Shaft couplings, collars, and U-joints join components and shafts together to transmit mechanical power. Shaft couplings join two shafts together to transmit rotary motion from one shaft to the other. They include elastomeric flexible couplings and fluid couplings. Elastomeric flexible couplings compensate for shaft misalignment and deformation and do not require lubrication. Fluid couplings use hydraulic fluid to transmit rotational power between shafts. Shaft collars attach components to a shaft, while U-joints are universal couplings that form joints in shafts to allow movement in any direction.