Why does angular misalignment produce a 1x (shaft rate) vibration in the axial direction? Why is this vibration out of phase on the bearings on either side of the coupling?
It’s easy to understand if you think about it from the point of view of a bolt holding the coupling faces together. As the bolt is forced to rotate through the gap in the coupling it gets stretched in the axial direction. (“axial” is in the direction of the centerline of the shaft). This stretching motion happens once per revolution of the shaft, hence 1x vibration.
If you look at the orange bolt in the animation, it looks like it is being stretched like a spring, once per revolution, in the axial direction.
Why does angular misalignment cause the vibration to be out of phase on the bearing faces on either side of the coupling? Phase is the relationship between 2 vibrations. Things that are in phase move in the same direction at the same time. Things that are 180 degrees out of phase move in opposite directions. As the bolt gets stretched, the ends of the bolt move in opposite directions, and hence the vibration in the axial direction is out of phase.
To be a little more precise about it, we can say that the angular misalignment causes a repetitive force at 1x in the axial direction. The vibration you will see in the spectrum however is related to the response of the machine to these forces. How the machine responds depends on many factors including its mass, stiffness, coupling type and degree of axial play in the rotor etc. Therefore, the patterns in your spectrum may contain harmonics of 1x, an elevated 2x, or frequencies related to the type of coupling you have such as 3x or 4x. Out of these, an elevated 2xA is common.
It’s always important to avoid over simplifying things. It’s also important to understand how things work at a deeper level. Know the best way to do that? Take a class with me!
Angular misalignment is just one of the many faults you can find with vibration analysis. Check out our course schedule https://zencovibrations.com/events/ for a live virtual course or on-site training near you. Our animations make learning easy! Get trained and certified vibration Cat 1, 2, or 3 in accordance with ISO 18436-2 today!
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