GB2099205A - Swinging-arm arrangement for a magnetic-disc storage apparatus - Google Patents

Abstract

A swinging-arm arrangement for moving and positioning a magnetic head or heads (9-12) over a rotating magnetic disc or a plurality of coaxially spaced discs (3, 4) in a magnetic-disc storage apparatus comprises a swinging arm (14) or assembly of swinging arms carrying the magnetic head or heads and mounted on a rotatable bearing pin (54) which is journalled in two ball-bearings (48, 49). A spring cup 62 inhibits axial play between the outer races 52 and 53. The ball-bearings are disposed on opposite sides of the swinging arm or assembly of swinging arms, resulting in a symmetrical and stable bearing arrangement which reduces deformation of the bearing pin. <IMAGE>

Description

SPECIFICATION Swinging-arm arrangement for a magnetic-disc storage apparatus The invention relates to a swinging-arm arrangment for moving and positioning a magnetic head or heads over a surface or the surfaces of a rotating rigid magnetic disc or a plurality of coaxially spaced rotating rigid magnetic discs in a magnetic-disc storage apparatus for the storage and reproduction of data in digital form, which arrangement comprises a frame, a swinging arm which is pivotable on the frame and which carries a magnetic head or a plurality of magnetic heads, or an assembly of swinging arms which ar pivotable about a common axis on the frame and which are spaced from one another along said axis and each of which carries a magnetic head or a plurality of magnetic heads, the swinging arm or assembly of swinging arms being pivotably supported on the frame by a bearing arrangement which comprises first and second ball-bearings, each having an inner race and an outer race, a rotatable bearing pin, means for securing theinner races of the ball-bearings in axially spaced relationship on the bearing pin, means for securing the swinging arm or assembly of swinging arms on the bearing pin, bearing-housing means which are adapted to engage the outer races of the ball-bearings and which are secured to the frame, and spring means for axially loading said outer races to inhibit axial play in the bearings.

Magnetic-disc storage equipment comprising one or a plurality of rigid magnetic discs, which each serve for the storage and reproduction of data in digital form on at least one side and generally on both sides, is employed on a large scale as peripheral equipment for data-processing apparatus such as computers.

The magnetic discs rotate at a high speed, for example 3,600 revolutions per minute, and the magnetic heads, which are secured to the ends of carrying-arm arrangements, are moved over the surfaces of the magnetic discs to locations which are preselected by the data-processing apparatus in order to store data in a specific track on a magnetic disc or in order to reproduce data from said track.

The carrying-arm arrangements comprise carrying arms which are either linearly movable in a radial direction or which are pivotable about an axis parallel to the axis of rotation of the magnetic discs. The invention relates to a carrying-arm arrangement of the latter type, herein referred to as a "swinging-arm arrangement". The magnetic heads should be moved as rapidly as possible over the magnetic discs in order to minimize the time required for locating the correct track and storing or reproducing the data. The magnetic discs are manufactured with an extremely high degree of flatness, the magnetic heads floating on a very thin film of air very close to the surface of the magnetic discs.

In many cases each magnetic head is secured to a resilient magnetic-head carrier in order to obtain the mobility which permits the magnetic head to float on the air film and in order to provide a resilient load which exerts an accurately defined pressure on the head. In order to minimize the external dimensions of magnetic-disc storage equipment, various manufacturers attempt to arrange the magnetic disc as close as possible to each other. The carrying-arm arrangements, especially when they are interposed between two magnetic discs, should therefore be as flat as possible and move as close as possible to the magnetic-disc surface. It is obvious that the carrying arm should never touch the magnetic disc surface, because this would render the magnetic disc unserviceable.

Carrying-arm arrangements for magnetic-disc storage equipment should therefore be manufactured with the necessary precision.

A swinging-arm arrangement of the construction described in the opening paragraph is known from United States Patent Specification 4,150,407. This known swinging-arm arrangement comprises one or more aluminium swinging arms in the form of bifurcated levers which are pivotable about an axis disposed between two ends. Arranged near one end of each swinging arm is a magnetic-head unit comprising a magnetic head and a resilient magnetic-head carrier, which carrier is rigidly connected to the swinging arm. On the other end of the swinging arm a control coil is arranged. This coil forms part of electromagnetically and electrically controllable actuating means by means of which it is possible to impart a pivotal movement to the swinging arm about its pivotal axis, so as to enable the magnetic head to be moved over the magnetic disc in a substantially radial direction.

In a version comprising a plurality of swinging arms the arms are arranged above and spaced from one another and they are combined to form an assembly of swinging arms which is pivotable about the pivotal axis. Each swinging arm carries a separate flat control coil, so that these coils are also combined to form an assembly of control coils which are spaced from each other. The assembly of swinging arms isjournalled on a frame by means of a bearing arrangement which comprises two ball-bearings and a bearing pin on which the inner races of the ball bearing are fixed by suitable means, axially spaced from each other. The outer races of the ball-bearings are fitted in a bearing housing which is screwed to the deck of the magnetic-disc storage apparatus, a compression spring between the outer races inhibiting axial play in the bearings.The control coils move in a permanent-magnetic stator. This stator comprises a stator frame and a plurality of flat, axially magnetized permanent stator magnets, which are connected to the stator frame and between which air gaps are formed for each flat control coil, so that each control coil is movable in an axial permanent magnetic field within an air gap between permanent stator magnets. On each side of the stator, iron stator end-plates are arranged which extend over the interposed control coils and stator magnets and on which the outermost stator magnets are mounted, so that the axial permanent magnetic stator fields are closed by the iron stator end-plates. Each control coil comprises turns of an electrically conductive material wound onto a plastics coil former, which is formed into a single structural unit with the control coil by means of a synthetic resin.The control coils are connected to the swinging arms by clamping and glueing, on the ends of the swinging arms which are remote from the magentic heads.

The magnetic heads are moved over the magnetic discs at a high speed in a substantially diametrical direction in orderto minimize the time required for locating a track and storing or reproducing the data.

Thus, the swinging arm or assembly of swinging arms is moved with jerky movements, which give rise to relatively large inertial forces. The bearing arrangement should therefore be robust and stable.

The invention provides a swinging-arm arrangement of the construction described in the opening paragraph, which is robust and stable and which comprises a small number of parts, so that it is also suitable for magnetic-disc stores of small dimensions intended for large-scale use, for example as peripheral equipment for personal computers.

According to the invention there is provided a swinging-arm arrangement for moving and positioning a magnetic head or heads over a surface orthe surfaces of a rotating rigid magnetic disc or a plurality of coaxially spaced rotating rigid magnetic discs in a magnetic-disc storage apparatus for the storage and reproduction of data in digital form, which arrangement comprises a frame, a swinging arm which is pivotable on the frame and which carries a magnetic head or a plurality of magnetic heads, or an assembly of swinging arms which are pivotable about a common axis on the frame and which are spaced from one another along said axis and each of which carries a magnetic head or a plurality of magnetic heads, the swinging arm or assembly of swinging arms being pivotably supported on the frame by a bearing arrangement which comprises first and second ball-bearings, each having an inner race and an outer race, a rotatable bearing pin, means for securing the inner races of the ballbearings in axially spaced relationship on the bearing pin, means for securing the swinging arm or assembly of swinging arms on the bearing pin, bearing-housing means which are adapted to engage the outer races of the ball-bearings and which are secured to the frame, and spring means for axially loading said outer races to inhibit axial play in the bearings, wherein the first and second ball-bearings are located on opposite sides of the swinging arm or assembly of swinging arms, wherein the bearing-housing means comprise first and second bearing housings for the first and the second ball-bearings respectively, and wherein the spring means are arranged between the outer race of one of the two ball-bearings and the frame in such a manner as to load said outer race axially towards the outer race of the other ball-bearing.

The stability of the bearing arrangement is obtained by pivotally supporting the swinging arm or swinging-arm assembly on both sides, so that the two ball-bearings are equally loaded. In comparison with the swinging-arm arrangement of the aforesaid known type, the symmetrical bearing arrangement reduces deformation of the bearing pin. A further advantage is that the overall mounting height of the swinging-arm arrangement can be reduced.

In one embodimentofthe invention the means for securing the inner races of the ball-bearings on the bearing pin and the means for securing the swinging arm or assembly of swinging arms on the bearing pin comprise combined means for axially urging the inner races of the ball-bearings towards each other so that the swinging arm or assembly of swinging arms is clamped between said inner races.

Each bearing housing may comprise a length of extruded tube of which the central bore receives the outer race of a respective one of the ball-bearings and which is formed with a plurality of parallel through-bores of smaller cross-section then the central bore, which through-bores are spaced around the central bore, and the bearing-housing means may be secured to the frame by self-tapping screws which are screwed into said through-bores.

An embodiment ofthe invention will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 is a plan view of a magnetic-disc storage apparatus incorporating a swinging-arm arrangement according to an embodiment of the invention, Figure 2 is a side view of the magnetic-disc storage apparatus of Figure 1, Figure 3 is an exploded view of the swinging-arm arrangment of the magnetic-disc storage apparatus shown in Figures 1 and 2, and Figures 4-6 show the swinging arm of the swinging-arm arrangement in different positions.

The magnetic-disc storage apparatus comprises a frame 1 on which a swinging-arm arrangement2 is mounted. Two rigid magnetic discs 3 and 4 are space coaxially from each other on a common rotary support 5 for rotation about an axis 6. The swinging-arm arrangement2 and the magnetic discs 3 and 4 are covered by a plastics cover 7. This cover protects the magnetic discs 3 and 4 against dust and other contaminants. For the sake of brevity those parts of the magnetic-disc storage apparatus which are irrelevant to the present invention are not described and are not shown in the drawing, except for a box 8 which accommodates the electronic circuitry necessary for the operation of the magnetic-disc storage apparatus.

The swinging-arm arrangement2 will now be described in more detail with reference to Figure 3.

The swinging-arm arrangement serves for moving and positioning four magnetic heads 9 to 12 overthe surface of two rotating rigid magnetic discs 3 and 4.

The magnetic heads are arranged one above the other in a series extending parallel with the axis of rotation of the magnetic discs 3 and 4 and are spaced from each other in conformity with the spacing betwees the magnetic discs 3 and 4. The magnetic heads are all carried on a single swinging arm 14 of substantially planar form which is pivotable about an axis 13 parallel with the axis of rotation of the magnetic head. The magnetic heads are all mounted on a single mounting device 15, which is formed with means, for example, faces, recesses, bores and threaded holes, for locating and mounting the magnetic heads on the swinging arm. The mounting device 15 is rigidly connected to the swinging arm 14 by glueing. The method proposed in the Applicants' prior United Kingdom Application No.8210483 may be used for this purpose.

Each of the magnetic heads 9 to 12 is arranged on one end of a resilient, elongate magnetic-head carrier 9a to 12a respectively, which at its other end is provided with a fixing part 9b to 12b respectively, which is provided with a fixing bush 9c to 12c respectively. The mounting device 15 is formed with a through-bore 16, in which the fixing bushes fit with some clearance. To secure a magnetic-head unit, comprising the magnetic head, the resilient head carrier and the associated fixing part, to the mounting device 15, the fixing bush on the fixing part is inserted into the through-bore 16 in the mounting device and a slightly oversized ball is forced through the fixing bush so that the clearance between the fixing bush and the wall of the through-bore is removed. Such magnetic-head units are commercially available.

The centre of gravity of the assembly comprising the mounting device 15 and the magnetic heads 9 to 12 arranged thereon is disposed substantially in the plane of pivotal movement of the swinging arm 14.

This also applies to the other parts of the magnetichead units. However, these parts have a substantially lower mass than the magnetic heads and the mounting device.

The mounting device 15 comprises a series of spaced flat projections 17, 18 and 19 constituting a comb, for supporting the magnetic heads 9 to 12 at regular distances from each other. The through-bore 16 extends through the projections 17,18 and 19.

The mounting device consists of a plurality of parts 20,21 and 22 which are detachably secured to each other and which each comprise a projection of the comb. The parts 20 and 22 are secured to the central part 21 by means of bolts 23 and 24. The part 21 is glued to the swinging arm 14 before the magnetichead units comprising the magnetic heads 10 and 11 are connected to the projection 18 by means of the fixing bushes 10c and Ilc. The units comprising the magnetic heads 9 and 12 are connected to the projections 17 and 19 of the parts 20 and 22 before these two parts are connected to the centre part 21 by means of the bolts 23 and 24. This facilitates mounting of the vulnerable magnetic-heads units on the mounting device 15.

The swinging arm comprises a first end 25 and a second end 26. The magnetic heads 9 to 12 are connected to the swinging arm at or near the first end 25. The swinging-arm arrnagement comprises a permanent magnetic stator27 having flat permanent stator magnets 28 and 29 which are spaced from each other in parallel planes and between which an air gap 30 is formed (see Figure 2). In this air gap there is arranged a flat control coil 31 of an electrically conductive material, which coil is attached to the swinging arm 14. The swinging arm 14 is pivotabiy journalled by means of a bearing arrangement 32. The flat control coil 31 is arranged in an opening 33 in the swinging arm 14. The swinging arm is punched from aluminium sheet, the opening 33 being formed during the punching operation.The bearing arrangement 32 is situated at or near the second end 26 of the swinging arm and the control coil 31 is arranged between the first end 25 and the second end 26 of the swinging arm.

The swinging arm is manufactured from a single flat piece of sheet material and the flat control coil 31 has a thickness, measured in the direction of the axis of the coil, which is substantially equal to the thickness of the sheet material from which the swinging arm is manufactured. The opening 33 in the swinging arm has a shape which corresponds to the shape of the outer circumference of the control coil 31, the coil fitting in the opening with clearance.

The control coil is glued in the swinging arm 14 and is attached to the wall of the opening 33 by a film of glue, which is not visible in Figure 3. The control coil is situated in the plane of the swinging arm, so that the swinging arm with the control coil glued therein constitutes a flat unit of substantially constant thickness. The turns 34 of the control coil are made of an electrically conductive material and are wound on a central flat plastics coil former 35. The electrically conductive material is not formed by a copper wire, as is customary for control coils, but by an aluminium strip 36 having a width equal to the thickness "d" of the coil and having a thickness which is substantially smaller than the width.The glue for attaching the control coil 31 to the wall of the opening 33 is mixed with aluminium powder in order to ensure a satisfactory heat transfer between the coil and the sheet material of the swinging arm.

The two outermost magnetic heads 9 and 12 of the series of magnetic heads 9 to 12 are shielded from spurious magnetic fields by mu-metal (RTM) shields 37 and 38 respectively which are connected to the swinging arm 14 via the mounting device 15 and each of which is so arranged that the respective outermost magnetic head 9. 12 will lie between the respective shield 37,38 and the magnetic disc over which that magnetic head isto be moved and positioned. The shields are formed with holes 39 and 40 and together with the parts 20 and 22 of the mounting device 15 they are connected to the central part 21 of the mounting device by means of the bolts 23 and 24 respectively. On the lower shield 38 an integrated semiconductor circuit 41 is mounted for processing signal from the magnetic heads 9 to 12.

Thus, these signals can be amplified before they are applied to the signal processing sections of the magnetic-disc storage apparatus. The semiconductor circuit 41 is mounted on a flexible plastics strip 43 provided with electrically conductive tracks 42, which strip is secured to the shield 38 at a first end 44 of the strip. At or near a second end 45 the plastics strip is connected to a stationary connector on the frame 1 of the magnetic-disc storage apparatus via a multi-pole connector 46. Between said first and second ends 44 and 45 the plastics strip 43 comprises a freely movable portion 47 which does not impede the movements of the swinging arm 14.

The bearing arrangement 32 which pivotably supports the swinging arm comprises a first ball-bearing 48 and a second ball-bearing49 having inner races 50 and 51 and outer races 52 and 53 respectively.

The two inner races 50 and 51 are secured on a rotatably bearing pin 54, axially spaced from each other, by means of a circlip 55 and a nut 56. The ball-bearings48 and49 are located on opposite sides of the swinging arm 14. The swinging arm is formed with an opening 57 for the bearing pin 54. On oppo site sides of the swinging arm, rings 58 and 59 are disposed which hold the swinging arm between the inner races 50 and 51 of the ball-bearings48 and 49.

The swinging arm is clamped between the inner races 50 and 51 by the force exerted between the nut 56 and the circlip 55. The outer races 52 and 53 of the ball-bearings are engaged by bearing-housing means which comprise a first and second annular bearing housings 60 and 61 respectively. To inhibit axial play between the outer races 52 and 53 of the ball-bearings these races are axially loaded by a cup spring 62 which is disposed adjacent the outer race 52 on top of the upper ball-bearing 48. The ballbearings are sliding fits in the bearing housings 60 and 61 and the cup spring 62 urges the ball-bearings axiallytowardseach other.Each bearing housing comprises a length of extruded tube of which the central bore 63 receives the outer race 52,53 of the ball-bearing 48,49 respectively, and which is formed with three parallel through-bores 64 of smaller cross-section than the central bore 63, which are spaced around the central bore. The bearing housings 60 and 61 are secured by means of self-tapping screws 65, which are screwed into the bores 64.

The permanent magnetic stator27 comprises a statorframe with two iron stator end-plates 66 and 67, one at each end of the stator, which stator endplates extend over the interposed control coil 31 and stator magnets 28 and 29. These magnets are glued to the stator end-plates. The stator magnets are axi ally magnetized in a direction parallel with the pivotal axis 13 of the swinging arm 14. The stator magnet 28 comprises two zones 28a and 28b which are oppositely polarized. The magnet 29 also com prises two oppositely polarized zones 29a and 29b.

At the side facing the control coil 31 the zone 28a is polarized oppositely to the zone 29a and the zone 28b oppositely to the zone 29b, so that the parts of the control coil 31 between the permanent stator magnets are disposed in permanent magnetic fields of oppositely polarity. The axial permanent magnetic fields are closed by the two iron end-plates 66 and 67. Instead of integral stator magnets 28 and 29 it is obviously also possible to employ a plurality of separate magnets. As an example, the stator magnet 28 may comprise two parts, the zone 28a belonging to the one stator magnet and the zone 28b to the other stator magnet. The end-plates 66 and 67 of the permanent magnetic stator 27 extend transversely of the pivotal axis 13 of the swinging arm 14, and the bearing housings 60 and 61 are secured to the stator end-plates 66 and 67 by the self-tapping screws 65.

Thus, the stator end-plates 66 and 67 also constitute frame platesforthe bearing arrangement32. In addition to the two stator end-plates 66 and 67, the stator 27 comprises three spacers 68,69 and 70 in the form of short sleeves. By means of three bolts 71,72 and 73 and associated nuts 74 to 76, the stator end-plates 66 and 67 and the three spacers 68 to 70 are held rigidly together in assembled relationship to form a rigid unit, which constitutes the stator frame of the permanent magnetic stator. Since the bearing hous ings 60 and 61 are secured to this stator frame the stator frame also constitutes a frame for the com plete swinging-arm arrangement, the stator end plate 67 also serving as a base plate by means of which the complete swinging-arm arrangement can be mounted on the deck 77 of the magnetic-disc storage apparatus.The means for connecting the swinging-arm arrangement2 to the deck 77 are not shown, but will generally comprise screw-threaded means.

The embodiment of the swinging-arm arrangement shown in the drawings is equipped with a tacho-coil 78, which is connected to the swinging arm 14 and which is movable in a permanent magnetic field in orderto generate an induction voltage which is a measure ofthe speed of pivotal movement of the swinging arm 14. For this purpose permanenttacho-magnets 79 and 80 are arranged on the outer side of the statorend-plate 67, which magnets are magnetized axially and oppositely in a direction parallel with the pivotal axis 13. Instead of two magnets a single flat magnet may be used with zones of opposite polarization. The stator end-plate 67 also serves for closing the axial magnetic fields produced by the two tacho-magnets 79 and 80 on one side.The bearing pin 54 has a free end 81 which via an opening 82 in the stator end-plate 67, extends beyond the outer side of this end plate. The tachocoil 78, which is flat like the control coil 31, is secured to the free end 81 of the bearing pin 54 by means of a nut 83 and can move in a permanent magnetic field produced by the tacho-magnets 79 and 80. At a distance from the stator end-plate 67 an iron tacho end-plate 84 is arranged parallel to the stator endplate 67. The tacho-coil 78 moves parallel to the flat tacho magnets 79 and 80, leaving an air gap between the stator end-plate 67 and the tacho end-plate 84, so that the axial permanent magnetic field of the tacho-magnets is closed by the stator end-plate on one side and the tacho end-plate on the other side.

The tacho end-plate 84 is secured to the underside of the deck 77 by means offourselftapping screws 85 to 88, but may alternatively be secured to the stator end-plate 67 by separate spacer means. The tachomagnets 79 and 80 as well as the tacho-coil 78 are disposed in a recess 89 in the deck 77.

The two stator end-plates 66 and 67 comprise substantially identical plates which are punched from a flat magnetizable sheet material. Thus they can be manufactured cheaply using simple means, in which respect it is important that only one type of stator end-plate need be manufactured. The requirements imposed on the manufacturing accuracy are not stringent, so that after punching the stator endplates need not be subjected to any machining operation. The stator magnets 28 and 29 extend slightly beyond the edges of the stator end-plates 66 and 67 respectively, substantially in the directions of movement of the control coil 31 (see also Figure 1).

A stop member 90 is provided which with two opposite sides 91 and 92 is cooperable with opposed side walls 93 and 94 respectively of a recess 95 in the swinging arm 14. The sides 91 and 92 of the stop member 90 limit the pivotal movement of the swing ing arm between a first position and a second position. Figures4 and 5 show the swinging arm 14 in the first or inner position and the second or outer position respectively. The stop member 90 is made ofrubberand is movable between an operative position, as in Figures4 and 5, and a maintenance position, as in Figure 6. The stop member, which is made entirely of elastic rubber, has the form of a rotatable eccentric with an aperture which is a moderately tight fit on the spacer 68. For this purpose the aperture 96 has a diameter which is slightly smaller than the outer diameter of the spacer 68.The aperture is located eccentrically of the stop member so that the stop member is eccentrically rotatable between the operative position and the maintenance position.

From the operative position to the maintenance position the stop member is rotated through 90 .

In the maintenance position of the stop member the side walls 93 and 94 of the recess 95 are cooperable with two further opposite sides 97 and 98 respectively of the stop member. These sides are situated nearer each other than the sides 91 and 92, so that for maintenance purposes the swinging arm 14 can be pivoted through an extended range to bring the magnetic heads 9 to 12 outside the peripheries of the magnetic discs 3 and 4. This position is also employed during assembly of the magnetic-disc storage apparatus to enable the magnetic heads to be mounted on the apparatus without being damaged.

In a magnetic-disc storage apparatus a plurality of swinging arm arrangements may be employed, for example on both sides of a deck or a frame plate, or superimposed above one another. Although in the embodiment described the swinging-arm arrangement comprises only one swinging arm, an assembly of such arms may be supported on the bearing pin 54 between the two ball-bearings48 and 49, the arms being spaced from one another along the bearing pin and each carrying a magnetic head or a plurality of magnetic heads. Many embodiments are possible within the scope of the invention. For example, the ball-bearings need not be mounted on the stator end-plates: they may be mounted on other frame portions, for example, on a separate bearing frame. In such an embodiment, in the same way as in the known swinging-arm arrangement described in the aforesaid United States Patent Specification 4,150,407, the stator and the bearing arrangement with the swinging arm or assembly of swinging arms may be mounted separately on the deck of a magnetic disc storage apparatus.

Claims (4)

1. A swinging-arm arrangement for moving and positioning a magnetic head or heads over a surface or the surfaces of a rotating rigid magnetic disc or a plurality of coaxially spaced rotating rigid magnetic discs in a magnetic-disc storage apparatus for the storage and reproduction of data in digital form, which arrangement comprises a frame, a swinging arm which is pivotable on the frame and which carries a magnetic head or a plurality of magnetic heads, or an assembly of swinging arms which are pivotable about a common axis on the frame and which are spaced from one another along said axis and each of which carries a magnetic head or a plurality of magnetic heads, the swinging arm or assembly of swinging arms being pivotably supported on the frame by a bearing arrangement which comprises first and second ball-bearings, each having an inner race and an outer race, a rotatable bearing pin, means for securing the inner races of the ballbearings in axially spaced relationship on the bearing pin, means for securing the swinging arm or assembly of swinging arms on the bearing pin, bearing-housing means which are adapted to engage the outer races of the ball-bearings and which are secured to the frame, and spring means for axially loading said outer races to inhibit axial play in the bearings, wherein the first and second ball-bearings are located on opposite sides of the swinging arm or assembly of swinging arms, wherein the bearing-housing means comprise first and second bearing housings for the first and the second ball-bearings respectively and wherein the spring means are arranged between the outer race of one of the two ball-bearings and the frame in such a manner as to load said outer race axially towards the outer race of the other ball-bearing.

2. A swinging-arm arrangement as claimed in Claim 1, wherein the means for securing the inner races of the ball-bearings on the bearing pin and the means for securing the swinging arm or assembly of swinging arms on the bearing pin comprise combined means for axially urging the inner races of the ball-bearings towards each other so that the swinging arm or assembly of swinging arms is clamped between said inner races.

3. A swinging-arm arrangement as claimed in Claim 1 or 2, wherein each bearing housing comprises a length of extruded tube of which the central bore receives the outer race of a respective one of the ball-bearings and which is formed with a plurality of parallel through-bores of smaller cross-section than the central bore, which through-bores are spaced around the central bore, and wherein the bearing-housing means are secured to the frame by self-tapping screws which are screwed into said through-bores.

4. A swinging-arm arrangement as claimed in Claim 1 and substantially as herein described with reference to the accompanying drawings.