Underdreven

Underdreven is a term used in engineering to describe a drive arrangement or condition in which the drive train delivers a lower output speed at the driven element than would be expected from the input speed, effectively multiplying torque at the driven shaft. The term is closely related to underdrive and is used alongside overdrive to describe the range of gear ratios or pulley arrangements within transmissions, gearboxes, and belt drives. In Scandinavian technical usage, the adjective is used in discussions of mechanical power transmission and is often contrasted with overdriven configurations.

In automotive engineering, an underdriven setup may result from a high final drive ratio or selecting lower gears, which increases torque at the wheels but reduces top speed and efficiency at highway conditions. In industrial drives, underdriven configurations with larger driven sprockets or pulleys relative to the driver can provide high starting torque for applications such as cranes, conveyors, or hoists, at the cost of reduced output speed and potential energy losses if the system is not matched to the load.

Common causes include intentional design choices to meet starting torque requirements, incorrect sizing of gears or pulleys, or faults such as belt slip or worn teeth that effectively reduce output speed. Diagnosis involves comparing input and output speeds under load, measuring torque, and verifying gear ratios and belt tension. Mitigation includes selecting appropriate gear or pulley ratios, ensuring proper belt tension and alignment, and addressing mechanical wear. See also: overdrive, gear ratio, torque, efficiency.