Fluke Defines Terms Associated With Vibration
Acceleration is not constant but will vary through the vibration cycle, reaching maximum levels as velocity reaches its minimum. This is typically when a particular mass has decelerated to a stop and is about to begin accelerating again. Next, an accelerometer is described as a transducer whose electrical output responds directly to acceleration. Accelerometers typically cover a much wider frequency range, along them to pick up signals not present with other types of transducers. Owing to the frequency range, accelerometers are suitable for most types of rotating equipment, making them the most used transducer for vibration measurements.
Alignment is a condition where components within a drive train are parallel or perpendicular, according to design requirements. A tester such as the Fluke 810 vibration tester can diagnose misalignment conditions where these components are no longer aligned according to design requirements, causing excessive bearing wear and power consumption in the machine. A bronze or stainless-steel attachment pad can be placed at appropriate measuring locations on machines using an industrial adhesive. The triaxial accelerometer is attached to these pads for measurement collection.
The pad may include an alignment notch to ensure the consistent orientation of the accelerometer to the three vibration axes (radial, tangential and axial). The pad ensures a good transfer of vibration data to the transducer by providing a strong and consistent mounting location. Axial is described as one of the three vibration axes; the axial plane is parallel to the centreline of a shaft or the turning axis of a rotating part. Next, balancing (mechanical) is described as adjusting the distribution of mass in a rotating element to reduce vibratory forces generated by rotation.
Meanwhile, condition monitoring (CM) relates to the measurement, recording and analysis of machinery parameters (such as acceleration) to determine machine health. Current condition is compared with when the machine was new. CM is also known as machinery health monitoring. Displacement is the word used when measuring machinery vibration; displacement represents the actual distance the vibration causes the part in question to move. It is measured in thousandths of an inch (mils) in the English system and in millimetres (mm) in the metric system. Failure is the event, or inoperable state, in which any item or part of an item does not, or would not, perform as specified.
Next, failure mechanism is the mechanical or physical parts that result in failure. Forced vibration is described as the vibration of a machine caused by some mechanical excitation. If the excitation is periodic and continuous, the response motion eventually becomes steady state. Frequency, the number of events that occur within a fixed time period, is also calculated as the reciprocal of time (that is, one divided by the time interval). Frequency is typically expressed in terms of Hertz (Hz) but can also be expressed as cycles per minute (cpm) or revolutions per minute (rev/min) when multiplying Hz times 60. It can also be represented as multiples of turning speed, or 'orders', where frequency in rev/min is divided by the turning speed of the machine.
The term 'frequency domain' is used when a vibration signal is represented as a time waveform if viewed on an oscilloscope, since vibration exists within the time domain. If plotted, the time waveform would represent a plot of amplitude versus time. If the waveform were transformed to the frequency domain, the result would be a spectrum representing a plot of amplitude versus frequency. Harmonic refers to a sinusoidal quantity having a frequency that is an integral multiple of a fundamental frequency. Harmonic distortion is described as distortion caused by the presence of frequencies not present in the input signal, in the output signal of a device.
Hertz is a unit of frequency. Imbalance is a condition on rotating equipment where the centre of mass does not lie on the centre of rotation. Imbalance can severely reduce bearing life as well as cause undue machine vibration. Isolation refers to a reduction in motion severity, usually by a resilient support. A shock mount or an isolator attenuates shock. A vibration mount or an isolator attenuates steady-state vibration. The meantime between failure (MTBF) is a measurement of reliability for repairable items - the mean number of life units during which all parts of the item perform within their specified limits, during a particular measurement interval under stated conditions.
The meantime to failure (MTF) is a basic measure of reliability for non-repairable items - the total number of life units of an item divided by the total number of failures within that population, during a particular measurement interval under stated conditions. In rotating machines, orders are multiples or harmonics of the running speed (or associated reference component). The term 'pitch' relates to rotation in the plane of forward motion, about the left-right axis. Radial is one of the three vibration axes; the radial plane represents the direction from the transducer to the centre of the shaft on rotating equipment.
For typical horizontal machines, radial equals the vertical axis. For horizontal machines, radial refers the horizontal axis to which the accelerometer is attached. Repeatability is described as the maximum deviation from the mean of corresponding data points taken under identical conditions. It is the maximum difference in output for identically repeated stimuli when there is no change in other test conditions. Replication relates to testing that reproduces a specified desired history. Resolution is described as the smallest input change that produces a detectable change in an instrument's output.
Root-cause analysis is determining what actually caused a failure. The running speed is the speed - usually expressed in rev/min - at which a rotating machine runs. It may also be expressed in Hz by dividing the rev/min by 60. Sensitivity describes the ratio between electrical signal (output) and mechanical quantity (input). Next, a solid-state sensor is a sensor with no moving parts. Tangential is one of the three vibration axes; the tangential plane is positioned 90deg to the radial plane, running tangent to the driveshaft. For typical horizontal machines, tangential equals the horizontal axis.
For typical vertical machines, tangential equals the second horizontal axis perpendicular to the mounting of the accelerometer. Unbalance refers to unequal mass distribution on a rotor. The mass centreline does not coincide with the rotation or the geometric centreline. This term can also be known as 'imbalance'. Velocity is the rate of change in position, measured in distance per unit of time. When measuring vibration signals, velocity also represents the rate of change in displacement and is expressed in inches (in) or millimetres (mm) per second. Finally, the term 'vibration' itself describes mechanical motion around an equilibrium reference point.
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