Understanding the Escapement

The escapement is the part of a watch most people have heard of and few can describe. Its job is two things at once: it locks the gear train so energy is released in portions rather than all at once, and it gives the balance a push on each oscillation so the balance keeps swinging. Every tick you hear is one locking and unlocking event.

The energy problem it solves

A mainspring wants to unwind at once. Left to itself it would spin the train down in a second or two. The escapement interrupts that release: it holds the train still, lets it advance by one escape wheel tooth, holds it again, and repeats — thousands of times an hour.

At the same time it must pay the balance back for the energy the balance loses to friction and air. Without that repayment the oscillation would decay in seconds. The impulse is that repayment, and it happens at a specific point in the swing.

The Swiss lever cycle

In a Swiss lever escapement an escape wheel tooth rests against a ruby pallet on a pivoted fork. The balance carries an impulse jewel in a roller. As the balance swings through centre, the impulse jewel enters the fork notch and rotates the fork, which unlocks one pallet. The escape wheel advances, its tooth slides along the impulse face of the pallet, and the fork pushes the balance on its way. The other pallet then catches the next tooth. The cycle repeats twice per oscillation.

• Locking — a tooth rests safely on a pallet face
• Unlocking — the balance, through the impulse jewel, rotates the fork and frees the tooth
• Impulse — the tooth slides along the pallet and drives the fork, which drives the balance
• Drop — a small free travel before the next tooth lands
• The fork is then held by the banking pins until the balance returns

The three settings that matter

Lift angle

Lift angle is how far the balance travels while it is being impulsed. It is a property of the geometry, typically 30° to 52° for a lever escapement, and it is not adjustable — you set it into your timegrapher so the instrument can convert the impulse duration into an amplitude reading. Enter the wrong lift angle and every amplitude figure you read is systematically wrong.

Lock

Lock is the margin by which a tooth rests safely on a pallet face, typically one to two degrees of escape wheel rotation. Too little and a shock can make the escapement skip, which shows up as sudden large gains. Too much and the unlocking consumes more of the balance's energy, and amplitude suffers.

Drop

Drop is the small free travel between releasing one tooth and catching the next. It is deliberately minimal. Excess drop wastes energy and increases the chance of the wheel overrunning; too little and the wheel will not release cleanly.

Other escapements

The coaxial escapement is worth understanding because it changes the lubrication requirement. Because impulse is delivered radially rather than by sliding along a pallet face, the lubrication demand at the impulse faces is reduced — which is the point of the design, and why service intervals can be longer.

What goes wrong, and what it looks like

• Knocking or rebanking — amplitude too high, impulse pin strikes the back of the fork horn; the watch gains dramatically and intermittently
• Skipping — lock too shallow; the escapement jumps teeth under shock
• Sticking — oil on the impulse faces, or a magnetised or dirty pallet
• Low amplitude — usually not the escapement at all: look for a power or friction problem first

That last point is the most useful thing to take away. When amplitude is wrong, the escapement is usually innocent. Check the mainspring, the train and the lubrication before you touch the escapement.