Abstract
Vibration analysis is a crucial tool in condition monitoring of industrial machinery, enabling early detection of mechanical and electrical faults before catastrophic failure occurs. The success of this method depends heavily on precise sensor placement, adherence to international standards, and careful analysis of vibration transmission paths. This paper presents five key principles for optimal sensor placement, dynamic vibration transmission, compliance with ISO 20816-3, and best installation practices, ensuring reliable vibration data for early fault detection.
Introduction
Condition monitoring through vibration analysis is a cornerstone of predictive maintenance (PdM) strategies. Effective implementation requires careful planning, correct sensor installation, and strict adherence to international standards. Improper sensor placement can result in misleading data, significantly reducing the effectiveness of monitoring programs. This paper aims to provide practical guidance for determining the most effective and accurate locations for vibration sensors.
Methodology
1. Sensor Placement
Accurate vibration analysis begins with correct sensor placement. Even high-quality sensors can produce unreliable data if improperly mounted. Optimal sensor locations are near bearings, with short and direct transmission paths. Placing sensors on distant or non-structural parts leads to reduced signal amplitudes and loss of high-frequency information crucial for early fault detection.
Sensor location away from the bearing housing or end shield.
Placing the vibration sensor in an improper location on the motor body can result in the failure to detect critical faults such as inner-race bearing defects, as illustrated in the figure below. Undetected faults of this type can escalate into severe failures, including rotor–stator contact, which may ultimately require stator rewinding. Such events typically lead to extended process downtime and significantly increased maintenance costs.
Opposite drive end (ODE) bearing inner race from the motor
The stator winding from the rotor scraping during testing as the rotor fell from the ODE bearing damage
2. Dynamic Vibration Transmission
The path from the vibration source to the sensor should be short and direct. Real-world machinery exhibits multiple structural resonances affecting vibration spectra. These include resonance modes in the end-cap region, resonances arising from the stator core laminations, resonances resulting from dynamic unbalance, and other vibration sources. The combination of these resonances at each frequency produces a highly complex vibration transmission pattern.
The Single Degree of Freedom (SDOF) model, defined by mass (m), damping (c), stiffness (k), and external force F(t), can describe vibration behavior. Indirect or elongated paths increase damping and the farther the sensor is from the bearing, the more the vibration behavior is influenced by various structural resonances and boundary conditions.
Near the bearing: When the sensor is mounted close to the bearing, the vibration transmission path is short and direct, minimizing attenuation effects caused by structural mass and stiffness. In this configuration, impact forces from defects such as pitting or spalling on the inner and outer bearing races are transmitted directly to the sensor with minimal loss. This installation location provides optimal conditions for early detection of bearing faults.
On the end-cap: In this region, the vibration path is considered indirect, as the vibration signal must propagate through the bearing housing and the mechanical structure of the end-cap. Thinner sections of the end-cap structure can induce local resonances, which nonlinearly amplify certain frequency components while simultaneously attenuating or filtering other critical frequencies, particularly in the high-frequency range that is essential for early bearing fault detection.
Farther from the end-cap, on the motor casing: Longer structural paths with multiple interfaces and connections increase damping and/or alter the structural stiffness. Consequently, overall vibration amplitudes are generally reduced due to damping. However, in some frequency ranges, overlaps with casing resonance frequencies may occur, resulting in unexpected peaks in the vibration spectrum.”
The equation below contains a standard SDOF model:
3. Compliance with ISO 20816-3
ISO 20816-3 recommends placing sensors on machine sections that respond effectively to dynamic forces from bearings. Measurements should include horizontal, vertical, and axial directions, avoiding weak or thin components such as fan covers. Proper adherence ensures accurate and representative vibration data.
4. Best Practices for Sensor Installation
Sensors should be installed close to bearings in regions with direct vibration paths. This setup captures high-frequency impacts and enables early detection of faults. Random or distant sensor placement undermines condition monitoring, reducing its effectiveness to that of reactive maintenance. consequently, vibration measurement points are located on the motor end-caps, but they are installed in positions where the vibration transmission path is direct and perpendicular to both the bearing housing and the shaft axis.
Example of proper end shield placement if you cannot mount directly on the bearing housings
Transducers positioned perpendicularly to the bearing. Alternatively, the bottom part of the split pedestal would have been acceptable
Sensor placement perpendicular to the bearing offset from a potential cavity
5. Field Verification and Industrial Case Studies
Field studies in pharmaceutical and metal production facilities show that continuous monitoring programs often fail to detect bearing faults due to improper sensor placement. Correct installation near bearings, particularly Babbitt-type bearings, ensures direct and perpendicular vibration paths relative to the shaft, maximizing measurement accuracy.
Results and Discussion
– Near bearings: Provides the best conditions for early fault detection.
– On end caps: Indirect path may induce local resonances, enhance some frequencies while filter others.
– On motor casing: Longer paths with additional interfaces cause damping, reducing critical signal amplitudes.
– Adhering to ISO 20816-3 and ensuring direct vibration paths improves detection of bearing and rotating component faults.
Conclusion
Correct sensor placement and standard compliance are critical for successful vibration-based condition monitoring. This paper presents five key principles: 1) sensor placement, 2) dynamic vibration transmission analysis, 3) ISO 20816-3 compliance, 4) optimal installation practices, and 5) field verification. Implementing these principles ensures early fault detection and enhances machinery reliability.
