Vibration Analysis Equipment: Sensors and Hardware (Series) - HECO

This blog post continues an 8-part series on vibration analysis written by Dr. Sara McCaslin & Nolan Crowley, Business Development Manager at HECO.

Dr. Sara McCaslin: Sara has a Ph.D. in mechanical engineering from the University of Texas at Arlington. Sara has also taught materials science, manufacturing, and mechanical system design at the University of Texas at Tyler.

Nolan Crowley: Nolan is a Business Development Specialist for HECO. Nolan has BS from Miami University along with extensive field experience with powertrains, electric motors, & vibration issues since 2007.

• Week 1: Vibration Analysis Training: Who’s Doing Your Analysis?
• Week 2: Vibration Analysis Equipment: Sensors and Hardware
• Week 3: Balancing Rotating Equipment: Static vs Dynamic
• Week 4: The Importance of Route-Based Data Acquisition
• Week 5: The Basics of Modal Analysis for Electric Motors and Powertrains 
• Week 6: How to Setup Remote Monitoring Vibration Monitoring
• Week 7: The Place of Motion Amplification in Modern Vibration Analysis
• Week 8: Bidding/Specifying Your Vibration Analysis Program

Subscribe to HECO’s blog, Here You Go to learn from this valuable 8-part series on vibration analysis.

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You’ve heard that vibration analysis can potentially increase uptime and reduce M&O costs. But what does it really take to set up a vibration analysis system? What kind of equipment do you need, and how is it organized? Read on for a quick introduction to vibration analysis equipment!

Vibration Analysis Components
Here’s one way to break down the components found in a basic system for vibration monitoring:

• Vibration sensors to obtain the data
• Data transmission and storage
• Vibration monitoring systems and visualization tools

Let’s start with vibration sensors.

Vibration Sensors
The sensors (transducers) used for measuring vibration fall into two categories: accelerometers and proximity probes. Accelerometers, as the name implies, use a piezoelectric or sensor to measure acceleration. They are directly mounted on the surface of the object whose vibration you want to measure. These are the most commonly used vibration sensors and work well for both low- and high-frequency vibration.

There are also non-contact proximity probes that measure the proximity of the sensor to a target. The change in proximity is used to approximate acceleration, which then provides information on acceleration. These sensors, however, are not recommended for use in vibration analysis for rotating equipment. When they are used, it is typically for evaluating the amount of vibration in a rotating shaft.

How Accelerometers Work
Let’s talk about how accelerometers work. And there are two important components within them: a seismic mass and a piezoelectric crystal. Next, there are two primary equations on which these sensors are based. Here they are in their most simple form:

Force = Mass x Acceleration

Stress = Force / Area

When the object on which the sensor is mounted moves due to vibration, the seismic mass inside the sensor also moves along with it. This movement is due to acceleration which becomes a force (Force = Mass x Acceleration) transmitted from the seismic mass to the piezoelectric crystal. Within the crystal, that force becomes stress (Crystal Stress = Force / Area over which the force acts). As the piezoelectric crystal is stressed, it generates a voltage that is proportional to the acceleration.

Vibration Measurement Locations
Here are some tips for specifying measurement locations, starting with a disclaimer of sorts: If a transducer can’t be placed in a standardized position, it should be mounted as close as possible. This usually happens when there is a safety concern or obstruction.

If you are obtaining measurements related to a bearing, vertical and horizontal measurements are going to be needed to obtain the right kind of data. For each machine component shaft, there should be at minimum one measurement obtained along the axial direction.

Choosing the Right Accelerometer
Omega has a great list of what you need to consider when selecting an accelerometer, and here are the key ones:

• What is the vibration amplitude and frequency range that needs to be monitored?
• What is the environmental temperature where it will be used?
• How much space do you have for it to be installed?
• Is electrical noise and/or electrical interference likely to be a problem?
• Is it going to be used in a wet or wash down area?
• Could it be exposed to corrosive chemicals?
• Does the operating environment require explosion-proof instruments?

National Instruments goes into a bit more detail on some of the technical specifications:

• Vibration Amplitude: If the amplitude of the vibration you’re trying to measure fall outside the range of the accelerometer, you’re not going to get good data
• Sensitivity: This represents the conversion between vibration and the electrical signal (voltage) generated by the accelerometer at a reference frequency; for low amplitude signals you need a low sensitivity accelerometer and vice versa
• Number of Axes: There are three planes of measurement that you can obtain vibration data from (vertical, horizontal, and axial) but not all accelerometers are designed for more than one plane of measurement
• Weight: The accelerometer should weigh no more than 10% of the structure you are obtaining measurements from

Finally, handheld pickups (proximity sensors) should not be used for vibration monitoring of powertrain equipment.

Mounting Accelerometers
We’ve talked about where transducers should be placed — let’s talk about how they should be mounted. The transducers should be mounted on a rigid part of the machine. This means they should not be mounted on flexible covers or shields such as fan covers or belt guards. Good mounting locations include earing housings, machine casings, and mounting blocks.

Magnetic mounts need to be securely mounted so they don’t rock, and the area where they are attached should be clean of debris and any excess paint. In addition, flat rare earth magnets are preferred and two pole horseshoe style magnets should be avoided.

Stud mounted transducers should be attached with a threadlocker and only after the area has been thoroughly cleaned and all removed. Adhesives, whether used to attach the accelerometer or a magnetic mounting pad, are going to have an effect on the mounted resonance frequency of accelerometers. Soft adhesives will reduce the upper-frequency limit of the transducer by 50% and hard adhesives by 20%.

Data Transmission and Storage
When data is obtained, it needs to be transmitted and stored for analysis. In the past, data transmission was only achieved using cables. Modern wireless technology, however, allows data to be transmitted via Bluetooth and wireless networks as well as cables. The vibration data that’s been transferred is then stored on some type of data logger or vibration database that provides access for it to be analyzed.

Vibration Monitoring Systems and Visualization Tools
As we’ve discussed, vibration monitors convert movement (acceleration) into an electrical signal. That signal, however, won’t do us much good unless we can turn it into frequency data for analysis.

Vibration monitoring systems answer that need by processing the data. These systems use principles of mathematics to change it from raw acceleration data into actionable information. Results of analysis take the form of time waveforms and frequency spectrums, both of which can be visualized with charts and plots. A vibration analyst can then take that information and use it to detect a variety of machine faults.

Keep in mind that the software analyzing data can be on a computer, on a specialized piece of hardware (i.e., an FFT analyzer), or on a handheld device. It’s usually the same software that visualizes the results of the analysis in the form of charts and plots.

Remote Monitoring of Vibration Data
Modern approaches to electric motor and powertrain reliability depend heavily on remote condition monitoring. Successfully monitoring makes use of your vibration data. For remote monitoring, there are tools like the ITT i-ALERT 2 and Waites Wireless vibration monitoring system. These come with specialized vibration sensors that transfer vibration data to a data logger using Bluetooth. From there, the data can be analyzed and transformed into actionable insights into the health, condition, and performance of critical components within your powertrain.

Conclusion
Setting up vibration analysis equipment requires several different components, including sensors, data transmission, data storage, and vibration monitoring systems. For a truly effective vibration analysis program, we recommend that it be configured and installed by a certified vibration analyst. They will be able to recommend a remote monitoring solution or help you build a custom solution with the right components.

At HECO, we provide the necessary tools and complete solutions for vibration analysis. We also have on staff a team of certified vibration analysts that can help you set up a vibration analysis system and vibration route for your facility as well as provide vibration analysis. Contact us today to find out how we can help you with vibration analysis!

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