Wanting to ensure the wristwatch has very accurate time keeping and yet is still reliable I felt the choice of escapement design was either a Daniels coaxial escapement or a modern lever escapement. Most other escapements were ruled out either because they are not self-starting, can become stuck if a shock to the watch is received or they do not have accurate enough time keeping.
The Swiss Lever escapement was chosen mainly due to there being more literature and support available as compared to the coaxial alternative. The Swiss Lever is also superior to the English Lever in several ways. Mark V. Headrick gives excellent discussion on optimum escapement design in his ebook, so this won’t be repeated here. As recommended by Headrick, I chose to model and simulate this escapement movement first to find an optimal design. Headrick does not explain the software or offer instruction on how to perform the simulation. I used Fusion 360 to model and simulate, finding the results to be perfectly acceptable.
The first design attempt (shown below) was a double roller. I used the same roller vs. escape wheel scale as illustrated in Headrick’s ebook. Although this worked fine in the model, the initial attempt showed the whole escapement was going to be too big for my wristwatch. Whilst I could simply reduce everything by a common factor, the escape wheel would become very difficult to manufacture.
Double Roller Escapement (this design was too big)
However simulation proved that the double roller in the ebook worked and the all the appropriate angles of draw, impulse and unlocking functioned as expected – perhaps another way of interpreting this is that my simulation environment worked as expected. As can be seen in the above image, the roller is almost as large as the escape wheel, and the palette stem is undesirably long.
At the same time as looking to reduce the size of the escapement, I decided to try modelling a single roller. In Watchmaking there is a chapter on escapements and Daniels talks about using a single roller with a guard pin. This design was favourable to me over the double roller as there are fewer components to manufacture. I read some of the theory about how to calculate various angles and lengths for the safety action, but in the end I decided to simply try a few different ideas within a simulation.
The final product, after reducing the size of the palette and using a single roller with a safety action, was the design below.
Final Design – Smaller, Single Roller Escapement
Here is the same design in different states of movement (bankings not shown):
Addenda were added to the teeth of the escape wheel as shown above in order to increase the total angle through which impulse is supplied. The simulation exactly matches the various stages of movement as suggested by Daniels (shown below for convenience).
Actions at various lever angles (note action is symmetrical for a total of 22 degrees)
The safety action was also tweaked to ensure no friction or binding would occur under normal operation. Scenarios were generated and tested to ensure it was impossible for the escapement to become stuck, for example if it received a sudden shock in any given position.
I did not attempt to reduce the size of the escapement further as the parts would become difficult to machine accurately. It may still be possible to reduce the size of the palette stem and roller, whilst keeping other parts the same size, but the results were quite satisfactory and will now easily fit into the movement being designed.
The final dimensions of the newly designed escapement are shown below. It remains to be seen how easy it will be to machine such small components to the tolerances required.
Dimensions of the Final Design (in mm)