Wrist mechanism of industrial robot

A wrist mechanism of an industrial robot is provided with a forearm, a wrist base mounted on the forearm so as to be rotatable about a lengthwise rotary axis of the forearm, a wrist body mounted on the wrist base so as to be swingable about a swing axis perpendicularly crossing the rotary axis of the wrist base, and a wrist top mounted on the wrist body so as to be rotatable about an axis perpendicular to the swing axis of the wrist body. The wrist base is characterized in that a motor for rotating the wrist tip and a motor for swinging the wrist body are arranged opposite to each other in a lengthwise direction.

BACKGROUND OF THE INVENTION 
This invention relates to a structure for miniaturizing a wrist of an 
industrial robot. 
FIG. 7 is a schematic diagram of a conventional industrial robot. This 
industrial robot generally makes motions similar to ones of portions of a 
human body from the shoulder to the wrist. 
In FIG. 7, a shoulder 2 is provided rotatably on a base 1. An upper arm 3 
is mounted swingably on the free end portion of this shoulder. A forearm 4 
is provided at the free end portion of the upper arm 3 so that the forearm 
4 can swing with respect to the upper arm 3. The forearm 4 is driven via a 
forearm driving motor (no shown), a lever 47 and a driving link 48. A 
wrist base 5 is provided on a front portion of the forearm 4 so that the 
wrist base 5 can rotate around the longitudinal axis of the forearm 4. A 
wrist body 6 is provided on a front portion of the wrist base 5 so that 
the wrist body 6 can swing around a pivot extending at right angles to the 
axis around which the wrist base 5 rotates. A wrist tip 7 is provided on a 
front portion of the wrist body 6 so that the wrist tip 7 can be rotated 
around an axis extending at right angles to the wrist body-swinging pivot. 
Various kinds of instruments 8, such as a tool, a welding torch or a 
handling device are attached to the wrist tip to carry out various 
machining and processing operations. The above description is of the 
construction of a commonly used wrist of an industrial robot. There is 
also a wrist of an industrial robot, the construction of which is 
partially different from that of the abovementioned industrial robot 
wrist. For example, the wrist base-rotating axis, wrist body-swinging axis 
and wrist tip-rotating axis referred to in the above statement extend at 
substantially right angles. The angles of these axis do not necessarily 
cross each other at right angles and they can be regulated in accordance 
with the working posture. 
Japanese Patent Laid-Open No. 59390/1984 discloses a construction of a 
conventional industrial robot wrist, as discussed above. According to the 
technique disclosed in this publication, motors for rotating a wrist tip 
and swinging a wrist body and reduction gears are provided within a wrist 
base so that this wrist can also be applied to a small-sized industrial 
robot. 
According to the prior art disclosed in the above publication, however, the 
two motors in the wrist base are parallel-arranged so that each of them 
orients at right angles to the lengthwise axis of the wrist. Therefore, it 
is difficult to miniaturize a wrist which generally has an elongated 
structure of a columnar shape. 
If these two motors are arranged so that they extend in the lengthwise 
direction of the wrist, the following problems which must be solved arise. 
(1) The transmitting of a driving force from a motor to a tip portion of 
the wrist is generally done by a belt (including a chain), which is 
generally extended in the lengthwise direction of the wrist in view of the 
necessity of miniaturizing the wrist. If these motors are arranged in this 
manner, it becomes necessary that the driving force from the 
lengthwise-arranged motors be transmitted to the belts after the direction 
of the driving force has been changed at right angles, so that the length 
of the driving force transmitting path increases. If the driving force 
transmitting path increases, the path along which an external force 
imparted to the tool at the free end of the wrist tip is transmitted also 
increases. This would cause the wrist to be displaced accordingly with 
ease, i.e., the rigidity thereof decreases. (2) If the two motors are 
arranged in the lengthwise direction of the wrist with no sufficient space 
left between the rear ends of the two motors and structural members of the 
wrist facing the rear ends, it becomes troublesome to take out the motors 
when the maintenance work for the wrist is carried out. Since the output 
shaft of a motor is generally meshed with a gear, the motor must be taken 
out by moving the motor in the axial direction thereof. However, when the 
length of the wrist is not set so large, the motor cannot be moved 
axially. In such a case, it is necessary that the motor be taken out by 
moving it in a direction perpendicular to the lengthwise axis thereof. 
This makes it necessary to disassemble the gear box meshed with the output 
shaft of this motor, so that the motor-removing operation becomes 
troublesome. Summary of the Invention 
An object of the present invention is to provide an industrial robot 
capable of being miniaturized by arranging a wrist tip-rotating motor and 
a wrist body-swinging motor in the lengthwise direction of the wrist, 
preventing a decrease in the rigidity of the wrist, and carrying out the 
maintenance work therefor with ease. 
A wrist mechanism of an industrial robot according to the invention 
comprises a forearm, a wrist base provided rotatably around a lengthwise 
axis of the forearm, a wrist body provided swingably around a pivot or 
shaft (swing shaft) disposed perpendicularly to the lengthwise axis of the 
wrist base, a wrist tip portion provided rotatably around a rotary shaft 
perpendicular to the swing shaft around which the wrist body is swung, and 
the wrist base has a motor for rotating the rotary shaft and a motor for 
driving the swing shaft to cause the wrist body to swing around the swing 
shaft, those two motors being arranged opposite to each other in a 
lengthwise direction thereof.

DETAILED DESCRIPTION OF THE INVENTION 
An embodiment of the present invention will now be described with reference 
to FIGS. 1-4. FIGS. 1 and 2 constitute one drawing. An example of an 
industrial robot comprises a base, a shoulder rotatably mounted on the 
base, an upper arm swingably mounted on the shoulder, a forearm swingably 
mounted on the upper arm and a wrist mechanism rotatably mounted on a free 
end portion of the forearm. The base, shoulder, upper arm and forearm 
corresponds to ones of the conventional industrial robot shown in FIG. 7, 
respectively. The wrist mechanism is installed with various kinds of tools 
to carry out various operation. The wrist mechanism comprises a wrist base 
5 rotatably mounted on the forearm 4 around a lengthwise axis of the 
forearm 4, a wrist body 6 swingably mounted on the wrist base 5, and a 
wrist tip 7 rotatably mounted on the wrist body 6. The robot carries out 
various operation with the wrist tip 7 being installed with a tool. The 
forearm 4 shown in FIG. 2 is supported on an upper arm as shown in FIG. 7 
by reference number 3 via a fulcrum 41a. The axial force of a driving link 
as shown in FIG. 7 by reference numeral 48 is transmitted to a fulcrum 
41b, and the forearm 4 is swung around the fulcrum 41a. The rear portion 
of the wrist base 5 is housed rotatably in the interior of the forearm 4. 
The rear portion of this wrist base 5 is formed cylindrically and 
supported on a rotation supporting bearing 42 provided in the forearm 4. A 
gear 45 on the side of a motor 44 for rotation is meshed with a gear 43 
formed on the rear end portion of the wrist base 5. The gear 45 on the 
side of the rotation motor 44 is connected to the same motor 44 via a 
harmonic final reduction gear 46. 
A method of transmitting power to the wrist body 6, which is provided 
swingably with respect to the wrist base 5, will now be described with 
reference to FIG. 1. A motor 61 for swinging the wrist body 6 (referred to 
as swing motor) is provided in the interior of the wrist base 5 so as to 
extend in the lengthwise direction thereof. An output shaft 62 of the 
swing motor 61 faces in the forward direction, and is provided with a 
bevel gear 63 at the free end portion thereof. A bevel gear 65 is also 
mounted on a first transmission shaft 64 which extends at right angles to 
this output shaft 62, and these two shafts 62, 64 are connected at right 
angles via the bevel gears 63, 65. A pulley 66 is mounted on the other end 
portion of the first transmission shaft 64, and a belt 67 is engaged with 
this pulley 66. This belt 67 runs in the lengthwise direction of the wrist 
base, and is engaged with an opposite pulley 68. This pulley 68 is mounted 
on a swing shaft 69 around which swing is effected, which shaft is 
supported on a bearing 52 provided in a housing 51 of the wrist base 5. A 
harmonic final reduction gear 601 is connected with this shaft 69, and the 
wrist body 6 is supported on a shaft support bearing 53 via this reduction 
gear so that the wrist body can be swung. The axis of this shaft 69 
extends at right angles to the axis around which the wrist base 5 is 
turned. 
A method of transmitting power to the wrist tip 7 which is provided 
rotatably with respect to the wrist body 6 will now be described. A 
rotation motor 71 for rotating the wrist tip 7 is provided in opposition 
to the swing motor 61 in the wrist base 5 so as to extend in the 
lengthwise direction thereof. An output shaft 72 of this motor 71 has at 
its free end portion a bevel gear 73 meshed with a bevel gear 75 mounted 
on a second transmission shaft 74 which extends at right angles to the 
output shaft 72, and these two shafts 72, 74 are connected to each other 
at right angles. The second transmission shaft 74 is provided at the other 
end portion thereof with a pulley 76 with which a belt 77 is engaged. The 
belt 77 runs in parallel with the above-mentioned belt 67 in the forward 
direction in the interior of the wrist base 5. A pulley 78, the other 
pulley around which this belt 77 is passed, is mounted on a third 
transmission shaft 79. This third transmission shaft 79 is positioned in 
parallel with the second transmission shaft 74. The third transmission 
shaft 79 is provided at its inner end portion with a bevel gear 701, which 
is meshed with a bevel gear 703 mounted on a rotary shaft 702. The two 
shafts 79, 702 extend at right angles to each other. The wrist tip 7 is 
mounted on the front end portion of the rotary shaft 702 via a harmonic 
final reduction gear 704. The wrist tip 7 is supported on a rotary 
movement supporting bearing 705 and rotated. 
The axes of the shaft 69 and third transmission shaft 79 are aligned with 
each other, and these aligned axes and the axis of the rotary shaft 702 
cross at right angles. The axes of the first and second transmission 
shafts 64, 74 are also aligned with each other, and a gear box 10, which 
is described later, can be turned around these aligned axes. 
The two output shafts 62, 72, two transmission shafts 64, 74 and bevel 
gears 63, 65, 73, 75 mounted on these output shafts are housed in the 
single gear box 10, which can be turned with respect to a bearing housing 
101 for the first and second transmission shafts 64, 74. The turning 
movement of the gear box 10 is normally prevented by a motor holder 104 as 
shown in FIG. 3. The turning of the gear box 10 is done so as to carry out 
a maintenance inspection operation easily. The gear box 10 is provided 
with an opening 102 through which the output shafts 62, 72 of the motors 
extend. This opening 102 is made so large that the bevel gears 63, 73 
mounted on the free end portions of the output shafts 62, 72 can be passed 
therethrough. 
The operation of this embodiment will now be described briefly. When the 
rotation driving motor 44 is energized, the wrist base 5 is turned with 
respect to the forearm 4. This turning movement is made around the 
lengthwise axis of the forearm 4. When the swing motor 61 is energized as 
the wrist base 5 is turned, the wrist body 6 is swung. This swinging 
movement is made around the shaft 69, which extends at right angles to the 
axis around which the wrist base is turned. When the motor 71 is energized 
as the wrist body 6 is swung, the wrist tip 7 is rotated. Accordingly, the 
wrist of this industrial robot has three degrees of freedom. A tool is 
attached to the free end of this wrist tip, whereby machining and 
processing operations are carried out. 
The maintenance work will now be described. FIG. 3 is a side elevation of 
what is shown in FIG. 1, and FIG. 4 illustrates an operation of the 
portion of the embodiment which is shown in FIG. 3. When it is necessary 
to take out the two motors 61, 71 during a maintenance operation, a cover 
103 for the wrist base 5 is removed first, and then the motor holder 104, 
the gear box 10 being then turned around the bearing housing 101. The gear 
box 10 is turned with the two motor 61, 71 fixed thereto. This turning 
movement is made around the aligned axes of the first and second 
transmission shafts 64, 74. 
During this time, relative movement occurs between the bevel gears 63, 65; 
73, 75 which are meshed with each other. Since brakes are generally 
provided in these two motors 61, 71, relative rotating movements occur in 
the free bevel gear 65, 75 positioned on the driven side. Since the angles 
of such rotating movements of the bevel gears are not more than 
90.degree., and since the speeds of the same movements are reduced (to, 
for example, around 1/50-1/100) by the harmonic final reduction gears 601, 
704 installed at the terminal ends on the power transmission paths, the 
angles of rotating movement of the wrist body 6 and wrist tip 7 
constituting driven members are very small, i.e., the wrist body 6 and 
wrist tip 7 are not rotated until they contact the mechanical stoppers. 
The meshing condition of the pairs of gears 63, 65; 73, 75 does not 
change. Namely, the bevel gears are not pressed against each other, nor 
they are disengaged from each other. Accordingly, if the gear box 10 and 
motors 61, 71 are turned back to the original positions, the positions of 
the driven members, i.e. the wrist body 6 and wrist tip 7 can be restored. 
In order to remove motors 61, 71 from the gear box 10, the gear box 10 is 
turned. The motors 61, 71 are then ready to be taken out easily from the 
opening 102 with the bevel gear 63 on the output shaft 62 and the bevel 
gear 73 on the output shaft 72 attached to the motors 61, 71. This means 
that it is unnecessary to draw back the first and second transmission 
shafts 64, 74 in the axial directions thereof for disengaging the bevel 
gears thereon from those on the output shafts. Accordingly, it is also 
unnecessary to remove the belts 67, 77 from the pulleys 66, 76. Thus, the 
motors or detectors (position detectors or speed detectors) provided in 
the rear end portions of the motors are allowed to project to the outer 
side of the wrist base 5, and the motors can be removed. This enables the 
maintenance, inspection and replacement of the parts to be carried out 
easily. 
According to the present invention, the motors in the wrist are arranged in 
the lengthwise direction thereof, so that the wrist can be miniaturized. 
Since the paths of transmitting the power from the motors are at right 
angles, the lengths of the power transmission paths increase due to the 
first and second transmission shafts 64, 74 which are part of these paths. 
However, the increase in the rigidity of the wrist, which is caused by the 
increase in the lengths of these transmission paths, can be minimized by 
providing the harmonic final reduction gears 601, 704 at the terminal ends 
of the transmission paths. Since the two motors are arranged in an opposed 
state, only one gear box for housing them need be provided. Moreover, 
since this gear box is formed so that it can be turned around the aligned 
axes of the first and second transmission shafts, the two motors are 
allowed to project or can be removed from the wrist base during the 
maintenance work without disassembling the gear box. This enables the 
maintenance work to be carried out easily. 
Next, another embodiment, wherein a power transmitting method is different 
in part from a method of transmitting power to the wrist body 6 swingably 
mounted on the wrist base 5 and to the wrist tip portion 7 rotatably 
mounted on the wrist body 6 as explained in FIG. 1, will be described 
referring to FIG. 5. The present embodiment is the same in operation and 
effects of the invention as ones of the previously mentioned embodiment. 
In the embodiment illustrated in FIGS. 1 to 4, the belt 67 and pulley 68 
are used as a power transmitting means from the first transmitting shaft 
64 to the swing shaft 69. In this embodiment, the first transmission shaft 
64 is connected to the swing shaft 69 through a fourth transmission shaft 
613 and bevel gears 611, 612, 614 and 615. Namely, the first transmission 
shaft 64 has the bevel gear 612 at a tip thereof and is connected to the 
fourth transmission shaft, which is perpendicular to the first 
transmission shaft 64, through the bevel gear 612 mounted on one end of 
the fourth transmission shaft 613. The fourth transmission shaft 613 has 
the bevel gear 614 at the other end thereof and is connected to the swing 
shaft 69 through the bevel gear 614 which is meshed with the bevel gear 
615 mounted on the swing shaft 69. 
Further, power transmission between the second transmission shaft 74 and 
the third transmission shaft 79 also is effected in a similar manner. 
Namely, the second transmission shaft 74 has a bevel gear 711 at its tip, 
and the bevel gear 711 is meshed with a bevel gear 712 mounted on one end 
of a fifth transmission shaft 713 disposed perpendicularly to the second 
transmitting shaft 713. The fifth transmission shaft 713 has, at the other 
end thereof, another bevel gear 714 which is meshed with a bevel gear 715 
of the third transmission shaft 79 perpendicular to the fifth transmission 
shaft 713, thereby connecting the third and fifth transmission shafts 79 
and 713. 
Next, a method of connecting electric wires both to the motor 71 and the 
motor 61 within the wrist base 5 is described referring to FIG. 6. 
The electric wires 801 are surrounded by flexible pipes 802, respectively. 
One end of each electric wire 801 is fixed to the wrist base 5 and 
connected to the motors 71, 61. The other end of each wire 801 is fixed to 
the forearm 4. 
The electric wires 801 pass through a hole which is formed in a gear 43 so 
as to be concentric with the wrist base 5. 
The electric wires 801 are provided along a central axis of the wrist base 
5, so that the wires are twisted as the wrist base rotates. Deformation 
caused in the electric wire 801 is mainly one caused by twisting. The 
wires are easily deformed, so that they are not easily damaged.