GB2120443A - Gramophone turntable apparatus - Google Patents

Abstract

Apparatus for use with a gramophone turntable for flattening a gramophone disc and preventing slippage includes a split bushing (25) releasably locatable on the spindle (40) of the turntable by engagement in a circumferential groove on the spindle. A thrust member (2) is screw threaded to the housing of the bushing for movement towards an underlying disc to apply pressure thereto. An annular washer (42) is located beneath the disc to raise the central portion slightly and the thrust plate bears via its marginal region radially outwards of the lower washer. <IMAGE>

Description

SPECIFICATION Apparatus for use with gramophone turntable This invention relates to apparatus for use with a gramophone turntable.

A gramophone record is located on a record player by placing the hole in the middle of the disc over the short spindle upstanding from the turntable. For high quality sound reproduction the playing area of the disc should lie flat on the turntable and usually a mat of rubber or other soft material is included on the upper surface of the platter to receive the disc. However, discs frequently become bowed, and in consequence the pick-up stylus moves up and down as the disc rotates to the detriment of the fidelity of the reproduced sound. To alleviate this movement, it has been proposed to apply pressure to the disc to force it flat against the turntable mat. Although modern gramophone discs are flexible enough to withstand such pressure, it is difficult to apply the pressure over the whole disc without the risk of damaging the recording grooves on the disc.

Methods have been proposed for applying pressure to the central region of the disc, i.e. that which usually bears the record label, but these lack an adjustment facility and have not in general been able to achieve a distribution of pressure across the whole of the playing surface of the disc.

According to the present invention there is provided apparatus for use with a gramophone turntable, comprising a thrust plate dimensioned and configured for application to the central, nonplaying, area of a gramophone disc, releasable engagement means adapted for releasable connection to a spindle of a gramophone turntable for rotation therewith, the thrust plate being mounted on the releasable engagement means such that it may be moved axially with respect to the spindle, and secured at a fixed axial position to bear on an underlying gramophone disc and apply a pressure thereto.

In a preferred form the apparatus is adapted for use with a spindle having a circumferential groove; the releasable engagement means comprising a resilient jaw or jaws adapted to snap-fit within the circumferential groove.

Preferably only the outer marginal region of the thrust plate contacts the underlying disc, and a lower thrust plate, less wide than the aforementioned thrust plate is located between the central area of the disc and the turntable.

The resilient jaw may be located at the lower internal margin of a split bushing constituting part of said engagement means and adapted for location over the spindle.

The thrust plate may be connected by a screwthreaded engagement to the releasable engagement means thereby enabling it to be moved axially and fixed at a relative axial position.

The split bushing may be slidably mounted in a housing member, the housing member forming that part of the releasable engagement means which is connected by a screw-thread to the thrust plate.

Preferably the split bushing is spring-loaded such that it is urged into a retracted position within the housing wherein the jaws are urged together.

An embodiment of the invention will now be described, by way of example only, with reference to and as illustrated in the accompanying drawings.

In the drawings: Figure 1 is an axial sectional view through apparatus for applying pressure to a gramophone disc in accordance with the present invention, Figure 2 is an axial sectional view through the apparatus in position on a gramophone turntable, and Figure 3(a) to (d) shows diagrammatically the stages in the procedure used in applying the apparatus to a gramophone turntable.

Referring to Figure 1, apparatus for applying pressure to a gramophone disc on a turntable is in the form of a multipart assembly 1. The assembly 1 comprises a circular thrust plate 2 having a radius approximately equal to that of the centre, label-bearing portion of a gramophone disc. The thrust plate 2 is fabricated from aluminium and comprises an outer annular portion 3 having a flat undersurface 4 and an upper surface which comprises an outer portion 5 parallel to the undersurface 4 and an angled stepped portion 6.

Inwards of the outer annular portion 3 is a gently sloping frusto-conical portion 7 terminating inwardly at a hub portion 8. The upper and lower surfaces of the frusto-conical portion 7 diverge from the periphery to the hub region of the plate, and are both sloped upwards from the periphery so that the frusto-conical portion 7 of the plate 2 is spaced above the plane of the outer annular portion 3. To lessen the weight of the circular thrust plate 2, the frusto-conical portion has six circular apertures 9 spaced equiangularly around the thrust plate 2.

The hub portion 8 of the thrust plate has a large circular recess 10 in its lower surface and an axial bore 11 of circular cross-section opening at one end at the upper surface of the hub and at the other end into the recess 10 with which it is coaxial. The upper surface of the hub portion 8 surrounding the bore 11 is chamfered.

The assembly 1 further includes a union nut 13 having a head portion and a shank portion and being fabricated from polyethylene. The union nut 13 has an axial bore 12 which is wider in its head portion and is countersunk where it opens on the upper surface of the head portion. The shank of the union nut 13 fits in the circular bore 1 1 through the hub portion of the thrust plate 2 and the undersurface of head of the union nut sits on the upper surface of the hub portion of the thrust plate. The shank meets the head portion of the union nut by a narrow tapered intermediate portion which is configured to correspond with the chamfer on the hub portion 8 of the thrust plate 2.

In location, the head of the union nut 13 extends outwardly beyond the hub portion of the pressure plate and the shank projects bevond the lower end of the circular bore 11 through the hub portion of the thrust plate 2. The union nut 13 is held in a fixed position with respect to the thrust plate 2 by means of an annular brass ring 14 fitted into a circumferential groove in the projecting portion of the shank of the union nut and abutting the horizontal surface which forms the base of the circular recess 10 in the thrust plate 2. That portion of the axial bore 12 from the free end of the shank of the union nut 13 to the wider portion has a screw threaded surface. The outer circumferential wall of the head of the union nut 13 is provided with a knurled surface.

An aluminium bolt 15 has a head 16 with a large circular recess 17 and a shank 18. The shank comprises a main portion 19 terminating at a short end portion 20 having a smaller diameter than the main portion 19. The shank has a first axial bore 21 opening into the recess 17 and extending approximately one third of the way along the shank 18, and a second axial bore 22 through the end portion 20 of the shank and the main portion 19 and terminating just short of the axial bore 21. The second axial bore 22 has a diameter somewhat less than the first axial bore 21 and where it extends through the end portion 20 the bore 22 is tapered such that it is wider towards its opening. The two bores 21 and 22 communicate via a narrower circular aperture 23 defined by an annular flange 24 extending from the inner wall of the shank portion 1 8 of the bolt.

The exterior circumferential surface of the main portion 1 9 of the shank of the bolt 1 5 is provided with screw thread corresponding to the thread on the union nut 13. The circumferential surface of the end portion 20 of the shank of the bolt has a circumferential groove which receives an annular brass abutment ring 34.

A split bushing 25 is slidably mounted within the axial bore 22 of the shank 1 8 of the bolt 1 5.

The bushing is fabricated from brass and comprises cylindrical portion 26 with four longitudinal slots dividing the bushing into four resilient segments joined at their upper ends. At their lower ends the four segments of the bushing together describe a generally fursto-conical portion 27 of the bushing terminating in a ring portion 28. The rim portion is made up of a separate rim on each of the four split segments of the bushing 25 and each rim has an inwardly projecting circumferential jaw 29. The end of the cylindrical portion 26 of the bushing remote from the jaws 29 has a narrow tubular spigot 30. The spigot 30 is received in a socket at the end of the stalk 31 of an operating member 32.The stalk 31 of the operating member 32 is disposed coaxially through the bore 21 and through the circular aperture 23 of the bolt 1 5 to abut the upper surface of the cylindrical portion 26 of the bushing 25. The operating member 32 includes a discshaped head portion which is accommodated within the circular recess 1 7 in the head of bolt 15. A coil spring 33 is located co-axially around the shank of the operating member 32 and bears at one end against the lower surface of the head of the operating member and at the other end against the upper surface of the annularflange 24 on the bolt 15.

The split bushing 25 together with its connected operating member 32 is slidably located within the bolt 15 and its movement therein is limited by the abutment of the cylindrical portion of the bushing against the annular flange 24 and the abutment of the head of the operating member against the lower surface of the recess 17.

The bolt 15 is screw-fitted within the axial bore 12 of the union nut 1 3 and is prevented from removal therefrom by the abutment ring 34.

In use the assembly is releasably connected to the spindle of a gramophone turntable upon each occasion of record is placed on the turntable and the spindle is specially modified to receive the assembly. Referring to Figure 2, and in particular, the enlarged view thereof the domed head of a spindle 40 is modified bythe milling of a circumferential groove 41. The groove is wedgeshaped having an upper wall perpendicular to the side of the spindle and a lower oblique wall. This wedge shape of the groove 41 corresponds to the shape of the projecting jaws 29 from the segments of split bushing 25. The assembly is used in association with a lower thrust plate 42 which comprises a circular disc of radius approximately equal to the circular recess 10 in the main thrust plate 2. The thrust plate 42 has a central aperture adapted to fit over the spindle 40.

The upper surface of the lower thrust plate 42 has a horizontal portion surrounding the central aperture and is countersunk where it meets the circular aperture. The outer portion of the surface of the thrust plate 42 slopes towards the lower surface to provide a tapered outer region.

The method of using the apparatus is illustrated in Figure 3 which shows four steps (a) to (d) of the procedure. Reference is also. made to Figure 2 which illustrates in detail the final disposition of the assembly The initial step in the procedure is the location of the lower thrust plate 42 over the spindle 40 to rest on the turntable mat. A gramophone disc 45 is then placed over the spindle in the normal manner and its central labelbearing area rests on the upper horizontal surface of the thrust plate 42. It will be appreciated that in this position the record is unstable.

The assembly 1 is then located by pressing down on the head of the operating member 32 against the bias of the coil spring 33 so that the rim portions of the jaws 29 of the split bushing are pushed further out of the bore 22 in the bolt 15 than in their neutral position shown in Figure 1.

The motion produced by pressing the operating member is limited to a relatively small distance by the abutment of the head of the operating member on the lower surface of the recess 1 7 in the head of the bolt 15. The resilience of the segments of the split bushing 25 causes the projecting portions of the jaws to move apart when released from the constraint of the walls of the bore in the bolt 15.

The assembly 1 is then inserted over the spindle 40. The jaws 29 are able to pass over the domed head of the spindle, the contour of the head assisting the widening of the jaws, until the jaws snap-fit within the circumferential groove 41.

Releasing the operating member 32 causes the spring 37 to urge the bushing upwards and hence assists in maintaining the jaws in their portion in the groove 41.

Thus secured, the assembly appears as shown in Figure 3(b); the record disc is no longer able to tilt in a plane since the outer annular portion 3 of the upper thrust plate bears on the peripheral edge of the label region of the disc in a horizontal plane.

As a result the peripheral regions of the disc are forced flat against the turntable mat and the record flexes such that its central region becomes bowed upwardly. It will be appreciated that although this may provide good playing conditions for the outer portion of the record the portion of the playing surface close to the central region is unsatisfactorily angled to the surface of the turntable.

Following the location of the split bushing the thrust plate is rotated about the central bolt 15 by rotating the union nut 13 fixed to the upper thrust plate 2. Rotation of the union nut 13 screws the thrust plate 2 downwards with respect to the bolt 1 5. As the thrust plate moves downwards increasing pressure is applied to the outer region of the label portion of the disc by the annular portion 3 of the thrust plate. The disposition of the record disc is then as shown in Figure 3(c).

Further rotation of the union nut 13 causes the thrust plate 2 to be screwed further downwardly with respect to the bolt 15 and hence with respect to the turntable, until the position shown in Figure 3(d) is attained. In this final position the annular portion 3 of the pressure plate 2 and substantially all of the playing surface of the disc are horizontal.

The assembly 1 is released by unscrewing the pressure plate 2 via the union nut 13, and then depressing the operating member 32 to allow the jaws 29 to widen so that the assembly can be lifted off the spindle.

It will be appreciated that the union nut may be tightened to an extent such that the optimum pressure may be brought to bear on the disc.

It is envisaged that the assembly may be used with most types of existing turntables. The turntables merely requiring a modified spindle or a groove cut in the existing spindle to receive the assembly.

Claims (10)

1. Apparatus for use with a gramophone turntable comprising a thrust member dimensioned and configured for application to the central, non-playing, area of a gramophone disc, releasable engagement means adapted for releasable connection to a spindle of a gramophone turntable for rotation therewith, and in which the thrust member is mounted on the releasable engagement means by a screw threaded connection such that it may be moved axially with respect to the spindle and bear on an underlying gramophone disc to apply a force thereon pressing the disc against the turntable.

2. Apparatus as claimed in claim 1 further including means adapted for location beneath said central portion of a gramophone disc when placed on a turntable and operative to raise the central position.

3. Apparatus as claimed in claim 2 wherein said means adapted for location beneath the central portion of a gramophone disc comprises an annular member adapted for location over the spindle of the turntable, and having a radial extent substantially less than that of said thrust member.

4. Apparatus as claimed in claim 3 wherein the thrust plate is configured such that only its outer marginal region contacts the underlying gramophone disc.

5. Apparatus as claimed in any one of the preceding claims wherein the releasable engagement means includes a plurality of jaws adapted to be moved apart for location on said spindle.

6. Apparatus as claimed in claim 5 wherein said jaws are located at one extremity of a split bushing adapted to receive said spindle.

7. Apparatus as claimed in claim 6 wherein said releasable engagement means includes a housing member within which the split bushing is slidably mounted, the thrust member being mounted on the housing member by said screw threaded connection.

8. Apparatus as claimed in claim 6 or claim 7 wherein the split bushing is spring loaded urging it into a retracted position within said housing member, and whereby said jaws are urged together.

9. Apparatus as claimed in any one of claims 5 to 8 wherein the jaws are resilient or resiliently mounted and adapted for use with a spindle having a circumferential groove, the engagement comprising a snap-fitting of the jaws into said groove.

10. Apparatus for use with a gramophone turntable substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.