Page "Drum brake" Paragraph 21

As the brake linings wear, the shoes must travel a greater distance to reach the drum.

When the distance reaches a certain point, a self-adjusting mechanism automatically reacts by adjusting the rest position of the shoes so that they are closer to the drum.

Here, the adjusting lever rocks enough to advance the adjuster gear by one tooth.

The adjuster has threads on it, like a bolt, so that it unscrews a little bit when it turns, lengthening to fill in the gap.

When the brake shoes wear a little more, the adjuster can advance again, so it always keeps the shoes close to the drum.