A differential is a gear train with three drive shafts that has the property that the rotational speed of one shaft is the average of the speeds of the others. A common use of differentials is in motor vehicles, to allow the wheels at each end of a drive axle to rotate at different speeds while cornering. Other uses include clocks and analog computers.
Differentials can also provide a gear ratio between the input and output shafts. For example, many differentials in motor vehicles provide a gearing reduction by having fewer teeth on the pinion than the ring gear.
Differential unit for a rear-wheel drive car, built by ZF circa 2004
Differential gears (in yellow) in a punched tape reader, built by Tally circa 1962
Illustration of a ring-and-pinion differential for a rear-wheel drive vehicle
Schematic diagram of a ring-and pinion differential
Rocker-bogie
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The rocker-bogie system is the suspension arrangement developed in 1988 for use in NASA's Mars rover Sojourner, and which has since become NASA's favored design for rovers. It has been used in the 2003 Mars Exploration Rover mission robots Spirit and Opportunity, on the 2012 Mars Science Laboratory (MSL) mission's rover Curiosity, the Mars 2020 rover Perseverance and ISRO's Chandrayaan-3 rover Pragyan in 2023.
A rocker bogie
Rocker bogie on Curiosity