The pinion gear is the power entry point of the differential. Bolted to the end of the driveshaft yoke, this tapered bevel gear meshes at a 90-degree angle with the larger ring gear inside the axle housing. As the driveshaft spins, the pinion gear turns the ring gear, translating longitudinal driveshaft spin into lateral axle shaft rotation to drive the wheels. The gear teeth are cut in a spiral (hypoid) pattern to reduce noise and handle load.
Pinion bearings must keep the gear aligned. If these bearings wear out, the pinion gear shifts under load. The teeth will not mesh correctly, producing a loud whining or howling sound that changes pitch when you accelerate or coast. Left unchecked, the gears will chip and lock up the axle.