Air Compressor Vibration Analysis

Measuring vibration requires all three directions: horizontal, vertical, axial. Measuring only one direction easily misses problems. Misalignment faults show up prominently in axial direction. Imbalance is more pronounced in radial. Three-direction data together for judgment is accurate.

On airend, focus on drive end bearing housing. Horizontal direction sensor mounted on side of bearing housing, axis perpendicular to rotor. Vertical direction on top. Axial on end face, sensor axis parallel to rotor. Non-drive end some manufacturers also measure, some figure drive end can represent overall condition and skip it.

Motor has more measurement points. Drive end and non-drive end both measured. Each position three directions, six points total. Motor non-drive end has the fan. Vibration at this position sometimes relates to fan dynamic balance. Looking only at drive end data will miss this type of problem.

Sensor must directly contact metal surface. Measuring on top of paint or through guard cover gives low readings, can’t be used for judgment. Measurement location should be fixed each time. Best to mark bearing housing with marker pen. Different people measuring, data can still match up. Magnetic base must be firmly attached. Data measured while hand-holding isn’t stable.

ISO 10816 is most commonly used vibration evaluation standard. Uses vibration velocity RMS value to grade, unit in/s RMS.

For 20 to 100 hp power range units, vibration velocity below 0.07 in/s is excellent. New machines with good bearing condition and high alignment precision can reach this level. 0.07 to 0.18 is good. Most normally operating units are in this range. 0.18 to 0.28 is acceptable range. Can continue running, gotta watch the trend. Over 0.28 is unacceptable. Keep running and bearing life will be shortened.

Large power units have relaxed limits. 100 to 400 hp units, below 0.11 counts as excellent. Rotor weight is big, same residual imbalance produces larger vibration response. That’s why limits are relaxed. Some sites use small unit standards to judge large units. Everything tests as exceeding limits. Actually wrong standard was used.

Vibration acceleration metric mainly looks at high frequency, used to judge bearing condition. Velocity metric covers mid-frequency range, reflects overall machine vibration level. Use both metrics together. Velocity normal but acceleration elevated, usually bearing starting to have problems.

Getting vibration data, just looking at amplitude isn’t enough. Gotta do spectrum analysis. Time domain waveform becomes frequency domain plot. Each frequency component’s amplitude is clear at a glance. Different faults look different on spectrum.

1x frequency vibration, most common cause is imbalance. Rotor mass distribution uneven, heavy side during rotation produces centrifugal force, wobbles once per revolution, frequency equals rotation frequency. 1x vibration increases with speed squared. Speed doubles, vibration amplitude quadruples. Shaft bow is also mainly 1x. Bent shaft is like built-in eccentricity, rotating effect is similar to imbalance. Distinguishing these two faults requires looking at phase. Imbalance phase is stable. Shaft bow phase changes with speed.

2x frequency vibration, high possibility pointing to alignment problems. Coupling angular misalignment, two shafts have an angle. Rotor wobbles up and down twice per revolution, produces 2x. Parallel misalignment is two shaft centerlines parallel but not coincident, also 2x characteristic. Bearing housing looseness also shows as 2x. Foot suspended or bolt loose, unit taps twice per revolution.

Bearing characteristic frequencies are high frequency components. Rolling bearings have four characteristic frequencies: outer race pass frequency, inner race pass frequency, ball spin frequency, cage frequency. Spectrum showing these characteristic frequencies or their harmonics means corresponding component has damage. Outer race fault easiest to identify, frequency stays fixed. Inner race fault spectrum has running speed sidebands, because inner race rotates with rotor, damage point periodically enters and exits load zone.

Below 0.5x running speed vibration components can’t be ignored. Sleeve bearing machines watch out for oil whirl, frequency between 0.42 to 0.48x running speed, precursor to oil film instability. Screw compressors occasionally also have subsynchronous vibration. This type of low frequency vibration often has small amplitude, easy to overlook. Let it develop into oil whip and that’s a major fault.

After measuring vibration, compare data against limit table, what to do is clear. Within good range, no special action needed. Continue monitoring per original plan. Record data after each measurement. Over time plot trend chart. See if vibration is slowly climbing or staying stable.

Entering acceptable range, need to pay attention. Monitoring interval shortened. Originally once a month changed to every two weeks or weekly. At this point need spectrum analysis. Just looking at total vibration value doesn’t tell where problem is. Pull up spectrum, see if it’s 1x high or bearing frequency high, have some idea. Maintenance plan can start being scheduled.

Unacceptable range, must act immediately. First analyze cause. Spectrum data, phase data, temperature data, synthesize for judgment. Cause clear and not developing fast, can limit load operation and hold out until nearest shutdown window. Vibration still rising or spectrum shows bearing already has damage, shutdown for repair can’t wait. After repair must re-measure. Vibration back to good range before resuming normal operation.