In general
there are 4 alignment conditions:
1) Perfect alignment
2) Parallel or offset misalignment
3) Angular or Face misalignment
4) Skewed or combination misalignment (i.e., offset and parallel)
PERFECT ALIGNMENT :
When the two shafts are perfectly aligned they will be colinear and operate as a solid shaft when coupled. Without performing alignment procedure we saw rarely perfectly aligned shafts. Regular checking is necessary to maintain the perfect alignment condition. Regular checking is the key condition in preventing maintenance.
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OFFSET or PARALLEL MISALIGNMENT :
When there is a distance between shaft center lines, it is referred as Offset or parallel misalignment. Generally this alignment is measured in thousandths of an inch. In this condition the shafts are parallel to each other but not colinear. Offset may present in either the vertical or horizontal plane. Theoretically, offset is measured at coupling center line.
ANGULAR MISALIGNMENT :
Angular misalignment also referred as face misalignment. Angular misalignment is the condition when the shafts aren't parallel (Angular offset) however they are in same plane with no offset. Angularity is the angle between the two shaft centerlines, which usually expressed as a "slope", or "rise over run" of so many thousandths of an inch per inch (i.e., unit less) rather than as an angle in degrees. It should be determined both in the vertical and horizontal planes.
COMBINATION OR SKEWED MISALIGNMENT :
Skewed or Combination misalignment occurs when both Offset and Angular misalignment occurs. In this type of misalignment shafts are not parallel and also they wont intersect at the coupling. This type of misalignment can occur in both the horizontal and vertical planes or either in the horizontal or vertical planes. 