Alignment occurs when two lines that are superimposed on each other form a single line. Misalignment is a measure of how far apart the two lines are away from forming that single line. In one condition, the two lines can be parallel with each other, but at a constant distance apart. This is referred to as offset or parallel misalignment. In the other, one line will be at an angle to the other. This is referred to as angular misalignment.
Angular misalignment refers to the difference in slope of the two shafts. If the pump, base and foundation have been properly installed, the shaft centerline of the pump can be considered as level, and therefore, as the reference or datum line. The slope of the driver shaft can be calculated by determining the offset measurement at two different points, subtracting one from the other, and dividing the result by the axial distance between the two points.
The parallel misalignment can be considered as the distance between the driver shaft centerline and the pump shaft centerline at any given point along the length. Consequently, it is worthwhile to take the necessary measurements on the top and on the bottom for vertical offset, and also on each side for the horizontal offset.
When a foot-mounted process pump must operate at elevated temperatures, some adjustment will be necessary to allow for the thermal growth that takes place between the cold condition and the high operating temperatures. As the pump heats up, the shaft centerline will be moved up, creating an offset with the motor shaft. One method of handling this situation is to misalign the motor by the amount of growth anticipated from the pump prior to starting it up.
A second method is to start the pump and motor following a cold alignment, without any adjustment. As the pump heats up and expands, it will gradually move up, out of alignment with the motor. When the pump is fully up to temperature, the unit is stopped and hot alignment takes place. For both of these methods, a flexible coupling, capable of accommodating the total amount of anticipated misalignment, will be required.