# Diagnosing the Machine Set Rotor Using Its Relative Vibrations
## 3 Qualities and Symptoms of the Wear Margin of the Machine Set Rotor  
### Quality of the Machine Set Rotor Wear Margin  
The wear margin of the machine set rotor as a functional unit created conceptually for diagnostic purposes is not determined in the specifications of machines. Therefore, for diagnostic purposes, appropriate quality and quality measures should be indicated for the machine set rotor wear margin and associated with appropriate measures of diagnostic symptoms. Wear margin quality measures can be different for machine set rotors supported in rolling bearings, slide bearings and for machine set rotor with a crankshaft. Generally, if the wear margin of the machine set rotor has a nominal value, then such rotor will be affected by radial, axial and tangential forces, determined in specifications of the component machines; these forces will be dependent on the current efficiency of the driven machine (or the current load of the driving machine). The wear margin of the machine set rotor can achieve the nominal value if:  
* the shaft line axis of the machine set rotor will be a straight line;
* centers of the masses of the machine set rotor will overlap with the rotation axis of the machine set rotor;
* the rotor discs will be permanently connected with the shaft, and the masses and moments of inertia of the rotor discs and the shaft rigidity will conform with the specification. For real rotors, certain deviations are allowed from the above conditions. The following types of deviation are in use: shaft line alignment [19], rotor unbalance [20] and transfer function [21]. The alignment of the machine set rotor, its unbalance and transfer function should be considered as quality of the wear margin of the machine set rotor. The consequences of incorrect alignment and unbalance of the machine set rotor are additional variable and constant forces acting on the rotor. These contribute to the generation of relative vibrations of the rotor and absolute vibration of bearing support. The values of the generated vibration depend on the transfer function of the machine set rotor and the transfer function of the machine set body [22; 23; 24]. The causes of rotor unbalance are described in Refs. [24; 25; 26]. The machine set rotor will be properly aligned if:  
* rotor components (shafts and discs, including coupling discs) of the machine set will not have errors of shape and position;
* there will be no coaxiality errors of holes for bearings, errors of shape and hole dimensions, errors of shape and dimensions of shells and rolling bearings;
* support bearings will have oil film of proper thickness. Errors in couplings connecting the rotors of machines in a set consist of manufacturing and assembly errors and the wear of the couplings. In Ref. [27], the types of coupling errors are distinguished. Non-coaxiality of the support bearings leads to changes in the values of the reaction forces in the bearings and the deformation of the machine set rotor shaft. Additional constant reaction forces in the bearings are created, and the shaft is additionally bent. Consequently, the bearings become overloaded, and stresses in the shaft lead to fatigue damage. The possible causes of bearing non-coaxiality are:  
1. alignment errors--a typical alignment error leads to a constant (non-rotating) bending line of the shaft centerline [27]. Journals are displaced, particularly the first journals on the coupling side, mainly slide bearings (displacement within the bearing clearance). These deviations, referred to as angular offset and parallel offset, are measurable on the disconnected shafts of the machine set rotor. 2. displacement of the shaft bearing support of one of the machines. The displacement of bearing support may be caused by deformations of the support, machine body or foundation. Deformations of machine foundations are specific for a given production system and, in many cases, are given in the system specification. For instance, deformed foundation can be due to deformations of the ship's hull [19].