Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENq~
This invention relates to an electrically driven screw-driver
and more particularly to an electrically driven screw-driver which is
very compact in size and convenient in portability and in which only
a prefastening, or screwing step may be carried out by the mechanical
force of an electric motor while a finishing step may be performed
manually.
BACKGROUND OF I~E INVENTION
Electrically driven screw-drivers (using an electric motor
as a power source) may mostly be classified into two types, namely
one which is directly connected to a commercial current source (100 V)
and the other in which a current is supplied to a motor after reduction
of its voltage with a transformer. In either case, a connection cord
is indispensable for connecting the motor to the electric power source.
These screw-drivers may utilize a higher electric output but may be
operated portably in only the range of an extensible cord length.
Thus, these types of electrically driven screw-drivers may be essentially
used for an assembling process in a factory.
There is also known another type of electrically driven screw-
driver which is of a charging type including a battery-replacement type.
This type of screw-driver has a disadvantage of a lower output but has
an advantage of portability due to the absence of a connecting cord.
However, the conventional charging type of the screw-driver requires
a possibly highest output of a motor and therefore a charging unit of
a large capacity, resulting in a big and inconvenient appliance.
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Thus, it may well be stated that there has been no screw-driver of a
charging type, which are small enough to be portable in a pocket,
convenient in handling and resistant to a severe operational condition.
As mentioned hereinabove, a number of batteries are necessary
as a power source for obtaining a higher and practical output for
fastening, which prevents the screw driver from being compact and light.
Even a battery of a limited capacity may be used for obtaining a higher
output if the large number of reduction steps are utilized in a re-
duction system connected to the motor. However, such higher reduction
ratio brings about a decrease in rotation number of an output shaft,
leading to an impractical appliance. Thus, the portable screw-driver
must be sized and designed for balancing a necessary operational capacity
and a sufficient electric power to provide a corresponding output
(or a battery size).
In general, the operational capacity must be so large that the
appliance becomes inevitably too big to be carried by one hand. Thus,
there has been need for a very compact and light electrically driven
screw-driver, which is enough resistant to a severe operational condition.
In view of the foregoing, the inventor has studied for eliminating
the disadvantages of the conventional charging type of portable screw-
driver in the prior art and for developing a very compact, light but
convenient electrically driven screw-driver, and has now found out after
strict analysis of an operational procedure of the electrically driven
screw-driver (hereinafter referred to merely a "screw-driver" for simpli-
fication) that a screw may be fastened into an object, which has already
been threaded for receiving the screw, almost without load for about
90 % of the fastening procedure and that only a finishing step of about
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10% requires an instantaneously strong fastening force. Cn the contrary,
a so-called tapping screw, which is screwed into an ob~ect without a
threaded hole, may be gradually moved into the obJect against a consider-
able resistance while simultaneously making a thread. In this case,
most of the working amount is directed to the tapping step and a
relatively small working amount is directed to a finishing step requiring
a strong fastening force.
Since the single fastening procedure has two steps as described
hereinabove, it has been found out that about 90% of the total working
amount required in the prefastening or screwing step is advantageously
performed mechanically, while the r~maining 10% in the finishing step
of the strong fastening may well be performed manually without a
machanical force. Actually, an extent of output to be required in the
tapping work as described hereinabove may be sufficiently supplied from
a charging battery of a relatively small capacity by combination of a
small motor and a suitable reduction system. However, the finishing
step requires much more fastening force, resulting in a big appliance
with a higher power source if the fastening force is supplied electrically
and mechanically. The conventional portable screw-driver has always
utilized a mechanical and electrical force for performing the whole
procedure or two steps of operation as described hereinbefore, thus
never producing a very compact and l~ght screw-driver which is portable
in a pocket.
It will be appreciated from the foregoing that if only the
prefastening or screwing step occupying most of the total work is per-
formed mechanically but the finishing step requiring a strong fastening
force (which is consumed in a very short time) is carried out manually,
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the mechanical size and the battery capacity may be correspondingly
reduced, thereby producing a truly compact and light portable screw-
driver. It has been found out, however, that there are a number of
problems to be solved in order to embody the idea as described herein-
above. Firstly, when only the screwing step is carried out mechanically
with a motor but the finishing step of fastening is continued by a
turning movement with a hand gripping the screw-driver, the hand is
suffered from a strong reverse resistance for preventing the advancing
movement of the screw, so that the reduction gear (and the motor)
directly connected to the driver bit starts its reverse rotation, thus
never achieving the fastening operation. It has also been found out,
therefore, that any means for avoiding the reverse rotation is
necessary to be inserted between the driver bit and the output shaft
of the reduction system. In order to avoid only the reverse movement
of the driver bit, according to a principle of rheostatic braking
a positive pole of a DC motor as a power source may be short-circuited
to a negative pole instantaneously upon discontinuation of the rotation.
Secondly, there is a problem how to terminate the prefastening
or screwing step on an optimum timing. If the termination of the
screwing step is carried out by a manual ON-OFF switching operation
based on an operator's intuition, any skillful operator can not always
terminate the rotation on a constant timing. Further, the requirement
of such the operator's intuition fails to produce an automatic appliance.
The maximum output obtained on the driver bit may be ad~usted to a
degree of an opposite resistance generated at the end of the prefastening
or screwing step for mechanically discontinuating the rotation of the
driver bit. However, this operation generates an excessive mechanical
load on the whole appliance upon each repeated operation and thus may
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not be employed in the screw-driver for use in a severe operational
condition. Therefore, it is absolutely necessary for the driver bit to
be automatically discontinued on the spot for its rotation upon reaching
the predetermined fastening torque ( or the predetermined opposite re-
sistance), in order to terminate the screwing step on the optimum timing.
Since the opposite resistance may be varied depending on materials to
be fastened such as steel, wood or plastics, it will be appreciated
that the fastening torque is difficult to be controlled on the optimum
value for automatic discontinuation.
In view of the foregoing, it has now been found out that the
first problem may be solved by providing a ratchet mechanism (capable
of transmitting a rotation in only the unidirection but preventing the
same in the other direction) at a portion of a chuck having a removable
driver bit. Further, it has been confirmed that the ratchet mechanism
is conveniently provided with a switching means for preventing the
transmission of rotation power in the opposite direction upon either of
the forward or reverse rotation of the screw-driver.
It has also been found out that the second problem may be solved
by providing a torque controlling system having a clutch mechanism at
a so-called planetary reduction system which comprises planetary gears
operatively connected to a pinion gear, which in turn is connected to
an output shaft of an electric motor, and an internal gear meshing with
the planetary gears.
SUMMARY OF THE INVENTION
Accordingly, a general purpose of the inveniion is to provide
an electrically driven screw-driver which is useful in a general fastening
operation and is very compact and light, as well as portable in a pocket.
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In order to achieve the foregoing ob~ect, in accordance with the
invention, only the prefastening or screwing step occupying most of the
total fastening procedure is carried out mechanically and electrically,
while the finishing step requiring strong fastening force is performed
manually.
PREFERRED EMBODIMENTS O~ THE INVENIION
Thus, the electrically driven screw-driver according to the in-
vention is characterized in that a ratchet device is mounted to a portion
of a chucking shaft provided with a removable driver bit, said chucking
shaft being connected to an output shaft of an electric motor through
a planetary reduction system and that a torque controlling mechanism
having a clutch function is mounted to the planetary reduction system.
In the screw-driver according to the invention, the ratchet device
may comprise a ratchet gear provided at a shaft portion of a chuck,
a pair of pawls oppositely arranged for holding the ratchet gear there-
between, and an operating ring for engaging and disengaging either one ~
of said pawls with the ratchet gear through its circumferential rotation
and selective pressing of the pair of pawls.
Further, in the screw-driver according to the invention, the
planetary reduction system may comprise planetary gears capable of
self-rotating and revolving round through a power transmitted from a
pinion gear connected to the output shaft of the electric motor, an
internal gear meshing with the planetary gears and providing an orbit
for their revolution, and a cylindrical casing for rotatably receiving
the internal gear. In this case, preferably the cylindrical casing is
at its end provided with a recess for receiving a reversible switch for
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the electric motor, operation of said reversible switch being associated
with engaging and disengaging operations of the pawls with the ratchet
gear through the operational ring of the ratchet device. More preferably,
the cylindrical casing is provided with a hole for receiving a steel
ball while the internal gear at its circumference is provided with a
notch located oppositely to the hole of the cylindrical casing and further
a pressing means is provided for seating the steel ball in the hole onto
the notch with a predetermined pressure. In this preferred embodiment,
the pressing means may comprise a plate spring engaging at its end with
the cylindrical casing, and a sliding element which slides on the plate
spring to ad~ust steplessly the pressure of the plate spring on the
steel ball.
Alternatively, a cylindrical element having a M ange is fitted
into the cylindrical casing, said flange of the cylindrical element
being provided with a hole for receiving the steel ball and further the
internal gear at its one side is provided with a flange oppositely to
the hole of the cylindrical element, said M ange of the internal gear
being provided with a radially extending protrusion. In this case, a
pressing means is arranged for urging the steel ball in the hole of the
sylindrical element against the flange of the internal gear with a pre-
determined pressure. In this alternative preferred embodiment, the
pressing means may comprise a pressing ring fitted into the cylindrical
element, a coil spring inserted between the pressing ring and the
electric motor, and a pressing element for axially urging the electric
motor against the force of the coil spring. Preferably, the pinion gear
connected to the output shaft of the electric motor is supported
elastically in the axial direction.
:1~72~7'0
The invention will be illustrated in more detail by the preferred
embodiments but not limited thereto, with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWING
.
Figure 1 is a partially sectioned front elevation of the electrically
driven screw driver according to the invention;
Figure 2 is an enlarged sectional view of the planetary reduction
system used in the electrically driven screw-driver of the invention;
Figure 3 is a partial side view of the outer casing of the screw-
driver according to Figure l;
Figure 4 is a cross sectional view taken along the line IV-IV
of Figure l;
Figure 5 is a partially sectioned front elevation of the screw-
driver of another embodiment according to the invention; and
Figure 6 is a cross sectional view taken along the line V-V of
Figure 5.
DETAILED DESCRIPTION OF THE INVENTION
In Figure 1, a numeral reference 10 represents an electrically
driven screw-driver (hereinafter referred to merely a "screw-driver"
for simplification), an outer casing 12 of which is shaped in the pi~tol
form as a whole. In the gripping part 14 is removably and replaceably
accommodated a charging battery 16 which may be put thereinto from the
bottom. The charging battery 16 is connected electrically through a
reversible switch 20 to a motor 18 accommodated in a body portion of the
outer casing. Further, a connection of the charging battery 16 to a plug
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of a charging appliance (not shown) introduced into a co~,mercial electric
source allows repeated chargings.
In the body portion of the outer casing 12 is also received a
planetary reduction system 22 arranged in alignment with the ~lotor 18.
~s shown in detail in Figure 2, the planetary reduction system 22 comprises
essentially an internal gear 24 of a ring element which is provided on
its inner surface with a gear and planetary gears 26 coacting with the
inner surface of the internal gear 24 for self-rotating while revolving
round. ~hree planetary gears 26 may be provided, each of which is
mounted to a shaft 30 arranged at an apex of regular triangle which is
imaginatively drawn on a disc 28. The disc 28 at its center is provided
fixedly with an axially extending output shaft 32. Each of the three
planetary gears 26 coacts with a pinion gear 36 which is in turn fixed
to a rotating shaft 34 of the motor 18. The output shaft 32 is rotatably
inserted into and supported by a plain bearing 40 which is fixed near
a bottom center of a cylindrical casing 38 encircling the planetary
reduction system 22. Thus, as far as the internal gear 24 i~ fixed to
the cylindrical casing 38, the rotation power is transmitted through
the pinion gear 36, the planetary gears 26 and the disc 28 to the output
shaft 32. In this case, the system may work well as a reduction device.
In the planetary reduction system 22 incorporated in the screw-
driver according to the invention, the internal gear 24 is somewhat
smaller in its outer diameter than an inner diameter of the casing 38
and is rotatably received therein. Further, the internal gear 24 is
provided at its outer circumference with at least one notch 42 while
the cylindrical casing 38 is provided at its corresponding position to
the notch 42 with a hole 44 in order to fix the internal gear 24 to the
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cylindrical casing 38. Into the hole 44 is, for this purpose, removably
received a steel ball 46 of a predetermined diameter, as shown in Figure 2,
which ball is elastically supported by a plate spring 48 arranged in
parallel to the cylindrical casing 38 for preventing the removal of the
steel ball 46 from the hole 44. In order to ad~ust a pressure of the
plate spring 48 on the steel ball 46 as shown in Figures 2 and 3, a
slider 50 is arranged in slidable contact with the upper surface of the
plate spring 48. In this case, of course, the slider 50 may be smoothly
guided in the axial direction by a sliding groove (not shown) for pre-
venting the same from slipping out of the outer casing 12. It will be
appreciated from Fig. 2 that when the slider 50 is moved axially toward
the right the plate spring 48 urges the steel ball 46 more strongly into
the notch 42, whereas the movement of the slider 50 toward the left may
release the pressure gradually and steplessly. Thus, when the force
applied circumferentially to the internal gear 24 (the force is, as
described hereinafter, generated by an opposite torque transmitted from
the driver bit) exceeds the pressure of the steel ball 46 applied by the
plate spring 48, the steel ball 46 is lifted from the notch 42 and re-
leases the fixation of the internal gear 24 to the cylindrical casing 38,
thereby causing the idling of the internal gear 24 within the cylindrical
casing 38. Thls means the discontinuation of power transmission, as
described hereinafter.
A ratchet device connected to the output shaft 32 of the reducer
will now be described in detail with reference to Fig. 1 and Fig. 4.
In Fig. 1, the output shaft 32 is provided with a well-known quick-chuck 54
for convenient mounting and removal of the driver bit 52. A sleeve portion,
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where the quick-chuck 54 is put onto the output shaft 32, is provided
at its end with a ratchet gear 56, as shown in Fig. 4. Further, the
ratchet gear 56 supports a pair of pawls 58 of triangular plate pieces,
each of which swings on a shaft 60 in the axially outward direction.
The ratchet gear 56 is held between the pair of pawls 58, 58, their sharp
edges of which mesh with the former. The oppositely arranged pair of
pawls 58, 58 are engaged with a wire spring 62, which imparts an elasti-
cally restoring force to each of the pawls in the directions A and B,
as shown by arrows in Fig. 4. Further, a pawl-pressing lever 64 of an
arc shape is arranged circumferentially and slidably over a predetermined
angle, while the outer casing 12 is provided with a turnable cylindrical
cap 66 of a pot type into which the lever 64 is received. The pressing
lever 64 is determined in such a size that the top end thereof may
contact either one of the pawls 58 in the extreme limit of its turning
angle and urge the contacted pawl 58 swingably toward the opposite
direction relative to the arrows A and B aga nst the elastic force applied
by the wire spring 62, as shown in Fig. 4. A suitable click stop mecha-
nism may be provided for ensuring reliable stoppage of the cap 66 at each
position for forward, fixed or reverrse rotation.
In accordance with the ratchet mechanism as described hereinbefore,
the anticlockwise sliding movement of the lever 64 urges the pawl 58
(the left side in Fig. 4) swingably in the anticlockwise direction,
thereby disengaging the sharp edge of the pawl 58 from the ratchet gear 56.
If the ratchet gear 56 is turned clockwise, then the other pawl 58 (the
right side in Fig. 4) swings anticlockwise against the elastic force
applied by the wire spring 62 and is disengaged from one tooth of the
ratchet gear 56, thereby allowing one tooth of the ratchet gear to move
~17Z070
clockwise and unidirectionally. Then, the pawl 58 is engaged with the
next tooth under the elastic action of the wire spring 62. Such repeated
procedures allow the further clockwise and unidirectional movement of
the ratchet gear 56 (and hence the driver bit 52). On the contrary, even
if a turning force is applied to the ratchet gear 56 for anticlockwise
movement, the pawl 58 (the right side in Fig. 4) is prevented from
swinging by contact thereo~ with a shoulder of the other element, thereby
ensuring the engagemellt of the pawl 58 with the ratchet gear 56 and
preventing the anticlockwise movement of the latter. If the pressing
lever 64 is slided clockwise, then the ratchet gear 56 operates conversely,
thereby allowing the anticlockwise and unidirectional movement but pre-
venting the clockwise movement. Further, when the lever 64 is held at
the neutral position, the ratchet gear 56 is prevented from the turning
in either direction and fixed.
Now the operation of the screw-driver according to the invention
will be described hereinbelow.
At first, the ratchet mechanism is set to a forward turning position
and the slider 50 is ad~usted to give a suitable pressure on the steel
ball, thus starting a fastening operation. When the fastening operation
proceeds and approaches to the end of the prefastening or screwing step,
resistance to the fastening increases gradually to reach the predetermined
value of the torque. At this point, the load due to the resistance from
the driver bit 52 is transmitted successively through the output shaft 32,
the disc 28, the planetary gears 26, the internal gear 24 and the steel
ball 46 to the cylindrical casing 38. If the load exceeds the radial
pressure on the steel ball 46 applied by the plate spring 48, the ball 46
is forcibly removed from the notch 42, thereby releaslng the fixation of
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the internal gear 24 relative to the cylindrical casing 38. Thus, the
power transmission from the pinion gear 36 is interrupted to discontinue
the rotation of the cutput shaft 32 on the spot. Consequently, the
output shaft of the screw-driver discontinues its rotation automatically
at the optimum fastening torque which has been preset, thereby eliminating
any operator's skill and intuition.
After the procedure described hereinabove, the finishing step
of fastening is completed by turning the gripping part 14 manually,
because the driver bit 52 is operatively associated with the ratchet
mechanism in the forward direction. Generally, the turning angle of
the gripping part 14 is in the order of 30, but the ratchet mechanism
allows the fastening step of any angle to be completed by the repeated
movement of the gripping part while keeping the driver bit 52 fitted
into a slot of screw. On the contrary,strongly fastened screw may be
unfastened manually at first by changing the turning direction of the
ratchet mechanism and then withdrawn mechanically by the electric motor.
Figures 5 and 6 show another embodiment of the screw-driver
according to the invention.
In accordance with this embodiment, an operating element 69 for
a power switch 68 is arranged above the gripping part 14 of the outer
casing 12, while a reversible switch 20 for the motor 18 connected in
series to the power switch 68 is received in a recess 71 formed in the
cylindrical casing at its end. The reversible switch 20 is arranged
between a pair of operational pieces 70, 70 provided within the cylindrical
cap 66, as shown in Fig. 5, so that the engaging and disengaging
operations of the pawls 58 with the ratchet gear 56 may be associated
with the switching operation of the reversible switch 20.
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As the planetary reduction system 22, a double reduction system
is employed which comprises a power transmitting gear 72 arranged at the
shaft of the disc 28, a second planetary gear 74 meshing with the gear 72,
and a disc 76 fixed to the output shaft 32 and supporting the second
planetary gear 74. Within the cylindrical casing 38 is fitted a cylindri-
cal element 78 having a flange 77 adJacent to the internal gear 24.
The flange 77 is provided with a hole 80 into which is received a steel
ball 82, while the internal gear 24 at its one side is provided with a
flange 84 on which is extented an axial protrusion 86. Into the cylindrical
element 78 is fitted a pressing ring 88. ~etween the ring 88 and the
motor 18 is arranged a coil spring 90. Onto the outer casing 12 is screwed
a cap 92, rotation of which permits the steel ball 82 in the hole 80 to
be urged against the flange 84 of the internal gear 24 through the motor 18,
the coil spring 90 and the ring 88. Over the shaft 34 of the motor 18
is arranged a coil sprir.g 94 for supporting the pinion gear 36 eleastically
to the axial direction.
Upon operation of the screw-driver according to the embodiment
described hereinbefore, the cylindrical cap 77 is turned to preset the
ratchet mechanism and the reversible switch 20 at the forward turning
position, while the cap 92 is ad~usted to have a desired pressure on the
steel ball 82.
Then, the power switch 68 is turned on for starting the fastening
operation. When the operation proceeds and approaches to the end of the
prefastening or screwing step, the resistance increases gradually to
reach the predetermined value of torque. Then, the load due to the re-
sistance is transmitted from the driver bit 52 through the output shaft 32
and the disc 76 to the second planetary gear 74 (in this case, the load
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is enough heavy to be transmitted to the second planetary gear 74
rather than to the transmission gear 72) and further from the second
planetary gear 74 through the internal gear and the protrusion 86 to the
steel ball 82. When the load exceeds the pressure on the steel ball
applied in the thrust direction by the pressing ring, the steel ball 82
is pushed over the protrusion 86 of the flange 84 provided on the in-
ternal gear 24, thereby releasing the fixation of the internal gear 24
to the cylindrical casing 38. Thus, the power transmission from the
pinion gear 36 is interrupted on the spot to discontinue the rotation
of the output shaft 32. Consequently, the discontinuation of rotation
of the output shaft at the optimum fastening torque permits any operator's
skill and intuition to be eliminated and the finishing step of fastening
to be performed conveniently.
In addition, since the switching operation of the ratchet mecha-
nism by means of the cylindrical cap is associated with the switching
operation of the reversible switch for the motor, the handling and
operation of the screw-driver according to the invention is very
convenient. Furthermore, utilization of the double reduction system as
a planetary reduction mechanism permits the desired output to be readily
obtained.
In accordance with the electrically d~iven screw-driver of the
invention, the capacity and number of the charging batteries may be small,
resulting in a very compact and light appliance, which is very convenient
in portability.
The foregoing is to be considered as descriptive and not limitative
as many changes and modifications may be made therein without departing
from the concept of the invention.
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