The input shaft and output shaft are a “partnership” within a power transmission system; they are different in the direction of energy flow: the input shaft serves as the entry point for power, while the output shaft acts as the exit point.
A simple analogy helps illustrate this: the input shaft is like the pedals on a bicycle, receiving the force you (the power source) exert; the output shaft is like the rear wheel axle, transmitting the force—after it has passed through the gear system—to ultimately drive the wheel forward.
The fundamental difference lies in rotational speed and torque. Since power (torque × speed) remains essentially constant, the input shaft—typically connected to a motor or engine—operates at high speeds but bears relatively low torque; conversely, the output shaft—usually connected to the driven component—operates at lower speeds but handles higher torque. Consequently, the output shaft is designed to be more robust and stronger to withstand greater torsional forces. Sealing designs also differ: input shaft seals must manage heat generated by high-speed rotation, whereas output shaft seals require durability to resist contaminant ingress and accommodate potential shaft deflection.