Resilient shaft coupling

A resilient shaft coupling having two metal parts that include an inner coupling ring and, coaxial thereto, an outer coupling ring. The facing generated surfaces of the rings are formed with a number of substantially radial cams or dogs which mesh with one another alternately in the circumferential direction and extend to near the pertaining opposite generated surface. Each two circumferentially consecutive cams or dogs form a pair whose adjacent surfaces are concave and delimit a chamber whose longitudinal axis extends parallel to the rotational axis of the coupling, and in which a resilient coupling member is disposed. The coupling members are radially biased or pre-loaded by a predetermined amount, and are secured to the generated surfaces of the inner and outer ring coupling rings. With the coupling in its rest position, gaps or spaces are present between each resilient coupling member and each cam or dog, and permit the members to deform freely and thus to be stressed in shear. The advantage of this shear compression is that the coupling has a very reduced torsional stiffness at low torques, and operates without backlash or play at low torques.

BACKGROUND OF THE INVENTION 
The present invention relates to a resilient shaft coupling having two 
metal parts, one of which can be connected to an input, the other to an 
output, with torque being adapted to be transmitted between these parts 
via a number of circumferentially consecutive resilient coupling members 
that are made of rubber or the like; the metal parts comprise an inner 
coupling ring and, coaxial thereto, an outer coupling ring, with those 
generated surfaces of the rings that face one another having formed 
therein a number of alternately interengaging (in the circumferential 
direction) and substantially radially extending cams, dogs, or teeth that 
extend over the axial length of the coupling rings and to a location near 
the pertaining opposite generated surface; each two circumferentially 
consecutive cams or dogs form a pair whose facing surfaces are concave and 
delimit a chamber, the longitudinal axis of which extends parallel to the 
rotational axis; a resilient coupling member is disposed in the chamber, 
with the shape of the resilient coupling members before installation being 
substantially that of a circular rod of a length corresponding 
substantially to the axial length of the coupling rings; upon 
installation, the resilient coupling members are subjected to a 
predetermined radial loading. 
Resilient shaft couplings wherein the torque is transmitted between 
circular coaxial coupling hubs that have interengaging cams, dogs, or 
teeth via a number of circumferentially consecutive resilient rubber 
coupling members are known in a variety of constructions as so-called 
positive or dog couplings; the basic constructon thereof is disclosed by 
German Auslegeschrift No. 10 67 644 Croset dated Oct. 22, 1959, 
corresponding to U.S. Pat. No. 2,873,590-Croset dated Feb. 17, 1959. 
In this known coupling, chambers are provided each being adapted to receive 
a circular-section rubber coupling member disposed between every two 
adjacent cams or dogs; the chambers are of a size and design such that, 
with the coupling unloaded, the resilient coupling member in the chamber 
does not contact the cams or dogs that delimit the latter, i.e., each 
individual resilient coupling member extends in the circumferential 
direction with a reduced clearance between the cams or dogs. Consequently, 
when a torque is applied to the coupling, its torsional stiffness near its 
neutral point is undefined, but once the reduced clearance has been taken 
up, and the resilient coupling members experience compression, torsional 
stiffness rises very rapidly to high values as torque continues to be 
applied. 
In a coupling system of this type, it is impossible to provide low defined 
levels of torsional stiffness in the low-torque range after the clearance 
has been taken up. The result of this, for example in vehicle drives 
having a combustion engine, is the typical "transmission clatter" 
associated with starting at a low speed. 
It is an object of this invention to improve the design of a resilient 
shaft coupling of the positive type in such a way that its torsional 
stiffness is very reduced at low torques, that it retains this low 
torsional stiffness at zero average useful torque, and that it operates in 
this condition without clearance and without any impairment of torsional 
stiffness at full load as compared with known resilient positive couplings 
that have intermediate resilient coupling members. Furthermore, the novel 
resilient shaft coupling is intended to be highly reliable in operation, 
to require little servicing, and to be able to be manufactured 
economically.

SUMMARY OF THE INVENTION 
According to the invention, starting from a resilient shaft coupling of the 
aforementioned general type, the resilient coupling members are secured to 
the generated surfaces of both the inner coupling ring and the outer 
coupling ring and, with the shaft coupling in its rest position, spaces 
are present between each resilient coupling member and each cam or dog of 
a pair of cams or dogs; these spaces enable the resilient coupling members 
to deform freely, in response to an initial shear stress, up to a 
predetermined level of shear stress, at which point the resilient coupling 
members engage substantially simultaneously with a chamber-delimiting cam 
or dog. 
The shape of an individual resilient coupling member prior to installation 
basically resembles a circular-section rod. When the members have been 
installed, they are compressed relatively intensely by the radial loading 
force, so that their cross-sectional shape becomes substantially oval. 
As a simple way of securing the resilient coupling members, retaining 
strips, which extend parallel to the rotational axis of the coupling, are 
disposed to part of their depth in, and are secured to, the coupling 
member on the radially opposite surfaces thereof; and a portion of the 
strip which projects radially from the resilient coupling member is 
retained in a companion recess in the wall of the inner and outer coupling 
rings. Advantageously, the retaining strips are secured to the resilient 
coupling member by vulcanization. Advantageously too, the projecting 
portion of a retaining strip is of dovetailed cross-section, and the 
respective receiving grooves in the inner and outer coupling rings are 
adapted to this cross-sectional shape. 
The resilient deformability of resilient coupling members having 
predetermined external dimensions can be influenced as required; for 
example, the presence of a through-bore that is parallel to the rotational 
axis of the coupling, and is of a particular size, helps to increase 
resilience as compared with a solid resilient coupling member. 
According to another feature of the invention, to close the chambers at 
their end faces, flat end rings are releasably secured one each to the 
inner and outer coupling rings on both end faces of the latter, there 
being formed on the end rings teeth which are radially adjacent one 
another and which mesh alternately with a gap between them; each two 
teeth, that are disposed in circumferentially consecutive relationship on 
the same end face, form a pair and cooperate to delimit a chamber. In such 
an embodiment, with the coupling in its rest position, the gap between the 
teeth of a pair is at least large enough to ensure that when the 
predetermined shear stress level of the resilient coupling members is 
reached, i.e., after an appropriate rotation of the coaxial coupling rings 
relative to one another, the teeth do not abut one another. Also, these 
teeth can provide abutment means to stop rotation after the maximum torque 
has been reached. 
A main advantage provided by the invention as compared with a known 
resilient positive coupling is that very low torsional stiffnesses, and 
correspondingly high angles of rotation, are possible at low torques 
because at low torques the resilient coupling members, unlike what happens 
in a positive or dog coupling, can be stressed initially only in shear. 
This shear stressing is made possible by the gaps or spaces present 
between a resilient coupling member and the cams or dogs of a pair that 
delimits the associated chamber. The cams or dogs of the two coupling 
rings engage the resilient coupling members to inhibit any further free 
deformation thereof only after the permissible shear stress arising from 
this free deformation has almost been reached. As the torque loading of 
the coupling increases, the stressing of the resilient coupling members 
changes from shear to compression. Starting with this operating condition, 
appropriately high torques can be transmitted just as in the case of 
conventional resilient dog couplings. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to the drawings in detail, the metal coupling parts of the 
resilient shaft coupling are formed by an inner coupling ring 1 and, 
coaxial thereto, an outer coupling ring 2, the inner ring 1 being provided 
with a central bore 3 to receive an input shaft or output shaft (not 
shown). The outer ring 2 has on its periphery a flange ring 4 that is 
provided with a number of through-bores 5 which are disposed equidistant 
from one another in such a way as to be consecutive in the circumferential 
direction, with the axes of the bores 5 being parallel to the axis of 
rotation of the coupling; the bores 5 are adapted to receive securing 
screws for securing the coupling to another drive element. 
A plurality of substantially radially extending cams or dogs 7, which in 
cross-section are substantially sickle-shaped, are formed on the outer 
cylindrical generated surface 6 of the inner ring 1 and are consecutively 
disposed, equidistant from one another, in the circumferential direction. 
The free ends of the cams or dogs 7 extend to a location near the inner 
generated surface 8 of the outer ring 2. An equal number of cams or dogs 9 
can be provided to extend radially inwardly from the surface 8; the cams 
or dogs 9 have the same cross-sectional shape as do the cams or dogs 7, 
but with their sickle curvature oriented in the opposite direction. 
Each outer-ring cam 9 forms a pair with an inner-ring cam 7 which is next 
to the cam 9 in the circumferential direction. Each cam pair 9, 7 delimits 
an axially disposed chamber, six of which are shown in the illustrated 
embodiment. Disposed in each chamber is a resilient coupling member 10 
which is made of rubber or the like, and the axial length of which 
corresponds to the axial length of the chamber, i.e., the axial length of 
the coupling rings 1, 2. The resilient coupling members 10 have the shape 
of a circular-section rod before being installed; once installed, they are 
compressed radially and assume the oval cross-sectional shape visible in 
FIG. 1. This compression subjects the members 10 to a predetermined radial 
loading. Each member 10 is provided with a through-bore 11 which is 
circular to start with and which the compression makes oval, as can be 
seen in FIG. 1. The resilience of the members 10 and, therefore, the 
torsional stiffness of the coupling can be varied by varying the size of 
the cross-section of the bore 11. 
At their engagement or bearing surfaces, the members 10 are secured to the 
inner ring 1 and to the outer ring 2. To this end, each such surface has 
at its center, in a groove that extends parallel to the rotational axis of 
the coupling, a retaining strip 12, 13, a projecting portion of which is 
of dovetailed cross-section. The strips 12, 13 are rigidly secured to the 
resilient coupling member 10, preferably by vulcanization. The projecting 
dovetailed portions of the strips 12, 13 are retained positively in 
companion grooves 14, 15 in the inner and outer coupling rings 1, 2 
respectively. To introduce a member 10 into its operative position, the 
appropriately compressed resilient coupling member is introduced axially 
from one end face so that its two strips 12, 13 engage in the grooves 14, 
15. 
FIG. 1 shows the resilient shaft coupling in its unloaded normal position. 
In this position the members 10 do not fill the chamber completely in the 
circumferential direction; instead, there is a gap or space 16, 17 at the 
front and at the rear. When the coupling starts to take up a load, i.e., 
when, for example, the outer ring 2 rotates clockwise relative to the 
inner ring 1, the spaces or gaps 16, 17 enable the members 10 to deform 
freely at first so that they are stressed in shear. 
The size of the gaps or spaces 16, 17 is such that when the shear stressing 
of the members 10 reaches a predetermined level, the associated cams or 
dogs 7, 9 engage substantially simultaneously with the opposite generated 
surfaces of the members 10, whereafter the members 10 are no longer free 
to deform and can be stressed only by compression. 
In the first phase of coupling loading, the torsional stiffness of the 
coupling is determined only by the shear forces associated with the free 
deformation of the members 10. Since these forces are relatively low, the 
shaft coupling has a very low torsional stiffness during the starting 
phase, with this starting stiffness resulting solely from shear stressing 
of the resilient members 10 and being exactly definable, even for zero 
torque. Another advantage is that in this first loading phase, which is 
associated with very low torsional stiffness, the two metallic parts of 
the coupling engage one another without clearance or play. 
After the spaces 16, 17 have been taken up i.e., after the cams or dogs 7, 
9 of each pair have engaged with the associated resilient member 10, the 
members 10 can no longer be deformed freely, the stressing of the members 
10 changing from shear to compression. The coupling can now transmit the 
specified high torque with an appropriately high torsional stiffness. 
It has been found that the amount of free deformation of the members 10 can 
be slightly greater at the front than at the rear. To ensure that the cams 
or dogs 7, 9 nevertheless engage substantially simultaneously after 
termination of the free deformation phase of the members 10, the gap width 
of the space 16 at the front is slightly greater than at the back of the 
associated member 10. 
A flat outer end ring 18, and a flat radially inner end ring 19 of the same 
strength or thickness, are provided to close the chambers at both of their 
end faces. The end rings on the opposite side of the shaft coupling are 
similar and therefore do not have different reference numerals in the 
drawings. Each of the two rings 18, 19 is releasably secured to the rings 
2, 1 respectively by means of threaded bolts 20, 21 respectively which 
engage in tapped or threaded bores in the rings 2, 1, with those bores 
extending parallel to the rotational axis of the coupling. On their 
adjacent axial generated surfaces, the rings 18, 19 are provided with 
teeth 22, 23 which mesh with one another with clearance, and the free ends 
of which extend to near the pertaining opposite generated surface. 
As FIG. 1 shows, the teeth 22, 23 are trapezoidal, with the inclined flanks 
of any two consecutive teeth 22, 23 facing one another. Any two teeth 22, 
23 whose inclined flanks face one another form a pair and cooperate to 
delimit a chamber on their end face; with the coupling in its normal 
position, the teeth 22, 23 of a pair are disposed substantially 
symmetrically relative to the central axis of the chamber, and partly 
cover the front and rear ends of the associated member 10. A gap 24 is 
left between the teeth 22, 23 of a pair and is at least large enough to 
ensure that when the predetermined level of shear stress of the resilient 
members 10 is reached, the teeth 22, 23 do not abut one another. 
A resilient shaft coupling of this kind is of use only for unidirectional 
operation. If it is required to make use of the advantages of the shaft 
coupling for reversible drives, two consecutive individual couplings for 
opposite drive directions can readily be provided. 
The present invention is, of course, in no way restricted to the specific 
disclosure of the specification and drawings, but also encompasses any 
modifications within the scope of the appended claims.