8.3 ISO 18434 and Machinery Severity Criteria

Electrical NETA-style tables do not fully cover rotating machinery. Motors, pumps, fans, gearboxes, and compressors need a condition-monitoring mindset: baselines, like-for-like comparison, and severity zones that respect design differences. ISO 18434-1 (condition monitoring and diagnostics of machines—thermography) is the international framework Level II candidates should associate with machine thermography severity philosophy.

What ISO 18434-1 Is (and Is Not)

Exam posture: Treat ISO-style machinery criteria as structured guidance. Memorize the comparative logic and commonly taught severity bands; do not pretend a classroom table overrides the motor OEM’s maximum bearing temperature.

Reference Conditions: The Heart of Machinery IR

Severity for machines is usually relative to a reference, not an arbitrary color.

ISO-oriented practice prefers ΔT between corresponding components on machines operating under similar conditions. Absolute temperature alone can mislead when ambient, process fluid temperature, or enclosure heat differ.

What the Standards Actually Provide

ISO 18434-1:2008, Condition monitoring and diagnostics of machines — Thermography — Part 1: General procedures, covers how to plan and perform machine thermography and gives general guidelines for establishing thermal severity assessment criteria. ISO 18434-2:2019, Part 2: Image interpretation and diagnostics, covers how to read the resulting thermograms.

Neither part publishes a universal numeric severity table. There is no "ISO advisory band", no "ISO 18434 bearing zone", and no ISO-sanctioned list of housing temperatures. Certification syllabi that reference ISO 18434-1 — including the BINDT CM/GEN Appendix B and AINDT condition-monitoring outlines — list it under "general guidelines for establishing thermal severity assessment criteria", which is exactly what it is: a method for building criteria, not a set of numbers to memorize.

Exam posture: if a stem offers "the ISO 18434-1 severity zones" with specific degree values, that framing is wrong. The defensible answer is that ISO provides the framework and that numeric triggers come from the OEM, the equipment rating, or the plant's own written criteria.

Where Real Machinery Numbers Come From

Level II builds a severity criterion from anchors that are published:

Two cautions on applying these with a camera:

1. NEMA rises are measured by resistance, not by an infrared surface reading of the frame. A motor frame surface will read cooler than the winding. Use MG-1 to reason about margin, not to declare a winding exceedance from a frame thermogram.
2. Surface IR is not an embedded sensor. Where an RTD or thermocouple is installed, cross-check against it rather than asserting the internal temperature from outside.

Building a Defensible Criterion

Because no standard hands you the number, the Level II method is:

1. Establish the reference — sister machine, opposite bearing, or historical baseline at matched conditions
2. Record the OEM limit and the insulation or lubricant rating for that asset
3. Set action thresholds in the written procedure, justified by those ratings and by the plant's failure history
4. Document which criterion was applied in every report, exactly as with electrical work
5. Reassess triggers as trended data accumulates — ISO 18434-1's own posture is that criteria evolve with data

A plant that writes "investigate at 10 °C above the sister bearing, act at 20 °C or at 90 percent of the OEM limit, whichever comes first" has a legitimate criterion. The same numbers presented to a client as "the ISO standard" are a misattribution.

Valid Machinery Survey Conditions

Soft foot, misalignment, lubrication starvation, overload, and belt tension can all raise temperatures. IR classifies severity; root cause may need vibration, oil analysis, or alignment checks—Level II recommends the right next diagnostic, not IR-only dogma.

Worked Examples

Example 1 — Sister pump bearings. Pump A outboard bearing housing 12 °C hotter than Pump B outboard under same flow and speed. That sits at/above a common advisory elevation (~10 °C class). Document, verify lubrication and load, re-baseline soon; do not ignore simply because absolute is 55 °C.

Example 2 — Serious comparative rise. Gearbox input bearing 28 °C hotter than historical baseline at matched load. Treat as serious comparative severity: investigate promptly; consider operational risk even if still below a trip set point.

Example 3 — Absolute OEM. OEM max bearing temperature 90 °C (embedded sensor). IR surface reads 86 °C with good setup. Surface may under-read internal; report proximity to limit and recommend confirming with installed RTD if available—do not claim large margin.

Example 4 — False serious. Motor NDE bearing 15 °C hotter than DE because of a shaft-mounted fan design asymmetry documented by OEM as normal. Comparative “fault” without design knowledge is a trap—use OEM and baselines.

ISO Machinery vs NETA Electrical (Do Not Cross-Wire)

Applying electrical Priority 1–4 numbers to a pillow block without stating a machinery procedure is sloppy reporting. Applying only machinery advisory language to a 40 °C bus lug ΔT is equally wrong.

Reporting Machine Findings

Include: machine ID, operating load/speed/process state, reference used (sister/baseline/OEM), T and ΔT, severity category in your program’s language, recommended next steps (lube, vibration, alignment, outage), and parameter set. Pair IR images with visual photos of grease fittings, coupling guards, and nameplates when possible.

Common Traps

Summary for Recall

ISO 18434-1 frames thermography for machine condition monitoring around valid references, baselines, and severity assessment. Teach and examine with comparative zones: about ~10 °C above reference as a common program-defined advisory level elevation, with larger ΔT moving through intermediate to serious response themes. Absolute housing bands are secondary guidance; OEM limits and trends rule. Do not confuse machinery zones with NETA electrical priorities.