As illustrated in Figs. 6.3a and b,

As illustrated in Figs. 6.3a and b, the shaft centerlines are not concentric, but the shafts are parallel to each other. This condition of the shaft causes shaft offset called crank or eccentricity. This type of misalignment is known as “parallel shaft misalignment” [1, 2]. The eccentricity in the shaft would cause mass unbalance leading to vibration. The resulting mass unbalance produces centrifugal or unbalance forces, the magnitude of which depends on the shaft rotational speed. Large eccentricity in the coupling joint could build excessive unbalance force that could excite the coupling with unacceptable vibration. Such radial offset or eccentricity measured at the coupling flanges is called diametric eccentricity. Half of the diametric eccentricity is called “displacement” and is measured at the coupling rim assuming the flange faces are parallel