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Rotary Union Design Blog Series: Steps to Ensure Excellent Rotary Union Rotational Dynamics

To wrap up our rotary union design blog series, we will discuss best practices to optimize the rotation dynamics of a hydraulic rotary union.

Brady Haugo
Hydraulic Engineering Supervisor
July 8, 2020
Steps to Ensure Excellent Rotary Union Rotational Dynamics

To wrap up our rotary union design blog series (click here to read blog #1 and blog #2 in the series), we will discuss best practices to optimize the rotation dynamics of a hydraulic rotary union. Hydraulic circuit pressure and location, hydraulic seal material, hydraulic seal diameter, rotary union mounting, and rotation constraints are all variables that influence rotational dynamics. So what are some ways to help ensure excellent hydraulic rotary union rotational dynamics?

Keep the swivel mounting and torque restraints close to each other axially- this is one of the most critical factors. When they are on the opposite ends of each other, it can lead to bad rotational dynamics or having the rotary union move, chatter, make noise, and/or bind up. If they are closer together, nylon cap seals, large diameter seals, and high-pressure circuits can be used.

  • Torque arm geometry plays a significant role: single versus double, rotational dead-band, and binding issues play into this equation. Also, rod-end style joints versus an open slot design can change the rotation dynamics.
  • Using tight tolerances within the assembly eliminates binding and keep the assembly copacetic.

Keep the mounting and torque restraint assembly very rigid. UEA engineering can give estimates on unpressurized swivel torque for mounting and torque design considerations including: an estimate of max torque with worst-case scenario circuits pressurized is three times the unpressurized torque and an estimate of four times unpressurized torque for max static torque- this would only occur for a split second as it begins to rotate.

Use a combination of hard and soft PTFE circuit seals to reduce the rotational torque. Keep the hard seals around the high-pressure circuits and use PTFE seals for the low-pressure circuits.

Keep the circuit seal diameter as small as possible.

  • Slight flow restrictions allow for smaller diameter deep drills, ultimately reducing the seal diameter.
  • Ensuring the port size matches the flow through the circuit.
  • The center hole diameter directly affects the seal diameter. If the application requires a slip ring with the rotary union, deciding whether to pass the electrical connectors through the center of the rotary union will impact the center hole diameter. If passing the electrical connector from the slip ring through the rotary union is needed, it will require a larger center hole. The smaller the center hole, the smaller the rotary union diameter.
  • Utilize two smaller ports, instead of one large return port.

These general guidelines are not a one size fits all examination.  Much of the time when a customer requires a custom rotary union, there is not the space or resources available to completely optimize all the variables discussed here, however through design collaboration we will help the customer determine the most critical aspect of the assembly.  Whether its performance concerns, heat transfer, minimization of flow restrictions, assembly and maintenance concerns or a host of other key aspects of the assembly.  Zeroing in on what really matters to the customer is what UEA does best.

Due to the many variables in designing rotary unions, open communication is key between the customer and the engineering team. United Equipment Accessories can be that design partner, helping to minimize the many risks involved with dynamic systems while keeping costs in check. Contact us to talk with UEA’s engineers about your next rotary union solution!

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