Rotary transmission, also known as rotary union, is a machine element for connecting rotating pressure systems (e.g. rotation dryers, heating and cooling rollers) to fixed pressure lines, coal-cutting machines for mining. A fundamental distinction is drawn between single-channel rotary transmissions (medium "ON" or "OFF") and multiple channel rotary transmissions in which either one medium or several media can be conveyed in and out. A rotary transmission works in a similar or identical way to a mechanical seal.