Machinery Vibration Diagnostics 2

GEAR RELATED PROBLEMS

Normal Gear Spectrum

Typical Spectrum

Normal Spectrum shows 1x and 2x RPM, along with Gear Mesh Frequency (GMF). GMF commonly will have running speed sidebands around it relative to the shaft speed which the gear is attached to. All peaks are of low amplitude and no natural gear frequencies are excited.

Gear Tooth Wear

Typical Spectrum

A key indicator of gear tooth wear is excitation of the Gear Natural Frequency, along with sidebands around it spaced at the running speed of the bad gear. Gear Mesh Frequency (GMF) may or may not change in amplitude, although high amplitude sidebands surrounding GMF usually occur when wear is noticeable. Sidebands may be a better wear indicator than Gear Mesh Frequencies themselves.

Tooth Load

Typical Spectrum

Gear Mesh frequencies are often very sensitive to load. High GMF amplitudes do not necessarily indicate a problem, particularly if sideband frequencies remain low and no gear natural frequencies are excited. Each analysis should be performed with the system at maximum operating load.

Gear Eccentricity and Backlash

Typical Spectrum

Fairly high amplitude sidebands around GMF often suggest gear eccentricity, backlash or non-parallel shafts which allow the rotation of one gear to “modulate” the running speed of the other. The gear with the problem is indicated by the spacing of the sideband frequencies. Improper backlash normally excites GMF and Gear Natural Frequencies, both of which will be sidebanded at 1x RPM. GMF amplitudes will often decrease with increasing load if backlash is the problem.

Gear Misalignment

Typical Spectrum

Gear Misalignment almost always excites second order or higher GMF harmonics which are sidebanded at running speed. Often will show only small amplitude 1x GMF, but much higher levels at 2x or 3x GMF. Important to set the F_max high enough to capture at least 2 GMF harmonics if the transducer has the capability.

Cracked or Broken Gear Tooth

Typical Spectrum

A Cracked or Broken Tooth will generate a high amplitude 1x RPM of this gear, plus it will excite the gear natural frequency (f_n) sidebanded at its running speed. It is best detected in Time Waveform which will show a pronounced spike every time the problem tooth tries to mesh with teeth on the mating gear. Time between impacts () will correspond to 1/speed of gear with the problem. Amplitudes of impact spike in Time Waveform will often be much higher than that of 1x Gear RPM in FFT.

Hunting Tooth Problems

Typical Spectrum

Hunting Tooth Frequency (^fHT) is particularly effective for detecting faults on both the gear and the pinion that might have occurred during the manufacturing process or due to mishandling. It can cause quite a high vibration, but since it occurs at low frequencies, predominantly less than 600 CPM, it is often missed. A gear set with this tooth repeat problem normally emits a “growling” sound from the drive. The maximum effect occurs when the faulty pinion and gear teeth both enter mesh at the same time (on some drives, this may occur once every 10 or 20 revolutions, depending on the ^fHT formula). Note the ^TGear and ^TPinion refer to the number of teeth on the gear and pinion respectively. N_a = number of unique assembly phases for a given tooth combination which equals the product of prime factors common to the number of teeth on each gear.

FLOW RELATED PROBLEMS

Blade Pass & Vane Pass

Typical Spectrum	Machine Diagram

Blade Pass Frequency (BPF) = number of blades (or vanes) x RPM. This frequency is inherent in pumps, fans and compressors and normally does not present a problem. However, large amplitude BPF (and harmonics) can be generated in the pump if the gap between the rotating vanes and the stationary diffusers is not kept equal all the way round. Also, BPF (or harmonics) sometimes coincide with with a system natural frequency causing high vibration. High BPF can be generated if the wear ring seizes on the shaft or if welds fastening diffuesers fail. Also, high BPF can be caused by abrupt bends in linework (or duct), obstructions which disturb the flow path, or if the pump or fan rotor is positioned eccentrically within the housing.

Flow Turbulence

Typical Spectrum

Flow turbulence often occurs in blowers due to variations in pressure or velocity of the air passing through the fan or connected linework. This flow disruption causes turbulence which will generate random, low frequency vibration, typically in the range of 20 to 2000 CPM.

Cavitation

Typical Spectrum

Cavitation normally generates random, higher frequency broadband energy which is sometimes superimposed with blade pass frequency harmonics. Normally indicates insufficient suction pressure (starvation). Cavitation can be quite destructive to pump internals if left uncorrected. It can particularly erode impeller vanes. When present, it often sounds as if “gravel” is passing through the pump.

ELECTRICAL PROBLEMS

Stator Eccentricity, Shorted Laminations and Loose Iron

Typical Spectrum

Stator problems generate high vibration at 2x line frequency (2^F_L). Stator eccentricity produces uneven stationary air gap between the rotor and the stator which produces very directional vibration. Differential air gap should not exceed 5% for induction motors and 10% for synchronous motors. Soft foot and warped bases can produce an eccentric stator. Loose iron is due to stator support weakness or looseness. Shorted stator laminations cause uneven, localised heating which can significantly grow with operating time.

Eccentric Air Gap (Variable air gap)

Typical Spectrum

Eccentric Rotors produce a rotating variable air gap between rotor and stator which induces pulsating vibration (normally between (2^F_L) and closest running speed harmonic). Often requires “zoom” spectrum to separate the (2^F_L) and the running speed harmonic. Eccentric rotors generate (2^F_L) surrounded by Pole Pass frequency sidebands (^F_P) as well as ^F_P sidebands around running speed   ^F_P appears itself at low frequency (Pole Pass Frequency = Slip Frequency x # Poles). Common values of ^F_P range from approximately 20 to 120 CPM (.30 – 2.0 Hz)

Rotor Problems

Typical Spectrum

Broken or Cracked rotor bars or shorting rings, bad joints between rotor bars and shorting rings, or shorted rotor laminations will produce high 1x running speed vibration with pole pass frequency sidebands (^F_P). In addition, cracked rotor bars will often generate  ^F_P sidebands around the 3rd, 4th and 5th running speed harmonics. Loose rotor bars are indicated by 2x line frequency (2^F_L) sidebands surrounding the rotor bar pass frequency (RBPF) and/or its harmonics (RBPF = Number of rotor bars x RPM). Often will cause high levels at 2x RBPF with only small amplitude at 1x RBPF.

Phasing Problems

Typical Spectrum

Phasing problems due to loose or broken connectors can cause excessive vibration at 2x Line frequency (2^F_L) which will have sidebands around it at 1/3rd Line Frequency (1/3 ^F_L). Levels at (2^F_L) can exceed 25 mm/s (1.0 in/s) if left uncorrected. This is particularly a problem if the defective connector is only sporadically making contact and periodically not.

Synchronous Motors

Typical Spectrum

Loose stator coils in synchronous motors will generate fairly high vibration at Coil Pass Frequency (CPF) which equals the number of stator coils x RPM (# Stator Coils = Poles x # Coils/Pole). The coil pass frequency will be surrounded by 1x RPM sidebands.

DC Motor Problems

Typical Spectrum

DC motor problems can be detected by higher than normal amplitudes as SCR firing Frequency (6^F_L) and harmonics. These problems include broken field windings, bad SCR’s and loose connections. Other problems including loose or blown fuses and shorted control cards can cause high amplitude peaks at 1x through to 5x line frequency (3,600 – 18,000 CPM).

ROTOR  RUB

Rotor Rub

Typical Spectrum	Phase Relationship
Type ‘A’

Rotor Rub produces similar spectra to Mechanical Looseness when rotating parts contact stationary components. Rub me be either partial or throughout the whole revolution. Usually generates a series of frequencies, often exciting one or more resonance’s. Often excites integer fraction sub harmonics of running speed (1/2, 1/3, 1/4, 1/5, ….1/n), depending on location of rotor natural frequencies. Rotor rub can excite many higher frequencies (similar to wide-band noise when chalk is drug along a blackboard). It can be very serious and of short duration if caused by shaft contacting bearing Babbitt; but less serious when the shaft is rubbing a seal, an agitator blade rubbing the wall of a vessel, or a coupling guard pressing against a shaft.

RESONANCE

Resonance

Typical Spectrum	Phase Relationship

Resonance occurs when a Forcing Frequency coincides with a System Natural Frequency, and can cause dramatic amplitude amplification which can result in premature or even catastrophic failure. This may be a natural frequency of the rotor but can often originate from a support frame, foundation, gearbox or even drive belts. If a rotor is at or near resonance, it will be almost impossible to balance due to the great phase shift it experiences (90° at resonance; nearly 180° when it passes through). Often requires changing natural frequency location. Natural Frequencies do not change with a change in speed, this helps facilitate their identification.

BEAT VIBRATION

Resonance

Typical Spectrum

A Beat Frequency is the result of two closely spaced frequencies going into and out of synchronisation with one another. The wideband spectrum normally will show one peak pulsating up and down. When you zoom into this peak (lower spectrum), it actually shows two closely spaced peaks. The difference in these two peaks (F2 – F1) is the beat frequency which itself appears in the wideband spectrum. The beat frequency is not commonly seen in normal frequency range measurements since it is inherently low frequency. Usually ranging from only approximately 5 to 100 CPM.

Maximum vibration will result when the time waveform of one frequency (F1) comes into phase with other frequency (F2). Minimum vibration occurs when waveforms of these two frequencies line up 180° out of phase.

BELT DRIVE PROBLEMS

Worn, Loose or Mismatched Belts

Typical Spectrum

Belt frequencies are below the RPM of either the motor or the driven machine. When they are worn, loose or mismatched, they normally cause 3 to 4 multiples of belt frequency. Often 2x belt frequency is the dominant peak. Amplitudes are normally unsteady, sometimes pulsing with either driver or driven RPM. On timing belt drives, wear or pulley misalignment is indicated by high amplitudes at the timing belt frequency.

Belt / Sheave Misalignment

Typical Spectrum

Misalignment of sheaves produces high vibration at 1x RPM predominantly in the Axial direction. The ratio of amplitudes of driver to driven RPM depends on where the data is taken as well as on relative mass and frame stiffness. Often with sheave misalignment, the highest axial vibration will be at the fan RPM.

Eccentric Sheaves

Typical Spectrum

Eccentric and/or unbalanced sheaves cause high vibration at 1x RPM of this sheave. The amplitude is normally highest in line with the belts, and should show up on both driver and driven bearings. It is sometimes possible to balance eccentric sheaves by attaching washers to taperlock bolts. However, even if balanced, the eccentricity will still induce vibration and reversible fatigue stresses in the belt.

Belt Resonance

Typical Spectrum

Belt Resonance can cause high amplitudes if the belt natural frequency should happen to approach or coincide with either the motor or the driven machine RPM. Belt natural frequency can be altered by either changing the belt tension or the belt length. Can be detected by tensioning and the releasing belt while measuring response on sheaves or bearings.