Understanding Running Speed (1X)

1. Definition: What is Running Speed?

Running Speed is the fundamental frequency in vibration analysis that corresponds to the rotational speed of a machine’s shaft. It is the frequency at which the shaft completes one full revolution.

In vibration terminology, this frequency is almost always referred to as 1X. For example, if a fan is running at 1800 Revolutions Per Minute (RPM), its 1X running speed frequency is 1800 CPM (Cycles Per Minute), which is equivalent to 30 Hz (1800 / 60).

The 1X frequency serves as a primary reference point in nearly all diagnostic efforts. Most other vibration frequencies of interest are often related to it as multiples (harmonics) or fractions (sub-harmonics).

2. Why is 1X So Important?

The 1X frequency is of paramount importance because many of the most common and significant machine faults generate vibration at this specific frequency. A high level of vibration at 1X is often a clear indicator of a problem.

Common faults that manifest at 1X include:

• Unbalance: This is the most common cause of high 1X vibration. An uneven distribution of mass creates a centrifugal force that rotates at the same speed as the shaft, producing a sinusoidal vibration at 1X.
• Misalignment: While often showing a strong 2X component, angular and parallel misalignment can also produce significant vibration at 1X.
• Bent Shaft: A bent shaft behaves mechanically like a form of unbalance, creating a high 1X vibration peak.
• Eccentricity: An eccentric component, such as a pulley or gear, will create a 1X vibration peak due to the rotating high spot.
• Resonance: If a machine’s natural frequency is close to its running speed, even a small amount of forcing energy (like minor unbalance) will be greatly amplified, causing extremely high vibration at 1X.

3. Harmonics and Sub-Harmonics of Running Speed

Once the 1X frequency is identified, the rest of the spectrum can be interpreted in relation to it:

• Harmonics (2X, 3X, 4X, etc.): These are integer multiples of the running speed. They are often indicative of issues like misalignment (strong 2X), mechanical looseness (multiple harmonics), and other non-linear effects.
• Sub-Harmonics (0.5X, 1/3X, etc.): These are frequencies that are fractions of the running speed. They are often associated with oil film instability in journal bearings (oil whirl) or mechanical looseness in a bearing housing.

This type of analysis, where frequencies are described as multiples of a fundamental speed, is known as Order Analysis. For variable-speed machines, tracking vibration by “orders” is essential.

4. How is Running Speed Measured?

The running speed is typically determined in one of two ways:

1. From the Vibration Spectrum: In most cases, there will be a clear peak in the vibration spectrum that corresponds to the shaft’s rotation. This is usually the first significant peak an analyst looks for.
2. Using a Tachometer: A tachometer provides a direct and unambiguous measurement of the shaft speed. It generates a pulse for each revolution, which can be fed into the vibration analyzer. This not only confirms the 1X frequency but also enables advanced diagnostic techniques like phase analysis and order analysis.

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