25.8 Recording and Analysis Instrumentation

Back

25.8.1 Vibration Meters

Vibration meters are generally small, hand-held (portable), inexpensive, simple to use, self-contained

devices that give an overall vibration level reading (see Chapter 15). They are used for walk-around

surveys and measure velocity and/or acceleration. Generally, these devices have no built-in diagnostics

capability, but the natural frequency of an accelerometer can be exploited to look for specific machinery

faults. As an example, rolling element bearings generally emit “spike” energy during the early stages of

deterioration. These are sharp impacts as rollers strike defects (pits, cracks) in the races. A spike energy

meter is an accelerometer that has been tuned to have its resonant frequency excited by these impacts,

thus giving a very early warning of deteriorating bearings.

25.8.2 Data Collectors

Most vibration data collectors available today for use in machine condition monitoring and fault

diagnostics are microcomputer based. They are used together with vibration sensors to measures

vibration, to store and transfer data, and for frequency domain analysis. Considerably more data can

be recorded in a digital form, but the cost of these devices can also be considerable. Another

* A transducer is a device that senses a physical quantity and converts it into an electrical

output signal, which is proportional to the measured variable.

* The selection, placement, and proper use of the correct transducer are important steps in

the implementation of a condition monitoring and fault diagnostics program.

* The transducer must be correct for the task, properly mounted, in good working order

(properly calibrated), and fully understood in terms of operational characteristics.

* Traditional vibration sensors fall into three main classes: noncontact displacement

transducers, velocity transducers, and accelerometers.

* Transducers are selected to optimize sensitivity over the frequency range that is expected to

be recorded.

* Using several locations and directions when recording vibration information is

recommended.

* When selecting sensor locations, one must consider mechanical independence, the

vibration transmission path, and locations where natural frequency vibrations may be

excited.

25-14 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

advantage of most data collectors is the ability to use these devices to conduct on-the-spot

diagnostics or balancing. They are usually used with a PC to provide permanent data storage and a

platform for more detailed analysis software. Data collectors are usually used on general-purpose

equipment.

25.8.3 Frequency-Domain Analyzers

The frequency-domain analyzer is perhaps the key instrument for diagnostic work. Different machine

conditions (unbalance, misalignment, looseness, bearing flaws) all generate characteristic patterns that

are usually visible in the frequency domain. While data collectors do provide some frequency domain

analysis capability, their main purpose is data collection. Frequency-domain analyzers are specialized

instruments that emphasize the analysis of vibration signals. As such, they are often treated as a

laboratory instrument. Generally, analyzers will have superior frequency resolution, filtering ability

(including antialiasing), weighting functions for the elimination of leakage, averaging capabilities (both

in the time and frequency domains), envelope detection (demodulation), transient capture, large

memory, order tracking, cascade/waterfall display, and zoom features. Dual-channel analysis is also

common.

25.8.4 Time-Domain Instruments

Time-domain instruments are generally only able to provide a time domain display of the vibration

waveform. Some devices have limited frequency-domain capabilities. While this restriction may seem

limiting, the low cost of these devices and the fact that some vibration characteristics and trends show up

well in the time domain make them valuable tools. Oscilloscopes are the most common form. Shaft

displacements (orbits), transients and synchronous time averaging (and negative averaging) are some of

the analysis strategies that can be employed with this type of device.

25.8.5 Tracking Analyzers

Tracking analyzers are typically used to record and analyze data from machines that are changing speed.

This usually occurs during run-up and coast-down of large machinery or turbo-machinery. These

measurements are typically used to locate machine resonances and unbalance conditions. The tracking

rate is dependent on filter bandwidth, and there is a need for a reference signal to track speed

(tachometer input). These devices usually have variable input sensitivity and a large dynamic range.

* Vibration meters are generally small, hand-held (portable), inexpensive, simple to use, selfcontained

devices that give an overall vibration level reading.

* Most vibration data collectors available today for use in machine condition monitoring and

fault diagnostics are microcomputer based. They are used together with vibration sensors to

measures vibration, to store and transfer data, and for frequency domain analysis.

* Frequency domain analyzers are specialized instruments that emphasize the analysis of

vibration signals, and as such they are perhaps the key instrument for diagnostic work.

* Time domain instruments are generally only able to provide a time domain display of the

vibration waveform.

* Tracking analyzers are typically used to record and analyze data (locate machine resonances

and unbalance conditions) from machines that are changing speed. This usually occurs

during run-up and coast-down of large machinery or turbo-machinery.

Machine Condition Monitoring and Fault Diagnostics 25-15

© 2005 by Taylor & Francis Group, LLC

Навигация