English: differential used to control the take-up reel of a paper tape reader made by Tally circa 1962. A belt and pulley system powers the rear shaft at constant speed. This turns the right bevel gear, which spins freely on the middle or output shaft. The axle of the single planet gear is locked to the output shaft. The left bevel gear also spins freely on the driven shaft, unless it is stopped by a brake shoe attached to the front shaft. When there is slack in the tape, the brake is engaged, stopping the left bevel gear, and causing the planet gear to drive the output shaft, taking up the slack in the tape. When there is no slack, the brake is released, and the left bevel gear rotates freely, allowing the planet gear to spin and removing the torque from the output shaft.