Second design revision

A project log for ARX Hand Project X1M

An advance modular 3D printed mechanical prosthetic

SupercellSupercell 08/04/2020 at 00:440 Comments

Second revision helps reduce the issue that the previous prototype which had issues with strength when holding uneven loads across all 4 fingers when using a whippletree system. To help with the issue, instead of a single connection point for the main actuation cord for the whippletree block, a pulley system is used to help distribute the actuation force across the block. The pulley also has an added benefit of adding mechanical advantage to the system to help improve overall strength. 

To integrate a pulley system into the design without increasing additional part cost, the pulley system will have to function without ball bearings, and will mainly use sliding elements in the design. To use a thin fishing line that's around 0.5-0.8mm, high loads should not be focused on a small area, as it will result in wear of the surface. Without ball bearings, the cords will be sliding on the pulley surfaces, so therefore the effective diameter of the pulleys shouldn't be small as it would result in a low surface area. For pulleys that need to be small, they can be made in to pulley wheels that slide on an axle. Pulley wheels can use nylon as a low wear and low friction axle, and can be designed wide to distribute the load out onto a large surface area of the axle.

The additional mechanical advantage added to the system from the pulleys greatly increases usability of the whippletree. For how much mechanical advantage the system should have would depend on the actuation length it would result in. Having more mechanical advantage to provide greater strength is ideal, however, it adds increased length to the total length required to fully actuate the fingers. Without mechanical advantage, the fingers require around 20mm of actuation. For 2, 3 and 4 mechanical advantage, 40mm, 60mm and 80mm of actuation length is required respectively. The maximum actuation length that would be suitable for manual actuation was uncertain so a decision was made to implement a configurable 2 or 3 times mechanical advantage as it was the easiest to integrate into the system.