Actually, would this joint be considered a ratcheting joint or a joint with indents? 

This design looks into the use of a ratcheting mechanism for use within the wrist section of the arm. I'm not entirely certain on what mechanism would be ideal for prosthetic use, however, I had decided on a ratcheting mechanism for wrist design over a locking, friction or fixed joint. I felt a ratcheting mechanism would be the easiest for a user to operate with, and would be fairly easy to implement. A friction joint would have a similar user operation and better user adjustment, however, I felt it may be difficult to implement reliably and to have sufficient friction to resist unintended rotation. I do feel it would be possible to create a friction joint however, so may look into creating one in the future. For a locking joint, whilst it would be acceptable for a single hand amputee to operate, it may be difficult to operate the locking mechanism if they are a double hand amputee.

The design I came up with further develops on the method of using nylon filament as a flexible low wear material. Compared to the mechanism I had used in the thumb joint, this design takes a different approach to achieve a ratcheting mechanism. The ratcheting mechanism in the thumb joint relied on the flexibility of the nylon axle to act as the spring of the mechanism, and a piece of short nylon to act as the low wearing indent. There is a limit on how much ratcheting resistance the thumb joint design would allow so a different method would be required.

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As it would be ideal to have the wrist joint as compact as possible, I started from reusing the short piece of nylon filament used in the thumb joint. To make a reliable joint, wearing surfaces should be made with a low wear material, and ratcheting parts should be made from a flexible and durable material. Nylon fits the requirements so I decided to integrate the design around having the short nylon piece as the bearing material as well as the ratcheting component. Therefore, a section of the nylon will provide the bearing surface, whilst another section will give the flexible component that rides over an indent to provide the ratcheting feedback.

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For a wrist joint where there may be cables or wires that need to be passed between the hand and arm, the joint should have a open pathway at the center axis of rotation. Having cords at the center constrains the movement of the cords in only at a twisting motion, rather than an increase in cord length when routed outside of the center axis of rotation. 

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Having a basis of knowing how the mechanism should be designed, I settled on laying out the design similar to a ball bearing, where you have an inner and outer race along with ball bearings which will be nylon filament in my design. The only part that differs from a ball bearing is that the rolling elements will be fixed, and will be sliding instead. A major benefit to the layout is that the number of nylon elements can be reduced or increased depending on design requirement. Indents can be small to improve long term reliability whilst an increase in nylon elements can improve load bearing capabilities as well as distribute wearing effects of indents. Changes to indent size and quantity of nylon elements allows for a large variety of applications where this method may be suitable, however, there will have to be a balance of functionality and reliability.

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Printing out the design, I was well impressed by the torque required to overcome the indents which I had designed to be 1mm (which thinking now might be too aggressive). Despite requiring a wrench to overcome the indents, the mechanism seemed quite reliable as there wasn't a perceivable degradation from ratcheting the mechanism many times but should really be checked out with some equipment. I decided to reduce the indents to suit my requirements but with quick manual test, the design shows a lot of promise as a high torque ratcheting mechanism which can be used in many applications.