Technology

Experimental Measurement of the Dynamic Stiffness and Mass of Structures

The dynamic stiffness and mass of structures are crucial parameters when carrying out predictive maintenance and analysis. So much so, that the ISO 7626-1:2011 specifies the use of experimental methods to measure the mobility of structures and to estimate their dynamic stiffness and mass. This method can be used to estimate the stiffness and mass of complex structures by simple percussion tests, for example, in the measurement of the stiffness of floors in high-tech factories, the measurement of the dynamic stiffness of tools, and the measurement of the stiffness of bases for large machine tools.

The new version of our handheld analyzer impaq Elite makes it possible to easily determine dymanic stiffness and mass by including the built-in ISO 7626-1 test methods in the FFT program for quick data review. For example, when a user performs a FRF test on a structure, the analyzer will display the following four functions on the same screen with a single key press: 1. inertance: acceleration/force; 2. mobility: velocity/force; 3. compliance: displacement/force; and 4. stiffness: force/displacement.

These functions can be individually enlarged for further data review. In each function chart, the reference lines for flexibility and mass are also marked with a diagonal or horizontal line, so that the approximate range of flexibility and mass of the structure under testing can be viewed quickly. Of course, a cursor can also be used to read the exact values. In the near future, we will release this new version of the software on our website, and all users who have purchased FFT modules can download and update it for free from our website.

Example 1: Four frequency response functions are displayed at the same time and the reference lines of flexibility and mass are indicated.

Example 2: Mobility graph with flexibility and mass reference line

Example 3: Compliance (top) and Stiffness (bottom)

Multi-point Dynamic Balancing Method

Generally, when performing dynamic balancing calibration in the field, we often encounter a problem. That is, after the dynamic balancing calibration, the vibration value of the measurement point is reduced while the vibration of other points is limited after calibration, and sometimes it even becomes larger. This is because the vibration of the double frequency may not be caused by dynamic balancing, or coupled dynamic balancing problems and yet we try to solve this problem by the correction of a single-plane approach. To solve this problem, we can consider using multi-point dynamic balancing techniques to do the correction. The following example is used to illustrate the process:

Case A: The vibration value of point A is used as the calibration basis for dynamic balance. It can be seen from the graph that when the vibration of point A decreases, the vibration of point B decreases to a limited extent.

Case B: Conversely, the value vibration of point B is used as the calibration basis for dynamic balance. It can be seen from the graph that when the vibration of point B decreases, the vibration of point A decreases to a limited extent.

Case C: The vibration of point A and point B are simultaneously brought into the calculation of the dynamic balancing parameters for multi-point dynamic balancing. This results in effectively reducing the vibration of point A and point B simultaneously after the correction.

Our handheld analyzer impaq Elite and Fieldpaq II dynamic balancing kit both support multi-point dynamic balancing calibration procedures. Also, up to four vibration points can be used simultaneously for single plane or dual plane calibrations. For further details, please get in touch with one of our representatives .