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Rotary Union Design Blog Series: Rotary Union Rotation Dynamics

Rotational dynamics is another essential aspect of successfully designing a hydraulic rotary union. Just like an overloaded washing machine, a rotary union that vibrates and shakes will not have longevity or have acceptable performance.

Brady Haugo
Hydraulic Engineering Supervisor
June 23, 2020
Rotary union design rotation dynamics

As we discussed in the last blog about rotary unions and vibration, rotational dynamics is another essential aspect of successfully designing a hydraulic rotary union. Just like an overloaded washing machine, a rotary union that vibrates and shakes will not have longevity or have acceptable performance.

Devices that rotate require constraints on all degrees of freedom except one about the center axis. When this happens, the object can turn. The quality of rotation is dependent on the quality of how it is constrained. If the rotational object oscillates about a node, this can be caused by constraints that flex. If the constraints allow the object to vibrate at multiples of its system response frequency, this could indicate poor constraint performance.  

Rotational dynamics can also be affected by objects internal to the rotary union. Seals can cause the rotation to have either a smooth or choppy rotational characteristic. The seals within the swivel have an interference fit, meaning they are stretched slightly and are tight. The dynamics between a seal and the spool and housing also act as a type of modified response system. The seals try to grip one side harder than the other, so the dynamic motion stays on the side that wears better. If the seal slides on the side that it is not designed for, it will wear out very quickly, and the rotary union would start to leak. Ensuring that the seal maintains its proper position is critical. This is one of the key differences between hydraulic cylinder seals and rotatory union seals. While the two seals may look similar, cylinder seals do not need to grip one side and seal on the other. The linear motion of the rod does not have a rotation aspect to it.  

Because the inner and outer diameter of the seal both have a dynamic or almost dynamic interface, the elasticity of the seal itself can cause what is called stick-slip. According to Hydraulics & Pneumatics, “Stick-slip, in general, can be described as two surfaces first adhering to and then sliding over each other, with a corresponding change in the friction force. When applied to a seal, stick-slip occurs at low velocities or when a large change in friction force occurs relative to a small change in velocity, as may be found in hydraulic cylinders, often after a period of rest.”  

The effect of stick-slip is amplified when there is a significant difference between the static and dynamic friction of the seal.  This difference can set in motion the stick-slip, which is characterized by choppy rotation, one might claim that it can sound like a jackhammer as it rotates.  

To compound matters, the internal response system of the rotary union can affect the external response system of the rotary union. If stick-slip starts to occur, the rotational vibrations can excite the constraints of the rotary union and produce violent shaking. When this happens, the operator and bystanders can hear and feel it.

To throw in another variable, the pressure within the circuits that energize the circuit seals can affect the rotational dynamics, as well as the seal material. Higher pressure applications require harder seals. Harder seals are necessary so that the edges of the seal do not get squished out or extruded. This happens more frequently when the adjacent circuit has low pressure. Harder seals have less internal lubrication, so the seal tends to grab easier, which can increase the occurrence of stick-slip. Softer, PTFE seals have been standard because they do not have near the rotational friction as the harder seals do. The downside is PTFE seals can extrude and fail quicker under high pressures. The chemistry and geometry of the seal should be noted too but will not be a part of this article.

Seal diameter can be a multiplier of the disadvantages of the seal material. The larger the diameter of the seal, the more rotational torque it produces, and the quicker the seal will wear because a larger diameter seal turns farther and faster than a smaller seal. Also, a larger seal diameter will take longer to seal up. A real-world example of this is seating the bead on a car tire versus a lawnmower tire. It is easier to seat the bead on a smaller tire given similar volumes of air are used. Using the smallest possible seal diameter in rotational applications is always the best route.    

In review, the seal type and size can trigger a phenomenon known as stick-slip, which can excite the constraints of the rotary union. UEA hydraulic engineers work to ensure all rotational dynamics factors are considered for top performance from any UEA rotary union.

Check back for our next rotary union design blog post in this series that will focus on steps to ensure excellent rotary union rotational dynamics. Learn more about United Equipment Accessories’ line of rotary unions here.

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