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