal plates  8  and vertical plates  30 , complete the return path for vertical magnetic flux between the magnetic assemblages thereby forming a single efficient magnetic circuit. Loose limits can be placed on the vertical plate  30  positions without much affect on magnetic strength. Bolts placed through slotted holes in the vertical plates  30  and into the gussets  32  can provide further adjustment space against magnet element fabrication and assembly dimensional tolerances to achieve a more precise vertical gap  2  dimension. Vertical members  30  and gussets  32  also provide support against the compression force due to attraction of the vertical generator magnetic assemblages. 
   In a preferred horizontal generator embodiment, the vertical plates  30  are nonmagnetic, for example stainless steel, sheets formed into shallow channels attached to the nonmagnetic gussets  32 , for example those machined from aluminum. Vertical plates  30  can have slotted holes for attachment to the gussets  32  and for adjustability against manufacturing tolerances. Being nonmagnetic, the vertical plate  30  positions do not affect the magnetic strength of the horizontal generator. The dimensions of the vertical plate  30  or channel can be adjusted as needed against the tension force due to the repulsion of the horizontal generator halves. 
   For both vertical and horizontal magnetic field generating embodiments, the vertically magnetized permanent magnet elements generally form poles about the gap  2 , and horizontally magnetized permanent magnet elements generally serve to direct, reinforce, and concentrate magnetic flux density toward the center of the gap  2 . This dynamic of magnetic fields is often termed a superposition of fields emanating from the variously directed permanently magnetized regions. The superposition of fields results in the magnetic field in the gap  2 . 
   Because the superposition of fields creates the applied magnetic field, a variety of field directions and strengths may be created and used within the gap  2 . The permanent magnet segments may be in the form of plates, cubes, or rods as necessary for a given embodiment. The squares or rods are preferably formed from an assembly of sub-elements such as magnet blocks or elements having a 2×2×1 aspect size magnetized in the thin dimension because of the cost savings realized in using a number of identical or similar elements to build the magnetic assemblages. 
   Permanent magnet element count and weight are typically compared to the resulting magnetic strength and uniformity of the degausser to provide the primary optimization criteria for a given application. The magnetic media width in the gap  2  provides a preferred parameter for a worst case strength analysis, and media width as compared to the mid gap magnetic field strength provides a suitable uniformity parameter. 
   Further optimization often includes positioning the magnetic assemblages as closely together as possible. Generally, the magnetic storage media is carried through the magnetic field within the gap  2  in a bin or other carrier with the smallest possible thickness while having the size and strength needed to contain the media and extract it from the magnetic field. For example, the magnetic assemblages are held apart at a gap distance just over the one inch thickness of the standard hard disk drive form factor to allow for any thickness of the bin or carriage plus, in some alternatives, the thickness of a means to protect the permanent magnet materials from the to-be-erased magnetic media traveling through the gap  2 . 
   Even further optimization can be achieved by applying a horizontal field strong enough to erase a hard disk drive without rotation of the platters or disks contained therein. In these embodiments, the applied magnetic field need only have a uniform and sufficient magnetic strength as large as the smallest cross section of the hard disk drive. The bin or carrier would then move the drive through the magnetic field in the gap  2  in the direction of the media&#39;s longest dimension. For a typical “desk top” hard disk drive, the small cross section is just over one inch thick by four inches wide. By contrast, the longest dimension approaches six inches, which would impose a penalty on the amount of expensive and heavy permanent magnet material needed to generate a field of that size. 
   In accordance with these general parameters, various embodiments for the magnetic assemblages of permanent magnets are shown in  FIGS. 3 through 7  and are discussed in the &#39;882 application. For example,  FIGS. 3 and 4  depict horizontal magnetic field generator structures built from 2×2×1 permanent magnet elements with the segment thicknesses ranging from roughly 1 inch to nearly 2 inches (50 mm). Arrows on the element sides of  FIG. 3  and  FIG. 4  depict the constant direction of magnetization through the element thickness. The figures omit the support structures for clarity. 
   Inner horizontally magnetized segments  40  of the one-layer embodiment of  FIG. 3  have a non-square profile when viewed perpendicular to the magnetic media  42  path  46 . In contrast, outer segments  41  have a square profile, which is commonly associated with “classic” Halbach arrays.  FIG. 4  depicts a “two-layer” embodiment offering a slight strength increase even across a greater gap  2  than the “one-layer” embodiment of  FIG. 3 . 
   For example, 34 mm thick permanent magnet elements may be used in the embodiments of  FIGS. 3 and 4  to erase a typical “desk top” hard disk drive  42 . For the “one-layer” embodiment of  FIG. 3 , the gap  2  scales to about 1.2 inches and the magnetic field strength simulates to about 1.2 T. For the “two-layer” embodiment of  FIG. 4 , the gap  2  scales to about 1.4 inches and the magnetic field strength simulates to about 1.4 T. A typical hard disk drive  42  includes a circular projection of a spindle motor  44  through a circuit board side with various electronic components that factor into the difficulty of media transport in direction  46  through gap  2 . Preferably, a horizontal generator using the elements of this example uses at least four elements creating a gap  2  width of about 10.7 inches to counter fringing effects and yield a field uniformity of about a few percent over the four inch media width. 
   Referring to  FIG. 5 , an example “one-layer” assemblage vertical field generator  70  uses a larger magnetic assemblage needing only three elements of width to accomplish comparable strength and uniformity of field across the same media width as the four element wide horizontal generator of  FIG. 3 . Such vertical generators can be comprised of an upper assemblage  60  containing a central upper magnet segment  66  and a lower assemblage  62  containing a central lower magnet segment  65  magnetized in the same vertical direction. The horizontally magnetized lower outer segments  64  and  68  can be said to “pull” more magnetic flux out of the downward pointing magnetic pole of the middle segment  65 . The horizontally magnetized upper outer segments  67  and  69  can be said to “push” more magnetic flux into the downward pointing pole of the middle segment  66 . A “two-layer” three element wide vertical generator complimentary to  FIG. 4  provides similar degrees of strength versus uniformity and material savings advantages as seen with the previous example. 
   With reference to  FIGS. 6 and 7 , the magnetic field generators need not be limited to Halbach-like permanent magnet arrangements.  FIG. 6  illustrates in general the addition of eight permanent magnet elements  50  to a more conventional magnetic circuit  52 . This example illustrates a Halbach-style orientation of the added “helper” magnet elements  50  that effectively push or pull flux into or out the vertically magnetized segments of the magnetic assemblages about the gap  2 . 
     FIG. 7  illustrates a preferred embodiment having eight more side “helper” elements  54  added to the embodiment of  FIG. 6 . This illustrates an example of at least two side magnetic assemblages  54  extending between the at least two magnetic assemblages  5  to surround the magnetic media  42  on four sides when disposed in the gap  2 . Using the example size and strength elements discussed above, the side helper elements  54  are positioned beyond the gap  2  width for media passage with each element  54  covering approximately half of the gap  2  distance. The side helper elements  54  then virtually cancel fringing effects and provide a gap  2  width over 5.25 inches wide. The result is a horizontal field strength nearly equal to the example of  FIG. 3  using thirty-two permanent magnet elements instead of forty. 
   Various other arrangements of the side helper elements are possible, and the use of such side helper elements can provide magnetic strength and field uniformity benefits in a variety of applications. Permanent magnets of size aspects other than 2×2×1 could be fit at the gap sides or into voids left by shifting lateral magnets to the gap sides, or magnets at the gap sides could be placed three-high and overlap the steel plate edges. In another example, twelve helper elements could be applied either three-wide separated by the gap entrance or three-deep and touching along the gap sides without interfering with eight other two-deep helpers touching at the sides or two-wide helpers at the gap entrance. 
   Permanent magnet structures containing more conventional magnetic circuit techniques can also benefit from Halbach-like additions. Such structures can consist of permanent magnet poles attracted to steel plates to form parts of the magnetic circuits with additional permanent magnet elements applied to the outsides of one, two, three, or all four sides. The horizontal direction of magnetization “pushes” additional magnetic flux into the magnetic pole elements and further concentrates the flux density into the gap containing the working magnetic field. 
   Often, at least a slight strength advantage can be gained through the selection of certain magnetic materials. Preferably, the permanent magnet elements comprise a high energy grade of sintered neodymium-iron-boron elements, often denoted by an “N” prefix by vendors. The choice of material is informed by weighing the variables of cost, assembly effort, weight, and size, especially from the number of necessary elements, against the resulting magnetic field strength. In some cases, the achievement of ultimate strength can offset the cost of higher grade material. 
   The assembly of Halbach-like structures can take different sequences. One sequence preferred for horizontal generators is to assemble element pairs in attraction and then assemble a grouping of five elements with mix of attraction and repulsion to create a single assemblage. The two-layer embodiment can follow that with an attractive ten element step. Attraction to the steel plate aids in the repulsive assembly of five- or ten-unit groups to each other, which precedes the less magnetically repulsive step of placing the identical assemblage halves in mirror image to each other. That final step typically involves a large placement force. 
   Such assembly sequences typically involve tooling in the form of custom holding, positioning, clamping and alignment fixtures, which may take various forms as recognized by those skilled in the art. Except for the final application of structures to hold the assemblage halves opposite each other about the gap  2 , glue such as Versalock(r) glue can hold the various segments or elements to each other temporarily. Tooling for steps like the five-unit portion typically needs to provide clamping and alignment to all six sides before the glue sets. Some miss-alignment due to element dimensional tolerance and non-contact at glue joints may be tolerated and can be handled with adjustability in the support structure such as through using slotted bolt holes. Alternatively, a gap height adjustment mechanism can be provided to adjust for magnetic media form factor thickness and magnetic field strength. 
   A difficulty can arise in generating the appropriate magnetic field strength in the middle of a large gap. To achieve these field strengths, the field intensities may exceed the permanent magnet intrinsic coercivity over some portion of the permanent magnet elements that generate the magnetic field. Coercivity is a measure of the amount of outside magnetic field that can be applied to a permanent magnet to bring the overall field to zero and exhibit the same magnetic filed properties when the outside applied magnetic field is removed. Intrinsic coercivity is a measure of a permanent magnet material&#39;s internal magnetic properties to withstand an outside magnetic field before the magnetic field created by the material is irreversibly changed or removed. The intrinsic coercivity value is the strength of the outside field needed to irreversibly change or remove the magnetic field created by the material. Note, however, that the magnetic properties of the permanent magnet material can be restored in whatever direction, by the application of typically the same applied field strength used to de-magnetize the material. Thus, exceeding the coercivity or intrinsic coercivity of a permanent magnet can result in a partial or complete demagnetization of a portion of the permanent magnet, thereby affecting overall magnetic field strength. 
   A finite elements analysis (FEA) done using ordinary commercial software known in the art can identify but typically does not quantify that demagnetization effect. For example, a simulated horizontal generator embodiment using high energy grade N48 permanent magnet elements should produce about 1.4 T according to an FEA analysis, whereas the same physical horizontal generator embodiment results in about 1.03 T due to the flux leakage within, and partial demagnetization of, the permanent magnet materials. 
   Although some localized partial demagnetization can be tolerated, limiting the magnetic reversal and demagnetization can provide a distinct advantage. Permanent magnet materials are available in wide range of properties including the flux density, coercivity, and intrinsic coercivity of the material. One way to limit flux leakage and localized partial demagnetization is to select permanent magnet material grades with high coercivity at the expense of the flux density that the material can support. For example, material selection for maximum coercivity typically involves the selection of grades N42H or N45H that typically limit the regions where reverse field intensity exceeds coercivity causing flux leakage or localized partial demagnetization, whereas the selection of materials with grade N50 or even grade N48M typically increases the flux density but may increase the flux leakage or localized partial demagnetization. 
   Selection of material with a preferably high intrinsic coercivity over coercivity can provide an additional advantage by limiting the regional extent and degree of partial demagnetization by reverse field intensity. Such selection can also provide a stability of magnetic strength against the extremes of ambient environmental temperature, for example where storage temperatures may be higher than operating temperatures. The margin of intrinsic coercivity to coercivity can reduce any irreversible effect of an extreme ambient storage temperature thereby allowing a more or less reversible return to the original magnetic strength at operating temperature. Selective application of grade N42SH, for example, provides a significant increase of intrinsic coercivity with very little reduction of coercivity as compared with grade N42H. 
   Application of temperature resistant or high intrinsic coercivity suffixed grades materials, however, can come at a significant economic cost in addition to the penalty of reduced stored energy capacity. For instance, lower energy high temperature grades may cost approximately 30% more than highest energy low temperature grades due in part to the very rare additives used in the material&#39;s formulation. Simulations indicate that excess field intensity and flux density reversal are typically greatest in regions within the horizontally magnetized portions of the horizontal magnetic field generating assemblages. 
   Given these factors, a permanent magnet magnetic media degausser may include a plurality of permanent magnets  40  and  41  creating a magnetic field to erase magnetic media  42  wherein at least a first permanent magnet  40  has an intrinsic coercivity higher that at least a second permanent magnet  41 . Preferably, the first permanent magnet  40  directs its field directly into the second permanent magnet  41 , and the second permanent magnet  41  has a higher remanence than the first permanent magnet  40 . 
   Such selective application of suffixed grades to horizontally magnetized regions of the magnetic assemblages typically increases the overall financial cost by only a few percent with a potential improvement in magnetic strength of approximately several percent. Higher permanent magnet grades, as will likely become available, are equally applicable to the various embodiments of the invention. Furthermore, nominal characteristics and production tolerances of permanent magnet materials are such that the physical embodiment of materials with lesser ratings could measure stronger than the physical embodiment of material with higher grade rating. Thus, the prudent selection of materials at a given point in time may vary. 
   Passage of Magnetic Media Through the Field 
   Magnetic storage media can be moved through the applied magnetic fields by various means. For a stationary application where many magnetic media such as hard disk drives are directed to a centralized point for erasure, high throughput can be achieved with a motorized one-way conveyance of each disk drive through the gap  2  or a lineal series of gaps containing one or more magnetic fields. With reference to  FIG. 8 , such a device  101  may include at least one magnetic field generator  98  and/or  114  comprising magnetic elements arranged near a media conveyance path  46  generating a plurality of magnetic field directions in the magnetic media conveyance path  46 . The degausser device  101  also includes a cleated conveyor belt  103  defining magnetic storage media bins  100  wherein the cleated conveyor belt  103  moves substantially linearly through the magnetic media conveyance path  46 . In some alternatives, a passive belt  174  at least partially surrounds and is movable around the magnetic field generator(s)  98  and/or  114 . 
   The stationary application can employ enough permanent magnet material to allow for an increased gap  2  between the magnetic assemblages of the magnetic field generators  98  and/or  114  to fit hard disk drives, the cleated conveyor belt  106 , and the passive belt  174  to process hundreds of thousands of hard disk drives with reduced energy costs. For example, the peak pulling force needed to extract two typical hard disk drives simultaneously from two 1 T magnetic fields at a speed of four inches per second typically requires no more than 2 kW of peak power. Aside from motor heating, permanent magnets typically generate only a few watts of heat due to eddy currents in the hard disk drives moving through the magnetic fields. Electromagnet embodiments with a similar capability often consume over 25 kW, which mostly becomes heat that needs to be removed from the electromagnets. 
   The plurality of magnetic field directions created by the magnetic field generator(s)  98  and/or  114  preferably further include magnetic field directions in at least a substantially horizontal field direction and at least a substantially vertical field direction with respect to the magnetic storage media. A substantially vertical field direction is preferably generated by a preferred vertical magnetic field generator  98  including vertical magnetic flux return members such as a steel member  108  behind certain stainless steel “Z”-shaped members  110 . The vertical magnetic flux return members  108  can vary somewhat in their distance from the generator magnets, but likely will constrain the lateral width  106  of the conveyor belt  103 . Formed stainless steel members such as end member  112  can supplement the support of vertical magnetic flux return members  108 , which are analogous to the supports  30  communicating with gussets  32  seen in  FIG. 2 . The “Z” members  110 , flux return members  108 , end members  112 , and gussets cooperate with each other in stiffening against the attractive force between the vertical generator  98  halves. The “Z” members  110  can also serve as a mechanical constraint against the repulsive forces among the permanent magnet members of the vertical field generator  98 . 
   A preferred horizontal magnetic field generator  114  also utilizes “Z” members  116  and end members  118  formed of stainless steel sheet, but to attach the magnet assemblages in repulsion. As a result of the commonality of “Z” members  110  and  116 , steel plate holes  120  of the vertical generator  98  are placed inward to attach steel gussets between them and the vertical steel plate  108 , relative to the outwardly placed holes  122  of the horizontal generator  114  used to attach nonmagnetic gussets of similar form between them and the nonmagnetic channels  118 . 
   The steel plates  108  and “Z” members  110  and  116  can also support formed additional stainless steel support members (not shown) between them with web faces parallel to the supports  30  and in contact with the outer portions of the permanent magnet elements of segments  4  and  6  seen in  FIG. 1 . 
   Sideways forces, for example due to the interaction of typically asymmetric arrangement of ferromagnetic structures within hard disk drives with the magnetic fields, might cause accumulated miss-tracking of the conveyor belt  103  toward fringing regions of the applied magnetic fields in the gaps  2 . The belt  103  preferably engages a plurality of rotatable drums such as pulleys, drums, or rollers  132  and  136 . Belts  103  with side walls  104 , excess width  106 , or both may utilize crowned rollers  132  and  136 . Alternatively, sprockets may be applied to holes in the excess belt width  106 . In a further alternative, Vs formed on the inner surface of the belt  103  may interact with features such as grooves cut into the surface of the conveyor rollers  136  and  132 . In a preferred embodiment, assemblies including rollers mounted between plates  160  attached to the block  162  can constrain the conveyor belt  103  side walls  104  to limit lateral miss-tracking. Preferably, the blocks  162  slide laterally using slotted holes in the support brackets  164  to provide horizontal positioning adjustment near or against the side walls  104 . Additionally, the brackets  164  and blocks  162  may slide for vertical adjustment on other support structures such as those for the passive belt rollers  172 . 
   The belt  103  preferably has a composite web construction with cleats  102  applied by adhesive bonding, thermoplastic welding, or other fastening means. The belt  103  can overcome the attractive forces between the media and the magnetic assemblages  98  and/or  114 . To limit the amount of permanent magnet material needed, cleated belts  103  can be furnished with side walls  104  that insure constraint of the magnetic media within the effective field in the gap  2 . The side walls  104  can be slit at intervals to accommodate the radius of curvature for the conveyor rollers  132  and  136 . Such belt technology is commercially available in semi-custom or fully-custom form, for example from Sparks Belting or Midwest Industrial Rubber, so that the cleat  102  and side wall  104  thickness and position can be specified as needed to erase a particular magnetic storage media form factor. To withstand the shocky attractive force applied by a typical hard disk drive&#39;s traveling through the applied magnetic fields, the cleats  102  are preferably about 0.5 inches thick and bonded to the side walls  104 , although other sizes may be used for a given application. To strengthen the belt  103 , wider belt portions  106  can extend beyond the magnetic storage media cavity  100  and the side walls  104 . In some embodiments, the increased gap  2  thickness necessary to allow passage of the belt  103  with cleats  102  and side walls  104  with other protective measures justifies a doubling of the permanent magnet materials to provide sufficient erasure magnetic fields in the gap  2 . 
   Ferromagnetic components concentrated toward the upper surface of magnetic storage media can cause gravity-countering upwards attraction. A preferred protection for the upper assemblages of the magnetic field generators may include a passive upper belt  174  passing over a plurality of freely rotatable pulleys or rollers  172  disposed around at least a portion of the magnetic field generator(s)  98  and/or  114  such that the passive belt  174  moves in a direction substantially similar to the direction of the cleated conveyor belt  103 . Preferably, the passive upper belt  174  bears upon the upper rigid liner  170 . The protective liner(s)  170  typically include a single formed stainless steel sheet  130  extending through the gaps  2  to protect the magnet assemblies  98  and  114 . The liner  170  can be of minimal thickness. 
   Whenever upward media attraction occurs and media friction characteristics such as pointy circuit board components exceed the passive belt  174  to liner  170  stopping friction, the belt cleats  102  act on the media to drag it, in turn dragging upper belt  174  along upper liner  170  until the media clears the attractive region. Idler rollers  172  of the flanged type can guide the upper belt  174  when dragging occurs, due to its minimal motion. The idler rollers  172  may include minimal means for tracking and tensioning adjustment not shown for clarity. The passive belt arrangement requires no synchronization between the passive and driven belts  174  and  103 , and the upper belt  174  can be made quite thin to minimize gap penalty. Further, the passive belt  174  is relatively easy to replace at low cost in the event of wear. 
   Support rails  140  provide for the attachment of various components including, for example, support blocks  162 , axels for the upper belt rollers  172 , and end adjustment means for the upper liner  170 . Blocks between the field generators and attached to the upper surfaces of the rails  140  may support an additional upper liner  170  adjustment means. Laterally bracing members attached between the rails  140  additionally support an adjustment means of the lower liner  130 . Two pairs of legs with various bracing members attached near the ends of the rails  140  and a foot member between each leg pair provides for attachment of leveling pads for overall support. Bracing means between the leg pairs support a control enclosure for an emergency stop controller, protective fuses, motor drive, and similar controlling components. 
   The magnetic field generators  98  and  114  are typically heavy. Bracing members  180  can provide supplemental support at their attachment points near the bottom flanges of the rails  140 . Horizontal bracing members extending between the roller  172  support members to act against upper belt  174  tension can attach to the tops of the magnet assemblies  98  and  114 . Additional lateral horizontal bracing can be added in the direction of the upper roller  172  axes. Because the vertical support members and the magnet assemblies impart strength to such horizontal bracing, eye bolt blocks and eye bolts can be affixed near the top of the degausser  101  to facilitate its lifting by hoist during installation. 
   The degausser  101  may employ a motor operatively coupled to the drive roller  132  by belts, chains, gears, or the like. A preferred embodiment utilizes a motorized pulley, drum, or roller  132  instead of a belt driven by an external drive mechanism. Such an embodiment reduces in number and complexity the guards for any moving parts. Also, motors sealed within the drum  132  can sit in an oil bath to promote heat transfer and lubrication, resulting in nearly maintenance-free operation, needing oil-changes approximately every 40,000 operating hours. Another benefit includes that such motors are commonly available in the form of motorized crowned rollers to promote guidance without flanged sprockets or the like. 
   The pulley  132  diameter and width can be sized as appropriate to eliminate the need for more than one similarly sized idler  136  or pinch rollers to supplement the friction driving the belt  103 . Typically, the pulley length tends to be larger in size than the magnetic storage media form factor, facilitating excess belt width  106  for strength and supplemental tracking means. Lagging  134  placed on the surface of the drive pulley or drum  132  improves surface friction. Lagging  134  on a wide pulley can also provide sufficient thickness to accommodate grooved guidance means such as V profiles on the inside of the belt. 
   A fixed axel block  138  attached to the rail  140  supports the motor pulley axel  142  at each end of motor pulley  132 . A second axel block  144  that is adjustably translatable via rails  146  and a retainer  148  supports each axel end  150  of idler  136 . A bolt  152  acts through a threaded end block  153  on the adjustable block  144  against the tension of the belt  103  on the idler  136 , such that the pair of axel blocks acting on the idler  136  ends  150  provide for tensioning and tracking adjustments. 
   The motorized pulley  132  may be optimized in terms of power supply and operation parameters for a given location or type of operation. For example, a variable frequency drive suits an embodiment that might operate at speeds faster than suited to human loading. Further, variable frequency drives can be made to double voltage so that only a slightly different model can supply ¾ horsepower from a 120 volt North American power supply. Variable frequency drives can address other problems and requirements caused by the varying loads of media attraction, such as more or less active feedback speed regulation against load variations and electronic motor overload protection against low duty moderate overload. 
   Referring to  FIG. 9  and  FIG. 10 , the belt  103  and cleats  102  may cause pinch points and the potential for media jams at the magnetic assemblages  98  and/or  114 . Typically, a control mechanism  194  is operatively connected to the motor  199  of the motor pulley  132 . At least one sensor  190  and/or  192  is disposed in a spaced relationship with the belt  103  and operatively connected to the control mechanism  194  having various programmable control terminals such that when an object such as a disk drive, human extremity, or other object contacts the cleated conveyor belt  103  and a portion of the apparatus  101  operatively connected to the sensor(s)  190  and/or  192 , the control mechanism  194  stops operation of the motor  199 . Alternatively, at least one sensor  190  and/or  192  is disposed in a spaced relationship with the cleated conveyor belt  103  and operatively connected to the control mechanism  194  such that when a foreign object contacts the cleated conveyor belt  103  and another portion of the apparatus  110  operatively connected to the sensor(s)  190  and/or  192 , the control mechanism  194  stops operation of the motor  199  and enables reverse operation of the motor  199 . 
   Preferably, a first sensor  190  with rest position in contact with a first sensor cam  196  detects minute motions of a hinged door  195  caused by, for example, human extremities and miss-feeds of media  42 , and a second sensor  192  and a corresponding second sensor cam  197  placed downstream from the first sensor  190  triggers an emergency stop system. The first sensor  190  can disable the continuous forward motion of the conveyor  103  but allow reverse motion, for example through a rotary selector switch  198  to allow momentary reverse motion of the conveyor  103 . Such normal forward and jam-clearing momentary reverse actions can be at different speeds, for example at a productive preset forward speed and a safer slow reverse speed. Those skilled in the art of applying variable frequency drives will know that such functionality is inherently programmable in most standard models without recourse to external logic beyond the sensor connection and drive connection to a few switch contacts and accessories. 
   The second sensor  192  can be safety-rated to interact with similarly rated industrial controls that remove all power from the variable frequency drive. An advantage of the motor pulley  132  described above is that its gearing friction causes a nearly instantaneous cessation of motion in the event of a “power loss trip” of a typical variable frequency drive without resorting to complicated breaking mechanisms. Although activation of the second sensor  192  prevents motor functionality for jam clearance, the motor gearing friction is not so high as to prevent manual rotation. Thus, a jam or a near jam, where miss fed media lodges at the mouth of the magnet structure or merely approaches it and trips the second sensor, can be cleared without extraordinary measures. 
   Such embodiments for conveyorized erasure of magnetic storage media are preferably employed in production environments, such as for operations of large recycling services, that guarantee to protect information on a variety of magnetic storage media contained within the waste stream obtained from many clients. One advantage is the low energy demand of the motor and the lack of energy consumption and heat generated by using permanent magnets instead of electromagnets common to the art of conveyorized magnetic storage media erasure. 
   In other environments, fast operation is also desired, but media quantities tend to be smaller, and while motorized power operation is often desirable, assured operation in the face of power loss is typically more preferred. Such environments typically teach a preference for lower overall weight at the expense of the highest attainable magnetic strength, and discount extreme long life versus operational readiness. The approximate doubling of magnetic strength and the addition of a vertically directed magnetic field, as is available in certain embodiments, can address these concerns while simplifying media transport and separating components that are more sensitive to debris created by high volume media erasure. 
   An alternative embodiment of the invention with a “bottomless/topless” conveyance and less protection for the magnetic assemblages, thereby enabling a smaller gap  2 , can serve to erase individual hard disk drives near a point of use as they fail. Such an embodiment can also be if several hard disk drives need to be erased in a hurry, for example in the case of emergency destruction of information. Drive means for such a conveyance may be manual such that no power is needed, electrically operated by battery powered motor, or both with suitable transmission means to switch between the drive means. 
   A further alternative includes the use of a chain drive where the spacing between the links and rollers accommodates a particular magnetic storage media form factor or form factors up to some particular maximum. Links of such an oversized chain drive could be attached by plates, for example fixed plates forming media cavity bottoms and hinged plates forming media cavity tops. Both plates can be hinged to facilitate gravity assisted top loading and bottom ejection. Sprockets needed to drive such an outsized chain tend to be large. 
   The chain size can be greatly reduced by attaching it to magnetic storage media-sized bins or a similar structure. Preferentially, one chain per bin side avoids an undue increase in the field generating gap dimension. Those skilled in the art will realize that the pitch of such parallel chain drive demands precision, and shocky loads can cause problematic inelastic stretch. 
   Reciprocating Conveyor 
   With reference to  FIG. 11 , an alternative embodiment of a degausser  207  includes providing a reciprocal media conveyor generally designated with reference numeral  209  including a magnetic storage media bin  212  movable along the magnetic media conveyance path wherein the magnetic storage media bin  212  passes substantially through the at least one magnetic field generator  214  and/or  216  when traveling in any direction along the media conveyance path and is accessible at both ends of the magnetic media conveyance path. 
   In contrast to a typical two-pass process that rotates media outside of and between two passes through a horizontal magnetic field that requires operational floor space and a relatively long transport stroke, a reciprocal conveyor degausser  209  with unload and load stations at each end of a single pass process uses relatively little operational space while eliminating the long process stroke. Further, providing media unload and load stations at each end of a reciprocating transport can approximately double the media throughput over a two-way reciprocation of media while eliminating the need to reorient the media in between passes. Yet another advantage of the embodiment includes having quadrilateral symmetry such that many parts may be common to the various portions of the degausser  207 . The detailed description that follows with the accompanying figures often references or shows parts on one side or end while omitting them from the opposite side for clarity. 
   Although reciprocation of a media transport  212  between unload and load stations at each end of an erase process can be accomplished by a variety of means well known to the art including lead screws, drive chains, toothed belts, rack and pinion gearing and the like, such means have certain disadvantages. Magnetic field generators  214  and/or  216  that counter fringing through selective placement and orientation of permanent magnet material at the sides of the gap typically would preclude placing most such drive means at the sides of the conveyor  209 . Such embodiments can also incur the expense and slowness of lead screws or poor performance against shocky loads. Rack and pinion gearing presents a particular problem in that a rack attached to a media transport incurs considerable length. 
   A preferred embodiment of the single pass two station magnetic storage media degausser  207  provides at least one flexible drive element  204 , such as a rope or cable, with its ends attached to the media storage bin  212 , wherein the cable  204  wraps a plurality of times around a cable drum  206 . The flexible drive element or cable  204  is preferably a wire rope. An advantage of using a wire rope is that its dragging action in tension effectively counters the media attraction created in the magnetic field. Preferably, the wire rope  204  includes a number of fine strands of multiple wires each to increase its strength and resistance to shocky loads and to reduce the diameter of the drive components. Also, stainless steel or other generally nonmagnetic wire rope is preferred over typical wire rope. Coatings are also available such as vinyl to improve the friction between the wire rope  204  and the cable drum  206 . 
   Typically, the conveyor  209  includes grooved rollers acting on rods, V-guides, dove tail, re-circulating ball linear bearings, or the like. Preferably, the conveyor  209  also includes least top and bottom liners to protect the permanent magnet elements. Additional liners at the gap&#39;s sides can provide additional transport guidance. Features such as corner fillets and low friction sides or corners can enhance simple guidance along the fixed liners. 
   With reference to the drawings, and in particular  FIG. 11 , a preferred embodiment includes a reward frame channel  200  attached to formed end frame plates  202  that support various components such as lower liner support channels  203 . A corresponding forward frame member has been omitted to reveal a lower drive cable portion  204  bearing on the cable drum  206 . End frame plates  202  can support channels  205  reinforcing against the impact at the end of travel. An upper drive cable portion  208  at the near end  210  can pull the media transport shuttle  212  through the gaps of a horizontal magnetic field generator  214  and a vertical magnetic field generator  216 . 
   The cable drum  206  can be driven through a plurality of gears by various means such as a hand crank  220  or a gear motor  222  acting on a cable drum spur gear  224 . The cable drum  206  also may be operatively attached to a clutch  225  that operatively engages the cable drum  206  to the hand crank  220  and the motor  222 . The clutch device  225  includes an actuation knob  226  attached to a sliding plate  228 , sliding plate tracks  230 , a variable length link arm  232 , a pivoting clutch arm  234 , and a bearing block  236 . The clutch  225  is shown in the position for hand crank  220  operation. A hole in the sliding plate  228  allows the engagement of the crank  220  to a driving means, and a link arm  232  acting on the clutch arm  234  disengages the motor  222 . As an interlock against the gear motor&#39;s  222  hazardously driving the hand crank  220 , the hand crank  220  typically must be removed before the clutch  225  engages the gear motor  222  to the cable drum  206 . 
   Pulley assemblies at either end of the degausser  207 , including angled mounting brackets  240 , reverse the direction of the drive cable  208 . A nut (not shown) acting on the end of an adjusting bolt  242  against a bearing plate  244  tensions the drive cable  208  to help ensure adequate friction against the cable drum  206 . This adjustment mechanism allows for adjustment in the case of cable stretch and slippage and can be placed at either or both ends  210  or  213  of the degausser  207 . 
   Bumpers  250  are attached to support blocks  252 . A limit switch roller  254  is located to activate a switch  256  just before contact between the shuttle  212  and the bumpers  250 . The bumpers  250  define the limits of shuttle  212  travel between the unload and load stations at the ends  210  and  213 . Rollers  258  attached to the shuttle  212  engage rails  260  to limit the suttle&#39;s  212  lateral position within the fields generated by the magnetic field generators  214  and  216 . 
   The limit switches in combination with momentary push button switches can selectively disable the motor  222 . The normally open contact of an activated limit switch can illuminate a momentary push button. The push button can be configured to allow forward or reverse motion depending on the motor polarity determined by the active relay of an H-bridge connected SPDT relay pair. Push button switch illumination can signal which switch to activate to drive the shuttle in direction away from the activated limit switch. A third non-illuminated push button switch can be added to enable the directional push buttons, providing for two handed control. Thus, battery power and simple ladder logic can provide for the effective control of a motorized embodiment. 
   In practice, a suitable gear motor includes the Groschopp PM10818-RA4000M. Suitable automotive relays include the Omron G8JN-1C7T-MF-R-DC12. Alternatively, the control circuit can be adapted to electronic controllers including packaged controllers such as Winland WMC140-0120270-L0W, providing benefits such as soft start to reduce cable stretching shock. Other suitable motors and controls may be applied. 
   Bed liner sides  270  where media may be dropped into load stations can be formed of a relatively heavy gauge stainless steel for rigidity. The liner sides  270  can then support a vertical adjustment mechanism for a lower thin stainless steel liner above it. A formed end  272  of a thin stainless steel upper liner  274  can attach at various locations such as to a brace bar  276 , which can be provided with a vertical adjustment means (not shown). The liner  274  may also be attached loosely so it can float atop the shuttle  212 . Therefore, the shuttle  212  can transport magnetic storage media while liners protect the permanent magnet assemblages of the magnetic field generators  214  and  216  against attractive contact with the to-be-erased media. 
   The shuttle  212  can also act through the upper end of a pivot arm  262  on the vane  264  of a non-resetable mechanical counter  266  to provide a count of total operational cycles divided by two. A knob  282  can disengage a bidirectional latching one-way clutch mechanism operatively engaged to the cable drum  206  in the event of a jamming malfunction. 
   With reference to  FIG. 13 , a ferromagnetic plate  30  returns flux between the upper and lower portions of the vertical field generator  216 . Two nonmagnetic gussets  32  and the z-formed members  116  of the horizontal generator  214  and various permanent magnet containment members such as the z-formed members  110  of the vertical field generator  216  help support the magnetic field assemblages. Such support members can vary in form to fit the various possible forms of the magnetic assemblages. Such members can vary in material and function so as to provide magnetic flux return and compressive strength for the vertical generator or nonmagnetic members to provide tensile strength for the horizontal generator. Machined blocks  290  and  291  support and attach the magnet assemblies  214  and  216  to the frame channels  200 . 
   A pinion gear  300  on the output shaft of the gear motor  222  turns a spur gear  302  about the drive shaft  304 . Worm gearing in the motor  222  can provide a gear reduction, allowing the pinion gear  300  and the spur gear  302  to be of a similar size. The spur gear  302  is free to rotate about the drive shaft  304  on independent bearings. Four holes provided in the spur gear  302  mate with the clutch mechanism mounted on the pivoting arm  234 . A pivot block  306  for the pivoting arm  234  can contain ball plungers projecting from its upper or lower surfaces and onto the plate surfaces of the pivoting arm  234  bearing holes that provide detent for clutch positions as described in more detail below. A formed sheet metal member  308  attached to the channels  200  supports gearmotor  222  and additional members that rigidly attach to the pivot block  306  and bearing blocks for the drive shaft  304 . A heavy gauge lower bed liner  270  is located below the lower liner  310  and between the support and track  260 , and the mounting blocks  312  attach to the lower liner support channels  203 . Braces  276  support the ends of the upper liner  274 . Central support and optional adjustment means for the upper liner are not shown in  FIG. 13  for clarity. 
   The form and various functions of the angled mounting bracket  240  include supporting the limit switches  256  and the cable pulleys, including the pulleys  314  that form part of the position adjusting mechanism for tensioning the cable  204 . A formed mounting bracket  323  attached to one of the mounting brackets  240  and the end panels  202  at end  213  supports a counter  266 . A projecting rod  320  attached to the lower end of the pivot link  262  registers the shuttle  212  position with the counter vane  264 . A spring  322  extended between the frame and an extension  324  attached to the link  262  returns the link  262  to its rest position. 
   With reference to  FIG. 14 , the drive cable includes a long upper span  208  from the non-adjustable pulley  330  at end  210  to the shuttle  212  and the lower portion span  204  runs between the adjustable pulleys  314  and the cable drum  206 . The lower portions  204  typically skew toward the forward and reward ends of the drum  206 . Much of the shuttle  212  appears as a cavity with a side wall including a finger niche  340  provided for media removal, inner walls  342  acting on the media, and end walls  344  for cable attachment. Horizontal plate members  345  join the shuttle wall members. 
   The flat head bolt  242  and its associated nut act against the plates  202  and  244  on the machined block  350  that is secured between pulley housing panels  351 . The resulting pull on the adjustable pulleys  314  tensions the cable on the cable drum  206 . 
   The pinion gear  332  that is rigidly affixed to the drive shaft  304  turns a spur gear  224  attached behind the drum  206 . The pinion gear  332  operatively engages the mechanism, such as the turning crank  220 , to move the shuttle  212 . The cable drum  206  is typically hollow with an outer shell  352 , an inner axel  354 , and a bearing  356 . The drum&#39;s  206  diameter and the ratio of the pinion gear  332  to the spur gear  302  help determine the force needed to turn the crank  220 . Alternatively, gearing in a motor  222  can be selected to suit motor power and provide the desired speed. 
   A preferred bidirectional latching one-way clutch mechanism operatively engaged to the cable drum  206  will be described with reference to  FIG. 15 . The drive shaft  304  enters the housing  400  and is pressed into a rotatable disk such as a jam disk  402 . The rotatable disk  402  is operatively connected to a moving element such as the cable drum  206  or shuttle  212  through the drive shaft  304  as described above. The jam disk  402  is rotatably disposed in a housing  400  with an inner housing member  404  having at least a first tapered cavity  407  and a second tapered cavity  409  disposed about the rotatable jam disk  402 . A first ball  410 , a first spring  414 , and a first pin  406  are disposed within the first tapered cavity  407 , and a second ball  412 , a second spring  416 , and a second pin  408  are disposed within the second tapered cavity  409 . The pins  406  and  408  connect to an actuator arm  418  that is operatively connected to at least two limit detectors. The actuator arm  418  pivots about the axis of the drive shaft  304  and determines the positions of the pins  406  and  408 . 
   In a first position illustrated in  FIG. 15 , the first pin  406  is disposed toward a narrow end of the first tapered cavity  407  and away from the first ball  410  and the first spring  414 , thereby allowing the first ball  410  to rotate with minimal friction and allowing free rotation of the jam disk  402  and shaft  304  in a clockwise first direction indicated by the arrow  420 . A reversal of torque against the direction of the arrow  420  will urge the first ball  410  toward the narrow end of the first tapered cavity  407  between the jam disk  402  and the housing member  404 . The resulting friction of the jammed balls will effectively brake the jam disk  402 , preventing counterclockwise rotation in the second direction. 
   At the same time, the second pin  408  is disposed away from a narrow end of the second tapered cavity  409  and toward the second ball  412  and the second spring  416  thereby pressing against the second ball  412  and compressing the second spring  416  into the wider portion of its cavity  409  to disable the locking action of that ball  412 . Disposing the actuator arm  218  and pins  406  and  408  in a second position opposite to the first position allows rotation in the second direction opposite of the arrow  420 . 
   Engaging forks  430  are spring-loaded in grooved members  432 , such as detent blocks, and act on pivoting pins  434  pressed into the lateral ends of the actuator arm  418  to keep the pins  406  and  408  in either the first or second positions. In this preferred way, the grooved members  432  are operatively coupled to the actuator arm  418 . Ball plungers  436  are disposed to slidably engage the grooved members  432  such that when the ball plungers  436  engage a groove of the grooved members  432 , the actuator arm  418  is positioned in a neutral state wherein the first pin  406  and the second pin  408  are positioned in the first tapered cavity  407  and the second tapered cavity  409  to prohibit movement for the first ball  410  and the second ball  412  into the narrow end of the first tapered cavity  407  and the second tapered cavity  409 , thereby disabling their jamming functions. 
   Self centering jaws  440  can act on an actuator arm pin  442  affixed to an upward end of the actuator arm  418  to force the detent blocks  432  into that neutral state. The jaws  440  self-center through the downward direction of the slotted plate&#39;s  443  acting on the lateral pivots of the jaws  440  to rotate them about their mutual fixed pivot. Guidance of the plate  442  and linkage to the rod  444  allow a user to disable the ball jamming action, for example, in the event of a media jam where reverse motion of the shuttle  212  is needed to clear the jam. A spring can return the disengagement device to its neutral position between functions. Lateral pivots of the jaws  440  can be made loose enough to accommodate the rigidity of the plate  443 . 
   Rods  450  thread into engagement blocks  452 , which act on engagement plates  454  that are rigidly attached to the detent blocks  432 . Slots at a point of engagement between the detent blocks  452  and engagement plates  454  allow independent pushing action by either of the rods  450 . In this preferred way, the rods  450  have a first end operatively engaged to the actuator arm  418 . 
   With reference to  FIGS. 16 and 17 , a moving element such as the media transport shuttle  212  operatively engages linkages to push or pull the rods  450  into and out of the housing  400  with springs provided to help pull the rods  450  out of the housing  400 . Such linkages can be predetermined to push the rod  450  to rotate the respective engagement plate  454  and pivoting block  432  to the neutral position. From the neutral state of the actuator arm  418 , a greater push on the rod  450  enables the jamming action to suppress reversal except in the event of disengagement through the disengaging knob  282 . The shuttle  212  includes side walls  560 , and a mirror-imaged double sided cam  562  can be attached to each of the shuttle&#39;s  212  outer walls  344 . 
   Preferably, a cam plate or disk  526  operatively engages a second end of the rod  450  and a lever arm or link  540  operatively engaged to the cam plate  526  such that, when the moving element or shuttle  212  operatively engages the lever arm  540 , the cam plate  526  rotates thereby moving the rod  450  and the actuator arm  218 . More particularly, as the shuttle  212  approaches a bumper  250 , an inner face of the cam  562  can force a pin  564  pressed through the link  540  a short distance in direction  570 , effecting relatively little counterclockwise rotation of the block  568  rigidly attached to turn about a point  528  fixed to the frame, allowing the shuttle  212  to be driven in an opposite direction. As the shuttle departs, the outer face of the cam  562  forces the pin  564  in a direction  572  opposite to and greater than the motion of the link  540  in direction  570 , such that the shuttle  212  must travel to the opposite end where like events occur to disengage and reverse the process. The motion of the link  540  can be further controlled by its pin  564  riding along a linear slot fixed with respect to the frame, with its pin  566  pivotally attached to a block  568 . 
   An end cap  520  attaches to the outer end of the rod  450 , with two links  522  and  524  pivotally attached to the end cap  520 . Rotation of the disk  526  in different directions about a pivot  528  fixed to the frame of the degausser  207  operatively engages the links  522  and  524 . For example, relatively little counterclockwise rotation of the partial disk  526  can push a link  522  a relatively small distance, while pushing the rod  450  into the housing  400  just enough to disengage the bi-directional clutching mechanism. Conversely, relatively greater clockwise rotation of the partial disk  562  can push a link  524  a relatively greater distance, in turn pushing the rod  450  into the housing  400  enough to set the bi-directional clutching mechanism into a locking state opposite that prior to being disengaged by the push of the link  522 . 
   The different length slots in the partial disk  526  preferably engage a pin such that the links  524  or  522  ride along the slots while the disk pushes the opposite link  533  or  524 . Rotation of the partial disk  526  can realized through the link arm  540  operating on a member rigidly coupled to the disk  526 . 
   The bidirectional latching one-way clutch mechanism ensures the passage of media through all the applied magnetic fields, thereby avoiding a reversal of direction after partial exposure. The bidirectional latching one-way clutch mechanism can be applied in any situation deserving complete reciprocating motion between some linear limits, or by modification of linkages, allowing rotary motion to reverse between some angular limits without allowing reverse rotary motion until the achievement of such limits. 
   A further advantage of the bidirectional latching one-way clutch mechanism includes the provision of timing for the shuttle  212 . In other words, the one-way clutch mechanism is disengaged well after the shuttle  212  carries media through the magnetic fields. In some alternatives, the clutch mechanism may disengage just before a limit switch deactivates a motor driving the shuttle in that direction and before the shuttle actually hits the bumper, and clutch engagement against travel in the bumper direction will typically take place at greater distance from the bumper after the shuttle position has deactivated the limit switch. 
   With reference to  FIG. 16 , a clutch mechanism for selectively driving the shaft  304  with the crank  220  or the motor  222  is shown. The crank  220  is removeably attached to the shaft  304  through the hole in the sliding plate  228  to constrain the plate in position. Crank removal allows the plate  228  to slide by action on the knob  226 , thereby placing the variable length link  232  under compression and encouraging the clutch arm  234  to rotate about the clutch pivot  500 . 
   A clutch block  502  is rotatably mounted on a yoke  504  that is allowed to rotate slightly between the upper and lower plate members of the clutch arm  234 . The yoke  504  has a hexagonal inner profile slidable along a mating hexagonal profile portion on the shaft  304 . When sliding plate  228  shortens the distance between the end pivots of variable length link  232 , the clutch plate  234  rotates bringing the yoke  504  and the clutch block  502  closer to the spur gear  302 . If the dowel pins such as pin  506  attached to the clutch block  502  hit the spur gear  302 , the variable length link  232  can compress. Motor  322  output rotation brings the dowel pins into alignment with holes in the spur gear to establish a solid path through the gears, pin, and clutch block to the hexagonal profile portion of shaft  304 , thus driving the cable drum  206  and the shuttle  212 . 
   The clutch arm  234  can be comprised of two plates situated above and below the variable length link  232 , and the yoke  504  is allowed to rotate between them. Detents in the clutch arm  234  interact with bullet catches in the pivot block  306  to define the engaged and disengaged positions of the clutch. Such a clutch mechanism may be applied to the conveyor belt embodiment to allow for powerless, manual operation of the conveyor. 
   While this disclosure specifies orientations with respect to conveyance with the shortest media dimension or thickness axis vertical and media motion or direction of conveyance along the longest media direction, aspects of the invention can be applied to media oriented with the thickness axis horizontal with nearly equal practicality. Reorientation of the transport direction from horizontal to vertical and consequent reorientation of the media&#39;s longest dimension likewise of near equivalence to the preferred embodiments. Conveyance in the direction of the intermediate media dimension incurs a penalty in permanent material of less than direct proportion to the length/width aspect, given equal quantity of end material to counter fringing effects on magnetic strength. 
   Those skilled in the art will recognize that a wide variety of modifications, alterations, and combinations can be made with respect to the above described embodiments without departing from the spirit and scope of the invention. For example, many of the support structures for the conveyance means or magnetic field generators may be modified. Such modifications, alterations, and combinations are to be viewed as being within the ambit of the inventive concept.