in the showing of fig1 the various elements of the actuator assembly 10 are illustrated in an exploded view . the flange 11 and split cylinder inner core piece 12 are formed as a single part . the ways 13 , 14 are attached to the respective sides of the inner core piece and the integrally cast projections 17 , 18 by bolts 16 which have the heads recessed . the way 13 engages the core piece 12 and extension 17 at grooved raised mounting pads 19 , 20 ( fig2 ) that are cast as an integral part of the inner core , with holes 21 , 22 bored therethrough to receive the mounting bolts 16 . the openings 24 in flange 11 receive the bolts which mount the actuator assembly to a disk drive base casting 27 ( fig4 ). the carriage assembly 30 includes a body 31 to which a series of three arms 32 are secured . the upper and lower arms carry a single suspension 34 on which is mounted a slider 35 that carries the magnetic transducer . the intermediate arm extends between a pair of disks and carries two suspensions to enable heads to be flown adjacent each of the confronting data surfaces . the carriage body 30 has projections 35 which extend to and support the bobbin 36 about which the voice coil 37 is wound . the voice coil windings 37 are terminated at the terminal pad 38 adjoining the base of the upper projection 35 and connected to a flexible conductor 39 . the flexible conductor arcuate portion 40 connects to the voice coil wire terminations at terminal pad 38 and is secured to the bracket 42 carried by the core assembly 12 and at the other end terminates at plug 41 . the plug 41 receives the connector ( not shown ) through which the electrical signals or pulses are transmitted to the voice coil windings 37 . the carriage 30 is mounted for reciprocating motion in the actuator by three pairs of rollers which engage the ways 13 , 14 . two longitudinally spaced pairs ( not visible ) of fixed canted rollers engage the cylindrical way 13 and one pair of canted rollers 44 , 45 engage the way 14 . the roller pair 44 , 45 is mounted on the carriage 30 longitudinally intermediate the roller pairs engaging way 13 . roller 44 is fixed while roller 45 is pivotable and biased toward roller 44 so that any play or slack in the mounting system between the ways 13 , 14 and carriage 30 is removed . a crash stop 47 is mounted to the core assembly 12 by bolts which pass through openings 50 and are received in the threaded bores 51 formed in bosses 52 . the crash stop is formed of resilient sheet material and presents a pair of projections 48 . the carriage body 31 includes a pair of projections 54 ( and a similar pair on the bottom which are not visible ). in the assembled condition of the actuator , the crash stop projection 48 extends between the carriage projections 54 to limit carriage travel in each direction of reciprocation . the cylindrical sleeve 57 with end flanges 58 is formed of non - magnetic conductive material that functions as a shorted turn in the assembled condition . the collar 60 has a cylindrical projection 61 with a groove 62 formed therein for receiving an o - ring seal 63 ( fig2 ). the segments 65 form the radially magnetized permanent magnet and backing plate assembly . each segment 65 encloses 120 degrees about the inner core assembly such that three segments wholly surround the assembly 12 , voice coil 37 and air gap 74 of the voice coil motor . the magnet material is formed as a segment 66 as shown in fig3 and the backing plate portion of the segment 65 is formed of two identical parts 68 turned end for end , that the champfered surfaces 69 axially adjoin . using an appropriate fixture , the outer core pieces 68 are retained in axially spaced relation and the outer convex surface 70 of the magnet element 66 is bonded to the inner concave surfaces 71 of the backing plate elements 68 . it will be noted from fig2 that the magnet 66 is secured to the backing plate core elements 68 nearer one end 72 which is assembled in abutting relation to flange 11 . after the backing plate - magnet segment assembly 65 is formed , the magnet 66 is energized to become a radially polarized permanent magnet . the chamfered surfaces 69 and the axial spacing between the elements 68 of outer core segments 65 are provided to establish a reluctance gap that serves to more equally divide the flux that flows through each axial end of the core assembly to create a uniform magnetic field along the working length of the air gap 74 within which the voice coil 37 reciprocates . fig2 shows an axial section view of the inner core 12 , outer core segments 65 , collar 60 and shorted turn 57 . the inner core has a shoulder 75 extending from mounting flange 11 with a groove 76 . the shorted turn 57 is slipped over the cylindrical surface of shoulder 75 and a sealed relationship is assured by an o - ring 77 in groove 76 . the collar cylindrical inner surface 78 of projection or flange 61 is received over the distal outer axial surfaces of the halves of inner core 12 and the outer surface of flange 61 is received within the end of shorted turn 57 . a seal is assured by the o - ring 63 disposed in groove 62 . the backing plate - magnet segments 65 are assembled by placing the shorter projecting core segment end 72 on the flange 11 shoulder surface 79 and the opposite outer core segment inner end surface confronting the collar outer peripheral surface 80 . the outer core segments 65 can be adjusted axially to an abutting relation between segment axial end surface and the confronting surface of flange 11 and rotated where necessary with relative ease . however , the strong magnet makes removal from the flange surface 79 and collar surface 80 very difficult . the magnetic attraction is of such strength that no further means of securement is required to firmly attach the segments 65 to the other elements of the magnet and core assembly . it will also be observed that the outer core segments 65 are assembled with very little clearance between the shorted turn 57 and the permanent magnets 66 . the close tolerance which enables all the clearance between elements of the magnetic flux path to be contained in the working air gap 74 in which the voice coil 37 is positioned in the assembled condition . this close tolerance is enabled by the segment form of the magnet and outer core and would not be attainable if it were necessary to insert a strong cylindrical magnet or segments thereof axially within a cylindrical outer core . fig4 shows a disk file with a base casting or frame 27 and cover 82 forming a disk enclosure . the cover 82 and frame 27 are secured together by a series of clips 83 . a pair of actuators 10 are mounted on the base member 27 with carriage portions projecting through apertures to enable the transducer heads 35 to confront the data surfaces 84 of disks 85 mounted therein . as seen in fig5 the disk enclosure contains four magnetic disks . the actuator 10 at the left in fig4 and 5 accesses the data surfaces of the lower two disks , while the actuator at the right in these two figures accesses the data surfaces of the upper two disks . in addition to the elements shown in fig1 and 2 , the actuator 10 further includes a voice coil motor housing cap 86 and a voice coil motor housing cap seal 87 . the gasket 89 includes an aperture through which the plug 41 extends and is sealed against the cap 86 . the cap 86 is secured to collar 60 by a series of four end cap mounting screws 90 and in cooperation with the inner core assembly 12 , shorted turn 57 and collar 60 seal the actuator . as seen in fig4 cover 82 , frame 27 and actuators 10 ( with the magnetic segments 65 removed or not yet assembled ) form a substantially sealed head - disk enclosure that is isolated from the surrounding atmosphere except for the cover openings 91 that communicate with the enclosed volume through a breather filter . the breather filter is provided to accommodate changes in pressure and expansion and contraction of the air within the head - disk enclosure caused by temperature changes . this structure enables the disk drive to be assembled in a clean room environment to the phase of assembly wherein the head - disk assembly is completely enclosed and sealed such that any air entering must pass though the openings 91 and the associated breather filter without the magnet assembly segments 65 being present . after the head - disk assembly has been assembled to enclose the disk and transducer carriage assemblies , the unit can be removed from the clean room and the permanent magnet assemblies attached to complete the actuator fabrication . this sequence of assembly and device structure permits the exclusion of the permanent magnets from the clean room assembly area . the permanent magnets are also outside and isolated from the head disk enclosure space . it is thus not possible for particles released from the magnets to be in the disk enclosure . such particles would not only be likely to cause head crashes as the height of the particle exceeds the fly height of the head above the disk surface , but also precludes the possibility that such particles would erase or alter the data on the disk surfaces causing non - recoverable hard errors . particles emanating from the permanent magnets cause catastrophic results , since these are very strong permanent magnet materials in contrast to the magnetic domains on the disk which are small weaker magnets that permit relatively easy alteration . while the invention has been particularly shown and described with reference to a preferred embodiment , it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention .