What machines does ISO 20816-9 cover?

ISO 20816-9 applies to gear drives (gearboxes) used in industrial applications, including parallel shaft gearboxes, planetary/epicyclic gear trains, and bevel gear units. It covers speed-increasing and speed-reducing gearboxes typically found in power generation, process industries, and general manufacturing.

Why do speed categories matter for gearbox vibration limits?

Higher-speed gearboxes tend to produce lower overall vibration levels at the bearing housing because the forces are distributed over more cycles per second. Lower-speed gearboxes produce higher peak forces per revolution. Therefore, low-speed gearboxes ( 3000 RPM).

Why do planetary gearboxes have higher vibration limits?

Planetary (epicyclic) gearboxes have multiple gear meshes occurring simultaneously — the sun-planet meshes and the planet-ring gear meshes. This inherently produces more complex vibration signatures with higher overall levels. A multiplier of approximately 1.25× is applied to the parallel shaft limits to account for this.

What is gear mesh frequency and why is it important?

Gear mesh frequency (GMF) equals the number of teeth on a gear multiplied by its rotational speed in Hz. For example, a 50-tooth gear at 1500 RPM: GMF = 50 × 25 Hz = 1250 Hz. Sidebands around GMF indicate gear damage, while the GMF amplitude itself relates to gear quality, alignment, and loading. GMF and its harmonics are key diagnostic indicators.

What should I do if gearbox vibration reaches Zone C?

Zone C indicates the gearbox is not suitable for continuous long-term operation. Investigate root causes: gear tooth damage (pitting, scuffing, cracking), bearing degradation, misalignment with coupled machines, lubrication issues, or overloading. Perform vibration spectrum analysis focusing on gear mesh frequency and its sidebands. Plan corrective action and increase monitoring frequency.

Free Engineering Tool #025

Vibration Limits — Gear Drives

Evaluate vibration severity for gearboxes per ISO 20816-9. Supports parallel shaft, planetary/epicyclic, and bevel gear drives across speed categories.

ISO 20816-9 Zones A/B/C/D Gear Drives

Results

Vibration Zone

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Measured Value

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Recommendation

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ℹ️ Gear Mesh Frequency: For gear diagnostics, also analyze the gear mesh frequency (GMF = number of teeth × shaft speed in Hz) and its sidebands. Elevated sidebands around GMF indicate developing gear tooth damage.

Zone	Boundary	Description

ISO 20816-9 Scope

ISO 20816-9 specifies vibration evaluation criteria for gear drives (gearboxes) used in industrial applications. It covers parallel shaft, planetary/epicyclic, and bevel gear arrangements across various speed ranges.

Parallel Shaft — Medium Speed (1000–3000 RPM)

Base zone boundaries for parallel shaft gearboxes at medium operating speeds.

Zone Boundary	Velocity (mm/s RMS)
A/B	3.5
B/C	7.1
C/D	11.2

Speed Category Adjustments

Speed Category	A/B	B/C	C/D
Low (<1000 RPM)	4.5	9.0	14.0
Medium (1000–3000 RPM)	3.5	7.1	11.2
High (>3000 RPM)	2.8	5.6	9.0

Planetary / Epicyclic Gearboxes

Planetary gearboxes have multiple simultaneous gear meshes, producing inherently higher vibration. A factor of 1.25× is applied to the parallel shaft limits.

Gear Mesh Frequency

GMF and its harmonics are the primary diagnostic indicators for gear condition. Sidebands spaced at shaft speed around GMF indicate gear tooth damage patterns.

ℹ️ Note: The broadband velocity measurement used for zone classification does not distinguish between gear mesh vibration and other sources (unbalance, misalignment). Spectral analysis is essential for diagnosis.

⚠️ Important: Gearbox vibration is strongly influenced by load, alignment with coupled machines, lubrication condition, and gear tooth condition. Always compare measurements taken at consistent operating conditions.

Gear Drive Vibration Limits — ISO 20816-9 Free Tool

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