The forearm design was closely modeled after the 2000 graduate student design, once again incorporating the square aluminum tubing. The tubing with a 1/8-inch wall thickness provides a compact, lightweight supporting structure that completely encloses all forearm components, protecting both the equipment as well as the user from potential injury.

The preliminary design employed a 2.5-inch square tube, similar to the preliminary tube used in the lower arm. Uniform tubes reduced the costs for obtaining raw materials and permitted a compact rest position for the arm. The tubing supported two mounting plates, which secured the motors driving the differential gears at the wrist. Seen in Figure 6.1, one motor was offset behind the other motor, permitting both to fit within the enclosed tube. An aluminum drive shaft, supported by a bearing, connected the offset motor to its differential gear, while the pinion from the other motor was mated directly to its respective gear.



When the lower arm tubing was expanded for the final design to accommodate new motors and because the 2.5-inch square tubing was expensive and difficult to obtain, the forearm tubing was also enlarged to maintain uniformity. The 3.0-inch square tubing, which is readily available, was chosen in place of the 2.5-inch square tube. The 3-inch tubing permits the two wrist motors to be placed next to each other, eliminating the need for a second motor mounting plate, bearing, and drive shaft. Figure 6.2 illustrates the parallel position of the wrist motors. Fewer components within the forearm reduced not only materials and machining costs but also the overall weight of the design.

Two oil-lubricated ball bearings are press-fit into two wrist pillow blocks, machined from 0.25-inch aluminum plate, and attached to the forearm tubing to support the wrist differential. The bearings and wrist pillow blocks can be seen in Figure 6.3.





The wrist twist and bend motions are controlled by a differential gear set identical to the gearing on the 1999-2000 undergraduate students and the 2000 graduate student designs. The bevel-gear configuration provides two degrees of freedom within a compact volume.

In the final design, placing the wrist motors side-by-side increased the distance between the motor shafts. Spacers, appearing as red in Figure 6.4, were fastened between the differential and the outer bevel gears to accommodate the repositioning.

Although the differential gear set, purchased from Sterling Instruments, was one of the most expensive components purchased for the arm, the mechanical advantages were deemed to appropriately offset the high cost of the product. Providing two degrees of freedom within a small space was essential to maximizing the movement and functionality of the robotic arm design.



Table of Contents

Section 5: Lower Arm

Section 7: Gripper