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Site work by E. Scott Carleton, Matt Salac, Cale Magruder - April 28th, 2006

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The Project

Construct an arm out of aluminum weighing less than 20 oz. that reaches through a specified playing field and lifts a one pound weight two inches two consecutive times.

The Design

The structure consists of three parts, the base, the arm and the lifting mechanism. The base in anchored into the clamp, the arm is mounted on the base extending towards the weight, and the lifting mechanism hangs below the end of the arm at a height at which it can lift the weight. The base consists of two parallel L-sections partially trussed, partially plated. The arm is simply two ¾" strips of aluminum bent into a shape to allow them to bear a heavy load without deflecting. The lifting mechanism hangs below. The servo is mounted on two "danglers" on one side of the arm. The lifting arm (delron) is mounted on a pin suspended at the end of the arm to allow the lifting arm the ability to pivot about the pin. The gear ratio between the lifting arm and the servo is 5:1.

The plan was to keep the design simple and easy to construct. After a few static calculations, a general idea was gathered that the structure needed to resist a downward force of one pound at a relatively long (15"-20" depending on the length of the arm) distance. Upon constructing the L-sections, it was assumed that they would make a base strong enough to support the force of the weight on the lifting arm. The arm length was determined by the usable length of ¾" aluminum strips. This length was 17.5". The servo can supply about 3 lb-in (0.25 ft-lb) of torque. Over a distance of 0.25 inches, this results in a force of 12 lbs. According to calculations, a force of 9 lbs is needed to lift the weight at a distance of 9".

Problems Encountered

• Large moment created by lever arm and lifting mechanism due to distance.
• Gear slipping.
• Base bending.
• Arm is suffers lateral torsion.

Solutions

• Truss parallel face to the lifting arm to distribute the moment into compressive and tensile members.
• Use stronger "danglers" and brace gears.
• Add a an L piece to increase bases moment of inertia.
• Brace the arm with compressive members on the bottom.

Performance

The final weight of our design was 15.9 ounces. The constructed arm failed due to gear slippage and lateral torsional buckling upon trying to lift the weight. The braces helped, but were not enough.