How to Diagnose Machinery Vibration Using Bently Nevada Sensors
Industrial machinery communicates its health through vibration. A certain level of vibration is normal, but changes in these patterns can signal developing problems long before they cause a major failure. By monitoring and analyzing these vibrations, maintenance teams can shift from reactive repairs to a proactive strategy, saving both time and money. Bently Nevada has been a leader in this field for decades, providing the tools to measure and interpret equipment vibration.
The Fundamentals of Machine Vibration
To diagnose a machine's health, you should know the three main components of its vibration signature. These elements—amplitude, frequency, and phase—work together to tell a complete story about the machine's condition. A good grasp of these concepts is the first step toward accurate analysis.
Amplitude (How Severe is the Vibration?)
Amplitude measures the intensity of the vibration. It tells you how much a component is moving and is often the first indicator that a problem exists. It can be measured as displacement (how far it moves), velocity (how fast it moves), or acceleration (the rate of change in its speed). A high amplitude signals a significant issue, but it doesn't identify the root cause on its own.
Frequency (What is Causing the Vibration?)
Frequency indicates how often the vibration occurs, typically measured in cycles per minute (CPM) or Hertz (Hz). This is often the most important piece of diagnostic information because different mechanical faults generate vibrations at distinct frequencies. For instance, an imbalance might show up at the machine's rotating speed (referred to as 1X), while a gear issue would appear at a much higher frequency.
Phase (How is the Component Moving?)
Phase measures the timing relationship between two vibration signals or between one signal and a fixed reference point. It is typically referenced to a Keyphasor and provides directional information that often helps distinguish similar 1X faults. For example, both unbalance and misalignment can cause high 1X vibration, but their phase characteristics are very different, allowing an analyst to tell them apart.
By analyzing these three parameters, you can move beyond simply knowing a machine is vibrating and begin to pinpoint the specific mechanical fault responsible. This detailed information is the basis for any successful condition monitoring program.
How to Select the Right Bently Nevada Sensors
Selecting the correct sensor is an important first step because it determines what part of the machine's vibration you are measuring. Bently Nevada offers a range of sensors for specific applications, mainly in two categories: proximity probes that measure shaft motion and seismic sensors that measure casing motion. The right choice depends on the machine's construction and the potential problems you need to detect.
Proximity Probes for Shaft Movement
Proximity probes , which operate on the eddy current principle, are non-contact sensors that measure the distance between the probe tip and a conductive surface, like a rotating shaft. They are the standard for monitoring machines with fluid-film bearings, such as large turbines and compressors. These probes directly observe the shaft's movement relative to the stationary bearing, providing a clear picture of the rotor's dynamic behavior.
Before installation, you should verify that the probe system is functioning correctly. A portable calibrator like the TK3‑2E is used to verify the complete proximity system linearity and gap voltage (probe + extension cable + Proximitor), ensuring the measurements will be accurate. When selecting a probe, specific models like the 330180-X1-05 are chosen based on factors like the required cable length and environmental considerations. For applications with different mounting or hazardous area requirements, a probe such as the 330180-90-05 might be more suitable.
Seismic Sensors for Casing Vibration
Seismic sensors, which include accelerometers and Velomitor velocity sensors, measure the absolute vibration of the machine's casing or structure. They are typically used on machines with rolling-element bearings, like pumps, motors, and fans, where the shaft's motion is tightly coupled to the casing.
Accelerometers : These sensors measure acceleration and are highly sensitive to high-frequency vibrations. This makes them ideal for detecting early-stage bearing defects and gear mesh problems.
Velomitor Sensors : These are piezoelectric sensors that internally convert acceleration to velocity, distinct from moving‑coil velocity pickups. Velocity is often the best overall indicator of vibration severity for general-purpose machinery, making Velomitors a popular choice for broad condition monitoring.
The choice between a proximity probe and a seismic sensor is a basic part of the diagnostic process. One measures the source of the forces (the shaft), while the other measures the machine's response to those forces (the casing).
How the 3500 System Gathers and Protects Data
After the sensors are in place, their signals need to be collected, processed, and monitored. This is the job of the Bently Nevada 3500 Monitoring System, a rack-based platform that acts as the central point for machinery protection and condition monitoring. It is designed to comply with industry standards like API 670, making it a trusted choice for important equipment.
The 3500 rack is a modular system where different components work together to handle the data. A typical setup includes a 3500/15 Power Supply to energize the entire rack and its connected transducers. Sensor signals are fed into specific monitor modules; for instance, a 3500/42M Proximitor/Seismic Monitor is designed to accept and process inputs from both proximity probes and seismic sensors like accelerometers or Velomitors. This module converts the raw electrical signals into meaningful engineering units, such as micrometers or mm/s.
A primary function of the system is machinery protection. The monitor modules continuously compare the measured vibration levels against pre-configured alarm setpoints. If a vibration level exceeds a limit, the system can automatically trigger relays to alert operators or shut down the machine to prevent catastrophic damage. For advanced diagnostics and long-term trending, the 3500/22M Transient Data Interface (TDI) provides a high-speed Ethernet connection to System 1 software. This makes the same high-quality data used for protection available for detailed analysis.
The 3500 system provides a reliable, integrated solution for both protecting important machinery and collecting the detailed data needed for thorough diagnostics. This dual capability means assets are protected in real-time while also allowing engineers to identify and resolve underlying mechanical issues.
How to Identify Common Faults with Vibration Data
With data flowing from the sensors through the 3500 system , the final step is analysis. Different mechanical faults create unique vibration patterns, or "signatures." Recognizing these patterns in the data is important for an accurate diagnosis. The table below outlines the typical signatures for four common machinery faults.
Fault Type
Primary Frequency Signature
Phase Characteristics
Other Key Indicators
Unbalance
A high peak at 1X the running speed (1X RPM) in the radial (horizontal/vertical) direction.
The phase difference between horizontal and vertical readings at the same bearing is approximately 90°.
Vibration amplitude increases significantly with speed. The time waveform appears as a clean, repeating sine wave.
Misalignment
High vibration at both 1X and 2X RPM. High axial (thrust) vibration is also a strong indicator.
A phase shift of approximately 180° is often seen across the coupling in the same measurement direction.
The shaft orbit plot may appear elliptical. The coupling may run hot.
Mechanical Looseness
A series of harmonics (2X, 3X, 4X, etc.) of the running speed. In severe cases, sub-harmonics (0.5X, 1.5X) may appear.
Phase readings are often unstable and difficult to repeat.
The time waveform may show impacting or truncation (flattened peaks).
Rolling-Element Bearing Defects
High-frequency, non-synchronous energy. Specific fault frequencies (BPFO, BPFI, BSF) appear as the defect progresses.
Not typically used for diagnosis.
Early-stage faults are best seen in acceleration measurements. The time waveform may show periodic spikes of energy.
This table serves as a starting point for diagnosis. A skilled analyst will use multiple data plots—such as spectrums, orbits, and waveforms—to confirm a hypothesis and confidently identify the root cause of a problem.
Make Your Vibration Data Work for You
Learning machinery vibration is a skill that helps you make good maintenance decisions. With the correct Bently Nevada sensors and monitoring systems, you can find problems early, figure out what's wrong with them accurately, and make sure your vital equipment keeps working well. This proactive approach keeps your most precious assets running at their best and lasts longer, which saves you money on repairs.
FAQs
Q1: What is the difference between oil whirl and oil whip in fluid-film bearings?
Oil whirl is an instability caused by fluid that makes the shaft move around the bearing at a speed that is slightly less than half of the rotational speed (approximately 40–48% of 1X). Oil whip is a worse condition that happens when the oil whirl frequency matches the rotor's first natural frequency, or critical speed. This resonance can cause vibrations that are dangerously high and machines that break down quickly.
Q2: How does a machine's operating load affect its vibration readings?
The load that the machine is under can have a big effect on vibration. For some problems, like imbalance, the amplitude of the vibration may not change much with load. When there are problems like misalignment or gear failures, adding more weight often makes the forces stronger, which makes the vibration amplitude noticeably larger. This is why it's crucial to gather data under the same, well-documented conditions every time. This way, trend data will be accurate and comparable.
Q3: Why are there three different ways to measure vibration amplitude (displacement, velocity, and acceleration)?
Different frequency ranges and types of faults affect each measurement parameter. Displacement is best for low-frequency events, like when a machine is out of balance and moves slowly. This is because it shows how far the part is moving. Velocity is an excellent overall indicator of severity in the normal range of machine speeds and is closely related to tiredness. Acceleration focuses on high-frequency signals, which makes it the greatest way to find early-stage bearing and gear problems that cause small, sharp impacts.
