What causes belt vibration?

Belt vibration is typically caused by belt wear, misalignment, improper tension, belt defects (cracks, missing chunks), pulley eccentricity, or resonance. The belt frequency is the fundamental frequency at which defects on the belt surface will excite vibration as they pass over the pulleys.

What is the difference between belt frequency and shaft frequency?

Shaft frequency (1× RPM) is the rotational speed of the shaft. Belt frequency is the rate at which a single point on the belt completes one full loop. Belt frequency is always lower than shaft frequency because the belt circumference is longer than the pulley circumference. Belt defects appear at belt frequency and its harmonics.

How do I identify belt wear from vibration data?

Belt wear typically shows peaks at 1×, 2×, and 3× belt frequency in the vibration spectrum. A single defect (crack, hard spot) shows at 1× belt frequency. Multiple defects or overall wear shows higher harmonics. The peaks are usually visible in the radial direction perpendicular to the belt span.

What is the difference between timing belt and V-belt vibration patterns?

V-belts produce smooth belt frequency peaks and may show whip/flutter between driver and driven frequencies. Timing (synchronous) belts additionally produce tooth meshing frequency (number of teeth × belt frequency), which appears as a distinct high-frequency peak. Worn timing belt teeth produce sidebands around the meshing frequency.

What is belt resonance and how to avoid it?

Belt resonance occurs when the natural frequency of the belt span matches a forcing frequency (typically shaft speed or belt frequency). This causes excessive belt flutter and vibration. To avoid resonance, adjust belt tension (which changes the natural frequency), change span length, or modify operating speed. The natural frequency of a belt span depends on tension, mass per unit length, and span length.

Free Engineering Tool

Belt Drive Defect Frequency Calculator

Calculate belt frequency, driver/driven shaft frequencies, speed ratio, and belt harmonics for V-belt and timing belt drive systems.

V-Belt Timing Belt Harmonics 1×–3×

Results

Belt Frequency (1 revolution of belt)

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Driver Shaft Frequency

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Driven Shaft Frequency

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Speed Ratio (D₁/D₂)

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Driven Speed

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Belt Harmonics

Harmonic	Frequency (Hz)	CPM	Note

Whip / Flutter range: —

Belt Frequency

The belt frequency represents how often a single point on the belt completes one full revolution (loop). It is always sub-synchronous (lower than shaft speed):

D₁ — driver pulley diameter (mm)
n₁ — driver shaft speed (RPM)
L_b — total belt length (mm)

Shaft Frequencies

Belt Defect Indicators

A defect on the belt (crack, hard spot, missing chunk) will produce vibration at belt frequency and its harmonics:

1× belt freq — single defect, belt joint, belt splice
2× belt freq — two defects 180° apart, or belt resonance
3× belt freq — multiple defects, severe wear

Whip and Flutter

Belt whip or flutter occurs at frequencies between the driver and driven shaft frequencies. It is often caused by improper tension, excessive belt wear, or misaligned pulleys. The whip frequency range is bounded by the driven frequency (lower) and driver frequency (upper).

Example — SPA Belt Drive

Given: D₁ = 150 mm, D₂ = 300 mm, L_b = 2000 mm, n₁ = 1450 RPM

Belt freq = π × 150 × 1450 / (60 × 2000) = 5.69 Hz

Driver freq = 1450 / 60 = 24.17 Hz

Driven freq = 1450 × 150 / (300 × 60) = 12.08 Hz

Speed ratio = 150 / 300 = 0.50

⚠️ Note: Belt frequency is always lower than both shaft frequencies. If your spectrum shows peaks below shaft speed at non-integer fractions, suspect belt-related issues.

Belt Drive Defect Frequency Calculator | V-Belt & Timing Belt

Published by admin on February 15, 2026 February 15, 2026