Rat Jaw Mechanism
For the Mechanisms class mid-term I’m going to build the moving jaw mechanism for a wearable rat head. The mechanism I’ve decided on will be a pulley system that enables the motion of my jaw to control the motion of the rat jaw. The face of the rat head will not line up with my face, rather, it will point up from the top of my head so that when I walk on all fours it is in proper orientation, the way rats look. No electronics involved, as I want the natural action and nuance of real organic material. To that end, I must find an mechanical solution to amplifying the 2 axial rotational inches of movement my jaw is capable of to approximately 8 inches in the giant rat jaw. In order to visualize the problem I made some drawings on a white board, displayed below.
The first drawing shows the elements of the whole rat costume project, and the second drawing shows a rudimentary drawing of the elements of the jaw system. For the second drawing, I’m already seeing that two pulley systems on either side of my jaw may be more efficient and even allow for some side-to-side motion, like a real jaw, depending on how I design the point of rotation or joint where the rat jaw joins with the frame. Better drawings to come!
I’m planning to address the challenges of this system by optimizing the mechanism within following constraints:
- Weight; the whole mechanism, including mounting materials, must not weigh more than an estimate of 5lbs. I will be mounting other hardware (cameras, tubing, eye-wear, microphones, headphones, a small speaker, among others). Ideally the frame/structure would be able to withstand impact if I run into things, support the sum total weight, and yet be light enough that my neck won’t get strained. I have some ideas about strain-relief, connecting the headpiece to a harness I wear on my chest. I will no doubt have to custom build some pieces due to the fact that it is difficult to create a durable, sturdy form factor when dealing with wearables.
- Mechanical advantage; the use of pulleys rather than gears or motors will allow for a springiness in the action of the jaw, and a looseness in the fit. Gears would have to crafted well, fit together to work. Motors would require electricity, add weight, and probably seem more robotic than lifelike. Increasing mechanical advantage will be achieved by using several pulleys of different sizes in series– string or cable connected to my jaw will drive a large pulley, which will drive a small pulley, which will drive a large pulley, and so forth. I’m looking at compound bow design for this. The final step of the mechanism that activates the rat jaw will rely on a third class lever to give speed, which will be possible because I will have stepped up the strength of my jaw’s pull (25:1? 50:1?). The rat jaw will have a spring that will pop the jaw back into the closed position and hold it there.
- Materials; the erector-set-like pieces found on Pololu seem like a good choice for the concerns outlined above. I plan on ordering the pulleys from that company, building the mechanism on a table, and then transposing it to a headpiece I’ll construct off of a welding helmet headset without the mask.
The timeline for this aspect of the project is as follows:
- March 1st, order parts!
- March 3rd, have drafts of drawings and as much of the measurements and sorted
- March 4th, receive pulleys and material in the mail and begin assembly on tabletop
- March 7th, have tabletop assembly completed and be ready for head mounting
- March 9th, finish head mounting and troubleshoot/tweak
- March 11th, present in class
Many of the materials I will be using to build this prototype I already have, such as, string, cable, wood, tools and welding headgear. I’ve made the budget above to reflect the fact that some pieces will need to be altered and may break in the process. I have not budgeted any miscellaneous objects or hardware because I’m fairly certain I already have enough materials to complete this prototype.