**2. Materials and methods**

The original rig [8], shown in **Figure 2**, is used here and its main attributes described here for completeness. It was designed to model a typical propulsion system *Optimised PVDF Placement Inside an Operating Hydrodynamic Thrust Bearing DOI: http://dx.doi.org/10.5772/intechopen.110153*

used in marine settings. The system consists of a propeller (prop), shaft, journal bearings, and a thrust bearing attached to a supporting plate structure which represents the hull. The propeller provides axial thrust to the shaft which is transmitted to the hull (supporting plate) via the thrust bearing. The coupling, known as the "dog clutch", is rigid in torsion but flexible in bending thus allowing axial motion to be transmitted along the "floating" shaft. This helps to isolate any unwanted vibration from the motor. The 10 mm thick, 1500 mm wide supporting steel plate is fixed to a concrete block to take the thrust load generated by the propeller and also supports the motor as shown.

A two-blade and three-blade propeller of equal diameter (0.22 m), shown in **Figure 3**, were separately coupled to the propulsion system in the water tank (0.990.590.58 m) to observe different blade passing forces. A larger (diam = 0.3 m) alternative style bronze two-blade propeller was also used (**Figure 3**).

In addition to the four test configurations (No prop = NP, standard two-blade prop = TBP, bronze two-blade prop = TBBP and three-blade prop = ThBP), two PVDF arrangements were investigated with these test configurations. For each case, the nonrotating component into which the PVDF was embedded was constructed out of ABS (Acrylonitrile Butadiene Styrene) using a 3D printer. Since the forces, temperatures and operating times were relatively low, it was presumed that using the plastic

**Figure 3.**

*Propellers used in the experiment: (a) standard two-blade; (b) bronze two-blade; and, (c) three-blade.*

material inside a functioning, small-scale, thrust bearing would not be a problem. This was confirmed by our previous study [8].

Two different designs were investigated regarding the placement of the PVDF: Case 1 - the stationary component was a plain washer with the thrust washer in the rotating component sliding over top as in our first study [8]; Case 2 - was arranged such that the stationary component had the thrust washer and the PVDF embedded on top, with a plain washer in the rotating component sliding over top. Both arrangements (cases) had the PVDF fixed to the non-rotating body to permit the signals to be measured without the need for telemetry or slip rings (**Figure 4**).

In Case 1, (**Figure 4**), where the stationary component was the plain washer, recesses were made in the washer to insert two PVDF films as shown. Opposite the plain washer, a thrust washer, also constructed out of ABS, was used with two pads angled at two degrees to the surface. In Case 2, in **Figure 4**, the thrust washer was printed with two, two-degree tilted pads and the PVDF film was embedded into the tilted portion of the washer. Opposite the thrust washer, a plain washer was used in the rotating component. This configuration better resembles what is seen in a typical thrust bearing in a marine propulsion system.

The PVDF sensors were cut from a sheet of 110 *μ*m thick film obtained from Measurement Specialties. Contacts were made using conductive copper tape on the electrodes and the films were glued using West Systems 105 epoxy resin inside the two recesses in the plastic washer. The PVDF sensors were protected by covering the interacting surface of the washer with a thin layer of epoxy. Heat from a blow torch was briefly applied to the epoxy overlaying the PVDF sensors to remove any small bubbles in the thin layer before it was left to cure. Connectors were then added to the end of the cables from the PVDF sensors such that they could be operated independently.

A Brüel & Kjaer (B&K) 2635 charge amplifier was used to condition the signal from the PVDF film. In conjunction with the B&K Pulse Time Data Recorder software, a B&K LAN-XI data acquisition system (Type 3050-B-060) was used to analyse and collect data in the time domain from the PVDF. At a sampling rate of 16,384 Hz, any higher frequency noise from the sensors would be collected while retaining manageable file sizes.

The data was post-processed using Matlab. The raw voltage measured from the PVDF film were converted to force via the measured calibration factor as discussed in *Optimised PVDF Placement Inside an Operating Hydrodynamic Thrust Bearing DOI: http://dx.doi.org/10.5772/intechopen.110153*

**Figure 4.** *Thrust bearing comprising stationary and rotation components (a). Stationary bearing configuration: Case 1 - PVDF embedded in plain washer (b) and Case 2 - PVDF embedded in thrust washer (c).*

our previous papers [8, 25]. Although no physical trigger was implemented in the experimental setup, the time domain signals were aligned using Matlab's "alignsignals" function. The frequency-domain data was obtained using the inbuilt FFT function by dividing each track into 8 equal segments to obtain a better frequency representation of each time-domain signal. Each segment was windowed with a Hamming window, with 50% overlap to reduce the effect of windowing.

The set-up enabled the prop shaft to be spun in different configurations. The first being with no propeller at the end of the shaft to give a representation of the noise transmitted to the sensors from just the shaft spinning. Then, three different propellers were attached to the shaft in turn and rotated at speeds of 0 to 600 RPM in 20 RPM increments. Each configuration was performed for both Case 1's and 2's design.
