Stressed hoop slip clutch

A controlled slip torque transmitting apparatus comprises a first substantially cylindrical rotary member which is rigid and has a longitudinal axis of rotation. A somewhat compliant tubular rotary member snugly encircles the first member, the two members being rotatable relatively about that axis. The second member has at least three outwardly extending projections with free ends. An elastic band surrounds the projections so as to exert compressive loads on the body thereby pressing the body against the tubular member at the roots of the lobes so that the clutch has an inherent break away torque when the two members are moved relatively.

FIELD OF THE INVENTION 
This invention relates to a clutch. It relates especially to a clutch 
designed to allow relative motion between the clutch input and output 
while requiring a relatively constant predetermined relative torque to 
maintain such motion. 
BACKGROUND OF THE INVENTION 
Slip clutches are used in a variety of applications and to accommodate a 
variety of needs. These include torque transmission between shafts, torque 
transmission from shaft to gear and vice versa, braking of shaft rotation, 
among other torque transmission and/or torque limiting applications. 
Invariably, such clutches include first and second members movable relative 
to one another and means providing a selected amount of torsional friction 
between the two members so that relative motion of the two members occurs 
only when they are subjected to a relative torque which exceeds a 
predetermined "break away" torque associated with the clutch. 
Conventionally, the torsional friction may be provided by plates associated 
with the two members and pressed together face to face or by spring loaded 
shoes mounted to one member and arranged to frictionally engage the other 
member, for example. Indeed, there are myriad mechanisms for providing 
controlled slippage between two rotary members. However, these prior 
mechanisms usually have certain drawbacks of one kind or another. Some 
utilize springs and other small parts which are difficult to assemble; 
some have a relatively large number of parts which increases their cost 
and complexity and still others tend to be rather bulky so that they are 
difficult to incorporate to relatively small devices and machines where 
space is at a premium. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention aims to provide an improved slip clutch. 
Another object of the invention is to provide a slip clutch which may be 
composed of only three separate parts. 
Yet another object of the invention is to provide such a clutch which can 
be made quite small and compact. 
A further object of the invention is to provide a slip clutch whose parts 
can be made in quantity at relatively low cost using conventional 
machining or molding techniques. 
Still another object of the invention is to provide a slip clutch of simple 
design which can be used in a wide variety of torque transmission and/or 
torque limiting applications. 
A further object is to provide a clutch of this type which is 
bi-directional. 
Other objects will, in part, be obvious and will, in part, appear 
hereinafter. 
The invention accordingly Comprises the features of construction, 
combination of elements and arrangement of parts which will be exemplified 
in the following detailed description, and the scope of the invention will 
be indicated in the claims. 
Briefly, the clutch Comprises a first member which is cylindrical and of a 
rigid material and a second, somewhat Compliant tubular member which 
snugly surrounds the first member, the two members being rotatable 
relative to one another about their common axis. The second member is 
formed with at least three radially outwardly extending projections or 
lobes. The outer ends of the lobes define an imaginary circle whose 
diameter is appreciably larger than the diameter of the first member. 
The clutch also includes a third member, namely a flexible resilient 
normally cylindrical hoop whose inside diameter is somewhat less than the 
diameter of the circle defined by the lobes of the second member. The 
number, dimensions and spacing of the lobes is such that the hoop can be 
forcibly engaged over the ends of the lobes thereby causing the hoop to be 
distorted from its normal circular shape to a noncircular shape. The 
thus-distorted resilient hoop applies radial compression loads to the 
second member. Resultantly, the relatively compliant second member is 
pressed against the first member at the roots of the lobes so that 
appreciable torsional friction exists between the two members if there is 
an attempt to rotate one member relative to the other. As a consequence, 
when one member is rotated, the other member will rotate along with it 
unless a countervailing torque is applied to the other member which 
exceeds the "break away" torque of the clutch. The break away torque may 
be defined as the torque required to overcome the static friction 
presented by the opposing surfaces of the two members under the lead 
conditions imposed by the resilient hoop. 
Once relative rotation of the two clutch members does occur, a constant 
relative torque dependent upon the coefficient of kinetic friction at the 
boundary of the two members is required to maintain the relative motion of 
the two members. 
Depending upon the particular application, either member may receive the 
torque input or provide the torque output. For example, the first member 
may be coupled to a driven rotary input shaft and the second member may be 
coupled to an output shaft. In this case, the two shafts will rotate in 
unison unless the torque differential on the two shaft exceeds the break 
away torque of the clutch. In such an application, the output shaft may be 
connected to a roller around which web is being wound, the slip clutch 
being present to prevent undo tension on the web being wound. 
In another application, the second member may be provided with teeth so 
that it forms a spur gear which meshes with a driven gear. In that event, 
the driven gear will rotate along with an input shaft coupled to the first 
member so long as the break away torque of the clutch is not exceeded. If 
that torque is exceeded, there will be controlled rotational slippage 
between the gear and the shaft. 
In yet another application, means may be provided to retard one of the 
clutch members which will result in the other member being braked to a 
stop unless the relative torque on the two members exceeds the break away 
torque of the clutch. 
These, and many other uses of the clutch may be envisioned. 
The three major components of my clutch can be manufactured relatively 
easily using standard machining or molding techniques and the parts are 
particularly adapted for automatic assembly. Therefore, the clutch should 
be relatively inexpensive to make in quantity. 
Also, the clutch is quite compact and may be scaled up or down to suit 
particular applications.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 to 3 of the drawing, the clutch 10 specifically 
illustrated there is intended to couple torque between a rotary shaft S 
and a spur gear G shown in phantom in FIG. 2. It should be understood, 
however, that the clutch may be used in many other applications where it 
is desired to provide relative motion between two rotary members, while 
requiring a relatively constant predetermined relative torque to maintain 
such relative motion. 
Clutch 10 includes a first rotary member in the form of a rigid cylindrical 
sleeve 12 having a radial flange 12a at one end. For example, the sleeve 
may be of metal, e.g., steel, or of a strong rigid plastic material, which 
provides a smooth, relatively non-wearing outer surface on sleeve 12. A 
cylindrical passage 14 extends along the longitudinal axis of sleeve 12, 
the passage being sized to snugly receive the shaft S. Preferably, the end 
of the passage 14 at flange 12a is provided with a non-circular, e.g., 
square, counterbore 14a. In that event, the counterbore is arranged to 
receive a similarly shaped key S' on shaft S to rotatably lock the sleeve 
to the shaft. The axial position of the sleeve on the shaft may be 
maintained by any suitable means such as a set screw (not shown). 
The clutch also includes a second rotary member in the form of a somewhat 
compliant tubular body 16 having an axial cylindrical passage 18 sized to 
enable body 16 to be received on sleeve 12 so that one end of body 16 
seats against the sleeve flange 12a as best seen in FIG. 2. Preferably, 
the diameter of the passage 18 is only slightly larger than that of sleeve 
12 so that there is a relatively tight fit between the body and the 
sleeve. 
As shown in the drawing figures, the end segment of body 16 that faces the 
sleeve flange 12a is formed with engageable projection means, i.e., it is 
enlarged radially and shaped to form a spur gear 22. When the clutch 10 is 
in use, that gear 22 is arranged to mesh with the spur gear G shown in 
phantom in FIG. 2. 
The opposite end segment of body 16 is formed with at least three radially 
outwardly extending projections or lobes 24. The illustrated clutch has 
exactly three such lobes spaced at equal angles about the longitudinal 
axis of body 16. However, another such clutch could may have four or more 
lobes. 
As best seen in FIG. 3, the lobes 24 have the general shape of rectangular 
solids. Preferably, however, their radially outer surfaces 24a are curved 
about the axis of body 16 so that those surfaces in toto define an 
imaginary circle or cylinder. The diameter of that imaginary circle or 
cylinder may be more or less or the same as the diameter of the spur gear 
22. 
As noted above, the clutch body 16 should be somewhat compliant. 
Accordingly, it is preferably made of a strong, but compliant, plastic 
material which has a low wear rate when used with sleeve 12. 
The third major component of clutch 10 is a flexible, resilient hoop, loop 
or band 26 which is arranged to engage around the lobes 24 of clutch body 
16. 
The hoop is basically a compliant, thin-walled ring, although the thickness 
of the hoop is exaggerated in the drawing figures for ease of 
illustration. The hoop material may vary but should have good "creep" 
resistance at the stress levels to which it will be subjected when the 
clutch 10 is in use. Obviously, the material should have a yield strength 
significantly higher than the stress imposed upon it when the hoop 26 is 
forceably engaged over lobes 24 during assembly of the clutch. A suitable 
hoop material is steel. 
The length of hoop 26 usually corresponds to the lengths of lobes 24 of 
body 16 and the hoop has a central passage 26a whose diameter is somewhat 
less than the diameter of the imaginary circle or cylindrical defined by 
the lobe surfaces 24a. Resultantly, when hoop 26 is forceably engaged over 
the lobes, it is distorted from its normal circular shape into a 
non-circular shape as best seen in FIG. 1. 
Since the hoop 26 is flexible and resilient or elastic, the inherent 
restoring forces in the distorted hoop apply symmetricel compression loads 
to the clutch body 16 at the locations of lobes 24 as shown by the 
radially inwardly extending arrows in FIG. 1. These forces cause the wall 
sectors of the body passage 18 at the roots of the lobes to be pressed 
against sleeve 12 thereby producing torsional friction at those locations 
that resists relative rotation of sleeve 12 and body 16. However, relative 
rotation of those two members will occur if the relative torque imposed on 
the two members exceeds the above-defined break away torque of clutch 10. 
The components of clutch 10 are assembled by pressing them together. In 
other words, first the clutch body 16 is engaged on the sleeve 12. Then, 
the hoop 26 is forceably engaged over the lobes 24 of the clutch body, 
which action deforms the hoop from its normal circular shape to the 
distorted shape shown in FIG. 1. In most cases, the resulting frictional 
forces between sleeve 12 and body 16 and between lobes 24 and hoop 26 are 
sufficient to maintain the relative axial positions of those components 
and thus to maintain the clutch 10 in its assembled condition. However, if 
desired, the end of sleeve 12 opposite flange 12a may be provided with a 
circumferential groove 28 as shown in FIG. 3. When the clutch is 
assembled, that end of the sleeve will project beyond body 16 sufficiently 
to allow a C-clip 30 to be engaged in groove 28 to retain body 16 on 
sleeve 12. Since the hoop 26 always moves with the clutch body 16, the 
elastic forces caused by the distortion of the hoop are invariably 
sufficient to maintain the axial position of the hoop on the lobes 24 
around which it is engaged. 
When the clutch 10 is in use, any rotation of shaft S (FIG. 2) in either 
direction will cause a corresponding rotation of the gear G and vice 
versa. However, if a retarding force should be imposed on the driven 
member, e.g., gear G, sleeve 12 will slip relative to the clutch body 16 
when the break away torque of the clutch is exceeded. In that event, so 
long as the relative torque on the two members does exceed a determined 
constant value related to the coefficient of kinetic friction at the 
boundary between sleeve 12 and body 16, such relative motion will 
continue. That relative torque required for continued motion is usually 
somewhat less than the break away torque of the clutch. The break away 
characteristic of the clutch 10 may be changed or controlled by selecting 
a hoop 26 with the desired elastic properties. The break away torque is 
also affected by the number size and spacing of lobes 24 on body 16. 
Rotation of the clutch sleeve 12 relative to the clutch body 16 may cause 
some wear on the contacting surfaces of those members. As these contacting 
surfaces wear, the compressive loads imposed on body 16 by hoop 26 will 
maintain contact forces in the regions of wear, i.e., at the mots of lobes 
24. This may cause the hoop 26 distortion to decrease over time thereby 
reducing the loads applied to body 16. This effect may be minimized by 
proper selection of materials for sleeve 12 and body 16. 
Also, it is preferable that the interference fit between the hoop 26 and 
the clutch body 16 which causes the distortion of the hoop to its non 
circular shape be quite large relative to the amount of wear anticipated 
at the interface of the clutch sleeve and clutch body. With this 
condition, any wear at the sleeve/body interface would change the preload 
of the hoop 26 only by a small amount, and, therefore have minimal effect 
on the break away torque of the clutch. In this way, the clutch 10 may be 
designed to have a long useful life even when it is used in high or 
constant duty applications. 
It will thus be seen that the objects set forth above, among those made 
apparent from the preceding description, are efficiently attained. Also, 
certain changes may be made in the above construction without departing 
from the scope of the invention. For example, if the clutch is to be used 
for low relative torque applications, the body 16 may be quite compliant 
and the hoop 16 may comprise one or more elastic bands stretched around 
the lobes 24. Also, in a braking application, the gear 22 may be 
substituted for by a detent selectably engageable by a grounded part to 
arrest motion of the second clutch member. Therefore, it is intended that 
all matter contained in the above description or shown in the accompanying 
drawing shall be interpreted as illustrative and not in a limiting sense. 
It is also to be understood that the following claims are intended to cover 
all of the generic and specific features described herein.