Interesting Points about our Strucure

We designed our structure to minimize weight. We chose to go through the smaller hole because we figured that the base would use significantly less material than if we had gone through the larger hole.

In our first build, our arm went straight out from the wall with the lifting arm extended perpindicular to it. The load on the lifting arm caused a lot of torque which twisted the truss. To correct for this, rather than adding a lot more material as support, in our next build we had the arm go out from the wall at an angle to meet the weight head on, with the lifting arm extending out from the front of the arm.

We built our base at an angle to the clamps and plate it was mounted on so that it would meet the arm at a right angle. This minimizes the length that the base has to extend out to meet the arm, allowing it to be made from less material, and minimizing the moment caused by the arm on the base.

The greatest source of deflection in the base was due to the moment about the base along the axis of the arm. To counter this deflection, we used tension members running from the top to the far corner of the plate and compression members running from the front bottom of the base out to the far side of the arm. We also found that the load on the horizontal beams at the top of the base on which the arm was mounted caused the screws holding it in place to loosen. This slack allowed these beams to slide to a downward angle, contributing greatly to verticle deflection in the arm. The only way to correct this problem was to continuously retighten these screws.

Side view of the Structure

How the Mechanism Works

The mechanism in the crane functions by using the servo arm to push down at point A (to position B) on a compression member, which in turn transfers force to a lever arm at point C. The lever arm is pinned point D to the truss, allowing it to pivot and lift a weight on the other end of the arm from position E1 to E2.

The Mechanism

Theoretical Performance

Since the servo arm moves from +45° to the -45° position, the total movement of the pin connection between the servo arm and the compression member will be L*root(2), where L is the servo arm length. The connection between the compression member and the lifting arm will also be displaced by approximately the same distance. Given that the spacing of the joints on the lifting arm is 2" from compression member to pivot point and that the point of contact is approximately 5" away from the pivot point, the weight should lift up a distance of 5/2*L*root(2).

Under ideal conditions (i.e. when all of the screws in the truss are tightened), the lifting arm will deflect by less than 1/4". Using an L of 0.8", we calculate a net movement of slightly more than 2.5", taking into account 1/4" of deflection.

At the start point, the servo arm is at a 45 degree angle to the force application, allowing only F = sin(45°)*Theta/L = 0.88*Theta. With a minimum Thetastall = 45.82 oz.in at 4.8V, this gives an Fstall of 40.5 oz. When the pivot point on the lifting arm is properly configured, the maximum angle from the horizontal is only tan-1(0.566/2) = 15.79°, giving a net applied force of 40.5*cos(15.79°) = 38.97 oz. Given a weight of 16 oz, the moment applied is always large enough to lift the weight.

Last modified: Fri May 8 23:58:19 EDT 2009