Adaptable safety clutch

An overload clutch, particularly for use with manually operated drilling tools, has a release or disconnect moment adjusted at a predetermined velocity as a function of the applied torque, whereby in normal, malfunction-free operation the clutch does not release or disconnect. In the case of a malfunction, for instance if the tool is blocked or seized, a rapid rise in the required torque occurs and the clutch, having inertia or damped by a volume change, releases or disconnects and immediately interrupts the torque transmission from the tool motor to the drilling member. A purely mechanical solution for the safe use of such drilling tools is afforded by the provision of a two-part shaft with two co-axially guided shaft parts (3, 7). A drive element (2) is engaged in positively locked manner and acts on a drive side shaft part (7) by a mobile thread or a sliding link (4), so that the drive element (2) prestresses a spring (5) depending on the applied torque.

BACKGROUND OF THE INVENTION 
The present invention is directed to an overload safety clutch, especially 
for use in manually operated drilling tools, where the torque transmitted 
from a drive side to a driven side is effected by a connection provided by 
a prestressed or preloaded spring element. 
When using drilling tools, such as a drilling machine, a hammer drill, and 
diamond tip drilling tools, a large torque can be caused by the material 
being drilled. The maximum allowed torque for a tool of the type mentioned 
can be defined relatively accurately. The effect of the torque on the tool 
and especially in manual tools on the operator can be very different. If 
the operator stands on a ladder and operates the tool with only one hand, 
then he produces less opposing moment than if he is standing on the ground 
and applies pressure with his entire body against the drilling tool. In a 
hard material being drilled, the torque evidently increases more slowly 
than in the case when "hitting iron". In unfavorable applications and 
stability situations a very slight torque change, a reaction moment can 
result in accidents having severe consequences. 
To protect against excessive torque, it has been known to use friction or 
ratchet clutches representing a compromise between operator and tool 
protection. However, the result of such compromise, is unsatisfactory, 
since known overload clutches, if the full output range of the tool is 
used, can result in accidents, as mentioned above, in view of the reaction 
moment developed in the drilling tool in the event the drill bit is 
seized. 
To identify the malfunction based on the seizing of the tool bit and to 
trigger countermeasures in good time, for instance, by interrupting the 
drive train of the tool with a tool bit by a quick-acting electromagnetic 
clutch, various solutions have been known to avoid the problem before the 
operator is injured. As a rule, in electromechanical or mechanolelectronic 
solutions, where an acceleration component at the tool housing exceeding a 
predetermined threshold or a velocity component is determined on the basis 
of the reaction moment by means of a sensor responding to mass effects, 
such an indication is converted into an electrical signal and used for 
actuating or releasing the separation clutch. Examples of electromagnetic 
solutions employing velocity sensors are described in DE 33 46 215 A1 and 
WO-DE 88-00 109 and/or proposed in a modern solution using acceleration 
sensors and digital electronics in DE patent application P 43 44 817.8. In 
these electromechanical or mechanoelectronic solutions, a sensing system 
checks certain conditions, for instance, the rotational acceleration, a 
rapidly rising current value, a rising torque and the like. The 
electronics recognizes the malfunction as soon as the sensor signals 
excessive specific limit or threshold values and/or because of previously 
stored process development or operating conditions. The electronics then 
averts the dangerous consequences of the malfunction by suitable measures, 
such as uncoupling or braking the drive motor. 
All of the known or proposed solutions require, apart from the sensor 
system, an electronic recognition processing and amplification circuit. 
SUMMARY OF THE INVENTION 
Therefore, the primary object of the present invention is to provide an 
economical, purely mechanical solution to the known or proposed 
electromechanical or mechanoelectronic solutions. 
In accordance with the present invention, an overload safety clutch, where 
the transmission torque between the drive side and the driven side occurs 
by a connection produced by a prestressed spring, for the adaptation of 
the disconnect moment as a function drive moment, a drive side or driven 
side shaft is designed as a two-part shaft with both parts arranged 
co-axially and rotatable relative to one another, wherein an axial 
displaceability exists between the drive side shaft part and a drive 
element engaged in a positively locked manner, so that the prestressed 
spring presses increasingly or more forcefully against a snap-in means 
connected with the driven side shaft part in the event of an increasing 
drive moment. 
Compared with known electromechanical or mechanoelectronic solutions, the 
adaptable overload clutch embodying the present invention is based on a 
different functional principle. In the present invention, a purely 
mechanically acting overload clutch is involved where the disconnect or 
release moment is adjustable. The adjustment of such moment occurs 
automatically as a function of the respectively acting power torque. The 
release moment is set up somewhat greater than the existing operational 
torque, so that in normal operation a disconnect of the overload clutch 
does not take place. 
The drive element incorporating an inert mass or an adaptable dampening 
device preferably acts through an irreversible mobile thread or by means 
of a sliding link connection in a positively locked manner on the shaft 
part of the drive side. As can be noted in the following description, 
dealing with additional details, it is advantageous to operate the two 
shaft parts co-axially so that they overlap or telescope one in the other, 
with the driven side shaft part fitted into the drive side shaft part and 
the ratcheting or snap-in means formed of balls, rollers or the like 
arranged in the form of an annular shoulder and retained on the driven 
side shaft part, with the balls, rollers and the like penetrating through 
an overlapping end of the drive side shaft part, whereby the prestressed 
spring presses by means of a pressure ring against the annular shoulder. 
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 drawing and descriptive matter in which there is 
illustrated and described a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
In the moment-time graph of FIG. 1, the lower curve displays the possible 
actual course of the torque of a drilling tool operating without 
malfunction. The upper curve shows the maximum torque which can be 
transmitted by the adaptable safety overload clutch, wherein the course of 
the release moment or maximum torque up to the highest allowable value 
M.sub.max (compare FIG. 2) follows the shape of the power torque, because 
due to the invention, the release moment of the safety clutch follows a 
selectable or controllable adjustment velocity. It can be noted that the 
release moment is set somewhat higher than the applied torque, so that no 
disconnection of the clutch occurs in normal operation, as mentioned above 
and as can be seen in FIG. 1. 
The adjustment velocity can be limited, as shown in FIG. 2, through 
specific measures according to the invention to be explained later. If the 
torque increases faster than the maximum adjustment velocity permits, for 
instance when the tool bit is seized or jammed, an overshoot of the 
release threshold occurs before the adaptable overload clutch is able to 
form an adaptation to the actual situation as represented in FIG. 3. In 
this malfunction situation, the overload clutch will disconnect and 
suddenly interrupt the power train from the drive motor to the tool bit, 
whereby the tool operator is protected. 
The basic design principle of the inventive adaptable safety clutch 1 is 
illustrated in FIG. 4. As is known in sliding or ratchet clutches of 
similar construction, the torque transmission between the drive part 2 
(for instance a pinion), and the driven part 3 occurs by a lock produced 
by a spring 5 prestressed against a holding or snap-in ratcheting means 6. 
In conventional or ratchet clutches of this type, the release moment or 
the moment M which can be transmitted remains approximately constant. 
In the adaptable sliding overload clutch of the invention, the prestressing 
of the spring 5 is controlled. For this purpose, the driven or output 
shaft in the power train for the tool bit, not illustrated, is designed in 
two parts, it comprises, as shown in FIGS. 4-7, of two axially extending 
shaft parts arranged co-axially to one another. In the embodiments shown, 
the driven side shaft part 3 is located within the drive side shaft part 
7. The two shaft parts 3, 7 are thus rotatable relative to one another. 
The drive element 2 (possibly a gearwheel or a guide sleeve 22 as in FIG. 
7) engages the drive side shaft part 7 through a mobile thread, a link 
guide or the like at the drive side shaft part and is designated uniformly 
as a guide 4. The drive part 2 (gearwheel, guide) presses with an end face 
10 facing towards the driven side shaft part 3 along with the tool bit 
against the spring 5, and thus the spring is pressed to a greater or 
lesser extent against the stop or the snap-in ratchet means 6 which is 
connected with the driven side shaft part 3, as a component of the turning 
motion acting through the guide 4 due to the axial displaceability between 
the drive side shaft part 7 and the drive element 2, depending on the 
acting torque M. Because of the relative axial displaceability of the 
element 2 and the part 7 with respect to one another, because of the guide 
4, the spring is stressed to a greater or lesser degree by the applied 
torque and thus the releasing moment of the overload clutch is set up in 
adaptation to the moment desired at the driven side shaft part 3. 
Several force-vector diagrams are shown in the basic illustration of the 
adaptation overload clutch 1 of the invention as illustrated in FIG. 4, 
displayed as axially or circumferentially acting force components at 
specific effective points of the clutch for a specific acting torque M. 
To clarify the force or moment transmission behavior of the adaptable 
clutch system in FIG. 4 in the case of moment transmission, that is when 
operating without malfunction, (note FIG. 1) or at the moment of release 
(note FIG. 3), two vector diagrams (FIGS. 4a, 4b) of individual 
force/axial components are shown diagrammatically and are explained as 
follows. 
The force components acting on the adjustment or drive element 2 due to the 
moment can be resolved into an axial adjustment force Fv and a 
circumferential force Fuv. The axial component Fv is counteracted by an 
inertia or dampening force Fd. The force component Fd differs in magnitude 
depending on the operating situation, it remains approximately constant 
during operation without malfunction as shown in FIGS. 1 and 2 as well as 
in the vector diagram in FIG. 4a. The axial force Ffv transmitted to the 
spring 5 results in Ffv=(Fv-Fd). A computable spring force Ffa results, 
based on the Moment at the snap-in means in the moment transmission, acts 
on the driven side shaft part 3 through which the snap-in means effects 
the moment transmission between the shaft parts 3, 7 occurs. The 
circumferential force at the snap-in mechanism is designated by Fua. The 
size of the spring force axial component Ffv is changed according to the 
basic principle of the invention by means of the relative axial 
displacement of the drive element 2 by means of the guide 4 (sliding link) 
shown diagrammatically in FIG. 4. 
In the case of a malfunction, for instance in an "iron strike", the driven 
side shaft part 3 is seized or blocked. Due to mass inertia or an 
installed dampening device, the spring force component Ffv cannot follow 
the necessary torque rise. The axially acting inertia or dampening 
component Fd increases suddenly, so that the axial force component Ffa is 
no longer adequate for transmitting the force due to the pronounced 
increase of the moment required at the driven side shaft part 3 and 
thereby releases the clutch mechanism between thrust pressure ring 9 and 
the snap-in means 6, so that the transmission of torque from the tool 
drive to the driven side shaft part 3 is suddenly interrupted (FIG. 4b). 
As shown in FIGS. 5 and 6, the retaining or snap-in means 6 for the spring 
5 on the driven side shaft part 3 can be formed by a spring preloaded ball 
collar supported in the outside surface of the driven side shaft part 3, 
which penetrates the overlapping hollow shaft section of the drive side 
shaft part 7 and serves as an abutment for a pressure or thrust ring 9 
having a chamfered surface acting against the balls of the snap-in means 6 
and the end of the prestressed spring 5 presses against the thrust ring 9. 
The locked clutch with adaptable adjustment by varying the prestress on 
the spring 5 is provided between the chamfered inside surface of the 
thrust ring 9 and the balls of the retaining or snap-in means 6 supported 
in the driven side shaft part 3. A force transmission is possible in 
similar designs by means of rollers, rolls, or a set of teeth releasing as 
a function of torque or by means of a purely locked connection. 
As mentioned above, the arrangement of the adaptable clutch system is to 
enable a release moment which is higher than the applied torque. As 
discussed, this takes place by relative axial displacement between the 
drive element and the drive side shaft part 7 with simultaneous variable 
adjustment of the locked clutch connection by torque-dependent prestress 
of the spring 5. As a result, it is assured that the frictional overload 
clutch is subsequently prestressed in the normal case of increasing torque 
instead, as is the case in such clutches in the state of the art, of 
releasing or disconnecting at a specific torque. 
In the invention the displacement velocity for the adjustability of the 
torque is limited, and can be achieved by designing the adjustment or by 
having the drive element 2 provided with a specific mass 8 affording a 
mass inertia. Alternatively, the drive element 2 can be provided with a 
dampening device 11 for volume displacement as shown in FIG. 6, which 
becomes effective, if necessary, only beginning with a specific drive 
moment. In FIG. 6, the dampening component 12, shown schematically, is 
integral with the housing. 
The proven and tested laboratory embodiment of the inventive adaptable 
safety clutch is shown in FIG. 7. In this embodiment, the drive element 2 
comprises a splined shaft 20 with a flanged attachment 21 with a bolted 
counterflange 23 for the internal side retention of a ball bearing 31, for 
the driven side shaft part 3 secured by the same bolts with a guide sleeve 
22 for effecting the transmission of torque. The guide sleeve 22 has a 
slide groove 24 on its inside surface in which an entrainment wedge 70 is 
engaged so that it is displaceable in the axial direction. The wedge 70 is 
solidly connected with the drive side shaft part 7, shaped as an 
adjustment ring or sleeve. The axial displacement of the shaft part 7 
formed as an adjustment ring is effected by the guide 4 (sliding link) in 
which a slide lug 40, connected with the adjustment ring, engages. An 
axially extending volume of air 50 is located within the guide sleeve 22 
between the internal surface of the flange portion 23 and the oppositely 
located end face of the adjustment ring 7 with the volume of air serving 
as a dampening agent. 
As in the case of the two embodiments described and illustrated in FIGS. 5 
and 6, the snap-in means 6 against which the thrust ring 9 abuts, is 
shaped as a ring of spring preloaded balls which on one hand abut against 
a hollow shaft-like core section 32 of the driven side shaft part 3 and on 
the other hand penetrate a surrounding sleeve section 30, in which the 
guide 4 (sliding link) is located. 
The purely mechanical solution of the invention for protecting the operator 
in manually operating drilling tools against the consequences of an 
unexpectedly strongly increasing reaction moment by means of an adaptable 
safety overload clutch with a self-adjusting release moment provides, with 
high operational security, an advantageous alternative in many cases to 
the previously mentioned mechanoelectronic or electromechanical solutions 
of the problem. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the inventive principles, it will be understood 
that the invention may be embodied otherwise without departing from such 
principles.