Many of us know the sound and irritation of loading the washing machine too full. During the spin cycle of the wash, if the weight within the washing machine is off-center or too heavy, the machine will sound like it is shaking itself to pieces. This tends to terrify my 4-year-old daughter!
Other household appliances will show this same phenomenon. While not as dramatic, the fridge does something similar, when the compressor motor shuts down and there is a split second where the motor and compressor pump spins at a much lower speed. This lower speed will match the system response frequency of the dampeners and will shake much harder.
If this were an engineering class, the instructor would simplify this example down to 3 components, a weight, spring, and dampener. Together, these components make up a system, and its system response would be the frequency that the system vibrates at, given an input force. Appliances, such as of a fridge and washing machine, typically use rubber feet to act as the spring and dampener
When the engineers are designing a fridge or washing machine, they try and reduce the vibration that the unit imparts into the environment. They do this by measuring the frequency at which it operates at and tune the springs and dampeners to absorb the vibration. When the motor shuts off, the input energy changes, and the spring and dampeners no longer are tuned for the system and thus allowing higher vibrations.
Vibrations are especially a concern when designing a device to reduce sound, improve ergonomics and ensure it can last longer. Anyone who has operated a chainsaw can appreciate how dramatic vibrations can be to the human body. If you look under a modern car, you will see many examples of ways vibration is reduced. Weights suspended by rubber bushings can be found on drive shafts, CV joints, control arms, transmissions, and engines, all to ensure the car have a smooth and comfortable ride and extend its service life.
When designing rotary unions, vibration is important to consider. Rotational torque, circuit pressure, circuit seal material and geometry, and circuit number can all affect the internal system response frequency. Mounting plate geometry, mounting plate steel properties, mounting plate location, and torque arm length and location can affect the external system response of the swivel. The internal and external system response work either together or against each other to determine if the rotary manifold has good rotational dynamics.