Draw
The reliability of the modern lever escapement depends upon draw; the pallets are angled so that the escape wheel must recoil a small amount during the unlocking. The draw holds the lever against the banking during the detached portion of the operating cycle. Draw angle is typically about 15 degrees to the radial.
Early lever escapements lacked draw (indeed some makers considered it injurious as a cause of extra friction in unlocking); as a result a jolt could result in the escapement unlocking.
Read more about this topic: Lever Escapement
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