Understanding Subharmonics

1. Definition: What is a Subharmonic?

A Subharmonic is a frequency component in a vibration spectrum that occurs at an integer fraction of a fundamental forcing frequency. In machinery analysis, that fundamental frequency is almost always the running speed (1X).

Subharmonics appear at frequencies like 1/2X, 1/3X, 1/4X, etc., and are also referred to as sub-synchronous vibrations because they occur at a frequency below the main synchronous (1X) speed. The presence of a series of subharmonics (e.g., 1/2X, 3/2X, 5/2X) is a classic sign of specific types of machine faults.

2. Primary Causes of Subharmonics

Subharmonics are not as common as harmonics (2X, 3X), but when they appear, they are often a strong indicator of one of the following issues:

a) Mechanical Looseness

This is the most common cause of subharmonics, particularly the 1/2X component. When a component is loose, such as a bearing in its housing or a loose mounting bolt, it can introduce a non-linear “bouncing” or “rattling” effect. This type of impact can excite the system in a way that it responds at a frequency that is half of the main forcing frequency (1X).

A spectrum showing a 1X peak accompanied by a series of subharmonics at 1/2X, 3/2X (1.5X), 5/2X (2.5X), etc., is a textbook sign of severe structural looseness. The looseness allows for impacts that occur effectively every other revolution of the shaft, generating the 1/2X family of peaks.

b) Journal Bearing Instability

In machines with fluid-film or journal bearings, subharmonics are a critical indicator of oil film instabilities. These are self-excited vibrations that arise from the dynamics of the oil wedge supporting the shaft.

• Oil Whirl: This phenomenon typically occurs at a frequency between 0.42X and 0.48X of the running speed. It is caused by the oil film itself beginning to rotate (whirl) within the bearing clearance, pushing the shaft around. It is often a strong, distinct peak below 0.5X.
• Oil Whip: This is a more severe form of instability that occurs when the oil whirl frequency coincides with the rotor’s first natural frequency (critical speed). When this happens, the vibration can become very violent and destructive. The frequency becomes “locked” onto the natural frequency and will not change even if the machine’s speed is increased.

c) Other Causes

• Belt Drive Problems: A worn or damaged belt can sometimes generate subharmonic frequencies related to the belt’s rotational speed, which is typically lower than the driver or driven pulley speed.
• Blade or Vane Pass Frequencies: In some cases with pumps or fans, sub-synchronous peaks can be related to flow turbulence or rotating stall conditions, which may not be exact fractions of 1X but are still below the running speed.

3. Analysis and Confirmation

When subharmonics are observed in a spectrum, it is crucial to identify the exact frequency. A peak at exactly 0.50X is very likely related to looseness. A peak at 0.47X is almost certainly oil whirl. The presence of a full series (1/2X, 3/2X, etc.) strongly confirms looseness.

Analyzing the time waveform can also be helpful. Looseness often shows evidence of impacting or truncation, while oil whirl may appear as a more complex, modulating signal.

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