Sub-Synchronous and Synchronous Vibration Explained

Definition: What is Synchronous Vibration?

Synchronous vibration is any vibration that occurs at a frequency that is an integer multiple of the machine’s primary rotational speed. It is “in sync” with the rotation of the shaft. This is the most common category of vibration found in machinery.

• Vibration at exactly the running speed (1x) is synchronous.
• Vibration at twice the running speed (2x), three times (3x), and so on, are also synchronous vibrations, often referred to as “harmonics” of the running speed.

The vast majority of common machinery faults, such as unbalance, misalignment, and mechanical looseness, all manifest as synchronous vibration. For example, the vibration caused by unbalance will always be at 1x RPM, perfectly tracking any changes in the machine’s speed.

Definition: What is Sub-Synchronous Vibration?

Sub-synchronous vibration is any vibration that occurs at a frequency that is *less than* the primary rotational speed (1x). The prefix “sub-” means “below.” The presence of significant sub-synchronous vibration is often a serious warning sign, as it is typically caused by self-excited, unstable rotor dynamic phenomena rather than simple mechanical faults. Unlike synchronous vibration, the forcing function for sub-synchronous vibration is generated by the motion of the rotor itself.

Common Causes of Sub-Synchronous Vibration

Sub-synchronous vibrations are a major concern in high-speed turbomachinery equipped with fluid-film bearings.

1. Oil Whirl

This is the most common form of sub-synchronous instability. It occurs in fluid-film bearings when the hydrodynamic oil film that supports the shaft begins to “whip” around the bearing, pushing the shaft ahead of it. This creates a distinct vibration at a frequency of approximately 0.42 to 0.48 times the running speed (0.42x – 0.48x). Oil whirl is often load- and temperature-dependent and may appear or disappear as machine conditions change.

2. Oil Whip

Oil whip is a more severe and dangerous form of oil whirl. It occurs when the oil whirl frequency coincides with and “locks onto” the rotor’s first natural frequency (critical speed). When this happens, the sub-synchronous vibration amplitude can grow very large and will not go away with an increase in speed. In fact, the vibration will remain at the rotor’s critical speed frequency even as the machine accelerates further. Oil whip is a highly destructive condition that requires an immediate shutdown of the machine.

3. Rotor-to-Stator Rub

A rub between the rotor and a stationary part can sometimes induce sub-synchronous vibration. This often occurs at integer fractions of the running speed, such as 0.5x. The 0.5x component is a classic sign of a rub that is “bouncing” the rotor once every two revolutions.

How to Differentiate Them in an FFT Spectrum

Identifying these components in an FFT spectrum is straightforward:

• Synchronous Peaks: Look for the 1x RPM peak (the running speed) and any peaks that fall on exact integer multiples (2x, 3x, etc.).
• Sub-Synchronous Peaks: Look for any significant peaks that appear on the frequency axis *before* the 1x RPM peak. A peak at around 45% of the running speed is a classic sign of oil whirl.

Why the Distinction is Critical

Distinguishing between synchronous and sub-synchronous vibration is fundamentally important for diagnostics:

• Synchronous issues (like unbalance) are “forced vibrations.” They can often be corrected with mechanical adjustments like balancing or alignment.
• Sub-synchronous issues (like oil whip) are “self-excited vibrations” or instabilities. They indicate a problem with the fundamental design or condition of the rotor-bearing system and cannot be fixed by balancing. The solution might involve changing the bearing design, altering the oil temperature or pressure, or redesigning the rotor.

For this reason, a high-amplitude sub-synchronous peak is generally considered a more serious alarm condition than a high-amplitude synchronous peak.

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