flexible coupling - connector designed to accommodate misalignment between two rotating power transmission shafts. Flexible couplings minimize noise and vibration and prevent excessive bearing loads. Some types of flexible couplings use metallic or rubber diaphragms that require no lubrication. Of the kind that require lubrication, there are two principal types: grid type and gear type. The grid-type flexible coupling consists of grooved hubs attached to the shaft ends and connected to each other by a spring steel grid that fits in to grooves around the circumference of each hub. The grid flexes and absorbs shock that might otherwise be transmitted from one shaft to another. A fairly stiff grease can be used because the movement of the grid against the grooves serves to keep the lubricant well-distributed. A gear-type flexible coupling consists of geared hubs fitted to the shaft ends and connected to each other by means of a sleeve with internal gear teeth that engage the teeth of the hubs. The hubs rock back and forth within the sleeve to accommodate angular misalignment. Gear-type couplings depend on centrifugal force to distribute the lubricant and thus require a relatively soft grease with good migrating properties. Certain gear-type couplings can be oil lubricated.