[0001] 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.
[0002] 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. Thus, it may well be stated that there have 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.
[0003] 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
reduction 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).
[0004] In general, the operational capacity must be so large that the appliance becomes
inevitably too big to be carried by one hand.
[0005] Thus, there has been need for very compact and light electrically driven screw-driver,
which is enough resistant to a severe operational condition.
[0006] 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 sinplification)
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 10% requires an instantaneously strong fastening force.
On the contrary, a so-called tapping screw, which is screwed into an object without
a threaded hole, may be gradually moved into the object against a considerable 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.
[0007] 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 remaining 10%
in the finishing step of the strong fastening may well be performed manually without
a mechanical 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 light 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 performed mechanically
but the finishing step requiring a strong fastening force (which is consumed in a
very short time) is carried out manually, 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 hereinabove. 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 shortcircuited to a negative pole instantaneously upon
discontinuation of the rotation.
[0008] 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 tenninate 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 adjusted 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 not be enployed
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
resistance), 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.
[0009] 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.
[0010] Accordingly, a general purpose of the invention 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.
[0011] In order to achieve the foregoing object, 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.
[0012] Thus, the electrically driven screw-driver according to the invention 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.
[0013] 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 threrebetween, 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
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 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 adjust steplessly
the pressure of the plate spring on the steel ball.
[0014] Alternatively, a cylindrical element having a flange 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 flange 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 cylindrical element against
the flange of the internal gear with a predetermined 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 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. Now the
invention will be described in more detail hereinafter with reference to the drawing
which illustrate the preferable embodiments.
[0015] One way of carrying out the invention is described in detail below with reference
to drawings which illustrate preferred embodiments, in which:-
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 1;
Figure 4 is a cross sectional view taken along the line IV-IV of Figure 1;
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.
[0016] 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 pistol 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 of a charging appliance (not shown)
introduced into a commercial electric source allows repeated chargings.
[0017] In the body portion of the outer casing 12 is also received a planetary reduction
system 22 arranged in aligment with the motor 18. As 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. Three 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 is 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.
[0018] 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 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 adjust 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 preventing 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 release 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. This means the discontinuation of power transmission,
as described hereinafter.
[0019] 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, 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
elastically restoring force to each of the pawls in the directions A and B, as shown
by arrows in Fig. 4. Further, a pawl-pressing level 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 against the elastic force applied by the wire spring 62, as
shown in Fig. 4. A suitable click stop mechanism may be provided for ensuring reliable
stoppage of the cap 66 at each position for forward, fixed or reverse rotation.
[0020] 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 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 thereof with
a shoulder of the other element, thereby ensuring the engagement 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 preventing 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.
[0021] Now the operation of the screw-driver according to the invention will be described
hereinbelow.
[0022] At first, the ratchet mechanism is set to a forward turning position and the slider
50 is adjusted 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 exeeds 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 releasing the fixation of 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 output 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.
[0023] 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.
[0024] 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 operation of the
pawls 58 with the ratchet gear 56 may be associated with the switching operation of
the reversible switch 20.
[0025] 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 cylindrical 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. Between 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 spring 94 for supporting the pinion gear 36 elastically to the axial direction.
[0026] 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 adjusted to have a
desired pressure on the steel ball 82.
[0027] 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 resistance is transmitted from the driver bit 52 through the output
shaft 32 and disc 76 to the second planetary gear 74 (in this case, the load 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 internal 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
discountinue 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.
[0028] Tn addition, since the switching operation of the ratchet mechanism 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.
[0029] In accordance with the electrically driven 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.
[0030] Without further elaboration, the foregoing will so fully illustrate the invention
that others may, by applying the current or future knowledge, readily adapt the same
for use under various conditions of service.
1. An electrically driven screw-driver which is characterized in that a ratchet device
is mounted to a portion of a chucking shaft (32) to which a driver bit (52) is detachably
connected, that the chucking shaft (32) is connected to an output shaft (34) of an
electric motor (18) through a planetary reductin system (22) and that a torque controlling
mechanism having a clutch function is mounted to the planetary reduction system.
2. The electrically driven screw-driver according to claim 1, characterized in that
the ratchet device comprises a ratchet gear (56) provided at the cluching shaft, a
pair of pawls (58), (58) oppositely arranged for holding the ratchet gear (56) therebetween,
and an operating ring (66) for engaging and disengaging either one of said pawls (58),
(58) with the ratchet gear (56) through its circumferential rotation and selective
pressing of said pair of pawls (58), (58).
3. The electrically driven screw-driver according to claim 1 or 2, characterized in
that the planetary reduction system comprises planetary gears capable of self-rotating
and revolving round through a power transmitted from a pinion gear (36) connected
to the output shaft (34) of the electric motor (18), an internal gear (24) coacting
with the planetary gears (26) and providing an orbit for their revolution, and a cylindrical
casing (38) for rotatably receiving the internal gear (24).
4. The electrically driven screw-driver according to claim 3, characterized in that
the cylindrical casing at its end is provided with a recess (71) for receiving a reversible
switch (20) for the electric motor (18), operation of said reversible switch (20)
being associated with engaging and disengaging operations of the pawls (58), (58)
with the ratchet gear (56) by the operational ring (66) of the ratchet device.
5. The electrically driven screw-driver according to claim 3, characterized in that
the cylindrical casing (38) is provided with a hole (44) for receiving a steel ball
(56) while the internal gear (24) at its circumference is provided with a notch (42)
located oppositely to the hole (44) of said cylindrical casing (38) and that a pressing
means is provided for seating the steel ball (46) in the hole (44) onto the notch
(42) with a predetermined pressure.
6. The electrically driven screw-driver according to claim 5, characterized in that
the pressing means comprises a plate spring (48) engaging at its end with the cylindrical
casing (38), and a sliding element (50) which slides on the plate spring (48) to adjust
steplessly the pressure of the plate spring (48) on the steel ball (46).
7. The electrically driven screw-driver according to claim 4, characterized in that
a cylindrical element (78) having an inwardly directed flange (77) is fitted into
the cylindrical casing (38), said flange (77) of the cylindrical element (78) being
provided with a hole (80) for receiving the steel ball (82) that the internal gear
(24) at its one side is provided with a flange (84) oppositely to the hole (80) of
the cylindrical element (78), said flange (84) of the internal gear being provided
with a. radially extending protrusion (86), and that a pressing means is provided
for urging the steel ball (82) in the hole (80) of said cylindrical element (78) against
the flange (84) of the internal gear (24) with a predetermined pressure.
8. The electrically driven screw-driver according to claim 7, characterized in that
the pressing means comprises a pressing ring (88) fitted into the cylindrical element
(78), a coil spring (90) inserted between the pressing ring (88) and the electric
motor (18), and a pressing element (92) for axially urging the electric motor (18)
against the force of the coil spring (90).
9. The electrically driven screw-driver according to claim 8, characterized in that
the pinion gear (36) connected to the output shaft (34) of the electric motor (18)
is supported elastically in the axial direction.