THE KNOCK (REBATE)
The knock (rebate) is a malfunction of the swiss lever escapement which occurs when the amplitude of the balance exceeds the maximum value that the geometry of the escapement can absorb (generally between 280° and 300°, depending on the lift angle). Under these conditions, the balance, at the end of its travel, then strikes the back of the fork of the pallets (lever) before having completed its free oscillation, causing an unwanted additional impulse. This parasitic shock delivers a surplus of energy to the system, which runs away and accelerates uncontrollably — the watch then gains time markedly and irregularly.
Knock (rebate) most often occurs following a violent shock to the watch, or when the mainspring delivers an abnormally high torque — for example, right after a full winding on a movement whose barrel lacks regulation. Prolonged or repeated knock can damage the pallets of the lever or the teeth of the escape wheel, due to successive high-energy shocks.
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