Understanding the Keyphasor

Definition: What is a Keyphasor?

A Keyphasor is the Bently Nevada trade name for a sensor that provides a once-per-revolution timing reference signal from a rotating shaft. While “Keyphasor” is a trademark, the term is often used generically in the industry to refer to any similar phase reference transducer or tachometer. The sensor is typically an eddy-current proximity probe that observes a unique event on the shaft, such as a keyway, a notch, or a flat spot.

When this unique feature passes the tip of the probe, the probe’s output voltage generates a sharp, repeating pulse. This pulse serves as a precise digital timing mark that indicates the exact rotational position of the shaft at that instant.

Why is a Keyphasor Signal Essential?

The Keyphasor signal is arguably the most important signal in advanced vibration analysis after the vibration signal itself. It unlocks several critical diagnostic capabilities that are impossible with vibration data alone:

1. Phase Measurement

The primary function of the Keyphasor is to enable the measurement of phase. Phase is the timing relationship between the Keyphasor’s pulse and the peak of the 1X (running speed) vibration signal. This phase angle provides invaluable information about *where* the “heavy spot” (unbalance) is located on the rotor. It is absolutely essential for:

• Balancing: All balancing procedures, both on a machine and in the field, rely on phase measurements to determine where to add or remove correction weights.
• Fault Diagnostics: Certain faults, like misalignment or a bent shaft, have characteristic phase signatures that are used to confirm the diagnosis.

2. Accurate Speed Measurement

The Keyphasor provides a highly accurate measurement of the machine’s rotational speed (RPM), which is fundamental for relating vibration frequencies to shaft movements.

3. Order Analysis

By using the Keyphasor signal as a timing reference, a vibration analyzer can perform order analysis. Instead of displaying a spectrum with a frequency (Hz) axis, it displays an order-based spectrum. An “order” is a multiple of the running speed (1X, 2X, 3X, etc.). This is extremely valuable for analyzing machines with variable speeds, as the order peaks (e.g., the 1X unbalance peak) will remain at a fixed position on the plot even as the machine’s RPM changes, making trends and patterns much easier to identify.

4. Advanced Plot Generation

The Keyphasor signal is a prerequisite for generating some of the most powerful diagnostic plots in vibration analysis, including:

• Bode Plots: Used to identify critical speeds during startup and shutdown.
• Polar Plots: Another way to view startup/shutdown data, showing the change in the vibration vector’s magnitude and phase.
• Orbit Plots: Created from a pair of X-Y proximity probes, the Keyphasor pulse is used to create a blanking spot on the orbit, revealing the direction of shaft precession and helping to diagnose issues like oil whirl or shaft cracks.

Installation and Setup

Proper installation is critical. The probe must be mounted rigidly and aimed at a single, clean, and distinct event on the shaft. The gap between the probe tip and the shaft must be set correctly, and the probe’s voltage output must be configured to produce a clean, sharp pulse that can be reliably triggered upon by the data acquisition system.

← Back to Main Index