JUNE 23, 2020 by Brady Huago, Hydraulic Engineering Supervisor

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.  Read More