Bearing Condition Monitoring: Vibration Spectra, Ultrasound, and Oil Cleanliness

Bearings are the part condition monitoring most often has to catch. They carry most of the load in a rotating machine, and they give warning long before they fail, if you know how to read it. A mid-level technician can go past "the overall number went up." You can find running speed on a spectrum, tell a bearing tone from an imbalance peak, judge how far the damage has gone, and put vibration, ultrasound, oil and temperature data together into one call. Formal vibration analysis has its own training and certification, so treat this as the skill of reading the data and asking good questions. Reminder: readings are taken on running machines, with guards on. Never remove a guard to reach a measurement point.

What You Will Be Able to Do

• Choose the right vibration unit and avoid peak and RMS mix-ups.
• Find running speed and its multiples on a spectrum.
• Estimate bearing fault frequencies and recognize them.
• Judge the stage of a bearing fault and how urgent it is.
• Use ultrasound to grease a bearing the right amount.
• Read an ISO 4406 cleanliness code and act on a change.

Units and the Peak Versus RMS Trap

Vibration can be reported three ways. Each one shows some frequencies better than others.

Velocity is the normal measure for overall machine condition. Acceleration is where early bearing damage shows, because the first signs are tiny, very fast impacts.

The same vibration can also be reported as peak or RMS (root mean square, a kind of average). For a smooth single wave, RMS is about 0.707 times peak. Many US meters show inches per second peak. ISO limits use millimeters per second RMS.

Worked example. A meter shows 0.30 in/s peak. To compare it with an ISO limit:

1. Convert to millimeters: 0.30 x 25.4 = 7.62 mm/s peak.
2. Convert to RMS: 7.62 x 0.707 = 5.4 mm/s RMS.

If you put 7.62 against a limit written in RMS, you would rate the machine about 40 percent worse than it is. Check the meter setting and the limit's units before you compare them.

Reading a Spectrum

A spectrum is a graph of vibration size at each frequency. The horizontal axis is frequency and the vertical axis is amplitude, usually in velocity. Think of it as sorting one messy signal into separate notes.

Find Running Speed First

Everything else depends on the true running speed, called 1x.

A 4-pole motor on 60 Hz is often listed at 1,800 RPM, but that is its magnetic speed. Under load, the rotor slips, and it might turn at 1,770 RPM. So 1x is 1,770 / 60 = 29.5 Hz. If you assume 30 Hz, your multiples drift further off with every step. At 10x you would be 5 Hz from where the peak really is. On a variable frequency drive (VFD), the speed changes with the command, so read the real speed at the time of the reading.

Synchronous and Non-Synchronous Peaks

A peak at a whole-number multiple of 1x (1x, 2x, 3x) is synchronous. It is tied to shaft rotation, and it points to things like imbalance, misalignment or looseness.

A peak at a non-whole multiple, such as 3.6x or 5.4x, is non-synchronous. It cannot come from the shaft alone. It often points to a rolling bearing, or to another shaft in the machine, such as a belt-driven fan.

So before you call a bearing fault, rule out other speeds. A belt drive has at least two shaft speeds and a belt frequency.

Bearing Fault Frequencies

A defect on a bearing's race gets struck by each rolling element as it passes. That sets up a repeating impact at a frequency set by the bearing's geometry. Software lists the exact values for a bearing number. A useful first estimate needs only the number of rolling elements, N, and the shaft speed in Hz:

• Outer race (BPFO): about 0.4 x N x shaft speed.
• Inner race (BPFI): about 0.6 x N x shaft speed.
• Cage (FTF): about 0.4 x shaft speed.

BPFO and BPFI stand for ball pass frequency, outer and inner. FTF is fundamental train frequency, the speed of the cage.

Worked example. A motor at 1,770 RPM (29.5 Hz) has a bearing with 9 balls.

• BPFO is about 0.4 x 9 x 29.5 = 106 Hz, or 3.6x.
• BPFI is about 0.6 x 9 x 29.5 = 159 Hz, or 5.4x.

Your spectrum shows peaks at 29.5, 59, 107, 214 and 321 Hz. The first two are 1x and 2x. The peaks at 107, 214 and 321 are a series, each a multiple of about 107 Hz. That matches the outer race estimate and its harmonics, which are whole-number multiples of a frequency.

The estimate is only a guide. Real values shift a little with load and slip, and the exact number depends on the bearing. Confirm it against the bearing's real data before you write it up.

An inner race fault often shows sidebands spaced at 1x on either side of the main peak. The inner race turns with the shaft, so the damage moves in and out of the load zone once per revolution. An outer race fault usually sits still in the load zone, so its sidebands are weak or absent. That is a helpful clue, not a law.

How Far Has the Damage Gone?

Bearing faults tend to move through stages. Timing varies with load, speed and lubrication, so use the trend, not a calendar.

The late stage has a trap. Near the end, individual tones can fade as the damage spreads and smooths out. A reading that looks quieter on one line may be a machine that is close to failing, so read the whole spectrum and the trend. A falling high-frequency level with no repair on record is a question to ask, not good news.

Why Overall Numbers Miss Bearings

An overall reading adds up all of the energy into one number. Imbalance at 1x might be 0.14 in/s, and a stage 2 bearing tone might be 0.02 in/s. Because the energy adds as squares, the overall changes by a tiny amount and looks normal. The bearing is hiding under the bigger peak.

That is why many programs add a high-frequency measure. Names vary by maker, such as enveloping, demodulation, spike energy or shock pulse. These methods filter out low-frequency motion and pull out the repeating impacts. Use the same setting each time, and compare a machine only with its own history.

Ultrasound-Guided Greasing

Ultrasound instruments pick up the friction inside a bearing. It works as both a condition check and a way to avoid over-greasing.

1. Check the route sheet for the grease and the calculated amount.
2. If the fitting is outside the guard and your procedure allows greasing while running, attach the sensor to the housing.
3. Note the starting level, in decibels (dB).
4. Add one stroke, slowly. Wait for the grease to reach the bearing, which can take a little while.
5. Watch the level. If friction was the problem, it drops.
6. Stop when it stops dropping. If it starts to rise, you have added too much. Stop and note it.
7. Record the strokes used and the final level.

If the level does not change at all after the calculated amount, stop. Adding more grease will not help. The bearing may have damage rather than a lack of lubricant, the grease may not be reaching it, or the reading may be wrong. A rising level trend across several routes, even after correct greasing, also points to damage.

Warning: Never reach near moving parts to grease a machine. If the fitting is inside a guard, the machine must be stopped and locked out, and the ultrasound method does not apply.

Oil Cleanliness

Oil-lubricated bearings fail from what is in the oil. The oil film in a rolling bearing is very thin, often around a micron or less, so even tiny hard particles can dent the surface. Those dents later grow into fatigue damage.

An ISO 4406 code is three numbers, such as 18/16/13. They give the particle count at 4, 6 and 14 microns and larger, as range numbers. Each step up in a number means about twice as many particles.

Worked example. The target for a gearbox is 18/16/13, and the lab reports 21/19/16.

1. Each number is 3 steps higher.
2. 2 x 2 x 2 = 8, so the oil carries about 8 times as many particles as the target.
3. This is a real change, even if the oil looks clean.

Ask where the dirt came from.

Fix the source before you filter the oil. Filtering a tank with a broken breather only cleans the same dirt twice. The target itself comes from the equipment maker, your lubricant supplier, or your site's standard.

Putting the Evidence Together

No single method tells the whole story. When two or more methods agree, you can act with more confidence.

Worked example. A motor's drive-end bearing shows a growing 107 Hz series in the last three routes, and high-frequency acceleration has doubled. Grease does not lower the ultrasound level. The housing temperature is still normal. This is stage 2 to 3: real bearing damage, not yet near failure. Temperature tends to rise late, so a cool housing does not clear it. You write up the reading, the likely fault, the stage and the trend, and you ask to replace it at the next planned stop in three weeks. You shorten the check interval to weekly until then. After the repair, you take a new baseline.

Common Mistakes

Check Your Understanding

1. A machine runs at 1,200 RPM with a 12-ball bearing. Roughly where would an outer race fault show?

Shaft speed is 20 Hz. BPFO is about 0.4 x 12 x 20 = 96 Hz. Confirm it against the bearing's real data.

2. A reading of 0.20 in/s peak needs comparing with a limit of 4.5 mm/s RMS. Is it under the limit?

0.20 x 25.4 = 5.08 mm/s peak, and 5.08 x 0.707 = 3.6 mm/s RMS. It is under that limit.

3. A peak sits at 59 Hz on a motor turning 29.5 Hz, and a second peak at 74 Hz. Which suggests a bearing or another source?

59 Hz is 2x, so it is tied to shaft rotation. 74 Hz is about 2.5x, which is non-synchronous, so look at bearing data and any other shaft speeds.

4. You add the calculated grease, and the ultrasound level does not move. What do you do?

Stop adding grease. Note it and report it. The bearing may have damage, or the grease may not be reaching it.

5. A gearbox code goes from 17/15/12 to 19/17/14. How much dirtier is the oil?

Each number rose 2 steps, so about 4 times as many particles. Find the source.

Key Takeaways

• Check whether a reading is peak or RMS before you compare it with a limit.
• Find the true 1x first. Whole-number multiples point to the shaft, and non-whole peaks point to bearings or other speeds.
• BPFO is about 0.4 x N x shaft speed, and BPFI is about 0.6 x N x shaft speed. Confirm with real bearing data.
• Bearing faults show first at high frequency, and a late-stage bearing can look quieter.
• Grease with ultrasound one stroke at a time, and stop when the level stops falling.
• Each ISO 4406 step is about double the particles. Fix the source before you filter.