Pendulum Suspension and Setting in Beat

A pendulum clock's oscillator is the pendulum itself: a rod and bob swinging from a thin, flexible suspension spring rather than a pivoted staff, because a flexure hinge of this kind introduces far less friction than a pivot would at the amplitudes and rates typical of a pendulum, and because its geometry is highly repeatable swing after swing. The suspension spring is clamped at its top to a fixed point on the movement's back plate or a bracket, and at its bottom to the pendulum itself, and its correct, undamaged, unkinked condition is essential to accurate timekeeping since any inconsistency in the spring's flexing directly disturbs the pendulum's period.

The pendulum communicates with the escapement through the crutch, a light fork or pin arrangement attached to the escapement's pallet arbor that engages a loop or fork on the pendulum rod near its top, transmitting the small impulses that keep the pendulum swinging while otherwise letting it swing freely. This is the pendulum-clock analogue of the lever and impulse jewel in a watch, and the same underlying principle applies: the escapement gives the oscillator just enough energy to replace what friction and air resistance take away, without materially disturbing its natural period.

A pendulum clock is 'in beat' when its escapement releases exactly halfway between the pendulum's two extremes of swing, producing an even tick-tock rather than an uneven gallop; because a clock's beat depends on the pendulum swinging symmetrically about a true vertical, being in beat is closely tied to the clock being level and to the movement being correctly mounted, which is why levelling a longcase or bracket clock correctly (side to side, and often front to back) is one of the first things checked, and re-checked, whenever a pendulum clock is moved or fails to run correctly after being moved.

In many pendulum clocks the beat is adjusted by very slightly bending the crutch, or by rotating an adjustable back-plate bracket that carries the suspension spring, so that the escapement's rest position aligns correctly with the pendulum's centre swing when the case sits level; some designs instead provide a small beat-setting screw or an adjustable pallet arrangement for the same purpose. In every case the adjustment is small, and the clock's actual level placement is checked first, because a clock that appears out of beat is very often simply not standing level rather than mechanically maladjusted.

As with watch beat-setting, correcting a genuinely maladjusted crutch or suspension on an antique clock, as opposed to simply levelling the case, involves bending or repositioning small, sometimes fragile parts under the movement's back, and is properly bench work for a trained clockmaker; the useful skill for an owner is recognising an audibly uneven tick, checking level placement first, and knowing when to call in a professional if levelling alone does not resolve it.