Engineering Perspectives

The Kassel Hand’s Mechanisms

The original artifact of the Kassel Hand has four fingers that are still operable and a thumb that appears to have once moved but is currently broken. Here’s what we know about the working mechanisms: the four fingers are attached at their bases to four toothed wheels that sit on a common axis. The unknown wearer would have operated the mechanisms with a (presumably) intact left hand to push the fingers downward into the desired position. Examining the artifact reveals that when one presses down on the fingers, as each wheel turns, it moves against a pawl (locking spring), a flat pivoted lever which catches on the teeth and prevents the wheel from rolling. The four pawls are mounted on bases that hold them in place. The interior of the artifact also has two pairs of flat springs bolted between the mounted pawls. These apply downward pressure to the proximal end of each pawl to keep them from moving too far in either direction. The artifact has a large Y-shaped main lever connected to the external release switch. The main lever acts on the four pawls to release the toothed wheels and open the fingers. This Y-shaped main lever consists of two parts: a vertical shaft that extends from the release trigger on the exterior of the wrist through the interior of the hand’s shell; and a solid triangular plate with a wedge-like thickness that stretches horizontally from the vertical shaft of the main lever to just beneath the proximal ends of the pawls. The release trigger, once pressed, acts on the vertical shaft of the main lever, which pushes the solid triangular plate (the “paddle”) forward. The wedge-like body of the plate moves beneath the ends of all four pawls at once, moving like a shovel to force them upward, creating a controlled seesaw motion of the pawls that removes pressure from the toothed wheels and allows the fingers to move freely again.

Engineering Context: Requirements and Constraints

Engineering requirements are the foundational specifications and guidelines that quantify the objectives for a project. These requirements ensure that the design, materials, and construction methods align with the intended purpose of the product or system. They typically cover factors such as functionality, safety, durability, environmental considerations, cost, and compliance with relevant regulations. By defining clear engineering requirements, teams can effectively address potential risks, maintain quality, and achieve the desired outcomes while meeting the needs of stakeholders. Properly defined requirements serve as a roadmap throughout the project, ensuring alignment between design, development, and testing phases.

In the case of the Kassel Hand model development, the competing objectives were a model which looked like the original Kassel Hand, functioned like the Kassel Hand, and was made with a generalizable and widely available printing technology, unlike the Kassel Hand’s original wrought iron. The team navigated these conflicting objectives by setting requirements at high and low levels – this resulted in splitting the models into the Lookalike and Work-Alike versions, with each version prioritizing dimensional accuracy and functional reproduction, respectively. Within each design, subsystems (the mechanism, gauntlet shell, etc.) had requirements set – either in terms of known dimensions which must be reproduced (i.e., overall length), or known functionality (ratcheting fingers that simultaneously released when pulling a lever), and then other requirements relaxed – certain dimensions that could not be measured well were driven by adherence to other dimensions, and hypothetical limits of the original mechanism were reduced to cover only the historically contextualized Activity of Daily Living (ADL) to be tested.

Manufacturing constraints in engineering refer to the limitations and challenges that affect the design, production, and assembly of an object. These constraints can include factors such as material availability, production capacity, cost limitations, machinery and technology capabilities, and the skillset of the workforce. Additionally, constraints related to quality control, time constraints for production, and the need to comply with safety and environmental regulations are significant considerations. These constraints influence the design process, often requiring engineers to optimize designs for manufacturability, minimize waste, and ensure that production processes remain efficient and cost-effective. Balancing these constraints with performance requirements is key to successfully realizing a design.

While 3D printing offers incredible potential for generating models of artifacts like the Kassel Hand, it does come with several limitations and constraints. As with most manufacturing systems, 3D printers have limitations as a function of cost (printers range from hundreds to hundreds of thousands of dollars) and function of complexity (e.g., what sort of safety considerations are required). Within the realm of low-cost printers selected for the project, there are still constraints which must be accommodated. One constraint is the size of the print volume, which restricts the production of larger items or complex assemblies. Furthermore, it may restrict the orientation of specific parts, which may have undesirable impacts on part strength. Additionally, the materials available for 3D printing, though expanding, are still limited compared to traditional manufacturing methods, which can impact the strength, durability, and flexibility of printed objects. For this project, Polylactic Acid (PLA) was selected as it is low cost, easy to print on most low-cost machines, and does not have safety limitations (e.g., ABS). Print settings, such as speed, are another set of constraints, with tradeoffs in cost, strength, and surface finish.

While the concepts of requirements and constraints can be described separately, they are interconnected aspects of realizing a design. The interactions between requirements and constraints may necessitate design iterations or revision. For us, this required engaging both historical and engineering expertise to realize the Kassel Hand model.