Rotary unions are used to transfer media from a stationary input to a rotating outlet. They are usually made of brass, bronze, plastic, steel, or stainless steel. Rotary unions connect via flanges or threads and may use dual radial ball bearings to prevent side loading to seals. Seals are made from a variety of materials and feature o-ring and bellows-type designs.

Rotary unions include one or more ports which range in size from 1/8 in. to 6 in. Straight ports direct media flow straight through the union. Elbow ports cause the media to exit at an angle from the inlet direction.

Common thread types for rotating unions include national pipe thread (NPT), British standard pipe (BSP), and unified coarse thread (UNC).

Video credit: Rotary Systems, Inc. via YouTube/ CC BY 3.0

Applications

Rotary unions are used in the production of textiles, clutches and brakes, automobiles, chemicals, paper, rubber and plastics, and food and pharmaceuticals. They move media such as compressed or vacuum air, hydraulic fluid, steam, pressurized water, chemicals, gases, and slurries or solid materials.

Rotary unions are also used to move petroleum products, gasoline, diesel, natural gas, and aviation fuel. Devices that move hot oil, steam, pressurized water, and heated air are often made of stainless steel to provide increased corrosion resistance.

Fluorocarbon o-rings and bearing seals allow rotating unions to operate at higher temperatures and pressures. In applications where equipment misalignment is common, bellows-type seals provide added flexibility.

Specifications

Specifications for rotary unions include:

• Pressure range - Usually measured in pounds per square inch (psi).
• Speed range - Usually measured in revolutions per minute (rpm).
• Number of ports - Usually between 1 and 8.
• Seal type - Usually made from nitrile, fluoropolymer elastomers, ethylene propylene (EPDM), polyurethane, or silicone.
• Operating temperature
• Length
• Diameter

Features

In terms of features, some rotary union devices include an integral slip ring for electrical power or data transfer. Others include a thru-bore for mounting and wiring. Rotary unions with a slowing motion are used to regulate media speed. Devices with dry running protection can operate without lubrication.

References

Image credits:

Rotary Systems, Inc. | DSTI - Dynamic Sealing Technologies, Inc. | Scott Rotary Seals