Classic Escapement - Graham (Ideal)

Last crawled date: 1 year, 10 months ago

Based on the book CLOCK AND WATCH ESCAPEMENT MECHANICS by Mark V. Headrick (Copyright 1997) as a fine source, I try to present rigorously the essence of escapement design. The book is public (http://www.nawcc-index.net/Articles/Headrick-EscMechanics.pdf), so you can read and compare (with the book) everything I present here. My intention is to create a series of projects dedicated to this subject - as you will see - but only concerning the geometrical aspects. Actually, you can use my projects as accurate simulations for understanding the book. I recommend you to download the book in order to use it when your computer is offline.
All parts, assemblies, and simulations/animations are made with Inventor 2014, so some specific references are bonded to this software.
As you can learn from the book, there are three classical kinds of escapement mechanisms: Graham (dead-beat), Recoil, and the Swiss Lever.
The term „dead-beat” is explained in the book on page 21. Actually, due to their circular shape, the palette arms slid along the wheel teeth, so the wheel is stationary during the lock time. More details can be found here: https://youtu.be/D9UT6YwzmBU. By contrast, the recoil escapements permit a certain backward rotation of the wheel, so the reel is practically „recoiled” during the lock time. This effect could be noticed in my previous project https://grabcad.com/library/escapement-macdowall-305-1. A great recoil can be noticed also in my former (original) project https://grabcad.com/library/escapement-non-slip-pawls-solution-1.
We start here with the Graham (dead-beat) escapement. The first step is to create the geometry of two parts: the wheel and the palette, as the main interconnected parts of the mechanism. The geometry has a first version - the Ideal mechanism. The Ideal version is very tight, with no clearance, and its purpose is to establish the geometry of the two parts. After considering this version, we shall slightly modify the palette in order to make the mechanism functional. This is why I present two different projects: the Ideal Graham escapement (this one), and Modified Graham escapement (the next one).
This first project is explained in the book till page 11: „5: The Importance of the Simulation”.
Firstly you have to look at the picture showing the Inventor sketch „Wheel - Palette.jpg”. The picture explains how the geometrical construction is made. The facet of the wheel tooth is 0.5 degrees (not mm). You have to consider the diamond-square which has the four driven dimensions noted (90.00). It connects the palette with the wheel. The reason for a pair of these dimensions is explained in the book on page 10. The other pair is naturally correct since they are radius and tangent to the same circle.
Watching the Video you see immediately the movement of the palette, due to the pendulum which is joined to it. It is difficult to see that the wheel is also forced to rotate clockwise. If you look carefully a bit, you understand that, when the palette is rotating also clockwise (the second swing), nothing but an imposed motion applied to the wheel makes it rotate. When the mechanism is presented from the back you understand that a thread and a weight must be spooled on the reel to produce the motion. They are not present because threads/ropes are not allowed in Dynamic Simulation in Inventor. Instead, a torque (of 20 N·mm) is applied to the reel.
Anyway, the escapement does not work properly. Actually, the wheel is blocked between two teeth by the palette because there is no clearance to permit the wheel to move on. The palette (and the pendulum) are slowing their movement, and finally, they will completely stop. The problem will be solved in my next project (see https://grabcad.com/library/classic-escapement-graham-modified-1).
Included is the file „Output Grapher (5s).jpg” where you can see the slowing movement of both mobile parts. Enter the Environment / Dynamic Simulation and run once the simulation, then click on the Output Grapher tool to see this for yourself.
Look to the next project (Classic Escapement - Graham (Modified)) to see the correct movement of the parts.

Note
You will notice the small eccentric hole (Ø2 mm) on the wheel. This was placed there in order to get attention from the Dynamic Simulation module to automatically create a revolution joint for the wheel. Without this hole, the automatic creation of joints in Inventor will exclude the wheel because no automatic movement is supposed to gain this part. I did not want to change the automatic creation of joints in settings of the Dynamic Simulation for you, to have the possibility to change the parts and to enter/leave the Dynamic Simulation and to see the changes. The other way is to manually insert the joints, and entering/leaving the Dynamic Simulation seems to be more complicated.
Enjoy!