V

 
LOWER ARM

 

 

5.1 SHOULDER JOINT

            The 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.

            During 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.

            The 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.


            To 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.

            The 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.

            The 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

            In 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 Fearon.

      

In our 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 initial design.

            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

            The 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 arm.

            Combined 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.


            Flanged bearings 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 place.

            Upon 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.

Text Box: Figure 5.9 - Elbow Motor Mounting Block

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

            Connecting 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.