Understanding Defect Severity

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Portable balancer & Vibration analyzer Balanset-1A

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Vibration sensor

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Optical Sensor (Laser Tachometer)

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Balanset-4

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Reflective tape

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Dynamic balancer “Balanset-1A” OEM

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Defect severity is the classification of a detected fault according to its seriousness, its stage of progression, and the urgency of corrective action it demands. A severity rating blends several inputs — vibration amplitude, the rate at which that amplitude is changing, the type of fault, and the criticality of the equipment — into a single category that runs from incipient (minor, just detectable) to critical (severe, on the brink of failure). In doing so it converts the raw output of fault detection into actionable maintenance priorities, telling the team not merely that a problem exists but how soon it must be dealt with.

Accurate severity assessment is what makes a condition monitoring programme genuinely useful. It decides how quickly to act, what resources to commit, and how to weigh one ailing machine against another when several compete for the same limited maintenance window. Get it right and scarce specialist time flows to the machines that need it; get it wrong and you either chase phantoms or miss a failure in the making.

1. Severity Classification Scales

Most programmes grade faults on a five-level scale. The amplitudes referenced below relate to the alert, warning, alarm and trip bands that a programme sets against its own baseline and against vibration-severity standards.

• Level 1 — Incipient: a very early defect, only just detectable. Amplitude sits slightly above baseline, just entering the alert zone. Estimated 6–12+ months to failure. Action: note it in the database and continue routine monitoring.
• Level 2 — Minor: a confirmed early-stage defect. Amplitude is in the alert zone with a clear trend developing. Roughly 3–6 months to failure. Action: increase monitoring frequency and begin planning maintenance.
• Level 3 — Moderate: an active, progressing defect. Amplitude reaches the warning or alarm zone. About 1–3 months to failure. Action: schedule maintenance within weeks and monitor weekly.
• Level 4 — Serious: an advanced defect with significant degradation. Amplitude is in the danger zone and rising quickly. Weeks to failure. Action: arrange urgent maintenance, monitor daily, and plan a rapid shutdown.
• Level 5 — Critical: imminent failure with catastrophic risk. Amplitude is at or above trip levels. Days or hours to failure. Action: immediate shutdown and repair, with continuous monitoring for as long as the machine keeps turning.

2. Severity Assessment Factors

No single number tells the whole story. A sound severity call weighs five factors together.

Vibration amplitude

• The primary indicator for many fault types — higher amplitude generally means a more advanced defect.
• Always judged against both the relevant standard and the machine’s own baseline, never in isolation.
• Interpreted in light of fault type, since a given amplitude means very different things for a bearing fault and for unbalance.

Rate of change

• Slow change: lower severity — the condition is essentially stable.
• Accelerating change: higher severity — active deterioration is under way.
• Exponential growth: critical severity — failure is close.
• Often the rate of change matters more than the absolute level, which is why trend analysis and trending sit at the centre of severity judgement.

Fault type

• Bearing spalls: progress rapidly once advanced → high severity.
• Unbalance: usually stable over time → lower severity.
• Shaft cracks: can fail suddenly and without warning → high severity.
• Misalignment: chronic but manageable → moderate severity.

Spectral characteristics

• The number of harmonics present — more harmonics generally means a worse defect.
• Sideband complexity — extensive sidebands indicate an advanced fault.
• An elevated noise floor, which points to widespread or distributed damage.
• Multiple distinct fault frequencies, signalling several problems at once. Techniques such as envelope analysis often expose these patterns long before they appear in a plain velocity spectrum.

Equipment criticality

• The same vibration level is more severe on critical machinery than on a redundant spare.
• Severity should be adjusted to the consequences of failure, not just the measured signal.
• On critical equipment, treat a lower vibration reading as a higher severity, because the cost of being wrong is so much greater.

3. Severity-Based Actions

Each severity band maps onto a defined response, so the rating translates directly into a work plan.

• Incipient / Minor: keep to the routine monitoring schedule, note the defect for a future outage months ahead, and time any action for the next scheduled maintenance or within 3–6 months.
• Moderate: measure weekly, schedule a dedicated maintenance task within 1–2 months, order the spare parts now, and act within roughly 2–8 weeks.
• Serious: monitor daily or continuously, expedite scheduling to a window of days up to two weeks, assign priority resources, and act within 1–2 weeks at most.
• Critical: monitor continuously, shut down and repair immediately, mobilise an emergency response with all necessary resources, and act on a timeline of immediate to a few days.

4. Documentation and Tracking

A severity assessment is only as good as the record behind it. Two kinds of documentation matter.

Severity assessment records

• The defect, clearly identified and described.
• The severity level assigned, with the justification for it.
• The vibration data that supports the assessment.
• The recommended actions and their timeline.
• Approval and review signatures. Much of this naturally lives in a structured diagnostic report.

Severity tracking

• Track severity over time as the fault progresses.
• Update the rating with each new measurement.
• Escalate the level as vibration climbs.
• Keep a historical record of the progression — it sharpens future calls and feeds the prognosis of remaining life.

5. Severity in Practice and on the Shop Floor

Severity assessment is the bridge between detecting a fault and acting on it: the framework that turns measurements into priorities and parcels out resources where they count. In the field this judgement is rarely made from a single reading. After a balancing job, for instance, an engineer using a portable analyser such as the Balanset-1A will record the 1× amplitude and phase and the overall vibration level, then compare them against the machine’s baseline to decide whether a residual problem is incipient or serious enough to warrant intervention. That decision draws on every factor above — amplitude, rate of change, fault type and the criticality of the machine — and it is the discipline of weighing them together that lets a condition-based maintenance programme balance equipment uptime, maintenance cost and the risk of failure.

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