Measuring "How Much" in Displacement Units

Displacement measures the length of the "trip" back and forth from (in this case) +X to -X (2X would be the total distance travelled - the "peak-to-peak displacement"). One of these "trips" is known as a "cycle" of vibration. The sequence of images about to be presented show the bearing at various important points during a single cycle with the transducer oriented vertically (remember, the transducer only "sees" movement in the direction of its orientation, or axis).

Since this movement must occur over time, when we measure it we plot the amount (amplitude) on the y-axis and the time taken (period) on the x-axis. The resulting shape, in its simplest form, is called a "periodic signal", a "sinusoid" or a "sine wave". That is the S-shape you see below the word "cycle" in the image at right. Mechanical vibration generates a wave shape that is rarely as simple as what we see here but the intricacies of processing more complex data will be covered in later sections.

The "At-Rest" position ("0") is the position the bearing would assume if the machine was not running. During a single (1) cycle, the bearing passes this position twice - once travelling towards +X and once travelling towards -X.  We'll begin our sequence of images with the bearing just passing the '0' point moving towards  '+X'. The red ball (seen at the very ends of the sine wave) indicates the amplitude level during the cycle.