What is shaft relative vibration?

Shaft relative vibration is the dynamic displacement of a rotating shaft measured relative to its bearing housing, typically using non-contact proximity (eddy current) probes. It is expressed in micrometers peak-to-peak (μm p-p) and captures shaft motion that bearing-mounted sensors may miss.

What is the difference between proximity probes and accelerometers?

Proximity probes (eddy current sensors) measure shaft displacement relative to the bearing housing without contact. Accelerometers measure absolute bearing housing vibration. Proximity probes are essential for fluid-film bearing machines where shaft motion may not transmit fully to the housing.

What is the difference between ISO 7919 and ISO 20816?

ISO 7919 covers shaft relative vibration measured by proximity probes. ISO 20816 (which replaced ISO 10816) covers bearing housing vibration measured by accelerometers or velocity sensors. Both use zone classification A/B/C/D but measure different quantities.

What is a shaft orbit?

A shaft orbit is the two-dimensional path traced by the shaft centerline during rotation, constructed from two orthogonal proximity probes (typically X-Y). Orbit shape helps diagnose faults: circular orbits suggest unbalance, figure-8 patterns suggest misalignment, and banana shapes suggest rub.

What is acceptable shaft displacement?

Acceptable displacement depends on machine type and speed per ISO 7919. For general machinery at 3000 RPM, Zone A (newly commissioned) is typically ≤50 μm p-p, Zone B (acceptable for long-term operation) is ≤100 μm p-p. Higher speeds generally require tighter limits.

Free Engineering Tool #027

Shaft Relative Vibration Limits

Evaluate shaft relative vibration measured by proximity probes per ISO 7919 series. Determine vibration zone (A/B/C/D) for turbines, generators, and rotating machinery.

ISO 7919 Proximity Probes Zone A–D

Results

Vibration Zone (ISO 7919)

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Measured Displacement

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Zone A/B Boundary

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Zone B/C Boundary

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Zone C/D Boundary

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Shaft Orbit Note: This calculator evaluates single-axis (S_max) peak-to-peak displacement. For full shaft orbit analysis, two orthogonal proximity probes (X-Y) are required. The orbit shape provides additional diagnostic information — circular orbits suggest unbalance, figure-8 patterns suggest misalignment.

ISO 7919 — Shaft Relative Vibration

ISO 7919 defines evaluation criteria for shaft relative vibration measured by non-contact proximity probes (eddy current sensors). The vibration is expressed as peak-to-peak shaft displacement in micrometers (μm p-p).

Speed-Dependent Zone Boundaries

Zone boundaries depend on shaft rotational speed. The general relationship follows:

Where n is the shaft speed in RPM and A is a coefficient that differs for each zone boundary and machine type.

Zone Classification

Zone	Description	Action
A	Newly commissioned machines	Normal — no action required
B	Acceptable for long-term operation	Monitor — normal operation
C	Not acceptable for long-term	Plan maintenance — investigate cause
D	Damage may occur	Immediate action — shutdown if necessary

Simplified Zone Boundaries by Machine Type

Machine Type	A/B (μm p-p)	B/C (μm p-p)	C/D (μm p-p)	Typical Speed
General (≥10 MW)	50	100	170	1000–3000 RPM
Steam turbine/generator	50	100	170	1500–3600 RPM
Gas turbine	50	100	170	3000–15000 RPM
Hydro turbine/generator	80	160	260	75–750 RPM
Reciprocating	80	160	260	200–1000 RPM

Practical Example

Example — Steam Turbine at 3000 RPM

Given: Steam turbine, speed = 3000 RPM, measured shaft displacement = 75 μm p-p

Zone boundaries: A/B = 50 μm, B/C = 100 μm, C/D = 170 μm

75 μm > 50 μm (A/B) and ≤ 100 μm (B/C)

Result: Zone B — Acceptable for long-term operation

⚠️ Note: This calculator uses simplified zone boundaries from ISO 7919-1 (general guidelines). The actual ISO 7919 series (parts 2–5) provides speed-dependent curves for specific machine types. Always consult the relevant part of ISO 7919 for contractual or warranty evaluations.

Shaft Relative Vibration Limits per ISO 7919 | Vibromera

Published by Nikolai Shelkovenko on February 15, 2026 March 5, 2026