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Terminology
- Accelerometer
- A sensor that measures vibration or acceleration forces. Single integration of the acceleration signal will produce a velocity display and double integration of the acceleration signal will produce a displacement display.
- Angular Misalignment
- Two minor angles between the extension of each machine centerline and the centerline of the structure joining the two flexible elements. Produces a bending moment on each shaft, generating a strong vibration.
- Axial Displacement
- Change in the relative axial position of the adjacent shaft ends of two coupled machines, usually caused by thermal expansion.
- Bearing Misalignment
- A misalignment that results when the bearings supporting a shaft are not aligned with each other. The bearings may not be mounted in parallel planes, cocked relative to the shaft, or distorted due to foundation settling or thermal growth.
- Continuous Torque Rating
- Coupling manufacturer's declared maximum torque that the coupling is capable of transmitting continuously for unlimited periods.
- Coupling
- A device used to connect two shafts together at their ends for the purpose of transmitting power. Couplings do not normally allow disconnection of shafts during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded.
- Damping
- Dissipation of energy within a mechanical structure and its conversion ultimately into heat.
- Displacement
- An objects overall change in position. Displacement is the vector that specifies the position of a point in reference to an origin or to a previous position. The vector directs from the reference point to the current position.
- Distance Between Shaft Ends (DBSE)
- Distance from the extreme end of one shaft (including any threaded end) to the extreme end of the next shaft.
- Eddy-Current
- Electric currents induced in electrically conducting materials by fluctuating magnetic fields. An eddy current is caused when a moving (or changing) magnetic field intersects a conductor, or vice-versa. The relative motion causes a circulating flow of electrons, or current, within the conductor. These circulating eddies of current create electromagnets with magnetic fields that oppose the effect of the applied magnetic field. The stronger the applied magnetic field, or greater the electrical conductivity of the conductor, or greater the relative velocity of motion, the greater the currents developed and the greater the opposing field.
- Eddy-Current Probe
- A non-contact electrical device that measures the displacement of one surface relative to the tip of the probe. Construction consists of an electrical coil of various lengths and diameters. This coil located in the tip of the probe is energized producing an electrical field around the tip of the probe. When a conductive surface is placed in the field and the distance from the probe is noted, variations in this gap can be determined by the variations in the voltage flow to the probe tip.
- Ferrite Core
- A ferromagnetic material placed in a Proximity Probe Tip to tremendously assist the passage of magnetic lines of force. These lines of force are needed to create eddy currents in the material to be measured.
- Hall-Effect
- When a conductor or semiconductor with current flowing in one direction is introduced perpendicular to a magnetic field, a voltage could be measured at right angle to the current path. The Hall Effect refers to the potential difference (Hall Voltage) on the opposite sides of an electrical conductor through which an electric current is flowing, created by a magnetic field applied perpendicular to the current.
- High-Pass Filter
- A filter that passes signal frequencies above a specific, or cut off, frequency is called a high pass filter. They are used in instrumentation to eliminate low-frequency noise, and to separate alternating components from direct (DC) components in a signal.
- Lateral Offset
- Lateral distance between the centerlines of two shafts, which are not parallel, measured perpendicularly to the centerline and in the plane of the shaft end of the driving machine.
- Load Cells
- A transducer which converts force into a measurable electrical output.
- Low Pass Filter
- A filter that passes signals with less than 3 dB attenuation up to its cutoff frequency, and attenuates the signal above that frequency. The decreasing slope is called the roll off.
- Hot Alignment or Real-Time Alignment
- Ability to provide current alignment information while machine is in operation
- Magnetic Flux
- Product of the average magnetic field times the perpendicular area that it penetrates. Magnetic flux is simply a quantity of magnetic field passing through a certain area. It is represented by the Greek letter Φ (phi).
- Maximum Continuous Speed
- Highest rotational speed at which the coupling is capable of continuous operation.
- Mils
- A unit of measure for displacement (thousandths of an inch). Usually measured in mils peak-to-peak, which represents total displacement.
- Mils/Inch
- A unit used to describe the angle of one shaft centerline to the other. It is equivalent to milliradians. It can also be expressed as rise/run (1 unit = 17.45 mils/inch), as long as the rise is measured in mils and the run is measured in inches.
- Mutual Inductance
- The ratio of the electromotive force (EMF) in a winding due to the corresponding change of current in a neighboring winding.
- Normal Operating Point
- Point at which usual operation is expected.
- Parallel Offset
- Distance between the centerlines of two coupled shafts that are parallel but not in the same straight line.
- Peak (pk)
- The maximum value, either positive (pk+) or negative (pk-), that a waveform attains. Peak values can be expressed for voltage, current, or power.
- Peak Scaling, Peak-to-Peak Scaling, RMS Scaling
- Methods to display the amplitude axis of a spectrum.
- Peak-to-Peak
- The amplitude difference between the most positive and most negative value in the time waveform.
- Peak Torque Rating
- Maximum torque the coupling can tolerate for short periods.
- Phase (time lag or lead)
- The difference in time between two events such as the zero crossing of two waveforms, or the time between a reference and the peak of a waveform. The phase is expressed in degrees as the time between two events divided by the period times 360 degrees.
- Piezoelectric Devices
- The ability of some materials to generate an electric potential in response to applied mechanical stress. This may take the form of a separation of electric charge across the crystal lattice. If the material is not short-circuited, the applied charge induces a voltage across the material.
- Proximity Probe
- A displacement transducer that consists of a small coil of wire around a metal rod, which is connected to a special preamplifier. The voltage output of the preamplifier is proportional to the displacement between the end of the probe and a conducting surface. The response of the system is from DC, i.e. it measures static displacement, and dynamic displacement up to about 1000 Hz. They are also sometimes called eddy current probe.
- Rated Speed
- Highest rotational speed at which the coupling is required to be capable of transmitting the continuous torque rating while simultaneously subjected to the rated angular misalignment and the coupling rated axial displacement.
- Seismic
- In vibration instrumentation, it means related to an inertial mass. A seismic transducer utilizes the inertia of a small mass to generate a force when accelerated, or to generate a relative motion which is then sensed. Examples are the piezoelectric accelerometer and the velocity transducer.
- Stator
- The stationary housing, containing both power and signal coils, that is attached to the machine frame/coupling surrounding the rotor. Based on the principle of Mutual Induction, the coils (acting as a rotary transformer) provide power to the rotor/coupling’s electronics as well as receive the coupling’s transmitted torque signals. An air gap between the stator and rotor allows for misalignment and axial movement.
- Strain Gage
- A device used to measure deformation (strain) of an object. The most common type of strain gage consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.
- Thermal Growth
- Movement of the shaft center lines associated with (or due to) a change in machinery temperature between the static and operating conditions.
- Transducer/Sensor
- Any device that translates the magnitude of one quantity into another quantity. Three of the most common transducers used in vibration measurements are the accelerometer, velocity transducer, and eddy current probe.
- Velocity
- The rate of change in position. It is a vector physical quantity; both speed and direction are required to define it.
- Velocity Transducer
- An electrical/mechanical transducer whose output is directly proportional to the velocity of the measured unit. A velocity transducer consists of a magnet suspended on a coil, surrounded by a conductive coil. Movement of the transducer induces movement in the suspended magnet. This movement inside the conductive coil generates an electrical current proportional to the velocity of the movement. The signal can also be integrated to produce a displacement measurement.
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