The picture below shows the basic layout of the club tooth lever escapement. In this picture you are looking down from the top. For the sake of this article, I am leaving out the safety parts of the balance which include the pallet horns, guard pin and safety roller as they do not add the to function of the escapement.
The Escapement at rest
When the balance is at rest with no power applied to the escape wheel, the impulse pin comes to rest on the line of center or Dead point. The line runs through the center of the balance staff, the pallet fork staff, and the escape wheel staff. If the balance is pushed in either direction, the hair spring will cause the impulse pin to oscillate until it comes to rest in the position shown below. Understanding the Dead-point is essential for inspecting the escapement and for timing adjustments during service.
The Locked Position
In the picture below the pallet lever or arm is resting against the entry banking pin. The entry pallet is resting on the green tooth of the escape wheel “locking” the tooth on the “locking face” of the entry pallet. The pallet lever is shown somewhat clockwise from its Dead-Point position. The Hairspring is turning the balance assembly anti clockwise as shown by the arrow, moving the impulse pin toward the line of center, into the notch on the end of the pallet lever.
As the impulse pin enters the notch of the pallet lever it will engage with the right side of the notch and push the end of the lever to the right. This moves the entry pallet in the direction away from the escape wheel tooth, so that the tooth rides along the locking surface of the entry stone and passes over the entrance or lock edge and onto the impulse face on the jewel. It is at this point that the escape wheel is considered unlocked.
Driven by the power of the train, the escape wheel begins to rotate. The tooth-colored green pushes against the impulse face of the entry pallet stone moving the lever clockwise. Since there is a bit of clearance between the impulse pin and the notch the lever will initially rotate freely until the left-hand side of the notch presses against the impulse pin. At this point the impulse begins. The movement of the pallet lever now will push the impulse pin giving an impulse to the balance.
Each impulse phase has two distinct parts that provide one smooth impulse.
As the escape wheel turns, the heel of the tooth slides along the angled impulse face of the entry pallet stone pushing it outwards. This causes the pallet assembly to rotate clockwise. The left-hand side of the pallet arm notch now pushes the impulse pin, providing an impulse to the balance.
The drawing below shows in greater detail how the impulse is split between the angle face of the pallet jewel, and the angle face of the escape wheel tooth. This is distinctive characteristic of the club tooth escapement. This is the first phase, as the heel the tooth moves along the impulse face shown in diagrams.
When the heel of the escape wheel tooth reaches the let off corner of the entry pallet stone the angled impulse face of the tooth continues to push the pallet outwards. This is the second phase of the impulse.
In the drawing below the sloping face of the wheel tooth now provides an impulse. Diagram F shows the point when the tooth has left the let off corner of the exit pallet stone and it the beginning of drop.
Drop and Lock
As the tip of the escape tooth leaves the Let Off Corner of the entry pallet, the escape wheel is free to rotate or drop. As this happens the locking surface of the exit pallet contacts the tooth on the escape wheel. After a brief period of drop the escape will is now arrested. At this point there is a small gap between the lever and the banking pin.
Draw and Run to the Banking
The locking face of the pallet jewel is angled so that as the tooth presses against the locking face the pallet is drawn into deeper engagement with the tooth. This moves the lever further to the right until it is arrested by the exit banking pin. The stage is called runs to the banking.
When Run to the Banking is complete, draw keeps the lever firmly against the banking pin so that the pallet lever notch is always in the correct position to receive the impulse pin as it returns.
If the pallet lever is moved away from the banking pin (but not too far away enough to unlock the pallet), and then released it goes back to the banking pin under the influence of the draw.
Both pallet jewels are angled to produce draw.
Since the escape wheel rotates a little further during the draw this causes the escape wheel to recoil a small bit during the Unlock. This takes a little power from the balance but the subsequent more than makes up for it.
However, to much Draw can be detrimental to the overall action of the escapement by taking to much energy from the balance causing a loss of amplitude.
Too little Draw will cause problems with the safety system of the escapement which I will write about later.