Everything you need to know about rotary unions

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Images: sourced from www.moflon.com

Robot arms, spools, winders, rotary tables, and spindles are just but a few rotating components that can be found in motion applications. Designing rotary unions can be easy. However, when liquids and air need to be transferred from a stationary supply to a rotating component, for lubrication, heating, cooling or transmitting fluid power, the task becomes a little bit complicated. It is at this point that rotary unions come in handy. Rotary unions are referred to by a variety of names including rotary joints, rotary swivels, and rotary couplings.

The rotary union or rotary joint is a rotary sealing device that connects motion applications to fixed piping to transmit water, coolant, thermal oil, air, hydraulic oil or other media. They are designed to accommodate a wide variety of media conditions, including high and low temperatures, pressures, flows, including vacuum applications. In contrast to other fluid application mechanisms, the rotary unions work for applications that involve a combination of rotation angles; this includes the continuous rotation in one direction.

There are a variety of configurations and designs for rotary unions; in fact, they are often custom-made for specific applications. However, it’s important to note that all rotary unions are made up of four basic parts:

  • Housing
  • Bearings
  • Shaft
  • Seals

Rotary unions are used in a wide variety of industries, from condensed rotary unions ideal for semiconductor setups to huge rotary unions common in defense, energy, and industrial setups. Depending on the complexity and size of the device a rotary joint will be used in, they can be designed with more than one passage for multiple media transfer within a single device. These particular types of rotary unions can handle different speeds and pressure but take on a lower number for each category to factor in for the passages that are not included.

How do I know which rotary union is ideal for my application?

 There are several parameters to the rotating system that can help you determine the best rotary union for your application. The first thing to consider is the media through which the union will be used for. Rotary union, as explained earlier, can be used for vacuum, water, gas, steam or oil. It’s important that the fitting materials are selected according to the possible presence of corrosive or abrasive agents.

Therefore, the number of passages needed for the speed and pressure in your installation, and the operating temperature will allow you to calculate the size of the rotary union. Finally, if you have to transmit an electrical current or signal in your installation, you will need a rotary union equipped with a slip ring.

Slip ring or rotary union?

Slip rings and rotary unions are all designed to facilitate rotation in machines with rotating components. Rotary joints work by transmitting fluids while the slip rings transmit electrical currents or signals. In a slip ring system, electrical signals are transmitted from a motionless inlet. A typical setup will have a slip ring mounted onto a moving part. The wires that pass through the shaft helps in transmitting electrical currents.

Slip rings are common in a wide variety of applications including HD systems like surveillance cameras or renewable energies like wind turbines. In case you are looking to have electrical signals in your setup, it’s vital that you combine the use of a rotary union and slip ring to pass both electrical currents and fluids. These systems have the capability to integrate both fluid and electrical transmission. This kind of setup is known as combine rotary unions (rotary unions with integrated slip ring).

Calculating the dimensions of your rotary union

There are several different sizes of rotary union available. The following are essential characteristics used to calculate the most appropriate size for your application.

  • Operating pressure. Rotary unions are used in both pressure and vacuum systems. It’s, therefore, important to note the usual pressure of the media. This is generally between 0 and 1,380 bar or 0 to 20,000 psi, and the maximum operating pressure. In case your system operates under variable pressure, use the maximum pressure as your operating pressure.
  • The rational speed and direction. The speed is from 0 to 3,600 rpm. It is possible to get higher speeds well over 10,000 rpm depending on the manufacturer.
  • The nominal diameter. The DN of your installation is the internal diameter of the connecting pipe used in your specific application.
  • The operating and ambient temperature. For a healthy running operation, you need to determine the minimum and maximum temperature. These temperatures vary between -100 at 200 degrees Celsius.

Other Characteristics to consider when choosing a rotary union

When choosing a rotary union, it is important to consider the type of material used. Rotary unions are designed in several different materials, including aluminum, brass, steel, cast iron, or even bronze. These materials can be selected depending on the type of fluid that will be conveyed.

It is also important to consider the type of connection; male or female, flange, screw-in, directly connected, or welded on. There are several different choices of rotary unions; many manufacturers will also provide custom made rotary unions that are designed with or without manifolds.

How can I achieve optimal rotary union performance?

Well, achieving optimal performance is of great importance, especially when it comes to the management and operation of applications. Since manufacturers are ever designing new machines that operate at higher speeds and temperatures, increased efficiency, it is important to make considerations to the rotary joint to ensure it can effectively perform under these new conditions.

After installation of the rotary union, the primary wearing component is the seal ring. It is therefore important to change the seal ring after maximum wear has been attained. If this service is overlooked, the metal surfaces of the rotary union will begin rubbing against each other, causing irreparable damage.

Modern rotary union setups use carbon graphite seal rings designed with a spherical surface on one side with a plane surface on the opposite side. This is meant to merge with the flanged enclosures of the rotary joint. Proper maintenance of components is key in realizing the optimal performance of the entire system.