Sub-Synchronous and Synchronous Vibration Explained

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Sub-synchronous vibration is any vibration component whose frequency is less than the machine’s primary running speed (1×), and its appearance is one of the more serious signals a rotating machine can send. To understand why, it helps to set it against its opposite: synchronous vibration, which tracks the shaft at exact integer multiples of running speed. The distinction is not academic — it separates the everyday faults you can correct mechanically from the self-excited instabilities that demand a redesign or an immediate stop. This article defines both terms, lists the usual culprits, and shows how to tell them apart in an FFT spectrum.

1. What is Synchronous Vibration?

Synchronous vibration occurs at a frequency that is an integer multiple of the shaft’s rotational speed — it is “in sync” with the rotation. It is by far the most common category of machinery vibration.

• Vibration at exactly the running speed (1×) is synchronous.
• Vibration at twice running speed (2×), three times (3×), and so on are also synchronous, and are usually called harmonics of running speed.

The vast majority of common faults manifest this way. Unbalance, misalignment, and mechanical looseness all produce synchronous vibration. Unbalance, for instance, always shows at 1× RPM and perfectly tracks any change in speed — double the RPM and the unbalance peak simply moves to the new 1× frequency. Because the forcing is locked to shaft angle, these are classic forced vibrations.

2. What is Sub-Synchronous Vibration?

Sub-synchronous vibration occurs at a frequency below 1× — the prefix “sub-” simply means “below.” Significant sub-synchronous content is often a serious warning sign, because it is typically produced by self-excited, unstable rotor-dynamic phenomena rather than by a simple mechanical defect. The crucial difference is the source of the energy: in synchronous faults an external geometric error drives the rotor once per turn, whereas in sub-synchronous instability the forcing function is generated by the motion of the rotor itself interacting with its bearings or seals. That feedback loop is what makes these conditions a hallmark of rotor instability.

3. Common Causes of Sub-Synchronous Vibration

Sub-synchronous vibration is a major concern in high-speed turbomachinery running in fluid-film journal bearings.

3.1 Oil Whirl

This is the most common form of sub-synchronous instability. In a fluid-film bearing the hydrodynamic oil film that supports the shaft can begin to circulate and push the shaft ahead of it, a phenomenon known as oil whirl. Because the average velocity of the oil film is a little under half the shaft surface speed, the resulting whirl appears at roughly 0.42 to 0.48 times running speed (0.42×–0.48×). Oil whirl is often load- and temperature-dependent, and may appear or disappear as bearing load, oil temperature, or speed change.

3.2 Oil Whip

Oil whip is a more severe and dangerous evolution of oil whirl. It occurs when the whirl frequency rises to meet — and then “locks onto” — the rotor’s first natural frequency, or critical speed. Once locked, the sub-synchronous amplitude can grow very large and will not disappear as speed increases; instead the vibration stays pinned at the critical-speed frequency even as the machine accelerates further. This locked, escalating condition — closely related to shaft whip — is highly destructive and generally requires an immediate shutdown.

3.3 Rotor-to-Stator Rub

Contact between the rotor and a stationary part — a rotor rub — can also induce sub-synchronous vibration, often at integer fractions of running speed such as 0.5×. A clean 0.5× component is a classic sign of a rub that bounces the rotor once every two revolutions. Other sources of sub-harmonic response include severe looseness and certain rub-driven nonlinearities.

4. Telling Them Apart in an FFT Spectrum

Separating the two families on a spectrum is largely a matter of where the peaks fall relative to 1×:

• Synchronous peaks: locate the 1× RPM peak (running speed) and look for peaks on exact integer multiples — 2×, 3×, and so on.
• Sub-synchronous peaks: look for any significant peak that appears before the 1× peak on the frequency axis. A peak near 45% of running speed is a textbook indicator of oil whirl.

Because the diagnosis hinges on the exact ratio of the peak to running speed, a precise speed reference is essential — small errors in assumed RPM can blur a 0.48× whirl into something ambiguous. Order analysis referenced to a once-per-revolution tachometer pulse removes that ambiguity by expressing the spectrum directly in orders of running speed.

5. Why the Distinction is Critical

Knowing which family you are looking at determines the entire response:

• Synchronous issues (such as unbalance) are forced vibrations and can usually be corrected mechanically — by balancing, alignment, or tightening fasteners.
• Sub-synchronous issues (such as oil whip) are self-excited vibrations or instabilities. They point to a fundamental problem in the rotor-bearing system and cannot be cured by balancing. Fixes typically involve changing the bearing design (for example to tilting-pad bearings), adjusting oil temperature or pressure, increasing bearing load, or modifying the rotor.

For this reason a high-amplitude sub-synchronous peak is generally treated as a more serious alarm than an equally large synchronous peak. In practice an engineer first confirms the machine is well balanced and aligned — a portable analyser such as the Balanset-1A measures the 1× amplitude and phase needed to rule out, or correct, the synchronous causes — so that any sub-synchronous component remaining on the spectrum can be confidently attributed to an instability rather than to a curable mechanical fault.

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