| The steps in manual (non-automatic)
| |
| | Some accelerometers will operate up to 400
|
| machine vibration analysis are:
| |
| | degrees F, but most give up the ghost at
|
| Identifying vibration peaks in the
| |
| | about 200 degrees F.
|
| spectrum and relating them to forcing
| |
| | Care must be taken that the accelerometer
|
| frequencies Determining the severity of
| |
| | is not dropped onto a hard surface lest
|
| machine problems from the amplitudes and
| |
| | the piezo-electric element be damaged. If
|
| relationships between the vibration peaks.
| |
| | the element is cracked, the stiffness of
|
| Making the appropriate repair
| |
| | the internal assembly will decrease,
|
| recommendations based on the severity of
| |
| | reducing the resonant frequency of the
|
| the machine problem.
| |
| | accelerometer, and this can greatly change
|
| In order to do a proper job of vibration
| |
| | its sensitivity at high frequencies.
|
| analysis, several tools are needed: If the
| |
| | In preparation for the diagnostic
|
| vibration spectra are being analyzed on a
| |
| | techniques described in the next chapter,
|
| computer, a calculator and Vibration Test
| |
| | the first steps of analysis should be
|
| and Analysis Guide (VTAG) for the machine
| |
| | performed as follows: This procedure
|
| in question are required. If the vibration
| |
| | assumes the vibration spectra are printed
|
| spectra have been printed on paper, then a
| |
| | on paper. When viewing spectra on the
|
| straight edge and ten-point divider are
| |
| | computer screen, similar procedures are
|
| desirable. Previous vibration data and
| |
| | used, as explained in the software
|
| average vibration data are also helpful if
| |
| | instructions.
|
| available.
| |
| | Note that all the following steps are
|
| The VTAG contains important information
| |
| | greatly simplified if the spectra are
|
| about the design of the machine, the test
| |
| | order normalized.
|
| points and their locations, the frequency
| |
| | The first step in machine vibration
|
| ranges to be tested, and the forcing
| |
| | analysis is to identify the spectral peak
|
| frequencies to be expected. The VTAG
| |
| | corresponding to shaft rotation rate, or
|
| should be consulted before any vibration
| |
| | the so-called 1X peak. This will be the 1X
|
| analysis is attempted.
| |
| | in a normalized spectrum. It is important
|
| After determining the shaft rotation rate
| |
| | to check to be sure the normalization was
|
| and locating it on the spectrum (it will
| |
| | done correctly. It is also called the
|
| be the first order in a normalized
| |
| | first order peak. In multiple-shaft
|
| spectrum), the vibration analyst must
| |
| | machines, each shaft will have a
|
| check the validity of the spectrum. Data
| |
| | characteristic 1X peak, and these are then
|
| validity can be corrupted by such things
| |
| | located by the analyst.
|
| as incorrect labeling of accelerometer
| |
| | Mark the harmonics of 1X on the spectra.
|
| orientation or position, improper
| |
| | This is simplified if you use a ten-point
|
| accelerometer attachment, rapid
| |
| | divider.
|
| accelerometer temperature changes, and
| |
| | Identify the fan blade pass frequency and
|
| incorrect machine operating conditions.
| |
| | mark it on the spectra. This is the number
|
| When data are to be compared to previously
| |
| | of blades multiplied by the RPM.
|
| collected data from the same point,
| |
| | Note the harmonics of blade pass frequency
|
| similar test conditions must be
| |
| | if they are prominent.
|
| maintained, especially machine speed,
| |
| | Look for bearing tones, which are between
|
| load, and operating temperature.
| |
| | the harmonics of the 1X run speed and not
|
| The integrity of the accelerometer cable
| |
| | synchronous with it. Mark them on the
|
| is crucial to the collection of valid
| |
| | spectra. There are other machine
|
| data. If the central conductor in the
| |
| | components besides bearings that generate
|
| cable is intermittent or open, the
| |
| | non-synchronous tones. Probably the most
|
| measured signal will consist mostly of
| |
| | common one is belt drives.
|
| random noise, and if the cable shield is
| |
| | Identify and mark the 1X and harmonics of
|
| intermittent or broken, the data will be
| |
| | the pump on the spectra. The pump RPM can
|
| contaminated with 60 Hz noise and
| |
| | be found from the VTAG, or can be
|
| harmonics. (50 Hz in countries with 50 Hz
| |
| | calculated from the motor speed and gear
|
| power lines.) In electrically driven
| |
| | ratio as follows: If the motor is turning
|
| machines, the 60 Hz line frequency will
| |
| | 1780 and the gear ratio is 2.3 to 1, then
|
| produce a series of 120 Hz harmonics in
| |
| | the pump speed is: Identify and mark the
|
| the vibration spectrum, as explained in
| |
| | pump vane pass frequency and harmonics, if
|
| the section on electrically induced
| |
| | any, on the spectra. The vane pass is the
|
| vibration.
| |
| | number of vanes times the pump RPM.
|
| If an accelerometer is exposed
| |
| | Search the spectra for non-synchronous
|
| continuously to a higher temperature than
| |
| | components that could be bearing tones, or
|
| that for which it is rated, it will become
| |
| | consult the VTAG for bearing tone
|
| desensitized, and the data it senses from
| |
| | frequencies, and mark them on the spectra.
|
| then on will be worthless.
| |
| |
|
