In simple terms, vibration in rotating and/or reciprocating machinery refers to the back-and-forth movement or oscillation of the machines and components that make up a system1. While vibration in industrial equipment is often a normal part of operation, it can also be a symptom or cause of a problem. If left unchecked, increased vibration can lead to gradual equipment degradation, component damage, or even catastrophic failure. Proactive vibration monitoring is the best method for preventing catastrophic failures, reducing downtime, and lowering maintenance costs. vibration sensors are the most effective means for vibration monitoring.

What is a Vibration Sensor?

A vibration sensor is a device that measures the magnitude of vibration of a given piece of equipment over a period of time.

(Figure 1. Vibration signal plot of a gas compressor over a 0.4-second period.) 

Once processed, vibration signals provide precise information (frequency and amplitude) used to evaluate the condition of critical industrial system components. It is directly connected to the machine or component and, once installed, can detect vibration in various ways depending on the sensor type. Data provided by vibration sensors helps predict future maintenance needs, thereby streamlining and improving workflows while enhancing equipment reliability and efficiency.

Vibration sensors, used alongside speed/phase sensors or other vibration sensors, collect three types of data: frequency, amplitude, and phase. Frequency indicates how often vibration occurs (related to the source location of the machine fault); amplitude reflects the intensity of the vibration the equipment is experiencing (related to the severity of the fault); and phase indicates the position of the vibration peak relative to one rotation of the shaft or rotor, which can be used for fault diagnosis and rotor balancing.

(Figure 2. Spectrogram of the signal shown in Figure 1. Here, you can see the individual frequency components and their amplitudes that make up the signal.) 

(Figure 3. The orbit plot on the left is constructed using signals collected from two sensors installed 90 degrees apart, used to monitor the shaft vibration shown on the right. Note the phase difference between the two sensor signals.) 

Why is Vibration Monitoring Important for Industrial Systems?

Condition monitoring is vital in the management and operation of industrial systems. Proper condition monitoring maximizes equipment performance and minimizes risk, thereby extending asset life, reducing downtime and failure rates, increasing efficiency, and ultimately lowering asset lifecycle costs. For industrial equipment, vibration is one of the key variables requiring proactive condition monitoring.

• Root Cause Analysis: Vibration monitoring and analysis assist in root cause analysis; if equipment is damaged, determining the root cause of vibration can provide insight into the cause of damage and ideas for preventing recurrence.

• Predictive Maintenance: While root cause analysis is a significant advantage, vibration monitoring is particularly important in predictive maintenance—it is a means of preventing failure. Using vibration sensors, vibration data can be monitored in real-time. Early detection of increased equipment vibration helps formulate cost-effective maintenance plans, preventing damage from occurring or developing.

Applications of Vibration Sensors

Any business operating rotating and/or reciprocating equipment can benefit from vibration sensors. These sectors include:

• Oil and Gas

• Renewable Energy

• Thermal Energy

• Manufacturing

Wherever there are moving parts, vibration is generated and can be monitored. Vibration sensors are critical in practical applications such as compressors (centrifugal, axial, reciprocating, and screw) and turbines (gas, steam, hydraulic, and wind). Auxiliary equipment includes:

• Fans: Fans have a wide range of applications. Deposits can accumulate on blades, reducing efficiency and increasing stress on bearings and gearboxes, leading to premature failure.

• Electric Motors: In addition to centrifugal forces from rotating parts, their operation generates electromagnetic forces, all of which can lead to system instability. In industrial settings, vibration sensors are used to monitor motor vibration levels caused by broken rotor bars, damaged rings, bearing failures, etc.

• Pumps: Used to transport liquids between two vertical heights. Like motors, pumps generate centrifugal force through rotational motion. Vibration sensors allow technicians to monitor vibration frequencies caused by pump operation.

• Compressors: Mechanical devices that increase gas pressure by reducing volume. Similar to motors and pumps, they generate centrifugal force through rotational motion. Sensors allow for monitoring vibration frequencies generated by compressor components, bearings, and shafts.

Main Types of Vibration Sensors

There are three main types of vibration sensors, each with its own advantages and disadvantages:

1. Accelerometers: Directly measure absolute vibration acceleration.

2. Velocity Sensors: Directly measure absolute vibration velocity.

3. Displacement Sensors (Eddy Current or Proximity Sensors): Directly measure relative vibration displacement (DC for static displacement / AC for variable displacement).

Accelerometers

Accelerometers are ideal for detecting rolling element bearing and gear faults, or almost any medium-to-high frequency fault, including transient shocks. They are also excellent for detecting imbalance, misalignment, bent shafts, loose or damaged parts, and component resonance. They are the most commonly used sensor in vibration monitoring systems, offering wide frequency ranges, long service lives (>20 years), and specialized versions for high temperatures or small spaces.

Velocity Sensors

These are easy to install and ideal for measuring general vibration in rotating and reciprocating equipment. They are electrodynamic (self-generating signals, so no power supply is needed) and work well for medium-frequency measurements.

• Disadvantages: They are more expensive than most accelerometers and have a limited lifespan as the internal springs (diaphragms) wear out over time. Their frequency range is limited by spring/mass resonance, and they are not suitable for early detection of bearing failure.

Displacement Sensors (Motion Sensors)

These non-contact sensors directly measure displacement and are used to monitor shaft motion and internal clearances. They are the best choice for monitoring machinery equipped with sleeve (fluid-film) and thrust bearings and provide good signal strength at low frequencies.

• Disadvantages: They require permanent, high-cost installation. They are sensitive to the material composition and condition of the surface and require a fixed cable length from the sensor to the oscillator/demodulator.

Which Vibration Sensor Should I Use?

Selection must consider: vibration intensity, frequency range, temperature range, and bearing type (rolling or sleeve). External environments and electromagnetic fields must also be considered as they can affect readings. For small applications, the size and weight of the sensor are also factors.

How to Establish a Vibration Monitoring System

Successful installation requires understanding sensor requirements and limitations. Sensors should be installed as close as possible to the vibration source on smooth, unpainted, level, and oil-free surfaces.

1. Installing the Sensor: Proper installation ensures accuracy. Generally, the closer the sensor is to the shaft support (bearing), the better it measures vibration (especially high-frequency signals). Accelerometers and velocity sensors can be mounted via studs, adhesives, magnets, or probe tips.

2. Connecting the Sensor: Effective cable management is crucial. Factors include cable length, capacitance, routing, and grounding. Since cable capacitance increases with length, affecting the output signal, displacement sensors use fixed-length cables. Instrumentation cables should be laid in dedicated trays and never parallel to power lines to avoid electromagnetic interference.

3. Building the System: The setup depends on the sensors and their connections, ensuring data repeatability and connection reliability. The system features two interfaces: one connecting the sensor to the data acquisition unit, and another connecting the unit to the network for users and other systems.