Power tool safety mechanism

In a portable motor driven tool such as a drill, a handle is connected to the housing in a manner permitting limited relative movement, in particular pivotal movement, between housing and handle. Biasing springs maintain the handle in a first position, and a normally-closed switch in series with the tool motor has an operator element engaging the handle. If the tool becomes stuck in a workpiece during operation, the sudden reverse force acting between the handle and housing overcomes the spring bias force causing relative movement between handle and housing to open the switch and break the motor electric circuit thereby preventing safety hazards and damage.

BACKGROUND OF THE INVENTION 
This invention relates to the art of portable motor-driven tools, and more 
particularly to a new and improved safety measure for such tools. 
One area of use of the present invention is in portable motor-driven tools 
where the work output is provided by a rotating shaft, although the 
principles of the present invention can be variously applied. In the 
operation of portable motor-driven tools, for example drills, automatic 
screwdrivers, tappers, tube expanders and the like, it sometimes happens 
that during use the tool, such as a drill bit, becomes stuck or lodged in 
the workpiece. If this happens, the entire motor reaction force then is 
transferred to the housing which normally is held in the operator's hand. 
If the tool becomes stuck in the workpiece suddenly, the resultant sudden 
reverse torque may be sufficiently great to cause the operator to lose his 
grip on the tool and even to be forced off balance himself. The foregoing 
presents safety hazards and possibility of serious personal injury and 
also costly damage to equipment. 
SUMMARY OF THE INVENTION 
It is, therefore, a principle object of the present invention to provide a 
new and improved portable motor-driven tool with an effective safety 
measure operative if the tool becomes stuck or lodged in a workpiece 
during operation thereof. 
It is a further object of this invention to provide such a safety measure 
which stops power to the portable motor-driven tool if the tool becomes 
stuck or lodged in a workpiece during operation thereof. 
It is a further object of this invention to provide such a safety measure 
which is effective yet does not interfere with normal operation of the 
portable motor-driven tool. 
It is a further object of this invention to provide such a safety measure 
for a portable motor-driven tool which is relatively simple in 
construction and easy and economical to manufacture. 
The present invention provides in a portable motor-driven tool including a 
motor housing, a motor operatively disposed within the housing, an 
electric circuit for the motor and a handle for grasping by the user 
during operation of the tool, a safety measure comprising means connecting 
the handle to the housing in a manner permitting limited movement, in 
particular pivotal movement, of the handle with respect to the housing 
between first and second positions, and a normally-closed switch in the 
motor circuit having a switch operator element operatively connected to 
the handle such that the switch is closed when the handle is in the first 
position. The handle is maintained in the first position by biasing means. 
In response to a force acting between the housing and the handle 
sufficient to overcome the biasing means, such as when the tool happens to 
become stuck or lodged in a workpiece during operation, the handle is 
moved to the second position opening the switch to break the motor 
electric circuit. 
The foregoing and additional advantages and characterizing features of the 
present invention will become clearly apparent upon a reading of the 
ensuing detailed description together with the included drawing wherein:

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
Referring now to FIG. 1, a portable, motor-driven tool having a safety 
mechanism according to this invention is generally designated 10, and in 
the present instance the tool is in the form of a hand-held power drill. 
While the present invention will be illustrated in connection with a 
portable power drill, the invention is applicable to other varieties of 
portable motor-driven tools, for example automatic screw drivers, tappers, 
tube expanders and the like. The power tool 10 includes a housing 12 and 
an electric motor (not shown) operatively disposed withon housing 12 in a 
known manner. The output drive shaft of the motor within housing 12 is 
drivingly connected to a conventional chuck assembly 14 extending out from 
housing 12 and a standard drill bit 16 is received and held in the chuck 
14 in a known manner. 
The tool is provided with a handle generally designated 20 which is 
elongated and extends outwardly from housing 12. In the present 
illustration the drill 10 is provided with a trigger-like control switch 
operator 22 located on the handle 20 adjacent the end nearest to the 
housing 12. Switch 22 controls the on-off states of power tool 10 in a 
known manner, and also as in some commercially available hand power drills 
it can control the speed of operation. A relatively smaller auxiliary 
switch 24 can be included on handle 20 for providing additional control 
modes in a known manner. The portable power drill 10 can include an 
additional handle 26 which is elongated and fixedly attached at one end to 
the housing 12 so as to extend outwardly from the housing at a location 
generally opposite the location of handle 20. 
In accordance with the present invention, handle 20 is connected to housing 
12 in a manner permitting limited movement of handle 20 with respect to 
housing 12 between first and second positions. In particular, and 
referring now to FIGS. 2 and 3, housing 12 includes an interior chamber or 
open region designated 30 which communicates with an opening in the wall 
of housing 12. An electrical switch 32 is located within chamber 30 
adjacent the opening in the housing wall surface. Switch 32 is fixedly 
mounted in housing 12 by a suitable means such as the screw and bolt type 
fastners shown in FIGS. 2 and 3 and includes a switch operator element 36. 
Switch 32 is located and disposed so that operator 36 is located adjacent 
the plane of the opening in the wall of housing 20. Switch 32 can be of 
various commercially available types being small in size and having 
electrical current carrying capability required in tools of this general 
type. Switch 32 has an adjustment screw designated 38 for setting the 
initial position of operator 36. Handle 20 is positioned such that one end 
thereof is adjacent housing 12 to cover the opening which leads into 
chamber 30. In particular, handle 20 is formed to include a relatively 
flat planar end surface 40 extending over substantially all of the one end 
of handle 20. When handle 20 is in assembled position in the tool 10, the 
surface 40 is in operative contact with the switch operator element 36 and 
the marginal or peripheral surface 42 of handle 20 adjacent surface 40 is 
substantially flush with a corresponding outer peripheral surface 44 of 
the portion housing 12 surrounding the opening leading to region 30. 
Handle 20 is pivotally connected to housing 12, and one illustrative 
pivotal connection includes the arrangement of a central elongated recess 
48 provided along one edge of the end of handle 20 which is boardered or 
terminated at opposite ends thereof by a pair of upstanding relatively 
short leg elements 50, 51. Recess 48 is curved and extends from end 
surface 40 to peripheral surface 42. Housing 12 is formed to include a 
projection or extension 53 along the edge thereof leading from surface 44 
and adjacent the recess 48 in handle 20. The extension of flange 53 has a 
length slightly less than the length of recess 48 and is curved or rounded 
along the lower edge thereof as illustrated in FIG. 2. When handle 20 and 
the housing 12 are assembled together, the projecting portion 53 fits into 
the recess 48 formed in the handle, and the housing is provided with 
recesses at opposite ends of the projection 53 to receive the upstanding 
leg elements 50, 51. The handle and housing then are pivotally connected 
together by a shaft or rod-like element 55 which extends substantially 
along the entire length of the one end of the assembly and which is 
rotatably received in the extension 53 and the upstanding leg elements 50, 
51. The pin 55 is rotatably connected in these parts and fixed by suitable 
means against longitudinal or axial movement. Thus handle 22 and housing 
12 are pivotally connected together along one common edge portion of the 
two components. Other mechanical arrangements can of course be employed to 
provide a pivotal connection between housing 12 and handle 20. 
The apparatus of the present invention also comprises biasing means for 
maintaining the handle 20 in a first or normal position relative to 
housing 12. The biasing means is located adjacent the common edges of the 
handle end and the housing spaced from and opposite the location of the 
pivotal connection. In the present illustration, the biasing means 
comprises a pair of spaced apart coil springs 59, 60 each of which springs 
has one end operatively engaging housing 12 and the other end operatively 
engaging handle 20. As shown in FIG. 3, the one end of spring 59 is seated 
in a recess provided in the end surface 40 of handle 20, and the opposite 
end of spring 59 is received in a recess or short bore provided in the 
wall of housing 12 and which is in alignment with the spring-receiving 
recess formed in the end of handle 20. Similarly, the one end of spring 60 
is received or seated in a recess provided in the end surface 40 of handle 
20, and the opposite end of spring 60 is received in an aligned recess or 
short bore provided in housing 12. A pair of bolts 62, 63 are operatively 
connected to handle 20 and housing 12 adjacent the biasing springs 59, 60 
in a manner limiting the relative movement between the two parts. In the 
present illustration, bolt 62 is received in aligned bores in the handle 
and housing having a common axes coincident with the longitudinal axis of 
spring 59, the bolt 62 extending along and within spring 59. Bolt 62 is 
threadably connected in the wall of housing 12, and the head of bolt 62 
abuts an external surface of handle 20. Similarly, bolt 63 is received in 
aligned bores in the handle and housing having a common longitudinal axis 
coincident with the axis of spring 60, the bolt 63 extending along and 
within spring 60. One end of bolt 63 is threadably connected in the wall 
of housing 12, and the head end thereof abuts an external surface portion 
of handle 20. The heads of bolts 62, 63 thus are externally available to 
permit adjustments. 
The power drill of the present illustration is the type of tool wherein the 
motor drive shaft is coupled to a tool, i.e. the drill bit, to provide an 
output in the form of rotation of the bit. The motor drive shaft and tool 
or bit have a common axis. Handle 20 is pivotally connected to housing 12 
along a pivot axis which is parallel to the axis of the drive shaft and 
tool and which pivot axis is offset from the drive shaft and tool axis. 
The biasing springs 59, 60 engage the housing and handle at a location 
spaced from the pivot axis and spaced from the common axis of the drive 
shaft and tool. 
FIG. 4 is a schematic diagram of the electric power circuit of the tool of 
the present invention including the electric drive motor 65 of the tool 
which is connected by a line 67 to one terminal of a conventional plug for 
connection to an electrical power outlet. The other terminal of motor 65 
is connected through the series combination of the main control switch 22 
and the safety switch 32 of this invention to another line 69 which, in 
turn, is connected to the other terminal of the electrical plug. The 
broken line 71 in FIG. 4 represents an operative mechanical connection 
between these two switches as will be described in further detail 
presently. 
In operation, the portable motor driven tool 10 is held by hand, either by 
the operator grasping handle 20 with one hand or grasping both handles 20, 
26 with his two hands. Switch 32 normally is closed and the operator moves 
the main switch 22 to a closed porition by pressing it with his finger in 
a trigger-like fashion. With the plug being connected to an electrical 
outlet, the power circuit is completed and motor 65 is energized. The 
drill bit 16 is rotated, conventionally in a counter-clockwise direction 
for forward operation, and is moved toward and into a workpiece for 
drilling holes in the usual manner. 
During normal operation of tool 10, handle 20 is maintained in a position 
relative to housing 12 which keeps switch 32 closed. In particular, handle 
20 remains in a position such that a small gap is present between handle 
end surface 40 and the opposed surface of housing 12 as shown in FIGS. 2 
and 3. This is provided by the biasing means in the form of springs 59, 60 
which have force constants of sufficient magnitude. The springs 59, 60 
should exert a biasing force at least great enough to overcome an opposing 
force resulting from the weight of the tool 10, friction between drill bit 
16 and the workpiece and any other ordinary forces which arise during 
operation of the tool and act in a direction opposing the biasing force of 
the springs 59, 60. 
During normal operation of tool 10, the output force or torque from the 
motor is applied to the workpiece and results in work being done thereon, 
such as drilling holes, advancing or removing screws, expanding tubes and 
the like. Should the tool happen to become stuck or lodged in the 
workpiece, however, the entire motor reaction force is transferred to 
housing 12 which is held in the user's hands. By virtue of the safety 
measure of the present invention, this causes relative movement between 
housing 12 and handle 20 to open switch 32 to shut off the tool motor 
thereby avoiding serious injury to the user and those nearly as well as 
avoiding damage to the tool and associated equipment and materials. In 
particular, the motor reaction force transferred to housing 12 is 
considerably greater than the bearing force of springs 59, 60 with the 
result that the end surface 40 of handle 20 and the opposed surface of 
housing 12 are moved together closing the gap therebetween. The surface 40 
moves switch operator element 36 to a position opening switch 32 which is 
in series with motor 65 thereby breaking the motor power circuit and 
stopping motor operation. This of course removes the force or torque which 
otherwise could cause injury or damage in the situation where the tool is 
stuck or lodged in the workpiece. 
The mechanical interconnection designated 71 in FIG. 4 between switches 22 
and 32 illustrates one possible arrangement whereby opening of switch 32 
opens the main power switch 22. With such an arrangement when a lodges 
tool causes operation of safety switch 32, the switch 22 would be opened 
until the operator releases his grip on switch 22 and presses it again to 
restart the motor. Another possible arrangement would be a mechanical 
interconnection between switch 32 and the auxiliary switch 24 such that 
once switch 32 is opened it remains open until the operator presses the 
switch 24. 
Various modifications can be made in the arrangement and location of safety 
switch 32 in relation to housing 12 and handle 20 which satisfy the 
operational requirement that the reverse force or torque arising from a 
lodged or stuck tool causes relative movement between housing 12 and 
handle 20 to operate switch 32 to deactivate the tool motor. For example, 
the switch 32 could be located in a recess provided in handle 20 and 
disposed such that the switch operator element 36 is operatively 
associated with housing 12 and in a manner providing the foregoing 
operation. The location of the first or normal position of handle 20 
relative to housing 12 can be adjusted by rotating the bolts 62, 63. 
The safety mechanism of the present invention for portable motor driven 
tools is effective in preventing personal injury and property damage which 
otherwise can occur when the tool becomes lodged or stuck in a workpiece 
during operation. The foregoing is provided by an arrangement which is 
highly effective yet relatively simple in construction and easy and 
economical to manufacture in existing tool constructions. 
It is therefore apparent that the present invention accomplished its 
intended objects. While a single embodiment of the present invention has 
been described in detail, this is for the purpose of illustration, not 
limitation.