Rotary union

A rotary union is a union that allows for rotation of the united parts. It is thus a device that provides a seal between a stationary supply passage (such as pipe or tubing) and a rotating part (such as a drum, cylinder, or spindle) to permit the flow of a fluid into and/or out of the rotating part. Fluids typically used with rotary joints and rotating unions include various heat transfer media and fluid power media such as steam, water, thermal oil, hydraulic fluid, and coolants. A rotary union is sometimes referred to as a rotating union, rotary valve, swivel union, rotorseal, rotary couplings, rotary joint, rotating joints, hydraulic coupling, pneumatic rotary union, through bore rotary union, air rotary union, electrical rotary union, or vacuum rotary union.

A rotary union will lock onto an input valve while rotating to meet an outlet. During this time the liquid and/or gas will flow into the rotary union from its source and will be held within the device during its movement. This liquid and/or gas will leave the union when the valve openings meet during rotation, and more liquid and/or gas will flow into the union again for the next rotation. Often functioning under high pressure and constant movement, a rotary union is designed to rotate around an axis. A rotary union's design can be altered to change this or to increase the psi or rpm it needs to withstand as well as the number of valves required.

While rotary unions come in many shapes, sizes, and configurations, they always have the same four basic components: a housing unit, a shaft, a bearing (mechanical) (or bearings), and a seal. Rotary unions typically are constructed from stainless steel to resist rust and corrosion, but many other metals can be involved, like aluminum.

The housing is the component that holds all of the other elements of the rotary union together. The housing has an inlet port, which is a threaded port to which the hose supplying the medium will be attached. The rotary union may also have an outlet port if the same joint is being used both to supply fluid to a roll and to remove fluid from the roll. In smaller rotary unions the housing is stationary. In larger rotary unions the housing is usually bolted to the drum or roll using a flange. In these cases, the housing rotates at the same speed as the drum

The shaft is the component that carries the medium through the rotary union into the drum or roll. In many cases, the shaft will turn with the drum or roll. In some cases, like in larger flanged rotary unions, the shaft may be stationary while the housing rotates. The bearings and seal are typically assembled around the shaft.

The second most important part of the rotary union is the bearing. A rotary union may have only one bearing, but multiple bearings are much more common. Roller bearings; such as ball bearings and tapered roller bearings; or non-roller bearings, like graphite bearings and bronze bushings, may be used in a rotary union. The bearings are always used to allow a part of the joint, either the shaft or the housing, to rotate.

The heart of the rotary union is the seal. The seal prevents the medium from leaking outside the rotary union while in operation. Seal types can vary from pusher-type end-face mechanical seals, non-pusher-type end-face mechanical seals, lip seals, and o-ring seals. Most rotary unions have more than one seal.

Many rotary unions incorporate multiple ports, some of which are designed to handle different types of material simultaneously. A rotary union with a straight port transfers the substance directly through the rotary union. Other designs include an elbow port, which causes the material to flow out at an angle, and multiple ports. A multiple-port rotary union looks like a perforated cylinder. At the end of the cylinder is a threaded screw with a seal or seals that lock onto it. The transferred material flows into the cylinder and out of the input holes.