Worm gear

A worm gear is a specialized gear arrangement consisting of a worm (screw) that meshes with a worm wheel (gear). In slack adjusters for automotive braking systems, it converts rotational motion from the adjustment mechanism into precise linear or angular movement to maintain proper brake shoe-to-drum clearance. The unique helical thread design provides high reduction ratios in compact spaces and exhibits self-locking characteristics when not actively driven, preventing back-driving that could compromise brake adjustment.

The worm gear operates on the principle of screw mechanics where the rotating worm (with helical threads) engages with the teeth of the worm wheel. As the worm rotates, its threads push against the worm wheel teeth, causing the wheel to rotate at a significantly reduced speed relative to the worm input. This creates a high mechanical advantage (typically 20:1 to 300:1 reduction) while maintaining compact dimensions. The shallow engagement angle creates friction that prevents reverse rotation when input torque is removed, providing the self-locking feature critical for maintaining brake adjustment settings.