Either side of the pendulum rod is the metal of the yoke, and this imparts the energy to maintain the swing. If through wear the gap between them has become greater than intended, energy will be lost.
A worn crutch.
In some cases the sides of the yoke can be closed up. Where the drive is imparted through a slot in the pendulum, a slight alteration in suspension length will introduce a fresh unworn surface. If this is possible, then the bob rating nut should be adjusted to compensate for any suspension length alteration.
A wooded wallclock pendulum with a brass plate.
Repairing Your Own Clocks by Mervyn Passmore