Ultrasonic cleaning: sends high-pitched sound waves through a liquid solvent to remove dirt, grease and small metal particles quickly and effectively from small corners and crevices.

Unloading valve (hydraulics): a device that directs hydraulic fluid back to the reservoir at low pressure when there is no demand on the system.

Unworked pen/penetration: (ASTM D-217) the penetration at 77°F of a sample of grease that has experienced minimum handling in transfer to the test apparatus and that has not been subjected to the action of a grease worker.