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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 3, Detuning and Proving Resonance

Section 12, Watching Phase vs. RPM Change to Determine Resonant Ranges

Various designs of vibration instruments require different means to obtain and record phase readings. The instructions supplied by the manufacturer are considerably better than can be given here. Therefore, this textbook will give further instructions only on the swept filter/strobelight-type instrument as instructions for this type of instrument are often insufficient for resonance detection. For a while, later instrument designs made swept filter/strobelight-type instruments obsolete. However, many plants still have these instruments and are finding that in many situations, they are very practical for obtaining phase information for vibration analysis using phase. Gradually, non-swept filter-type instruments such as FFT's are being provided with strobelight accessories. The following instructions apply with very little adaptation.

This method works easiest on machines that can have their speeds slowly changed. It also works on constant speed machines that have vibration phase observed while the machine accelerates from startup to operating speed. In most situations, observing phase changes is easier during deceleration from operating speed when power is stopped.

For machines that accelerate or decelerate too rapidly for this phase test to be practical, the machine can be shut down and vibrations imparted through a variable speed vibration shaker (see section, "Use Of Variable Speed Vibration Shaker"). For most parts, the resonant frequency will be excited by the shaker at the same frequency as would be excited by the actual running machine. The exception is rotors in journal-type bearings. A rotor mounted in journal-type bearings will usually exhibit a slightly different resonant frequency when not running as compared to when running. (The effect of the running machine's oil film alters the rotor's resonance frequency slightly.) The same is true for resonance "bump tests." However, as previously mentioned, it is very rare for rotors to be resonant, and the shaker is very practical for the many other parts which do not rotate. To obtain phase, direct the strobelight to the shaker rotor.

Simplified Procedure:

(Read complete instructions before starting.) Using most common frequency tuneable instruments with phase indication:

A. Filter switch Out/Off (all pass), since vibration frequency will change with speed change.

B. Amplitude switch on relatively sensitive setting.

Note: reading is better in displacement units (as for a given displacement, velocity readings change as the speed is changed).

C. Pickup in contact with either a bearing cap showing highest vibration reading or any convenient bearing housing with appreciable vibration.

D. Direct the strobelight at reference mark on the rotor (or on the shaker). Try to choose a phase mark that will be visible throughout full 360° rotation (or if using other phase measuring method, watch digital phase changes).

E. Start rotor (or shaker). If acceleration can be controlled, attempt to accelerate slowly enough to be able to readily watch phase shifts. If rotor accelerates to operating speed at its own non-controlled rate, try to watch for possible phase shift as the rotor approaches operating speed. If the last few seconds of acceleration do not cause phase shift, this indicates that there is no resonant condition at operating speed.

If shifting occurs right up to operating speed, a resonant or partially resonant condition exists. Determine approximately how much the phase shifts from the previous non-resonant, non-phase shifting speed range, to determine how close the machine is to complete resonance.

F. If acceleration occurs too rapidly to make the above practical, first run the rotor at operating speed, cut the power, then watch for possible phase shifts as the rotor freely decelerates. Most often a freely decelerating rotor loses speed slowly enough to allow the phase shifts to be readily detectable.

Often swept filter/strobelight-type instruments use velocity-type vibration pickups. Velocity pickups have low resonant frequencies, usually approximately 600 cpm (or much lower for special low frequency pickups). For such, ignore the phase shift that occurs at the lower resonant frequency of the vibration pickup itself. Most later instruments use accelerometers. Accelerometers usually have relatively high resonant frequencies (often beyond all but such very high speed rotating machines as some gas turbines). However, their resonant frequencies can be in the range of such high frequencies as gearmesh.

Sometimes when drive motor electrical power is switched off, the phase instantly shifts a number of degrees. If this were the result of decelerating through a resonant zone, the shift would be gradual rather than instantaneous. The instantaneous shift is not the result of resonance, but instead the result of sudden loss of magnetic field on an armature that was slightly eccentric (or "running eccentrically").


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