Analysis

Analysis

The following procedure gives an overview of the procedure to take in the analysis of vibration frequency spectra.

Predictive Maintenance Procedure

Date:createdate \@ "MMMM d, yyyy"July 11, 1995

Procedure Name: Signature Analysis

Purpose: To provide a routine procedure for analysis of vibration in order to promote understanding of the relationship between vibration frequencies and their causes.

Activities:

Select the first plot of the machine which will be at the driver outboard and will be an enveloped acceleration reading. Identify the run speed accurately. If you have frequency information for the machine ensure that the reference speed is accurate - if not you must change the speed reference before continuing.

In the envelope spectrum see if any of the generated frequencies coincide with (or are close to) any significant spikes. Remember that the bearing frequencies may not be completely accurate if the bearing which has been nominated in the frequency setup has been replaced with an equivalent.

As a rule of thumb:

Ball Pass Frequency XE "Frequency" Outer Race ( Run Speed x No. Of Rolling Elements x 0.4

Ball Pass Frequency XE "Frequency" Inner Race ( Run Speed x No. Of Rolling Elements x 0.6

Cage Frequency XE "Frequency" (Run Speed x 0.4

Note that this vibration is not necessarily direction specific.

Once a spike at a bearing frequency has been identified you should check the baseline for this type of machine for the trend. If the trend is deteriorating then further checks are necessary. Be careful that you do not confuse a run speed harmonic or an electrical frequency with a bearing defect frequency. One common bearing frequency is just over 3 x run speed for BPOR on a 8 element bearing. The run speed of an electric motor cannot exceed the electrical speed so harmonics of run speed cannot have a frequency even slightly more than 3600 cpm (for a 1200 rpm motor) or 5400 cpm (for a 1800 rpm motor) or 10,800 cpm (for a 3600 rpm motor) - if the spike is even at a slightly higher frequency then it is likely caused by a bearing defect otherwise it is likely a run speed harmonic or an electrical frequency. Check the time domain signal for angel fish patterns.

In the single spectrum plot double click on the convert the display to dB(G) (referenced to 0.001 G). Identify the carpet amplitude in dB(G) - this is the average amplitude (excluding spikes). Identify the amplitude of the spikes above the carpet level. The following rules of thumb apply to enveloped acceleration levels in dB(G):

> 20 dB(G) rise in carpet level-under lubrication

10-15 dB(G) spike above carpet-minor marking

> 20 dB(G) spike above carpet-marked race.

If the spectrum shows multiples of run speed then there is an impact every rev of the rotor, with possible looseness if there are many multiples. If the bearing defect frequencies have sidebands of cage frequency then there is a FALSE BRINELLING problem. If the bearing defect frequency and harmonics have sidebands of run speed then there is probably a defect on the inner race. As the defect deteriorates then the carpet level will rise and the sidebands and harmonics will increase in amplitude up to a certain amplitude then stop. The carpet level will continue to rise as random marking occurs around the bearing and may rise to mask the spikes completely.

Move to the velocity spectrum for the same point but with amplitude set to linear. Check to see if there are any spikes in velocity at the bearing defect, harmonics of the defect and/or sidebands of the defect - look particularly for the third and fifth harmonics. If any spike at these frequencies exist then there is physical spalling of the race. If the amplitude of the spike reaches 1 mm/s then the spalling is severe.

In the velocity spectrum the following patterns indicate the associated defects:

Dominant Frequency XE "Frequency" Secondary Frequency XE "Frequency" Harmonics of Dominant Frequency XE "Frequency" SidebandsDominant DirectionDefectSuggested Maximum Amplitude @ Dominant Frequency XE "Frequency" 1xNilNilNilRadialImbalance6 mm/s1x1/2 or 1/3 xMultiple XE "Multiple" NilRadialLooseness3 mm/s1x3x2 or 3NilAxialMisalignment4 mm/sBPFO / BPFI1xMultiple XE "Multiple" 1x / CageRadialBearings0.5 mm/s7,200 cpmRotor Bar2 or 32xLF of RBFAnyElectrical5 mm/sAnyAnyNilNilAnyResonance XE "Resonance" 7 mm/sGear Mesh1x3 or 41xRadialGearing1 mm/s2x Belt1x2 or 3N/ARadialBelts5 mm/sIf there is a significant defect raise a work request.