Synchronous and Sub-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, bent shafts, 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.

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.
• Non-Synchronous Peaks: Look for peaks that are not an integer multiple of the running speed. These are often related to components like bearings or external sources.

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. They are generally predictable and stable.
• Sub-synchronous issues are often “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. These conditions can be unstable and highly destructive. Common causes include oil whirl/whip in fluid-film bearings and rotor-stator rubs.

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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