5.1 SHOULDER JOINT
shoulder joint was of primary importance in the design of our robotic arm. Due to the weight of the arm and its overall
length, the imposing moment on this joint is substantial. Preliminary calculations estimated this
moment to be 483 lb-in. This meant that
all components of the shoulder joint needed to be designed to support this
load. This was especially crucial in
choosing a motor to power the bending motion at this joint.
early brainstorming sessions, the thought of using cordless drill motors for
high torque joints seemed feasible. Upon
testing, however, it was apparent that excessive gearing would be required to
bring the rotational speed of the motor down to a reasonable speed. It was for this reason that a gearhead motor
was sought. The chosen motor is a 24V DC
gearhead motor that supplies 300 lb-in of torque and rotates at a maximum speed
of 33 rpm (see Figure 5.1). This motor
requires very little gearing to achieve the high-torque, low-rpm goal. This motor also features a right angle
gearhead, which makes transferring the power to the shoulder joint simple, and
eliminates the need for expensive bevel or miter gears.
power from the motor is transferred through a set of spur gears to reduce the
speed by a 3:1 ratio. The chosen gears
were generously donated by the Boston Gear Corporation. The gears are made of steel and have a
diametral pitch of 20 and a face width of 0.5 inches. The shoulder joint pinion and gear are Boston’s YA20-1/2 and YA60A models
respectively. The pinion has a pitch
diameter of 1 inch and a bore of 0.5 inches.
The gear has a pitch diameter of 3 inches and a bore of 0.5 inches.
save space and weight in the shoulder joint, the gear was incorporated into the
design of one of the mounting brackets (see Figure 5.2). In previous designs, the shoulder brackets
served only to support the arm, to mount the arm to the base, and to provide a
location to place the shoulder bearings.
In the new design, one of the brackets also serves as part of the
shoulder joint gearing. By modifying the
shoulder gear, the new design allows the gear to accommodate a bearing for the
shoulder shaft and be mounted to a small aluminum block to create a “bracket”
that is equivalent in size to the previous design.
shoulder joint bearings are model E7-S3F flanged ball bearings from PIC
Design. They have an outside diameter of
1.125 inches and a bore of 0.5 inches.
This accommodates the shoulder shaft perfectly. The bearings are rated much higher than they
will ever be tested in practical use.
shoulder shaft is a simple D-shape design with a diameter of 0.5 inches (see
Figure 5.3). The D-shape shaft fits into
a similar hole cut into one end of the lower arm tube. This effectively mounts the shaft to the
lower arm so they move as a single unit. The shaft rotates in the mounting
brackets by way of the shoulder bearings.
The motion of the arm is
caused by the shoulder bend motor driving the shoulder pinion around the
shoulder gear (see Figure 5.6).
5.2 LOWER ARM TUBE
trying to reduce the overall weight of the robotic arm in comparison to
previous designs, the structure of the lower arm
was rethought and redesigned. In the 1999-2000
Gateway team design, the lower arm primarily consisted of two 0.5-inch thick
aluminum plates (see Figure 5.4). Though
these plates were very strong, they added an unnecessary amount of weight to
the arm. By using square aluminum tubing
(see Figure 5.5), we were able to keep the structural integrity of the arm
while reducing the weight significantly.
This design is similar to that of recent OSU graduate student Chris
preliminary design, the shoulder bend motor was attached to the base of the
arm. Using a set of bevel gears, the
power was transferred to the shoulder joint.
This kept the weight of the motor off the arm, thereby reducing the
required torque to rotate the shoulder joint.
Upon changing the motor for this application, this mounting position was
no longer feasible for the shoulder bend motor due to space limitations. The solution was to house the motor in the
lower arm tube and transfer power to the joint using a pair of spur gears. Although this position adds to the necessary
motor torque, it also reduces the cost of gearing significantly over the
large size of the motor chosen to power the bending motion at the shoulder
joint required the lower arm tube to be machined extensively. Since the motor could not be placed inside
the tubing completely, a large contoured hole was machined out of the tube to
accommodate the protruding parts of the motor.
To provide for easy servicing, a slot was cut into the tubing to allow
the motor to be slid into place and bolted securely. This allows for easy assembly and service of
the shoulder motor and joint (see Figure 5.7).
5.3 ELBOW MOTOR
motor initially selected for the elbow joint was the 18-volt Black & Decker
Firestorm drill motor, the same motor as originally chosen for the shoulder
twist and bend motions. Ordering the
same motor for all joints would decrease the difficulty of obtaining all
necessary motors, but this proved not to be the best design for the robotic
with a controls encoder, the motor extended approximately 8 inches and weighed
over 1.76 pounds. Initially, the design
team sought to place the motor at the baseplate and transfer the power up to
the elbow joint through transmission pulleys, minimizing torque requirements at
the shoulder bend motor. However, since
the motor-encoder assembly extended nearly the entire length of the lower arm
tubing, the drill motor was mounted within the tubing directly aligned to the
elbow joint, as seen in Figure 5.8.
Bevel gears reduced the speed of the elbow rotation to below 12 rpm.
were press fit into the lower arm tube to support the elbow shaft. Outside of the bearings, snap rings were
fastened to the shaft to secure it from sliding and to lock the bearings in
further research, the motor from a Black & Decker Model 9074 3.6V cordless
screwdriver was selected to replace the drill motor. The screwdriver motor was smaller and lighter
than the drill motor; the complete motor assembly weighed approximately 1
pound, reducing the motor weight by approximately 40%. However, the 3.7-inch long motor still
produced 40-in-lb of torque, enough to operate the elbow. Furthermore, the screwdriver operated at a
maximum of 180 rpm, thus requiring less gear reduction than the drill motor.
The gearing for the screwdriver involved
two planetary gear sets contained within a plastic chuck. To maintain the lightweight, compact internal
gearing arrangement and to avoid machining additional parts, the entire chuck
and gear assembly was removed from the screwdriver, mounted to a fabricated
motor mounting block. The mounting
block, seen in Figure 5.9, was designed to hold the gears within the chuck,
properly mate the screwdriver motor to the gearing, and secure the entire drive
assembly to the lower arm tubing. The
block was made as thin as possible to minimize weight. Two machined retaining plates were attached
to the motor mounting block to secure the motor and to prevent unwanted motor
rotation, seen in Figure 5.10.
The adapter for the screwdriver bits was removed from
the press-fit clamp within the screwdriver gearing and replaced with a steel 0.25-inch diameter shaft, upon which the elbow
worm was aligned and pinned. A 20:1 gear
ratio was selected to drive the elbow joint.
The 16-pitch, 0.625-inch diameter elbow worm was chosen from the Boston
Gear catalog for its strength, small size, and bore diameter, which permitted
the worm to be attached to the screwdriver motor assembly. The associated 1.25-inch pitch-diameter gear
provided the necessary gearing reduction while permitting enough room for the
worm/motor assembly to be completely mounted within the lower arm tubing. The assembly was mounted at a 21.53° offset from horizontal, as seen in Figure 5.11.
The elbow shaft was modified both to secure the worm gear
and eliminate the retaining clips. The
0.5-inch shaft was revised into two mated components each with a 0.75-inch
diameter flange which, when assembled in the lower arm, were secured between
the elbow bracket and elbow bearings, effectively preventing the shaft from
moving laterally. The worm gear, with a
0.25-inch bore diameter, was clamped between the two elbow shaft components and
pinned upon the male member. The entire
shaft and gear assembly is seen in Figure 5.11 and 5.12.
D. Elbow Brackets
the forearm and the lower arm are two 0.125-inch-thick aluminum elbow
brackets. Incorporating a 135-degree
bend, the brackets significantly increase the range of rotational motion of the
forearm. Combined with mounting the
brackets on the outside of the aluminum tubing, bracket design increases
rotational motion from 180 degrees in the 1999-2000 undergraduate students’ and
2000 graduate student’s designs to 255 degrees.
The angle also allows the forearm to rest directly atop the lower arm,
permitting the entire arm to fold into a compact rest position. Each bracket includes a jog to accommodate
the flanges from the elbow shaft.