What is the purpose of the spider gears in an open differential?

In an open differential, the spider gears serve the purpose of allowing the wheels to rotate at different speeds while still transmitting torque to both wheels. An open differential consists of a ring gear, a pinion gear, a carrier, and spider gears (also known as side gears). The pinion gear, driven by the driveshaft, turns the ring gear. The carrier is attached to the ring gear and rotates with it. The spider gears are mounted on the carrier and mesh with the side gears, which are connected to the axles of the wheels. When the vehicle is traveling straight, the spider gears do not rotate relative to the carrier. They simply transmit equal torque to both side gears, causing both wheels to rotate at the same speed. However, when the vehicle is turning, the outer wheel must travel a longer distance than the inner wheel, requiring it to rotate faster. In this situation, the spider gears rotate on their axes within the carrier, allowing the side gears (and thus the wheels) to rotate at different speeds. This differential action prevents wheel slippage and allows for smooth cornering. The downside of an open differential is that if one wheel loses traction (e.g., on ice or mud), it will spin freely, and very little torque will be transferred to the wheel with traction. This is because the differential always transmits equal torque to both wheels, and the wheel with less traction requires very little torque to spin. For example, if a tractor with an open differential has one wheel on ice, that wheel will spin, and the tractor will not be able to move forward, even if the other wheel is on solid ground.