Mechanical wrist mechanism

A three-axis wrist mechanism has three independent rotational axes with two points of mutual intersection which are offset from each other. The offset provides a generous bend radius for the wrist. First and second housing structures contain the first and second axes while the third axis is disposed within a tool support member. The first housing structure is adapted to be mounted to an outer tubular member of the arm of a robot to rotate independently of the rest of the mechanism. Gear trains of the mechanism provide the independent rotation of the second housing structure and the tool support member to permit selective positioning of a tool-mounting surface of the tool support member within a work envelope. The gear trains are arranged so that an unobstructed passage is maintained between an arm-mounting surface of the first housing structure and the tool-mounting surface of the tool support member for accommodating hoses and/or wires which are used to transmit fluid and power to a tool mounted on the surface of the tool support member.

TECHNICAL FIELD 
This invention relates to mechanical wrist mechanisms and, more 
particularly, to three-axis wrist mechanisms which have an unobstructed 
passage extending between its opposite ends. 
BACKGROUND ART 
Wrist mechanisms are designed to orient its end effector as demanded by 
workpiece placement. Wrist mechanisms provide three articulations that 
offer motions commonly called pitch, yaw and roll. Similar articulations 
of the arm deliver the wrist assembly anywhere in the robot's sphere of 
influence. Thereafter it requires three more articulations of the wrist 
mechanism for universal orientation of the end effector. 
Many mechanical wrist mechanisms are multi-axes mechanisms. For example, 
U.S. Pat. Nos. 3,231,098 and 3,315,542 both disclose such mechanisms. Many 
of such mechanisms, however, do not provide an unobstructed passageway 
which provides protection for hoses and wires which respectively provide 
fuel and power to a work tool mounted on the free end of the wrist 
mechanism. 
U.S. Pat. No. 4,218,166 discloses a multi-axis wrist mechanism which 
provides independent movement about two axes and which maintains an 
unobstructed passage between a stationary base and a work tool. 
U.S. Pat. No. 4,402,234 to Malarz et al discloses a wrist mechanism having 
three independent rotational axes which are mutually intersecting. A 
gearing mechanism controls the independent rotation of components 
supported for rotation about the three axes and also provides an 
unobstructed internal passageway for housing hoses and/or wires. The use 
of such a wrist mechanism, however, is somewhat limited and/or difficult 
due to the relatively small internal bend radius of the wrist mechanism. 
This imposes limitations on the amount of rotational movement that the 
housed hoses and wires will permit. Such a small bend radius furthermore 
makes programming of the robot to which the wrist mechanism is attached 
more difficult. This occurs because the freedom of movement of the wrist 
mechanism is limited which the programmer must take into consideration. 
Other U.S. patents which disclose wrist mechanisms of lesser relevance 
include U.S. Pat. Nos. Re. 25,889; 2,514,250; 2,861,700; 2,871,701; 
3,043,448; 3,066,805; 3,108,498; 3,182,813; 3,784,031; 4,001,556; 
4,068,536; 4,099,409; 4,149,278; 4,353,677; 4,356,554; 4,365,928; 
4,370,836 and 4,392,776. 
DISCLOSURE OF THE INVENTION 
An object of the present invention is to provide an improved three-axis 
mechanical wrist mechanism with an unobstructed passageway and including 
three independent rotational axes which have a pair of mutually 
intersecting points to provide greater flexibility in positioning a wide 
variety of tool members in a work envelope. 
Another object of the present invention is to provide an improved 
mechanical wrist mechanism having three independent axes of rotation while 
maintaining an unobstructed passage therethrough and having a generous 
internal bend radius which simplifies programming of the robot to which 
the wrist mechanism is attached and also offers the use of larger gearing 
so that the wrist mechanism can support and manipulate relatively heavy 
tools attached thereto. 
In carrying out the above objects and other objects of the invention, a 
mechanical wrist mechanism constructed in accordance with the present 
invention comprises three independent rotational axes, the first pair of 
said axes having a first point of mutual intersection and a second pair of 
the axes having a second point of mutual intersection offset from the 
first point. First and second housing means are rotatably supported on 
respective ones of the first and second axes. The first housing means 
includes an arm-mounting surface. A tool support member is rotatable on 
the third axis and includes a tool-mounting surface. Gear means are 
provided for independent rotation of the support member and second housing 
means about their respective axes for selectively establishing the 
position of the tool-mounting surface within a work envelope. A 
continuously unobstructed passage means is disposed between the 
arm-mounting surface and the tool-mounting surface. The passage means 
includes cavities in the first and second housing means and the tool 
support member for the enclosed containment of work tool related 
structures. 
Preferably, the passage means includes a pair of substantially identical 
plastic guide members defining the cavities which include the first and 
second points of mutual intersection. 
Also, preferably, the first pair of axes are angularly displaced by a fixed 
angle greater than 30.degree.. 
Yet, still preferably, the offset between the first and second points is 
selectable to allow generous bend radii of hoses and the use of larger 
gears and wider internal passages. The offset is determined by passager 
diameter and bend radius of the central axis. 
The mechanical wrist mechanism as constructed above provides: a greater 
bend radius to accommodate the twisting motion of the various cables, 
wires and conduits necessary for the work tool; makes the programming of 
the robot to which the wrist mechanism is attached simpler; and allows the 
wrist mechanism to carry a relatively heavy payload. 
Other advantages of the present invention will be readily appreciated as 
the same becomes better understood by reference to the following detailed 
description when considered in connection with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to FIG. 1 there is shown a three-axis wrist mechanism, 
generally indicated at 10, which is connected to an outer arm 12 of a 
robot, generally indicated at 14. It is to be understood, however, that 
the wrist mechanism 10 may also be connected to a boom or support arm of 
many other different types of robots in order to move an end effector or 
work tool within a work envelope, as shown in FIG. 1. 
As illustrated in FIG. 1, a work tool 16 is mounted at the free end of the 
wrist mechanism 10. The tool 16 may comprise a welder or paint sprayer or 
other device which is useful in the assembly or finishing of a product. 
As shown in FIG. 2, the wrist mechanism 10 includes an outer knuckle or 
first housing structure 18, an inner knuckle or second housing structure 
20 and a spindle or tool support member 22. The spindle 22 includes a tool 
mounting surface or face 23 on which a wide variety of tools can be 
readily mounted, such as the tool 16. 
A tubular retainer 24 is attached to the center outer knuckle 18 between 
the outer knuckle 18 and the inner knuckle 20. Similarly, a tubular 
retainer 26 is attached to the inner knuckle 20 between the inner knuckle 
20 and the spindle 22. Likewise, a tubular mounting ring 28 is provided at 
the free end of the outer knuckle 18 to facilitate coupling of the wrist 
mechanism 10 to the arm 12 of the robot 14. 
Referring now to FIG. 3, a mounting ring 28 and the outer knuckle 18 have a 
longitudinal axis 30 about which are circumferentially spaced a plurality 
of input or drive tubes indicated in phantom at 32, 34 and 36 of the arm 
12. The mechanism 10 is readily adapted to be secured to the tubes 32, 34 
and 36. For example, the innermost drive tube 32 is connected by a drive 
pin 38 to a bevel gear 40 which is rotatably supported by ball bearings 42 
in the mechanism 10. One bearing 42 is shown of the type that can carry 
rolling force moments. Alternatively, two axially spaced radial bearings 
may replace the bearing 42. 
The drive tube 34 is similarly connected by a drive pin 44 to an 
intermediate bevel gear 46 and is rotatably supported by bearings 48. The 
outer drive tube 36 is connected to the mounting ring 28 by a dowel 50. In 
turn, the mounting ring 28 is fixedly secured to the outer knuckle 18 by 
screws 52, only one of which is shown in FIG. 3. 
A second bevel gear, generally indicated at 54, meshes with the bevel gear 
40 and therefore rotates in response to rotation of the input tube 32. The 
bevel gear 54 includes a pair of bevel gear halves 56 and 58, which are 
threadedly engaged and which are pinned together by dowels 60, only one of 
which is shown in FIG. 3. The bevel gear halves 56 and 58 are provided so 
that there is uniformity of gear parts. 
Likewise, the gear halves 56 and 58 are rotatably supported by bearings 62. 
The bearings 62 allow the inner knuckle 20 to rotate about a second 
longitudinal axis 63. The axes 30 and 63 intersect at a point 69, the 
angle of intersection 73 preferably being fixed at 50.degree.. 
The bevel gear half 58 also meshes with an output bevel gear 64 which, in 
turn, is fixedly mounted to the spindle 22 by screws 66, only one of which 
is shown in FIG. 3. The gear 64 and the spindle 22 are also pinned 
together at 69. In turn, the output bevel gear 64 and the spindle 22 are 
rotatably supported by bearings 68. The bearings 68 allow the spindle 22 
to rotate about a third longitudinal axis 65. The axes 63 and 65 intersect 
at a point 71. The point 71 is offset from the point 69 to increase the 
inner bend radius of the wrist mechanism, simplify programming of the 
robot to which the wrist mechanism is attached, and to facilitate the use 
of relatively larger gears. Larger gears enable the wrist mechanism to 
drive a larger payload without compromising the size of the passageway 
through the mechanism. The passageway is described in greater detail 
hereinbelow. 
The angle of intersection 77 between the axes 63 and 65 is preferably equal 
to angle 73. Accordingly, the orientation in space of the axis 65 changes 
by an angle 67, equal to the sum of angles 73 and 77, as the axis 65 moves 
between the solid line position to the phantom line position in FIG. 3. 
The spindle 22 can be moved from its solid line position to its phantom 
line position in FIG. 3 by rotation of the inner knuckle 20 180.degree. 
while maintaining the position of the outer knuckle 18. 
The input bevel gear 46 meshes with a second intermediate bevel gear 70. 
The intermediate bevel gear 70 is rotatably supported by bearings 72. The 
bevel gear 70 is fixedly connected to the inner knuckle 20 by screws 74, 
only one of which is shown in FIG. 3. The retainer 26 is also fixedly 
connected to the inner knuckle 20 by screws 76, only one of which is shown 
in FIG. 3. The retainer 24 is fixedly connected to the outer knuckle 18 by 
screws 78, again, only one of which is shown in FIG. 3. 
The wrist mechanism 10 has a longitudinal passage 84 which is unobstructed 
along the axes 30, 63 and 65. This unobstructed passage can accommodate 
hoses and wires and/or cables. A first plastic guide 80 is disposed within 
the passage 84 and is fixedly connected at the inner surface of the outer 
knuckle 18 by screws, only one of which is shown at 82. A second plastic 
guide 84 is fixedly secured at the inner surface of the inner knuckle 20 
by screws 88, only one of which is shown. The inner surfaces of the 
plastic guides 80 and 86 help to define the unobstructed passage 84 
through the wrist mechanism 10. The guides 80 and 84 not only help to 
control the bend radius internal to the wrist mechanism, but also helps to 
contain the lubricant of the gears of the wrist mechanism and provides a 
smooth surface for hoses and cables to slide upon. 
The spindle 22 is adapted to support a work tool at its mounting face 23, 
such as a welder or paint sprayer or other device which is useful in the 
assembly and/or finishing of a product. Threaded holes, only one of which 
is shown at 90, are provided for this purpose. 
Seals 92 may be positioned between the opposing surfaces of the retainer 26 
and the spindle 22 and also between the retainer 24 and the intermediate 
bevel gear 70 and the inner knuckle 20. The seals 92 prevent contamination 
of the bearings 68 and 72 when the wrist mechanism is used in a hostile 
environment. 
From the position shown in FIG. 3, the tool mounting surface 23 can be 
rotated through 360.degree. by rotation of the inner knuckle 20. By 
judicious rotation of the inner knuckle 20, the axis 65 can be selectively 
positioned in an envelope defined by outer limits which are displaced from 
the axis 30 by the difference of the measure of angles 73 and 77 at one 
extreme and by the measure of the sum of the angles 73 and 77 at the other 
extreme (i.e. from 0 [parallel to axis 30] to 100.degree., if both angles 
73 and 77 are equal to 50.degree., as shown by angle 67). Within this 
envelope the tool-mounting surface 23 can be rotated substantially 
360.degree. so that a tool disposed on the mounting surface 23 can be 
positioned in a wide variety of positions. 
The wrist mechanism 10 is particularly useful for robots which are 
controlled to perform various manufacturing and production processes, such 
as spray painting and welding. The wrist mechanism 10 can also be used 
with a clamp device which could be operated to position a component during 
assembly. 
The rotary motion of the input tubes 32, 34 and 36 can be accomplished by 
any available well-known rotary drive devices, such as electric motors, 
pneumatic motors or hydraulic motors. If desired, the input of drive tubes 
32, 34 and 36 can be manipulated manually. It is apparent that the 
rotation of these input tubes can be controlled in a desired pattern by 
using a computer having an appropriate program to control their rotation. 
These devices are only cited by way of example and it will be apparent 
from the foregoing description that the wrist mechanism 10 can be useful 
in many instances where a universal positioning device is necessary. 
The above-noted construction of the wrist mechanism 10 allows for a more 
generous bend radius than provided by prior art wrist mechanisms and also 
simplifies programming of a robot controller which controls the rotation 
of the input drive shafts. Further, this construction allows the use of 
larger gears without sacrificing the size of the wrist or the unobstructed 
passageway. In turn, the larger gears are able to drive a larger payload 
on the free end of a wrist mechanism of a given volumetric size. 
The invention has been described in an illustrative manner and it is to be 
understood that the terminology which has been used is intended to be in 
the nature of words of description, rather than of limitation. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that within the scope of the appended claims, the invention may be 
practiced otherwise than as specifically described.