Industries

Modal Test on Structure Parts of Ship Stern Shaft Bearings

The traditional bearings of ship stern shaft adopts oil-sealed type, now several ship companies and design & research institutes have developed water-sealed stern shaft bearings. The inner circle of its external bearing uses rubber material. Because the characteristics of rubber and metal are completely different, the testing engineer adopts experimental modal analysis method to test on structural parts of the stern shaft bearings.
 
    
(Note: the two above are not true trial bearing pictures, only similar ones.)
The testing engineer adopts AVANT MI-7008 data acquisition & signal analyzer and its related test software, which Hangzhou Econ Technologies Co., Ltd. produces, and also uses special modal impact hammer with a force transducer and multi acceleration transducers, and then do modal test.
Considering the limited test time, the engineer only distribute test points on one section and collect modal data. Figures as following:
Test points 1, 2, 3 on the tube’s outer surface, the metal surface; test points 4, 5, 6 on the tube’s inner surface, the rubber surface. 
Distribution model of test section

 

 
Because of adopting single transducer to do modal test, the point 2 and 5 only do data acquisition for radial transfer function, the rest for axial and radial transfer function respectively. (Proposal: when doing modal test, in order to understand test article’s vibration mode completely, engineers usually adopt triaxial transducer to do data acquisition for all test points’ three direction transfer functions. But sometimes only analyzing one direction vibration mode, engineers can get transfer function from single transducer.)
Modal data acquisition:
Hardware parameters:
Choose a force transducer; Channel 1 is connected with an impact hammer for generating excitation signals. Channel 2 and Channel 3 are connected with acceleration transducers for receiving response signals. The test uses an ICP force transducer with 100mv/N sensitivity and acceleration transducers with 100mv/g sensitivity.
Analysis parameters:
5120Hz sampling frequency, 2048 sampling points, exponential average.
Operation mode: Channel 1 is set to trigger with an impact hammer. 
Test points settings:
All test points’ motion direction, current test points.
Window settings:
Excitation Channel: force window, response Channel: exponential window
Screenshots of modal data acquisition section (time domain signals, frequency response, auto power spectrum, coherent signals)

 

Time domain signals (excitation signals, response signals )
 
    
 
Frequency response function
    
Coherent signals

 
Modal parameter identification
The engineer imports all recorded transfer function by using special modal identification software and gets all orders trial parameter including modal frequency, modal vibration mode, modal damping ratio and etc., from curve fitting identification.
Modal parameter identification as follows

 

Figures of vibration mode section

 
         
First-order vibration mode                       Second-order vibration mode                            Third-order vibration mode
         
First-order vibration mode                      Second-order vibration mode                     Third-order vibration mode

Seventh-order vibration mode

 
During the whole testing, AVANT MI-7008 data acquisition and signal analyzer shows its features of high reliability, high accuracy and easy-to-use. The engineer connects it with up-to-date ICP force transducer and an acceleration transducer, and uses modal data acquisition software to record data, and then import them to the post-processing software to get results and test reports, which can be finished in a short time.