# Diagnosing the Machine Set Rotor Using Its Relative Vibrations
## 6 The Arrangement of Sensor Heads for Vibration Diagnostics of Marine Propulsion System Rotors  
The screw propeller propulsion is the basic propulsion of the ship. In the traditional propulsion systems placed in the hull, the ship's propeller fixed on the shaft converts the energy of the rotary motion into the energy of the thrust on the ship hull. The shaft bearing located in the ship's stern frame called stern tube bearing is a non-split bearing and is fitted with sealings. The effective moment on the propeller and the thrust force generated by the propeller are not constant because the propeller rotates perpendicularly in a non-uniform water stream, resulting from the asymmetry of the ship hull and other protrusions in the vicinity of the propeller's flow field. The main component of axial forces has a frequency known as the blade passing frequency (BPF), which is equal to the shaft rotational frequency multiplied by the number of blades of the propeller and an integer representing the order of BPF [61]. To enhance the ship's safety and the dependability of the propulsion system, the methods of calculating torsional vibration [62] and methods of optimal shafting arrangement [63] are improved, and propeller shafts, shaft bearings and the propeller are subject to technical supervision and periodical surveys [64, 65]. The propeller, propeller shaft, bearings and sealings are affected by unavoidable wear, which may be accelerated due to additional events. Damage to the stern bearing is largely a result of bearing displacement due to hull deformation caused by ship loading condition and sea waves. The most common line is the diesel engine propulsion line with or without gear. The diesel engine propulsion line without a gear has one rotor of the machine set, and the thrust bearing is set in the engine. The diesel engine propulsion line with a gear has two rotors of the machine set; the thrust bearing is usually placed in the gear, and the engine contains a locating bearing. The engine's crankshaft has two ends: the free end and the flywheel end. The engine body on the free end side is often closed with a cover. After removing the cover, the frontal surface of the crank shaft free end is accessible despite the possible presence of a gear for driving pumps and a vibration damper. Similarly, in the case of a gear connected with a fixed pitch propeller, the removal of the gear end cover gives access to the gear shaft end frontal surface. The herein proposed system for diagnosing propulsion systems with combustion engines includes an arrangement of sensor heads, as shown in Figure 22.  
Figure 21: Configuration for calibrating the position head: 2—engine cover; 10—position head; 11—calibration bushing; \(d\)—diameter of calibration bushing equal to diameter of new shaft in the measurement plane; \(l_{0}\)—distance between internal frontal surfaces of calibrating bushing flanges. In the case of diesel engine propulsion lines without a reduction gear, radial vibration from heads 2 and 3 depends on the wear margin of the bearings, deformations of the hull and machine support. Additionally, the radial vibration from head 3 is dependent on the propeller wear margin. The axial vibration from heads 1 and 2 depends, inter alia, on the wear margin of the main radial bearings of the engine, the thrust bearing and the propeller. The torsional vibration from head 1 depends on the moments acting on the machine set rotor (driving engine, friction moments of all component machines and the effective moment on the propeller), rotor wear margin (shaft cracks, loosened press fits, coupling damage, etc. ), wear margin of bearings located in the nodes of torsional vibration, wear margin of torsional vibration damper, etc. In the case of diesel engine propulsion lines with a reduction gear, the head 1 of free end sensors can be used as the head of the gear shaft free end and with the head 3 of the stern bearing sensors can serve for diagnosing the wear margin of the propeller shaft, the propeller and the thrust bearing.