I'm hoping to initiate a series of articles dealing with various topics in the Machine Condition Monitoring space. The intent is to keep most of these pretty short, and as entertaining as possible.
So now that we have all managed to struggle through the Critical Speed concepts for phase and magnitude, there is another notion of Phase that the Machine Guys (and in particular - the Structural Guys) employ.
We have established that the Rotational concept of phase on machinery always relates to angular position of the spinning shaft or component in the machine, referenced to a tachometer point on the shaft that is designated as zero degrees. However, if there are multiple sensors on a machine or structure it is often required to determine the relative movement BETWEEN these sensors. In this scenario one sensor is designated a REFERENCE, and other sensors are known as RESPONSE. Phase is then determined by the DIRECTION of relative movement between the reference and the response nodes on the structure.
A simple mental pictures to consider that may help visualize this: Imagine a beam suspended on springs. Mount a sensor at each end of the beam, sensing vertical movement. If the beam is forced downward in the middle, both ends of the beam are moving in the same direction (downward) and the phase between these two sensors would be zero. Now, put a fulcrum under the center of the beam (like a teeter-totter) and push down on one end of the beam. The opposite end of the beam moves UP! The relative movement is opposite, and in this case equal, and the phase is therefore 180 degrees! Simple, right?? Now move the fulcrum closer to one end of the beam. Force the ‘long’ end downward – the ‘short’ end moves upward, but the distance or MAGNITUDE of this movement is much smaller. Regardless, the phase remains opposite, and therefore 180 degrees, regardless of the relative magnitude!
We’re keeping this piece short and sweet, but tuck this concept away… we’ll be coming back to this many times in future discussions!