Friction clutch including rolling bodies for transmitting a limited torque

In a friction clutch a first disc-shaped driving member and a second disc-shaped driving member are held in frictional contact by a spring so that the driving members rotate coaxially, each driving member has a facing surface directed toward the other. Rolling members are positioned between the facing surfaces and seat in opposed indentations when the driving members rotate as a unit. When a predetermined torque is exceeded, the biasing action of the spring is overcome and the driving members move away from one another in the axial direction. During such axial displacement, the rolling members move out of at least the indentations in one of the driving members and aid in the separation of the driving members.

SUMMARY OF THE INVENTION 
The present invention is directed to a friction clutch for the transmission 
of a limited torque including two frictionally engageable disc-shaped 
driving members which can be disengaged by axial displacement against a 
spring biasing action. 
In motor-driven, hand-held tools, such as hand-held drills or hammer 
drills, a safety clutch must be provided in the rotary drive between the 
motor and the tool for safety reasons so that the continued transmission 
of torque between the motor and the tool can be discontinued if the tool 
becomes jammed in the work material. 
For this purpose, mechanical safety clutches have been used almost 
exclusively. Such clutches have been catch clutches or friction clutches. 
With the known constructions of these clutches, there has been the 
significant disadvantage that, after exceeding the maximum or 
predetermined torque of the clutch, the driving members can rotate 
relative to one another, however, this relative rotation occurs under a 
no-load torque which is still relatively high as compared to the release 
torque with the result that though the clutch has been released, the 
operator is still exposed to the high no-load torque transmitted to the 
handle of the device when the motor continues to run. 
In one known friction clutch, disc-shaped driving member is fixed on a 
drive shaft so that it rotates with the shaft. A second disc-shaped 
driving member provides the power take-off feature and is supported 
concentrically and axially movably relative to the first driving member. 
For the transmission of torque, the second driving member is pressed by a 
compression spring against an end face of the first driving member with 
the intermediate arrangement of a friction disc. If the limited torque 
defined by the spring action and the friction coefficient of the friction 
disc is exceeded, the two driving members rotate relative to one another 
and, during such rotation, the spring maintains the frictional engagement 
of the two driving members with the friction disc so that a 
disadvantageously high no-load torque exists between the driving members, 
as mentioned above. 
The primary object of the present invention is to provide a safety clutch 
for the transmission of torque distinguished by a minimum no-load torque. 
In accordance with the present invention, rolling bodies are positioned 
between the disc-shaped driving members with the rolling bodies seated in 
recesses or indentations in the facing surfaces of the driving members 
while their frictional surfaces are in contact. If the limited torque to 
be transmitted by the clutch is exceeded, then the driving members rotate 
relative to one another and the rolling bodies move out of the recesses in 
at least one of the driving members with an axial shifting of the driving 
members taking place against the biasing action of a spring. 
With the clutch engaged, the driving members are in mutual frictional 
contact due to the spring force acting on them so that the degree of 
frictional engagement can be controlled by the appropriate choice of the 
spring and a suitable choice of the material for the frictional surfaces 
of the driving members. In the engaged state, each rolling body is 
arranged between the facing surfaces of the driving members and is seated 
passively in a pair of cooperating recesses or indentations in both 
driving members. Advantageously, the recesses have the same depth and 
shape. 
If the tool is jammed during use, a very high torque suddenly acts on the 
driving members and this torque overcomes the frictional engagement 
between the driving members and causes them to rotate relative to one 
another. At the commencement of this relative rotation, the rolling bodies 
are displaced out of the recesses in at least one driving member and cause 
an axial lifting or displacement of the one driving member relative to the 
other. Further, contact between the friction surfaces of the driving 
members is also immediately interrupted and the transmission of torque 
between the driving members is discontinued. 
When the driving members continue to rotate relative to one another, the 
rolling bodies roll between the driving members and maintain them in the 
disengaged position. Accordingly, the rolling bodies form a roller bearing 
for the driving members which rotate relative to one another with the 
relative rotation between the driving members taking place under rolling 
friction and, therefore, without the significant frictional restraint, 
that is, minimum no-load torque is obtained. Frictional reengagement of 
the driving members while they rotate relative to one another can be 
prevented even at low working speeds by means of indentations of recesses 
with a saw-tooth profile. With such a profile, the steep side is arranged 
in front of the rolling direction of the rolling bodies. As a result, the 
rolling bodies jump over the deepest portion of the indentation 
maintaining the friction contact surfaces in spaced relation to one 
another. 
To obtain a defined and uniform disengagement of the driving members and 
also to ensure planar-parallel guidance of the driving members relative to 
one another in the disengaged state, it is preferable if at least three 
rolling bodies are provided at equiangular spacings and at equal radial 
distances from the center or axis of the driving members. 
During relative rotation of the driving members, experience has shown that 
the rolling bodies which have lifted out of the indentations maintain the 
same angular spacing as they continue to roll. With time, however, the 
angular spacing may vary due to contamination or other impairing 
influences, therefore, another feature of the invention involves the 
provision of a cage for guiding the rolling bodies. The cage may be 
constructed as a star having arms maintaining the rolling bodies in the 
desired angular spacing. An advantageous and simple construction of the 
cage, however, has been found to be in the form of a ring having bores for 
receiving the rolling bodies in the desired angular spacing and also in 
the radial spacing of the indentations from the center of the driving 
members. Accordingly, the receiving bores maintain the rolling bodies or 
balls at the requisite radial distance from the center of the ring or of 
the driving members. Such an arrangement is especially advantageous in 
high-speed devices in which the clutch members are subjected to 
significant centrifugal force. 
Spherical rolling bodies have proved to be problem-free, while cylindrical 
rolling bodies are also suitable, especially for the application of great 
disengaging forces. Ball-shaped rolling bodies have the significant 
advantage of a good rolling behavior. Moreover, balls can be mass-produced 
and, therefore, are inexpensive and can be obtained in any desired 
quality. 
For guiding the balls in the circumferential direction, at least one of the 
driving members is provided with an annular guide track. With such an 
arrangement, maintaining the desired radial spacing of the rolling bodies 
by means of the cage may be unnecessary. 
To obtain high contacting forces, it is preferable if the driving members 
have conical friction surfaces. The angular disposition of the friction 
surfaces can be advantageously selected so that a self-locking engagement 
is provided between the driving member. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its use, reference 
should be had to the accompanying drawings and descriptive matter in which 
there are illustrated and described preferred embodiments of the invention 
.

DETAIL DESCRIPTION OF THE INVENTION 
The friction clutch embodying the present invention is illustrated in FIG. 
1 and consists of a drive member 1 fixed on a shaft 2 by means of a 
half-round key 3. The drive member 1 rotates with the shaft and is held so 
that it cannot move axially relative to the shaft. Concentrically mounted 
about the shaft 2 is a driven member 4. External toothing 5 is located 
about the outer periphery of the driven member 4. A sliding bearing 6 is 
positioned between the drive member 1 and the driven member 4 and serves 
for the rotatable support of the driven member relative to the drive 
member. Further, this sliding bearing ring 6 also ensures the ability of 
the drive member 1 and driven member 4 to effect relative axial movement. 
A compression spring 7 laterally encircles the shaft 2 and is supported on 
the drive member 1 and biases the driven member 4 into frictional contact 
with the drive member. Driven member 4 has a frusto-conically shaped 
friction surface 8 in contacting engagement with a corresponding 
frusto-conical friction surface 9 on the drive member. The friction 
surfaces 8 and 9 afford the transmission of torque between the drive 
member 1 and the driven member 4. 
Each of the drive member 1 and the driven member 4 have a surface extending 
transversely of the axis of the shaft 2 and facing the other. Each of 
these surfaces has V-shaped indentations 11, 12 in the drive member 1 and 
driven member 4, respectively. In the engaged state of the clutch shown in 
FIG. 1, the V-shaped indentations are located opposite one another. Each 
pair of indentations 11, 12 are distributed equiangularly about the 
surfaces of the members 1, 4 in three equiangular spacings. A rolling body 
in the form of a ball 13 is positioned in and extends between each pair of 
the indentations. Balls 13 are mounted in annular cage 14 located between 
the facing surfaces of the drive member 1 and driven member 4. Receiving 
bores 15 are provided in the cage and hold the balls 13 at the desired 
equiangular spacing of the indentations. The cage 14 is rotatably arranged 
in the space between the facing surfaces of the drive member 1 and the 
driven member 4. 
An annular guide track 16 formed in the facing surface of the driven member 
4 serves as a guide for the balls 13 when the clutch is in a disengaged 
state. The annular guide track 16 is spaced radially outwardly from the 
center of the shaft at the same distance as the indentations 11, 12 and, 
accordingly, leads the balls into the three indentations 12 formed in the 
driven member 4. 
In FIG. 2 the three equiangularly spaced indentations 11, 12 and the 
similarly spaced balls 13 are shown. Furthermore, the shape of the guide 
track 16 is illustrated and it can be seen in FIG. 1 that the guide track 
16 has a smaller depth than the indentations 12. 
As shown in FIG. 1, the drive member 1 and the driven member 4 are in 
engagement so that they rotate together as a unit. When the torque being 
transmitted through the clutch, as defined by the frictional engagement 
between the friction surface 8, 9 is exceeded, the drive member 1 and the 
driven member 4 begin to rotate relative to one another. Accordingly, the 
balls 13 are forced out of indentations 11, 12 and cause the drive member 
and driven member to be forced axially apart so that the engaging contact 
between the friction surfaces 8, 9 is interrupted. As the drive member 1 
and driven member 4 continue to rotate relative to one another, the balls 
13 roll in the guide track 16 and on the same circle on the facing surface 
of the drive member 1. In this way, the drive member 1 and driven member 4 
are displaced out of engaging contact and can rotate relative to one 
another under a minimum no-load torque. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the inventive principles, it 
will be understood that the invention may be embodied otherwise without 
departing from such principles.