DCA31bumptest
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Determination of Structural Resonant Frequencies Performing an Impact or “Bump” Test with the DCA-31 What is a bump test? In order to evaluate the vibration behavior of machines, it is often desirable to measure the natural frequencies or “resonances” inherent in the mechanical structure of the machine. The classical way of doing this is to impart a measured force into the structure with an instrumented hammer, or other vibration exciter, and measure the response of the structure at the same time the excitation occurs. The vibration force data are analyzed with a dual- channel FFT analyzer which calculates the transfer function, or frequency response function between the two channels. This is an essential step in the technique of experimental model analysis. Usually, in most cases of machine vibration, it is not so important to determine the exact magnitude of the transfer function; the most important information is in the frequencies of the modes of vibration of the structure, and to a lesser extent, the damping associated with these modes. The DCA 31 can be used to perform an impact test without the need for a special hammer with a force transducer attached. The technique involves simply hitting the structure with a suitable impacting device, such as a wooden 2x4, a hard rubber mallet, or in some cases, a sledge hammer. If a metal hammer is used, it is a good idea to pad the striking surface to avoid introduction of excessively high frequency energy that could damage the machine. This application note outlines the procedure to do an impact test using a DCA 31. Fig. 1. The bump test shows structural modes at 52.0 Hz and at 113.5 Hz. The mode at 52 Hz is more lightly damped than the mode at 113.5 Hz as evidenced by the ratio of the width of the hump’s base to its height. The Bump Test — Simplified. The machine to be tested should not be running. A vibration sensor is placed on the desired machine. If using a triaxial sensor, you will have to specify the active channel. An impacting device is used to hit the machine in a random manner to excite the structural resonance(s) and allow it to ring. Think of a gong that has just been hit with a mallet. The gong will produce a tone (ring) at a frequency equal to its natural frequency. During the test, the resulting spectra are captured and saved in the data collector. Natural frequencies are seen as high level humps in the vibration spectrum, usually with a broad base at the bottom. The example shown in Figure 1 shows a machine with two structural modes. After natural frequencies are identified, steps should be taken to ensure the machine running speed (1X) and any principal forcing frequencies (e.g. pump vane rate, compressor thread rate, gear mesh rate) are not within 10% of the natural frequency. Note: Prior to starting a bump test, the analyst should locate a device to deliver a series of impacts to the structure. Your choice of an impacting device will depend on the mass and stiffness of the structure and must be able to impart enough energy to allow the structure to “ring”. The analyst should also select a suitable location on the structure to mount the
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Determination of Structural Resonant Frequencies
Transcript of DCA31bumptest
Determination of Structural Resonant Frequencies Performing an Impact or “Bump” Test with the DCA-31
What is a bump test? In order to evaluate the vibration behavior of machines, it is often desirable to measure the natural frequencies or “resonances” inherent in the mechanical structure of the machine. The classical way of doing this is to impart a measured force into the structure with an instrumented hammer, or other vibration exciter, and measure the response of the structure at the same time the excitation occurs. The vibration force data are analyzed with a dual-channel FFT analyzer which calculates the transfer function, or frequency response function between the two channels. This is an essential step in the technique of experimental model analysis.
Usually, in most cases of machine vibration, it is not so important to determine the exact magnitude of the transfer function; the most important information is in the frequencies of the modes of vibration of the structure, and to a lesser extent, the damping associated with these modes.
The DCA 31 can be used to perform an impact test without the need for a special hammer with a force transducer attached. The technique involves simply hitting the structure with a suitable impacting device, such as a wooden 2x4, a hard rubber mallet, or in some cases, a sledge hammer. If a metal hammer is used, it is a good idea to pad the striking surface to avoid introduction of excessively high frequency energy that could damage the machine. This application note outlines the procedure to do an impact test using a DCA 31.
Fig. 1. The bump test shows structural modes at 52.0 Hz and at 113.5 Hz. The mode at 52 Hz is more lightly damped than the mode at 113.5 Hz as evidenced by the ratio of the width of the hump’s base to its height.
The Bump Test — Simplified. The machine to be tested should not be running. A vibration sensor is placed on the desired machine. If using a triaxial sensor, you will have to specify the active channel. An impacting device is used to hit the machine in a random manner to excite the structural resonance(s) and allow it to ring. Think of a gong that has just been hit with a mallet. The gong will produce a tone (ring) at a frequency equal to its natural frequency.
During the test, the resulting spectra are captured and saved in the data collector. Natural frequencies are seen as high level humps in the vibration spectrum, usually with a broad base at the bottom. The example shown in Figure 1 shows a machine with two structural modes. After natural frequencies are identified, steps should be taken to ensure the machine running speed (1X) and any principal forcing frequencies (e.g. pump vane rate, compressor thread rate, gear mesh rate) are not within 10% of the natural frequency.
Note: Prior to starting a bump test, the analyst should locate a device to deliver a series of impacts to the structure. Your choice of an impacting device will depend on the mass and stiffness of the structure and must be able to impart enough energy to allow the structure to “ring”. The analyst should also select a suitable location on the structure to mount the
253 Winslow Way West Bainbridge Island, WA 98110 U.S.A. Tel.: (206) 842-7656 Fax: (206) 842-7667 E-mail: [email protected]
Application Note DCA-31 Bumptest © 2004 – DLI Engineering Corp. - All rights reserved.
Printed in U.S.A.
sensor (typically a piezo-accelerometer), and another location to impact the structure in order to optimize the response.
Setting up the DCA 31 for a Bump Test 1. While in real time mode, the DCA 31 can record one axis at a time. Connect your accelerometer to the machine or structure ensuring that it is not near a hollow cavity such as a motor cooling fan cowling.
2. Start the DCA 31 and select the following Instrument Setup settings. Press the soft key for “Apply” when done. Once back at the main menu, select Data Collection.
Instrument Setup
Bump Test
Instrument Setup
Bump Test
Memory Internal Date/Time n/a
Timeout 5 mins Barcode
Entry n/a
IrDa Disabled Transducer Triax
Baud Rate 115,200 Memory n/a
Default Units English Dbase Cache On
Date Format MM/DD/YYYY Purge Cache On
3. From the data collection screen, select the soft key labeled “Config”. Ensure the following settings are made. Press the soft key labeled “Apply” when done.
Configuration Bump Test Configuration Bump Test Freq Units Hz Acquisition Single Shot
Fast Averaging Off Auto-ranging Conservative
Auto Store On
Machine Speed Normal
Time 1s Settling Delay as needed Percent 50% Memory N/A Auto Advance On Fill Signature Off Dbase Advance Off
Trig Level Fixed
4. From the Data Collection Screen, select the soft key labeled “Offroute” and from the next screen select the soft key labeled “User”. Make the following settings in the User menu.
User Settings Bump Test
User Settings Bump Test
Change Name
edit position if desired
Fmax 200 Hz
Collection Sig Only Lines/Samples 400 / 1024 Display format Spectrum
Window Rectangular
Units Accel (G) Averages 10
Coupling ICP Overlap 50%
Input channel X
Type Spectral
Detection RMS Ext trigger
slope n/a
Sensitivity 100 MV/G Level n/a Hi pass filter 10 Hz
Bin Zeroing On
5. Leave the DCA 31 with the User menu selections open. This is where you will initiate the start of data collection. See the next section for performing the impact test
Performing the Impact Test 1. Ensure nothing is touching the machine or structure during the test. Begin to strike the structure. Strike with approximately the same force each time. Varying the amount of force can invalidate the test.
2. From step 5 on the previous page, press either Read-OK button to initiate the test. The test comes in two parts: auto range & data collection.
3. During the auto-range step, strike the structure at a steady rate of about 2 impacts per second. This step sets the amplifier gain to maximize the dynamic range for the measured response.
4. Once the data collection step begins, continue to strike the structure in a random manner (not with a steady striking frequency) until the DCA 31 has collected all 10 averages. At this point, the DCA 31 will display the final averaged spectrum.
Note: While collecting data, if you see the prompt “Input over range”, this means the force of your strike has exceeded the instrument settings made during the auto-range portion of the test. Try to strike with slightly less force during the data
253 Winslow Way West Bainbridge Island, WA 98110 U.S.A. Tel.: (206) 842-7656 Fax: (206) 842-7667 E-mail: [email protected]
Application Note DCA-31 Bumptest © 2004 – DLI Engineering Corp. - All rights reserved.
Printed in U.S.A.
collection portion than you did during the auto-range portion of the test.
5. Use the right and left arrow keys to move the cursor across the x axis to determine the natural frequencies. Use the up and down arrows to change the scale on the Y axis. You should see a portion of the spectra that rises out of the noise floor. If you do not see any humps in the spectrum, try a wider frequency band (Fmax) in the User setup menu.
6. It is a good idea to repeat the test several times to ensure the results are valid. In the upper right corner the cursor frequency and amplitude are displayed in units of Hertz and G (gravity) acceleration. If desired the label under the frequency/amplitude display can be edited by changing the position name in step 4 on the previous page. To capture the screen in a bitmap, hold down the “0” and the “7” keys simultaneously. Select “Bitmap” from the printer pull down. The screen capture will be saved as dump.bmp file on the PCMCIA card.
Figure 3 shows what happens when a principle forcing frequency is in close proximity to a structural natural frequency (330 Hz). The pump vane (PV) rate is about 10% lower than the structural natural frequency. The average plus sigma level for pump vane rate is 116 VdB or 0.352 in/s pk. If a bump test were performed on this machine you might see a similar spectrum without the discrete shaft rate harmonic peaks. The broad base on the hump implies that the structure is highly damped.
