CRANE
my undergraduate finale
The task: Design and build a crane, comfortably operated by a single user and with a base no more than 4’ long, which can safely lift a human payload up a concrete staircase to a designated target zone as shown in the diagram.
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The timeline: April 20 (project announced) to May 8 (showcase) – 18 days
This larger-than-life project was the sixth and final assignment of my senior Design for Fabrication (DFF) class. More than that, it closed the loop (in the most epic way possible) on what started me down the path of mechanical engineering nine years earlier: this scratch-built hydraulic crane I made because I didn’t have space in my schedule to take a robotics elective.
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In fact, this full-circle experience was foreshadowed by the fifth assignment in DFF, which was to build a small single-operator crane capable of lifting a cinderblock onto a table. At the time, we had no knowledge of the incoming project six! We were certainly in for a surprise when our professor announced a human payload for the next iteration…
Fortunately, I had the amazing opportunity to team up with Patrick Mulcahy, a brilliant peer of mine whose meticulous dedication to his craft had inspired me throughout our undergraduate careers. Furthermore, his years of competitive sailing experience meant that he brought an incredible wealth of rigging knowledge to the project. Paired with my decade of woodworking experience, the synergy of our teamwork was unlike anything I’d encountered in a project before, and we were able to tackle a very ambitious and intricate design on a tight timeline by leveraging our specialties.
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Most of our peers were considering some permutation of a lever-based design akin to a pivoting seesaw. Patrick and I feared that such a construction would be difficult to quickly and precisely start, stop, and control due to the large mass of the payload and the significant cantilever required to clear the staircase, resulting in a very high moment of inertia. Additionally, this large cantilevered load meant that any bearings or other supports for such a pivot would be subjected to enormous off-axis torque during operation.
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Instead, we opted for an XY gantry design. By eliminating the need to pivot altogether, the entire structure could be much stiffer and more thoroughly reinforced, and by completely decoupling the vertical and horizontal motion, the passenger would enjoy a safe and controlled ride.
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Using the required range of motion and dimensions of the payload as a guide, Patrick and I sketched out a rough design for our crane. Due to the enormous size of the build, we knew that we would have to do most of the assembly outside on the patio, but in the shop I was able to build the gripper, the rolling cart from which the gripper would be suspended, and the main I-beam on which the cart would roll.
By the way, the gripper works purely by gravity and friction: the tension in the links is much greater than the weight of the payload due to the shallow link angle, much like the high tension in the string holding up a picture frame from a nail. We can thus generate a large normal force pressing against the payload using only its own weight, and with rubber pads on the gripper, we get a theoretical safety factor of more than 4 (assuming a 100kg payload and 0.6 friction coefficient between rubber and wood).
While I was busy making the structural elements, Patrick was in charge of the all-important rigging and controls system of the crane, the network of cables and fittings that would allow my wooden parts to move in response to operator inputs. He devised a brilliant and extremely elegant method for decoupling the vertical and horizontal motion of the gantry using two lines of rope—let’s call them the lifting line and the translating line—secured by locking cleats.
The lifting line connects the gripper to the rolling cart via a 12:1 pulley reduction to lower the operator forces, and the translating line is anchored directly to the rolling cart. When the translating line is cleated, the cart cannot roll, and the operator can raise or lower the gripper by reeling the lifting line in or out. Conversely, when the lifting line is cleated, the gripper cannot raise or lower, and the operator can roll the cart back and forth by pulling the translating line one way or the other.
The magic that makes this possible is the fact that as the cart rolls, the length of the lifting line remains constant, so rope can feed through the pulley reduction at a constant rate. The whole process is like putting your finger through a loop in a taut string anchored at both ends: as you move your finger back and forth, the loop moves with you, and string is constantly spooling and unspooling around your finger as a result. The initial tests of this ingenious mechanism turned quite a few heads in the makerspace. Everyone seemed to think Patrick was some sort of wizard, and they were probably right.
At this point, we moved construction outside and built the base and truss walls close to where they would be deployed. Since we wanted to be able to take everything apart after the project so the wood could be reused, we opted for plywood gusset plates bolted to the truss members. We attached the main I-beam with three lengths of 2x4 which fit into notches in both the beam and the truss walls, with screws holding everything in place. Tipping the I-beam into a vertical position took many helpers, as did tipping the entire crane into its final orientation.
With mere hours before the showcase, we noticed a crack forming in the I-beam. On the advice of one of our peers, Brandon Sun, we quickly bolted plywood gussets over the crack to locally reinforce the beam. This did the trick and significantly cut down on the deflection at full payload extension. We were able to run a few tests at full weight before the event started, and we were fairly confident it would work at that point.
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Per the letter of the assignment (requiring a teammate to be the payload), I was the first one in the box. With our crane in position at the base of the staircase, weighed down with bags of concrete to prevent it from tipping as calculated in an earlier static analysis, Patrick began to pull on the lines. While slightly terrifying to be lifted off the ground, I must say that the whole thing felt very secure and controlled, just as we had hoped. The crowd went wild when I made it safely to the target zone--we had certainly gone in a very different direction than the rest of the class design-wise and I think people were excited to see our machine in action.
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After my run, our professor, Brandon Stafford, agreed to be lifted up the stairs. If that's not the ultimate act of trust in his students, I don't know what is. I couldn't possibly imagine a more cathartic way to finish off my undergraduate degree than this:
