GB2172066A

GB2172066A - Coiled wire clutch for a magnetic-tape apparatus

Info

Publication number: GB2172066A
Application number: GB08605217A
Authority: GB
Inventors: Ghislanus Matheus A Aldenhoven
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1985-03-07
Filing date: 1986-03-03
Publication date: 1986-09-10
Also published as: GB8605217D0; NL8500643A; DE3606807A1; JPS61149141U; FR2578604A1; GB2172066B

Abstract

A clutch (12), which is intended for use in a magnetic-tape apparatus, comprises two coaxial clutch sections (13, 18) which are rotatable relative to each other about an axis of rotation (14), a wound clutch wire (18) is provided which forms part of a first clutch section. A second clutch section (13) comprises a circularly cylindrical circumferential surface (13A) around which the turns of the clutch wire (18) are arranged. The clutch wire (18) is made of a plastics and the turns have substantially the same inner diameter. At both ends the clutch wire (18) is connected to a spring element (20) which exerts such a tensile force on the ends of the clutch wire (18) that the turns of the wire lie against the circumferential surface (13A) of the second clutch section (13) under tension. <IMAGE>

Description

SPECIFICATION Clutch in particular for a magnetic-tape apparatus The invention relates to a clutch, in particular for a magnetic-tape apparatus, comprising first and second coaxial clutch sections which are rotatable relative to one another about their axis, and a coiled clutch wire which forms part of a first clutch section, a second clutch section comprising a circularly cylindrical circumferential surface around which turns of the clutch wire are arranged.

A clutch of this type is disclosed in United States Patent Specification 4,353,226. Such a clutch can transmit a specific maximum torque by the friction between the clutch wire and the circumferential surface of the second clutch section For example, when used as a friction clutch in the drive mechanism of a magnetic-tape apparatus the clutch can provide the necessary tape tension during winding of the magnetic tape.

Another use is as a torque limiter, so that, for example, during winding of the magnetic tape only a part of the available motor torque is utilised. When the tape transport stops the maximum torque is exceeded and the two clutch sections slip relative to each other, so that the motor keeps rotating and another part of the torque may be employed by a stop mechanism to stop drive. In the known clutch the clutch wire is constituted by a steel spring. Owing to the nature of the material such a steel spring cannot be manufactured with sufficiently accurate tolerances. In practice, this generally results in different inner diameters between the respective spring turns of the known clutch, which gives rise to problems as regards the magnitude and the constancy of the torque to be transmitted.The known clutch is provided with an adjusting mechanism for adjusting the maximum torque to be transmitted, but such an adjusting mechanism renders the coupling intricate and requires additional space. Moreover, such an adjusting mechanism in fact does not solve the problems caused by a different action of each individual spring turn.

It is an object of the invention to construct a clutch of the type defined in the opening paragraph in such a way that without any further adjustment the magnitude and constancy of the torque to be transmitted are maintained over a comparatively long period.

To this end the invention is characterized in that the clutch wire is made of a plastics, the turns have the same or substantially the same inner diameter, and at its two ends the clutch wire is connected to a spring element which exerts a tensile force on the ends of the clutch wire such that the turns of the clutch wire surround the circumferential surface of the second clutch section with pretension.

Preferably, the clutch wire is injectionmoulded from a plastics material, for example polyacetal, so that the diameters of the individual turns can be defined accurately by means of a mould. Thus, the individual turns of the clutch wire can perform identical functions, so that after mounting of the spring, which ensures that the turns act on the circumferential surface of the second coupling member with suitable pretension, an accurately defined torque can be transmitted.

The functions are then performed separately, the clutch wire transmitting the torque and the spring element providing the necessary pretension on the wire. By the use of a polyacetal clutch wire and a second clutch section of, for example, nylon it is possible to ensure that the bearing characteristics of the two clutch sections relative to each other is improved. This results in a longer life, which is important for use as a friction clutch, and in addition the clutch produces hardly any noise during operation. Another advantage is that the magnitude of the torque to be transmitted depends directly on the force of the spring element used and does not depend on the coefficient of friction of the turns of the clutch wire.

A preferred embodiment of the invention is characterized in that at one end of the clutch wire the first clutch section is rigidly connected to a gear wheel, which gear wheel comprises a fixing portion for attaching one end of the spring element to the gear wheel.

In this way it is possible to integrate the clutch wire with a gear wheel situated on the input side of the clutch.

A preferred embodiment of the invention is characterized in that at one end the clutch wire is rigidly connected to a sleeve provided with a fixing portion for attaching one end of the spring element to the clutch wire. In this respect it is advantageous if the sleeve, the clutch wire and the gear wheel constitute an integral unit. Thus, the first clutch section can be made of a plastics in one piece, so that the clutch can be manufactured in an accurate yet simple manner.

A preferred embodiment of the invention is characterized in that the spring element is a helically wound spring whose turns concentrically surround the turns of the clutch wire.

The use of such a helical spring results in a compact clutch which can be manufactured accurately.

In this respect a clutch in which a winding mandrel is rigidly connected to the second clutch section and in which the first clutch section is situated below and is coaxial with the winding mandrel, can be constructed advantageously if the outer diameter of the runs of the spring is substantially equal to the diameter of the circumference of the winding mandrel. Thus, the outer diameter of the clutch is in fact not larger that the diameter of the winding mandrel, which facilitates mounting of the assembly comprising the winding mandrel and the clutch.

An embodiment of the invention will now be described in more detail, by way of example, with reference to the accompanying drawings. In the drawings: Figure 1 is a plan view of a magnetic-tape apparatus provided with a clutch in accordance with the invention, and Figure 2 shows a part of the apparatus in an enlarged-scale sectional view taken on the lines Il-Il in Fig. 1.

Fig. 1 shows a magnetic-tape apparatus 1 by means of which signals can be recorded on and/or played back from a magetic tape 2, which is wound on two reel hubs in a substantially rectangular cassette 3 and which between the reel hubs extends along the long front side of the cassette. The apparatus 1 comprises a frame with a deck plate 4 at the top, the upper surface of this deck plate constituting a supporting surface for the cassette 3. When the cassette is placed on the supporting surface the reel hubs are coupled to the winding mandrels 5 and 6.

Near the front side of the cassette the apparatus further comprises a head-mounting plate 7 carrying magnetic heads, which comprise an erase head 8 and a combined recording/playback head 9. At its front side the cassette comprises a side wall formed with a number of openings (not shown) which allow the magnetic heads 8 and 9 to be brought into operative contact with the magnetic tape by moving the head-mounting plate 7 towards the cassette, and which allow the magnetic tape to be pressed against a capstan 11 by a pressure roller 10, which capstan can be coupled to the drive mechanism of the apparatus and extends perpendicularly to the headmounting plate 7.

The winding mandrel 6 is coaxial with clutch 12 and is rigidly connected to the clutch at the upper end of a second clutch section 13 of the clutch 12, which clutch section comprises a spindle, which is made of plastics material, such as nylon, in the present embodiment. At its lower end the clutch section 13 comprises a gear wheel 15 which is adapted to drive the winding mandrel 6 directly. Preferably, the gear wheel 15 is constructed as an integral unit with the remainder of the clutch section 13.

The clutch section 13 comprises a circularly cylindrical circumferential surface 13A around which a first clutch section 16 is arranged.

The clutch section 16 comprises a sleeve 17, which adjoins the winding mandrel, and a coiled clutch wire 18 having a plurality of helical turns, suitably 5 or 6 turns, surrounding the circumferential surface 13A. The sleeve 17 and the clutch wire 18 are manufactured integrally from a plastics, preferably by injectionmoulding from polyacetal, which ensures that the clutch wire 18 and the circumferential surface 13A have satisfactory bearing properties relative to each other. This is also important in order to obtain a clutch which operates as silently as possible. At the end which is remote from the sleeve 17 the clutch wire 18 is rigidly connected to a gear wheel 19 and is preferably integral with the sleeve 17 and the gear wheel 19.

In the sleeve 17 an opening 17A is formed in which the first end of a spring 20 is fitted, which spring helically surrounds the turns of the clutch wire 18, the other end of the spring being fitted around a pin 21 on the upper side of the gear wheel 19. Thus, the opening 17A and the pin 21 form means for attaching the spring 20. The spring 20 is a spring element which exerts a tensile force on the ends of the clutch wire 18 such that the turns of the wire act with pretension on the circumferential surface 13A of the second clutch section 13. In this respect it is important that as a result of injection-moulding the turns of the clutch wire 18 have at least substantially equal diameters.Further, it is important that the inner sides of the turns of the spring 20 adjoin the outer sides of the turns of the clutch wire 18 and the outer diameter of the turns of the helical spring is substantially equal to the diameter of the circumference of the winding mandrel 6, so that the clutch 12 does not extend beyond the circumference of the winding mandrel 6. This is important in order the facilitate mounting of the unit comprising the winding mandrel 6 and the clutch 12. As a result of its compact construction this unit occupies only a minimal space in the magnetic-tape apparatus.

The clutch 12 may be compared to the customary torsion-spring clutch in which, in accordance with the invention, the torsion spring which is normally used in such a coupling is replaced by the combination of the plastics clutch wire 18 and the spring 20 surrounding it. This is important because it enables the functions to be performed separately. The clutch wire 18 performs the friction work during operation of the clutch and the spring 20 exerts the necessary tension on the turns of the clutch wire in order to ensure that these turns lie sufficiently tightly against the circumferential surface 13A. The use of the spring and the accurately defined inner diameter of the turns of the clutch wire ensure that the tension with which the turns of the clutch wire lie against the circumferential surface is defined accurately and that each turn performs a similar function. Alternatively, by providing a number of pins 21 the pretension can be made adjustable, but as a result of the choice of a spring with an accurately defined spring force this step may be omitted, which is particularly advantageous in the case of mass production of such a clutch.

When the clutch wire has a suitable number of turns, for example 5, the torque to be transmitted by the clutch 12 is substantially independent of the coefficient of friction of the plastics material used. The torque to be transmitted depends directly on the force of the spring 20 and does not depend on said coefficient of friction.

The properties of said clutch, which are comparable to those of a conventional torsionspring clutch, are such that, if the gear wheel 19 is driven by the drive mechanism of the magnetic-tape apparatus (in a manner not shown) in the direction indicated by the arrow A in Fig. 1, which is generally so during recording or playback, the turns of the clutch wire 18 cause a torque to be transmitted whose magnitude depends on the pretension of the turns and which is given by the force of the spring 20. The turns of the clutch wire 18 then drive the second clutch section 13, causing the winding mandrel 6 to be rotated and the magnetic tape to be driven by the capstan 11.An advantage of this is that the clutch 12 always transmits an accurately defined maximum torque, which ensures that the clutch always maintains the required tape tension and the tape is accurately wound by the winding mandrel 6. The gear wheel 15 provides the possibility of the winding mandrel 6 being driven directly by the drive mechanism of the magnetic-tape apparatus 1, thus bypassing the clutch 12, which may be desirable for fast winding of the magnetic tape towards the winding mandrel 6.

It is to be noted that the clutch in accordance with the invention may also be employed as a torque limiter, in which case only a part of the motor torque produced by the drive mechanism is used for winding. When a maximum torque is exceeded the two clutch sections slip relative to each other, which happens, for example, when the end of the tape is reached. If in such a situation the clutch section slip relative to each other, the motor can continue rotating and another part of the motor torque may be utilised by a stop mechanism of the magnetic-tape apparatus to stop the drive mechanism.

Thus, the clutch in accordance with the invention is generally applicable in all those cases where in the same way as in a torsionspring clutch an accurately defined torque is to be transmitted by friction, the advantage being that adjustment of the clutch with respect to the maximum torque to be transmitted is not necessary and that during the life of the clutch the maximum torque to be transmitted remains constant.

It is to be noted that instead of the helical spring 20 shown it is alternatively possible to employ a tension spring to provide the required pretension on the turns of the clutch wire 18.

Claims

1. A clutch, in particular for a magnetictape apparatus, comprising first and second coaxial clutch sections which are rotatable relative to one another about their axis, and a coiled clutch wire which forms part of a first clutch section, a second clutch section comprising a circularly cylindrical circumferential surface around which turns of the clutch wire are arranged, characterized in that the clutch wire is made of a plastics, the turns have the same or substantially the same inner diameter, and at its two ends the clutch wire is connected to a spring element which exerts a tensile force on the ends of the clutch wire such that the turns of the clutch wire surround the circumferential surface of the second clutch section with pretension.

2. A clutch as claimed in Claim 1, characterized in that at one end of the clutch wire the first clutch section is rigidly connected to a gear wheel, which gear wheel comprises a fixing portion for attaching one end of the spring element to the gear wheel.

3. A clutch as claimed in Claim 1, characterized in that at one end the clutch wire is rigidly connected to a sleeve provided with a fixing portion for attaching one end of the spring element to the clutch wire.

4. A clutch as claimed in Claims 2 and 3, characterized in that the sleeve, the clutch wire and the gear wheel constitute an integral unit.

5. A clutch as claimed in any one of the preceding Claims, characterized in that the spring element is a helically wound spring whose turns concentrically surround the turns of the clutch wire.

6. A clutch as claimed in Claim 5 in which a winding mandrel is rigidly connected to the second clutch section and the first clutch section is situated below and is coaxial with the winding mandrel, characterized in that the outer diameter of the turns of the helical spring is substantially equal to the diameter of the circumference of the winding mandrel.

7. A clutch as claimed in Claim 5 or 6, characterized in that the inner sides of the turns of the spring adjoin the outer sides of the turns of the clutch wire.

8. A clutch substantially as herein described with reference to the accompanying drawings.