Safety clutch for power-operated hand-held tool

In a motor operated hand-held tool, such as a hammer drill, a drive train transmits driving force from a motor to a drive spindle. The drive train includes a safety clutch positioned on a shaft for interrupting the transmission of driving force if a torque overload occurs. The safety clutch has a hub encircled by a gear ring with spring-biased balls releasably interengaging the hub and ring for transmitting the driving force. The hub is engaged with a thread on the shaft and under normal operation a spring prevents the hub from being rotated relative to the shaft and being displaced axially along the shaft. During torque overload conditions the hub acting against the spring moves axially along the shaft until the hub and gear ring are released.

SUMMARY OF THE INVENTION 
The invention relates to a power-operated, hand-held tool, such as a hammer 
drill, with a safety clutch located in the drive chain between a drive 
motor and a work spindle with the clutch interrupting the transmission of 
force in the event of a torque overload. The safety clutch includes a hub 
positioned on a shaft and a ring or rim releasably connected to the hub by 
clutch members. 
In power-operated, hand-held tools, such as hammer drills, the tool becomes 
jammed and, therefore, the transmission of the driving force is blocked. 
Jamming of the tool can be caused, for example by contacting reinforcing 
steel when drilling through concrete. When a tool is used on wood, the 
danger of jamming is caused by knots in the wood. To protect the drive 
motor and also the person using the tool, safety clutches are usually 
mounted in such tools and interrupt the transmission of force if a torque 
overload occurs. 
Known safety clutches are designed so that the torque remaining after the 
safety clutch has been disengaged can be easily handled by a person using 
the tool. It has been found, however, that extremely high torque values 
occur up until the time the safety clutch operates. Such high torques act 
suddenly on a person using the tool and can lead to the possibility of an 
accident, especially if the tool operator is working at a dangerous 
location, such as on a scaffold, a ladder or the like. 
The primary object of the present invention is to improve the operational 
safety of a hand-held tool of the above-mentioned type. 
In accordance with the present invention, a shaft threaded for at least a 
portion of its length mounts the safety clutch and the clutch includes a 
hub having projections which engage into the thread of the shaft. For the 
transmission of torque, a spring element biases the hub on the shaft and 
prevents it from rotational movement relative to the shaft which would 
result in axial movement along the shaft. 
Accordingly, the hub of the safety clutch is not rigidly connected to the 
shaft on which the clutch is mounted. When the work spindle and, as a 
result, the shaft in engagement with the spindle are blocked from 
rotation, the safety clutch does not have to be released immediately. In 
accordance with the present invention, when a torque overload occurs the 
hub of the safety clutch can be displaced axially along the thread on the 
shaft. When the hub moves along the shaft, the spring element biasing the 
hub is tensioned to an increasing degree. The safety clutch is disengaging 
when the hub contacts a rigid stop or when the increasing force of the 
spring element exceeds the axial force generated by the threaded 
connection between the hub and the shaft. By appropriately sizing the 
spring element, the torque increases slowly as it approaches the torque 
required for the disengagement of the safety clutch. During this time, the 
tool operator can either turn off the power to the drill or can apply an 
appropriate countertorque. Therefore, the possibility of an accident is 
practically eliminated. 
In connecting the hub with the shaft, the projections on the hub engaging 
the thread on the shaft can be in the form of an internal thread. When 
such an internal thread is used, it must be noted that if the work spindle 
is blocked, the response time of the safety clutch depends on the pitch 
angle of the thread, that is, the response time increases as the pitch 
angle decreases. To provide for the possibility of using a smaller pitch 
angle on the thread, the friction between the shaft and the hub must be 
reduced. To effect such a reduction, it is advantageous if the projections 
on the hub are in the form of the balls of a ball spindle. By means of 
such a ball spindle, the sliding friction is replaced by a significantly 
lower rolling friction. Since the rolling friction is less dependent upon 
the state of lubrication and, thus, indirectly less dependent upon the 
temperature, it is possible to achieve a higher operational safety of the 
device in accordance with the present invention. 
After the hub has been screwed in the axial direction along the shaft due 
to the torque overload or blocking of the rotation of the work spindle, it 
is necessary to return the hub in the axial direction along the shaft to 
place the tool in condition ready for operation. To assure that the return 
of the hub is effected automatically by the force of the spring element, 
it is advantageous if the threads of the shaft of the hub are dimensioned 
so that they are not in the self-locking range. Accordingly, if the power 
to the tool is switched off, the hub can be returned to its original 
position by the spring element. Considering the frictional conditions 
common in metallic contact, a thread of coarse pitch is required for a 
secure operation. 
The spring element biasing the hub must have a great working storage 
capacity and, especially, a relatively great working traverse. Since the 
space available for such a member is limited, it is advantageous if the 
spring element is a compression spring. By selecting the spring 
characteristic and providing an appropriate amount of tensioning, the 
delay time until the response of the safety clutch to torque overload can 
be fixed quite accurately. 
During normal operation, the shaft and the hub rotate at the same speed. To 
prevent any unnecessary friction, it is advantageous that the spring 
element is axially supported on the shaft. When the work spindle is 
blocked, the shaft is also held from rotating. Through appropriate 
frictional conditions, it can be provided that the spring element rotates 
at a slower speed than the hub, so that the relative motion at the contact 
surfaces with the shaft or with the hub and the resulting wear are 
reduced. 
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 
In FIG. 1 a hammer drill embodying the present invention is illustrated and 
includes a housing 1 having a handle 2 located at one end. A power feed 
line 3 extends into the handle 2 and supplies the electrical energy for 
operating the drill. Further, a trigger 4 is mounted in the handle by 
means of which the hammer drill is operated. A drive motor known per se 
and not shown, is located within the housing 1. Drive shaft 5 coupled to 
the motor shaft or formed integrally with it extends upwardly through the 
housing. A pinion 5a is mounted on the end of the drive shaft 5 more 
remote from the drive motor. Pinion 5a is in meshed engagement with a gear 
6 secured on a crank shaft 7. Reciprocating motion is imparted to an 
excitor piston 9 by the crank shaft 7 through a connecting rod 8 
eccentrically attached to the crank shaft. Piston 9 is guided within a 
cylinder 10 rotatably supported in the housing 1. The percussive force 
generated by the excitor piston 9 is transmitted to a percussion piston 12 
by an air cushion 11 located between the two pistons. 
Shaft 5 includes a splined shaft portion 5b on which a slide wheel is fixed 
for rotation with the shaft, however, it is also axially slidable along 
the shaft. Slide wheel 13 is in meshed engagement with a gear ring 14. A 
safety clutch 15 is arranged in the interior of the gear ring 14. Safety 
clutch 15 consists of a hub 15a and balls 15b located in radially 
extending bores open at the outer circumferential periphery of the hub 
15a. In the event of a torque overload, the balls 15b move against the 
biasing action of the springs in the bores interrupting the transmission 
of force between the gear ring 14 and the hub 15a. Hub 15a is positioned 
on a shaft 16. A portion 16a of the shaft 16, on which the hub 15a is 
positioned, is threaded. In FIG. 1, the hub 15a has an internal thread 15c 
in threaded engagement with the threaded portion 16a of the shaft 16. 
Accordingly, due to its threaded interengagement, the hub 15a can be 
screwed on, that is, moved axially along, the shaft 16. A spring element 
17 in the form of a compression spring laterally encircling the threaded 
shaft portion 16a biases the hub 15a against axial movement along the 
shaft 16. The threaded shaft portion 16a and the internal thread 15c on 
the hub are dimensioned so that the two parts are not in the self-locking 
range. Accordingly, the compression spring 17 can overcome the friction 
existing between the hub 15a and the shaft 16 and automatically return the 
hub 15a into the illustrated position when the torque overload is 
released. 
A bevel pinion 16b is fixed on the end of shaft 16 adjacent the cylinder 
10. Bevel pinion 16b engages bevel gear teeth 10a formed on the cylinder 
10. Accordingly, rotational motion is imparted to the cylinder 10 by the 
drive shaft 5 via the slide wheel 13, the gear ring 14, safety clutch 15 
and the shaft 16. A work spindle 18 is fixed to the end of the cylinder 
extending from the front end of the housing 1, that is the end opposite 
the handle 2. The spindle rotates with the cylinder. If, during operation, 
the work spindle 18 is blocked from continued rotation due to jamming of a 
tool 19 positioned in the work spindle 18, the shaft 16 is also brought to 
a standstill. If the safety clutch does not release, then the hub 
continues to rotate. With the shaft 16 held against rotation, the hub 15a 
is screwed around the external thread 16a of the shaft 16 against the 
biasing action of the compression spring 17. For continued compression of 
the spring 17 an increasing torque is required. After the hub 15a 
executes a certain number of rotations about the shaft 16 which are 
adjustable by the spring characteristic of the balls 15b and the 
compression spring 17, the torque developed exceeds the torque needed to 
release or disengage the safety clutch. The time required to attain the 
disengaging action of the safety clutch 15, results in a time delay for 
the response of the safety clutch. Accordingly, it is possible for the 
operator to release the trigger 4 and thereby switch off the tool. When 
work is performed which requires a torque slightly lower than the torque 
for releasing the clutch, the operator can prepare himself for the 
occurrence of such peak torques or forces. After the tool has been turned 
off, the hub 15a and with it the gear rim 14 and the slide wheel 13 are 
returned to the illustrated initial position by the compression spring 17. 
The detail of another embodiment of the engagement between the hub and the 
shaft is shown in FIG. 2. Safety clutch 25 includes hub 25a and spring 
biased balls 25b arranged in radially extending bores open at the outer 
circumferential periphery of the hub. Hub 25a is connected to a shaft 26. 
In the region of the connection of the hub 25a to the shaft 26, the shaft 
has a spindle-like thread 26a. The connection between the hub 25a and the 
spindle-like thread 26a is effected through projections or balls 25c. In 
principle, this arrangement operates exactly in the same manner as the 
embodiment illustrated in FIG. 1. Due to the significantly lower rolling 
friction in this arrangement, however, the pitch angle of the thread can 
be chosen smaller so that, for the available axial shifting path, more 
revolutions and, thus, a longer delay time is required. 
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.