Sensor Operating Theory

Alexander M. Tomsick - P.E. Director of Engineering

This video is part of a series of videos being produced to talk about the Voyager FDS sensor. This video describes the sensor’s function as well as introduces the idea of dynamic strain. The previous video of this series discussed static strain and how it's calculated. This video discusses the distinction between static and dynamic strain and why the Voyager FDS sensor only measures dynamic strain.

Figure 1: Voyager FDS Sensor

The Voyager FDS sensor, shown in Figure 1, was designed for health monitoring of fluid film bearings on turbomachinery by measuring the dynamic strain of a bearing housing due to the load transmitted from the rotating shaft. To accomplish this, the sensor is mounted adhesively to the face of the bearing housing with the sensor measurement axis, denoted by the arrow on the top of the sensor, pointing towards the center of the shaft. This adhesive mounting allows the Voyager FDS sensor to experience the strain that the bearing housing undergoes in the radial direction.

To recall, strain is the measure of a body's change in length compared to its original length in response to a load, and can be calculated with Equation 1.

Static strain occurs in a body when the magnitude of a load applied to a body does not change over time. While static strain is of interest in many disciplines, for vibration analysis on turbomachinery, it is not useful. Dynamic strain, which can be measured by the Voyager FDS sensor, occurs in a body when the magnitude of the applied load changes over time.

Beam Acted on by an Oscillating Load

An oscillating force (F) acts on the beam shown in Figure 2. The beam has an original length (L) of 12 inches. When the oscillating load is at its maximum, the beam elongates to a length of 12.25 inches. This means the beam's length changes by (ΔL) +0.25 inches. When the oscillating load is at its minimum, the beam compresses to a length of 11.75 inches. This means the beam's length changes by (ΔL) -0.25 inches. Across the loading cycle, the beam experiences a total change in length of (ΔL) 0.50 inches peak to peak.

Figure 2: Beam Acted on by an Oscillating Load

Using this information, the dynamic strain in the beam can be calculated as:

Strain Time Waveforms:

The Voyager FDS sensor was designed to be adhesively mounted to bearings that will experience dynamic strain during operation. Once mounted, the sensor allows the analyst to measure the strain amplitudes over time and plot them as a time waveform. An example of a strain time waveform is shown in Figure 3.

Figure 3: Strain Time Waveform

The data from the Voyager FDS sensor can be viewed in any of the typical vibration plots used for troubleshooting rotating equipment.

Bearing Dynamic Strain

Hydrodynamic bearings will experience dynamic strain of the bearing and bearing housing in response to the loads, which are transmitted from the rotating shaft. While some static loading and therefore some static strain will exist in the bearing housing, vibration is oscillatory. Since the Voyager FDS sensor is a vibration-specific sensor, it will only measure the dynamic strain of the bearing housing.

Figure 4. Strain of Bearing Housing Acted on by an Oscillating Load

Figure 4 shows the strain distribution of a bearing housing that is acted on by an oscillating load at the bottom of the bearing journal. The load modeled for this example compresses the bearing housing at the bottom of the bearing journal, then decreases to zero at a consistent interval. Due to this time-dependent load, the strain at the bottom of the bearing housing will vary with time. To measure the dynamic strain on the bearing housing with the Voyager FDS sensor, it is important to mount the sensor as close to the inside diameter of the bearing as possible. This is recommended as the maximum strain amplitude the sensor will measure will be lower the further away from the bearing surface the sensor is placed.

Topic of Next Video

The next video in this series is going to show an example of dynamic strain being collected on a cantilever beam using the Voyager FDS sensor with a portable data collector. This video will detail the setup of a portable vibration data collector for data collection with the Voyager FDS sensor and serve as an example of real data being collected using the sensor.