An adjustable torsion-spring slipping clutch, especially useful for tape recorders, having first and second coaxial clutch sections which are rotatable about an axis of rotation. The second clutch section includes a sleeve which has a substantially circular cylindrical shape and is adjustable in diameter. A coaxially arranged cylindrical helical torsion spring is connected to one end to the first clutch section and its turns are wrapped around the sleeve, adjustment of sleeve diameter determining the torque to be transmitted between the clutch sections.

BACKGROUND OF THE INVENTION 
The invention relates to an adjustable torsion-spring slipping clutch, 
which in normal operation provides a constant torque drive from a faster 
rotating driving section to a drive section; and in particular to such a 
clutch useful for apparatus for recording and/or playing back signals on a 
magnetic tape, the clutch comprising first and second coaxially arranged 
clutch sections which are rotatable relative to each other about an axis 
of rotation, and a coaxially arranged cylindrical helical torsion spring 
which at one end is connected to the first clutch section, the second 
clutch section comprising a sleeve which is substantially circular in a 
cross-section perpendicular to the axis of rotation and around which turns 
of the spring are wrapped for transmitting torque between the clutch 
sections by the friction between the spring and the sleeve, which turns 
are adjustable in diameter for determining the torque to be transmitted 
between the clutch sections. 
A clutch of this type is known from the published Austrian Patent 
Application A 8026/77, to which U.S. Pat. No. 4,258,556 corresponds. In 
this known clutch the sleeve of the second clutch section is conical, the 
spring being axially movable relative to the sleeve so as to enable the 
diameter of the spring turns to be adjusted. Depending on the apex angle 
of this cone some friction may occur between the turns of the spring under 
certain circumstances, which inter-turn friction may give rise to a 
certain inaccuracy in the operation. 
SUMMARY OF THE INVENTION 
The object of the invention is to construct such a clutch so that precise 
constant torque is transmitted under all circumstances. According to the 
invention the sleeve of the second clutch section has a substantially 
circular cylindrical shape and the diameter of the sleeve is adjustable. 
Because of the circular cylindrical shape of the sleeve all spring turns 
can continuously cooperate with the sleeve, each turn as far as possible 
contributing equally to the transmission of the torque. This optimum 
cooperation between each turn and the sleeve enables a constant accurately 
adjustable torque to be transmitted between the clutch sections over a 
long period. Thus, once it has been adjusted, the clutch in accordance 
with the invention will perform in a very reliable manner for a long 
period of time, without any further readjustment. 
In a clutch in accordance with the invention it is advantageous if the 
sleeve, for adjusting its diameter, is rotationally adjustable to any of a 
plurality of positions relative to another part of the second clutch 
section. This provides a satisfactory adjustability in combination with a 
compact construction of the clutch. In a suitable embodiment of the clutch 
the sleeve is made of a material which for adjusting the sleeve diameter 
is elastically movable in a radial direction relative to the axis of 
rotation. In this way the sleeve diameter can be adjusted in a reliable 
and simple manner. It has been found that such an adjustment can be 
carried out in a suitable manner if the sleeve comprises a plurality of 
segments of similar shape, which are arcuate in a cross-section 
perpendicular to the axis of rotation and which are each movable in a 
radial direction relative to the axis of rotation. The segments enable the 
circular cylindrical shape of the sleeve to be maintained upon adjustment, 
so that an optimum cooperation between the turns and sleeve is obtained at 
various sleeve diameters. 
According to another preferred embodiment the segments extend in an axial 
direction and at one end are connected to an annular support, which 
support is provided with adjusting means for the rotational adjustment of 
the sleeve relative to the other part of the second clutch section. In 
addition to a robust construction this provides a simple adjusting 
facility for the segments. 
In the preferred embodiment incorporating the features just described, the 
sleeve is concentrically arranged around a hub of the second clutch 
section, which hub at its periphery has a plurality of convexly curved 
spiralling ramp surfaces of similar shape which cooperate with the sleeve. 
In a cross-section perpendicular to the axis of rotation extending along 
an arc, the distance of each ramp surface from the axis of rotation 
gradually increases in a circumferential direction. The ramp surfaces 
enable the diameter of the sleeve to be adjusted to a multitude of 
gradually increasing values. Preferably the number of segments of the 
sleeve is equal to the number of ramp surfaces of the hub, and the inner 
surface of each segment, in a cross-section perpendicular to the axis of 
rotation, has a concave arcuate shape which corresponds to the convex 
arcuate shape of the ramp surfaces. During adjustment an optimum 
cooperation of the segments with the ramp surfaces is thus maintained, 
which is important in order to maintain the cylindrical shape of the 
sleeve. In view of this adjustment it is found to be advantageous if each 
segment comprises an inwardly radially extending shoulder at that end of 
its arcuate inner surface which is furthest from the axis of rotation, the 
distance of the shoulder from the axis of rotation substantially 
corresponding to the distance from this axis of that end of the surface of 
the segment which is nearer the axis of rotation. During adjustment the 
shoulder assist in maintaining the cylindrical shape of the segments. In 
this connection it is advantageous that the ramp surfaces and the segments 
each extend over an arc of substantially 120.degree.. The segments are 
then divided over the clutch circumference in an optimum manner, so as to 
obtain a wide range of sleeve diameter adjustment, while maintaining the 
cylindrical shape. 
The invention is described in more detail on the basis of an embodiment 
shown in the drawing, but to which the invention is not limited.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The apparatus 1 shown in FIG. 1 is an apparatus by means of which signals 
can be recorded and/or played back on a magnetic tape 2, which is wound on 
two winding hubs in a substantially rectangular cassette 3 and which 
between the winding hubs passes along a long side of the cassette. 
The apparatus comprises a chassis with a deck plate 4 at the top, an upper 
surface of said deck plate constituting a positioning surface for the 
cassette 3. When the cassette is placed on the positioning surface the 
winding hubs are coupled to the winding mandrils 5 and 6. 
The apparatus furthermore comprises a head plate 7, near the long side of 
the cassette, which plate carries magnetic heads, which comprise an erase 
head 8 and a combined recording/playback head 9. Near the long side the 
cassette has a side wall formed with a plurality of openings which, by 
moving the head plate 7 in the direction of the cassette, enable the 
magnetic heads 8 and 9 to be brought into operative contact with the 
magnetic tape, and which enable the magnetic tape to be pressed by a 
pressure roller 10 against a capstan 11 which extends perpendicularly to 
the head plate 7 and which is engageable with the drive of the apparatus. 
The winding mandril 6 concentrically surrounds and engages with the end 
portion of a hollow spindle 12 (see FIG. 2), which is rotatable, about an 
axis of rotation 13, on a journal 13A which is rigidly mounted on the deck 
plate 4. At its end which is remote from the mandril 6 the spindle 12 is 
rigidly connected as a unit to a gear wheel 14, which during fast winding 
of the magnetic tape is directly coupled to the drive of the apparatus. 
The spindle 12 together with the gear wheel 14 constitutes a first clutch 
section 15 which during recording or playback acts as the driven clutch 
section. In the embodiment shown the spindle 12 with the gear wheel 14 is 
made of a polyacetal plastic having the trade name "Delrin". 
The spindle 12 furthermore serves for journalling a cylindrical hub 16 of a 
second clutch section 17 which acts as drive clutch section during 
recording or playback. The hub 16 extends axially inside the gear wheel 14 
and inside the gear wheel 14 it is surrounded by a ring 18 which is 
concentric with the hub and which is integral with the gear wheel 14. At 
its end which is remote from the gear wheel 14 the hub 16 is rigidly 
connected to a connecting element 19 comprising a flat annular portion 20 
which extends perpendicularly to the axis of rotation 13. The element 19 
further comprises a rim portion 20A, which is concentric with the axis 13 
and adjoins the portion 20 (also see FIG. 4). The rim portion 20A is 
connected to an annular portion 21 of a gear wheel 22, which portion 21 is 
perpendicular to the axis of rotation 13. Said gear wheel 22 is rigidly 
connected to the hub 16 by the element 19 and viewed in FIG. 2, is 
situated at only a slight axial distance from the gear wheel 14. In the 
present embodiment the hub 16, the element 19 and the gear wheel 22 form a 
unit which is made of a polyamide plastic, known as "nylon". Engaged 
around the hub 16 is a substantially circular cylindrical sleeve 23, which 
comprises a plurality of separate segments 24 which have a similar arcuate 
shape in a cross-section perpendicular to the axis of rotation 13 and 
which, as is shown in FIG. 3, preferably each extend over an arc of 
substantially 120.degree.. The segments 24 are made of an elastic 
material. In the present embodiment the polyacetal plastic with the trade 
name "Delrin" is used for the segments, which plastic has satisfactory 
antifriction properties and moreover is comparatively hard. The segments 
each extend in an axial direction into the space enclosed by the ring 18 
and each are connected, as is shown in FIG. 3, at only two mutually spaced 
locations 24B to an intermediate ring 25A; the segments 24 are free at 
their other ends. In order to obtain an optimum movability for each 
segment in the radial direction each intermediate ring 25A, viewed in the 
circumferential direction, is connected only halfway at a location 25B to 
an annular support 25. One side of the support 25 is positioned against 
the flat annular portion 20. The support 25 is provided with adjusting 
means, constituted by two diametrically disposed adjusting members 26, 
which each extend in the radial direction beyond the circumference of the 
flat portion 20. For this purpose an arcuate recess 27 is formed in the 
portion 20 and the rim portion 20A for each member 26, which recess 
extends over an arc of approximately 120.degree. in the present embodiment 
(see FIG. 4). In order to enable the respective members 26 to be fitted 
from the side nearer the gear wheel 14, openings 28 are formed in the 
portion 21, one such opening being formed adjacent one end of each recess 
27, which openings, for the passage of the members 26, are shaped to suit 
the dimensions and shape of the members 26. At least one of the members 26 
is provided with teeth 29 on the side which faces the flat portion 20, 
which teeth cooperate with teeth 30 on the arcuate wall of the recess 27. 
The members 26 serve to enable the sleeve 23 by rotation around the hub 16, 
to be adjusted to and retained in any of a plurality of positions relative 
to the hub 16 (and thus relative to the other part of the clutch section 
17). The adjustment is possible due to a loose contact between the teeth 
29 and 30 (FIG. 4). 
On its outer periphery the hub 16 comprises three similar convexly curved 
spiralling arcuate ramp surfaces 31 which each extend preferably over 
substantially 120.degree.. Each surface 31 is formed so that, in a 
cross-section perpendicular to the axis of rotation 13 and reckoned in the 
direction of rotation A, the distance of the ramp surface from the axis of 
rotation 13 gradually increases (FIG. 4). The ramp surfaces act as cam 
surfaces to the diameter of the sleeve 23 upon the rotational adjustment 
of the sleeve around the hub 16. In order to obtain an accurate 
cooperation, the inner surfaces of the segments 24, on the inner 
circumference in a cross-section perpendicular to the axis of rotation 13, 
each have a concave arcuate shape which corresponds substantially to the 
ramp surfaces 31. Each segment, at that end of its arcuate inner surface 
which is further from the axis of rotation (i.e. the forward end relative 
to the direction of rotation represented by the arrow A) preferably 
comprises an inwardly radially extending shoulder 24A, whose distance from 
the axis of rotation 13 substantially corresponds to the distance from 
this axis of that end of the inner surface of the segment which is nearer 
the axis of rotation (the rearward end relative to the arrow A). The 
difference in radial distance between the two ends of the inner surface of 
each segment and the axis of rotation is approximately 0.4 mm in the 
present embodiment. However, depending on the required adjustment range 
this difference may have another value. 
During rotation the segments 24 slide over the ramp surfaces 31. When the 
adjusting members 26 are situated adjacent the openings 28 in the initial 
position, each segment coincides fully with an associated one of the ramp 
surfaces, while the shoulder 24A of the segment is positioned against the 
beginning of the next ramp surface. As the members 26 are moved further in 
the direction of the arrow A an increasingly thicker part of each segment 
24 slides over the part of the associated ramp surface which is furthest 
from the axis of rotation 13, as a result of which the segment is moved 
outwards in a radial direction and the sleeve diameter increases. As 
during the movement of the adjusting members 26 each shoulder 24A is also 
moved outwards in the radial direction by the next ramp surface, the 
cylindrical shape of the segments and therefore the cylindrical shape of 
the sleeve 23 is maintained during the adjustment. As stated, the segments 
are made of an elastic material and are moreover supported elastically 
with the aid of the intermediate rings 25A, so that when the members 26 
are moved back in the opposite direction to the arrow A the segments move 
back and the sleeve diameter is reduced. 
Wrapped around the sleeve 23 is a cylindrical helical torsion spring 32, a 
number of turns of which are accommodated in the space bounded by the 
sleeve 23 and the ring 18. An end portion 33 of the spring extends through 
an opening in the ring 18 and has a hooked extremity which is retained 
between the ring 18 and the gear wheel 14. Thus, the spring is connected 
to the first clutch section 15 with the end portion 33, at least in a 
tangential direction. The spring is wound in an anti-clockwise direction, 
i.e. in the direction indicated by the arrow A in FIG. 4. The end of the 
spring 32 which is remote from the end portion 33 is disposed freely. 
Thus, the spring is in contact with the second clutch section 17 via the 
spring turns only, all spring turns performing a similar function. In the 
present embodiment the spring is made of CrNi spring steel. 
Together with the clutch sections 15 and 17 the spring 32 constitutes a 
wrapped spring slipping clutch. This clutch has the property that if the 
spring-wrapped clutch section 17 is driven in the direction of the arrow A 
via the gear wheel 22, which is generally the case during recording or 
playback, the spring 32, with respect to the winding direction, is 
slightly unwound by a torque which is transmitted by the friction between 
the spring turns and the sleeve 23 and whose magnitude depends on the 
instantaneous stress in the spring turns. The spring 32 in turn moves the 
driven clutching section 15, so that the winding mandril 6 is rotated and 
the magnetic tape is wound from the capstan 11. 
If during winding the maximum transmissible torque is exceeded, slip will 
occur between the spring turns and the sleeve 23. Said maximum 
transmissible torque depends on the stress in the turns of the spring 32 
which surround the sleeve, which stress depends on the diameter of the 
sleeve 23. 
As stated previously, the diameter of the sleeve 23 can be changed by 
adjusting the members 26, a larger sleeve diameter resulting in a higher 
stress in the turns of the spring 32 and a higher maximum transmissible 
torque. In order to obtain a suitable lubrication of the sleeve 23, it may 
be advantageous, in addition to the use of a sleeve material which is 
suited to the function of the sleeve, to use silicone oil. 
The wrapped spring clutch described in the foregoing enables the 
transmission torque to be adjusted simply, which adjustment can moreover 
be performed in a particularly accurate manner. The construction of the 
adjusting members 26 and the cooperating teeth 29 and 30 ensures that a 
specific adjustment of the clutch is always maintained. 
In the case of a wrapped spring clutch which is employed in an apparatus of 
the type described such an adjustment may be necessary, because the 
components of the clutch, in particular the spring, exhibit tolerances 
which should be eliminated in order to obtain the most accurate tape 
transport. 
Owing to its small axial dimension the present clutch may be used in an 
apparatus of the type described without any problems. 
It is to be noted that the present clutch, in view of its unidirectional 
action through the use of the spring 32, also has the advantage that in 
the opposite direction, i.e. in the case of differences in relative speed 
between the gear wheels 14 and 22 in the direction opposite to the arrow A 
in FIG. 4, a free-wheel possibility is obtained. As a result of this 
several variants are possible for coupling the drive of the apparatus to 
the respective winding mandrils 5 and 6 and the capstan 11. 
Furthermore, it is to be noted that, depending on the requirements imposed 
on the clutch, it is alternatively possible, instead of a sleeve 23 
comprising separate segments 24, to employ a sleeve having a continuous 
cylindrical wall and made of a highly elastic material. Instead of three 
arcuate segments which extend over 120.degree. each, it is alternatively 
possible to employ two or four or more segments, which together again 
cover 360.degree.. The use of three segments represents an optimum 
situation in respect of the desired wide range of adjustment of the clutch 
and the retention of the circular cylindrical shape of the sleeve 23. 
Because of the properties described above the wrapped spring clutch is also 
suitable for use in other equipment where it is necessary to correct 
certain tolerances and where a transmission with an accurately adjustable 
torque, in combination with a unidirectional action, is required.