Analysis of most frequent cases of vibration in propulsion systems

Resumen Análisis de casos más frecuentes de vibraciones en sistemas de propulsión Date Received: October 20th, 2012 Fecha de recepción: 20 de Octubre de 2012 Date Accepted: April 2nd, 2013 Fecha de aceptación: 2 de Abril de 2013 Analysis of most frequent cases of vibration in propulsion systems 1 Tecnavin S.A. Guayaquil, Ecuador. e-mail: navser@gye.satnet.net Ship Science & Technology Vol. 7 n.° 13 (65-74) July 2013 Cartagena (Colombia) 66 Frequently, noise and vibration problems occur in propulsion systems. Because of this, this study presents analyses of cases of problems that have been solved, as well as the example of a propulsion system without any type of inconvenience. At the end, we present a statistical analysis of the percentage of principal inertial masses that participate in each model to have as reference when defining a propulsion system. This study does not seek to become a publication with formulations; rather, it seeks to share experience developed in propulsion systems. The main components of a propulsion system to be considered in this analysis are as as shown in Fig. 1. Lateral vibrations in a propulsion line can be caused by: the gyroscopic effect of the propeller, thrust imbalance, inadequate distance between supports or lack of rigidity in bedplates and/or buttresses. When these vibrations occur in a propulsion system, they can cause fracture, failure in system components or on the ship’s structure, producing: • Complete destruction of the propulsion system; • Reduction of the service life of shafts and/or their components; • Fatigue fracture on support brackets and/or engine mountings; • Increased seal wear and damage; • Excessive noise, vibrations on the hull and superstructure. The natural frequency of lateral vibrations and critical frequencies of the system were calculated through the Finite Elements method, considering the following terms of reference: • The bocin support center close to the propeller, taken from a distance of one shaft diameter, measured from the aft end of the bocin support, close to the propeller; • The center of support for the other bearings was taken at the center of the length of the bocin support; • The propeller inertial mass was calculated through integration of radial sections; • The propeller added mass was estimated using the methodology proposed by Parsons M.G. et al., (1980) and with the aid of the PRAMAD software. • Modeling of the line was considered up to include the thrust bearing. Torsional vibrations in a propulsion system can be produced by any of these possible causes: Introduction General components of a propulsion system Lateral vibrations in the propulsion system Engine Propulsion Shaft

Frequently, noise and vibration problems occur in propulsion systems.Because of this, this study presents analyses of cases of problems that have been solved, as well as the example of a propulsion system without any type of inconvenience.
At the end, we present a statistical analysis of the percentage of principal inertial masses that participate in each model to have as reference when defining a propulsion system.This study does not seek to become a publication with formulations; rather, it seeks to share experience developed in propulsion systems.
The main components of a propulsion system to be considered in this analysis are as as shown in Fig. 1.
Lateral vibrations in a propulsion line can be caused by: the gyroscopic effect of the propeller, thrust imbalance, inadequate distance between supports or lack of rigidity in bedplates and/or buttresses.
When these vibrations occur in a propulsion system, they can cause fracture, failure in system components or on the ship's structure, producing: • Complete destruction of the propulsion system; • Reduction of the service life of shafts and/or their components; • Fatigue fracture on support brackets and/or engine mountings; • Increased seal wear and damage; • Excessive noise, vibrations on the hull and superstructure.
The natural frequency of lateral vibrations and critical frequencies of the system were calculated through the Finite Elements method, considering the following terms of reference: • The bocin support center close to the propeller, taken from a distance of one shaft diameter, measured from the aft end of the bocin support, close to the propeller; • The center of support for the other bearings was taken at the center of the length of the bocin support; • The propeller inertial mass was calculated through integration of radial sections; • The propeller added mass was estimated using the methodology proposed by Parsons M.G. et al., (1980) and with the aid of the PRAMAD software.• Modeling of the line was considered up to include the thrust bearing.

General components of a propulsion system
Lateral vibrations in the propulsion system The excitations most frequently used in torsional analysis are generated by the propeller and the internal combustion engine.
• For propeller excitations, the Classification Societies recommend values in percentages of propeller torque.• For propeller engine excitations; normally, this information is provided by engine manufacturers.If this information is missing, we may use the harmonic tangential components provided by Lloyd's Register.
Torsional vibrations occurring in a propulsion system can cause fracture, failure in system components, gear damage, premature destruction of flexible couplings.
Frequently, fractures in shafts or crankshafts, due to torsional effect, occur in 45° direction.
The methodology to model a propeller system for torsional analysis is based on the equation: are the inertial mass matrices, damping, stiffness, and excitation, respectively.
The natural frequencies of torsional vibrations and the responses of the forced analysis of the system were calculated through the matrix solution method, considering the following terms of reference: • Include the frontal damper with its inertia, stiffness, and relative damping; • Include inertial mass and cylinder absolute damper, crankshaft stiffness; • Include the flexible coupling with its inertial mass, stiffness, relative damping, and energy dissipation limit; • Include the gearbox with its inertial mass, stiffness, diameters, stiffness of gear teeth.Avoid synthesizing the gearbox branches; • Propulsion shafts: Add as much inertial mass as necessary in case of section changes.
The precision of the calculations of system response will depend on the reliability of the input data, which is why it is recommended to request information on inertial mass, stiffness, and damping from the equipment manufacturers.
For the added mass and the propeller damping, the methodology proposed by Parsons M.G. et al., (1980) and the PRAMAD software were used.
Fig. 2 presents a model example of inertial mass without branches.
The vibrations in a propulsion system can be caused by lateral, torsional, axial effect or by its possible frequency couplings.
Adequate configuration of elements of a propulsion system, separation of supports, stiffness of supports and/or struts must be conducted taking into consideration the recommendations of Classification Societies and the manufacturers of the system's components.

Vibration analysis in a propulsion line
In the cases shown ahead, frequencies were calculated in the three orientations: lateral, torsional, and axial, to after that verify the possible coupling of frequencies of the system with the structure.
The case studies analyzed in this study are as indicated: 1 Frequency analysis was carried out for lateral and torsional vibrations.
Through analysis, it was found that the flexible coupling is sub-dimensioned, which is why we recommend: • Changing the flexible coupling according to the system's needs; • Operating the system in restricted manner until the flexible coupling mentioned can be replaced.
• Diminish the power developed by the controllable pitch propeller, under conditions of misfiring.
Case The original propeller presented cracks and section detachment in several blades.
Frequency analysis was carried out for lateral and torsional vibrations.
Analysis revealed that by maintaining the system's  Lateral analysis shows that the system's natural frequencies and critical frequencies are within the working range.
Based on this analysis, the support structure for the first suport was inspected, finding that the structure was corroded.
It was recommended to relocate the supports to minimize the system's frequencies from being within the working range.
Case study 4: Alteration of the distance between supports on a tugboat Type: Tugboat Length: 34.90 m Engine: 1566 KW MCR RPM: 600 -1800

Propeller: FPP + Fixed Nozzle
This case was analyzed because of the presence of persistent noise in the propulsion system within the interval of 650 to 700 rpm.Vibration measurements were made with a triaxial accelerometer.With these vibration readings, wear was detected on gears of the gearbox (excessive backlash).
Frequency analysis was made for lateral and torsional vibrations, taking into consideration the wear of the gear teeth and the possible relocation of the stuffing box support.
Through a sensitivity analysis, increased wear was found on the gear teeth, a possible relocation of this support in 300 mm would offer the possibility of resonance of the lateral frequency with the torsional frequency.
Lateral analysis shows that the system's natural frequencies and critical frequencies are within the working range.
Based on this analysis, the support structure for the first break was revised, finding that the structure was corroded.
It was recommended to relocate the supports to minimize the system's frequencies from being within the working range.

Repair recommendation
• Inspect the stuffing box and restore the bearing to its original position.• Inspect the structural condition of the nozzle.
• By relocating the stuffing box bearing, as suggested, we expect to increase the system's lateral frequency from 1527 rpm (mode 1 order 1Z) to 1580 rpm (mode 2 order 2Z+1), expecting slight vibration at speeds close to 1600 rpm.
Case study 5: Alteration of the distance between supports on a tugboat Frequency analysis was conducted for lateral and torsional vibrations.
Inspecting the system, it was found that the countershaft presents abrupt section change, close to the gearbox flange, which causes stress concentration, increasing the of the countershaft fracture risk.
From vibration analysis, we found that within the range of operation close to 900 rpm, there Result of the analysis of possible coupling between lateral and torsional vibrations of the system assumed: Frequency analysis was performed for lateral and torsional vibrations.Frequencies were estimated on the struts by using the finite elements method.

Fig. 2 .
Fig. 2. Propulsion System Torsional Mass diagram . Inadequate selection of the flexible coupling on a tanker 2. Change of propeller on a tanker 3. Inadequate separation of supports on a passenger ship 4. Alteration of the distance between supports on a tugboat 5. Restriction in the operation range on a yacht 6. Inadequate stiffness in struts and inadequate separation of supports 7. Inadequate selection of flexible coupling and diameter of propeller shaft in fishing vessel 8. Example of a propulsion system without vibration problems Case study 1: Inadequate selection of flexible coupling on a tanker Type: Tanker Length: 103.35 m Engine: 1300 KW MCR RPM: 500 Propeller: CPP This case was analyzed by request of re-engining.
study 2: Change propeller on a tanker Type: Tanker Length: 120.55 m Engine: 2574 KW MCR RPM: 200 -620 Propeller: FPP This case was analyzed by request of change of heavier propeller.