A high speed rotary union is a fluid connector that that allows transfer of liquid, steam, hydraulic fluid and gas from a tube, pipe or hose to a rotating part such as cylinder, drum or spindle. A rotary union is also called rotating union, rotary unions, rotary joints, swivel joints, rotary valve, vacuum rotary union, and hydraulic coupling.
A rotating union is used where liquid have to transferred from a stationary source to a rotating part. It is used in oil, steel, gas and mining industries to meet the ultraclean demands of semiconductor and pharmaceutical production. In renewable-energy industries, it is used in thermal solar plants and wind turbines.
Unions may have one or more passages. A rotary unit with two passages, could have the same media enter and outlet both passages. On the other hand, two passages could pass on the same media at two dissimilar pressures.
Primary considerations for choosing a rotary union may comprise number of passages, extent of the union, and types of threaded links on the machine attachments. Size refers to the flow and ports passages, as well as the equipment’s overall dimensions. Other criterion is pressure. Depending upon the design, rotary unions can operate compressed air to around 150 psi, and vacuum as low as 28 in. Hg.
Temperature is another crucial design factor. Rotating unions are comprised of various machineries; each has its own individual temperature limit. Temperature limit of unions with standard deep-groove is 250°F (120°C). Heat-stabilized bearings can extend up to 320°F (160°C).
Speed and track of rotation are critical operating factors of rotary union. It’s quite common for rotary unions to work at speeds of 3,500 rpm and more; however the high the machine speed, the better the alignment between machine and rotating union.
NPT threads are appropriate for slower speed machines. However, beyond 1,500 rpm, tapered NPT threads do not provide exact arrangement of the rotating union. In these cases, machine threads are recommended.
Direction of rotation is equally important. High-speed machine-tool spindles accelerate at fast rates. The direction of the thread should be always opposite to the spindle’s rotation direction. For low-speed devices, this may not be so grave. But, in order to fit any rotary union to a mating mechanism, the direction of both parts of thread must be steady.
Correct installation of rotary union will extend its service life. Despite specific production and use of highly defiant materials, rotating union is subject to wear, which is unavoidable due to its function. Factors that may restrict the service life of a union can be pressure, speed, flow, temperature and flow velocity of the media.
Mounting process is also a key factor for the long life of a rotating union. It is crucial that the bearing can be easily turned around in escalated condition without warping. Inappropriate hose length or bolting parts can cause warping of the bearings already at this stage.