What is Eccentricity?

Answer 1

Eccentric means, literally, "off center". The term originated with astronomy. Later, it came to be used in mechanics, to describe a wheel or gear which rotates about an axle which is not at its center. (To understand how eccentric motion can be useful. See animation of a automobile cam in related link below.) Around 1620, the term was first applied figuratively, to describe a thing which was odd or unusual. Around 1832, it began to be used to describe a person with unusual ideas or behavior.

Eccentric means deviating from the recognized or customary character or practice.

With regard to people, eccentric means behaving in odd, unusual ways without the implication of mental illness.

Answer 2

Orbital eccentricity is a very important measure of how elliptical the path is that is followed by an orbiting body. Everything that orbits around another body follows a path that is a conic section. Very few orbits are perfectly circular. Almost all orbits are elliptical to one extent or another. Ellipses are conic sections. Mercury is currently the planet with the most eccentric orbit around the sun. The Wikipedia link below contains a link giving the eccentricities of all the planets in our solar system.

The eccentricity of something can never be 1
Shaft Eccentricity plays a very important role as part of a Turbine Supervisory Instrumentation (TSI) System on large steam turbines and should be included in retro-fit plans when at all possible.

Operators use eccentricity measurements to determine when a combination of slow roll and heating have reduced the rotor eccentricity to the point where the turbine can safely be brought up to speed without damage from excessive vibration or rotor to stator contact.

Eccentricity is the measurement of Rotor Bow at rotor slow roll which may be caused by any or a combination of

1. Fixed mechanical bow
2. Temporary thermal bow
3. Gravity bow

In extreme cases of thermal/gravity bow, caused by a sudden trip of the unit and failure of the turning gear to engage, the rotor may be positioned and stopped 180o out of phase (bow up) to allow gravity to work entirely on the bow and substantially shorten the time required to reduce the bow.

Eccentricity is measured while the turbine is on slow roll (1 to 240 RP).

turbine is on slow roll (1 to 240 RPM below the speed at which the rotor becomes dynamic and rises in the bearing on the oil wedge) and requires special circuitry to detect the peak- to-peak motion of the shaft. This is accomplished using circuitry with long update times selectable between 20 seconds (> 3 RPM) and 2 minutes (<3 RPM).

As the eccentricity measurement is not required after a turbine is brought to speed and under load provisions are made to lock the measurement to zero. This can be accomplished without external contacts through the use of a speed measurement channel with underspeed or overspeed alarms.