The design of a compliant horizontal clutch and braking system for chronographs

Abstract

The precision and intricacy behind mechanical watches has long captivated horologists and is driven by centuries of innovations. Mechanical watch complications such as chronographs, can further increase mechanical complexity and present challenges due to the reliance on numerous rigid components requiring precise movements. These movements can result in wear, friction, and reliability issues building up over use. Compliant mechanisms offer a promising solution by utilising material flexibility to enable motion with minimal wear, backlash, and a reduced part count. These advantages however come with challenges, such as a more complex integrated design process, limited mobility, and fatigue considerations.

This thesis therefore presents the design, modelling and evaluation of a compliant bi-state switching mechanism designed for fabrication from a silicon wafer and integrated with the horizontal clutch and braking system sub-components of mechanical chronographs. The functional requirements of the chronograph sub-functions include reliable switching between the engaged and braking states, secure gear engagement with minimal misalignment during the engaged state, and precise timekeeping through minimal brake timing delay and slippage of the chronograph seconds hand during the braked state. The design is modelled and validated using both an analytical pseudo-rigid body model and a numerical finite element model. A scaled PETG-based proof-of-concept was then fabricated to verify the performance.

The PETG proof-of-concept demonstrator experimentally showed a robust switching success rate of 99.8\%, stops the seconds hand within 2.4 $ms$, and prevents slippage up to a 2.5g loading case meeting the functional requirement. The gear engagement functional requirement however was not fully meet with a success rate of 95\%. The underlying cause of this issue was identified and recommendations for redesign were provided. Additionally, the simulation model results indicate that the design offers a durable performance with von Mises stress levels within desired limits of under 300 $MPa$. Future works for this design include to develop a functional one-to-one silicon wafer prototype and integrate the mechanisms with additional chronograph sub-functions, such as the minute counter and reset mechanism.

This thesis provides two key contributions. First, it introduces a novel bi-state compliant switching mechanism based on a latch-lock design. This design hold potential applications in MEMS devices, mechanical logic designs, and robotics and is capable of scaling with minimal modifications or impact on performance. Second, as silicon wafer fabrication was desired, a practical methodology was developed to translate the silicon wafer requirements and expected behaviour from the experimental PETG results and scaled demonstrations.