“Introduction:

Shaft Center line position provides key machinery condition information on fluid film bearing machinery. Malfunctions including misalignment, bearing wear, and potential stability problems can be detected and when combined with analysis of dynamic vibration, a more informed assessment of machinery condition can be derived. However, the understanding and use of this important tool is not clearly understood by many plant vibration personnel. This is evident in the many questions asked during training sessions and most importantly in times of trouble when the consultant asks “What were the initial gap voltages?” and the answer is ” Not sure, maybe I&C has a record of that”. With responses such as this, it is clear why the trending of gap voltage and calculation of centerline position is not always included as part of the normal machine assessment and trending programs. Since understanding is directly proportional to use, it is the intent of this paper to take some of the mystery out of this measurement by reviewing basic machinery convention, showing how a shaft centerline plot is constructed, and showing examples of centerline position use.

Basic Machinery Convention: Whether we are setting up an instrument for a dynamic vibration test or getting ready to construct a shaft centerline plot, basic machinery information needs to be collected and proper convention followed. 

The 1st item we need to establish is view direction. View direction in our industry is always from the outboard of the driver looking toward the driven equipment. Making a sketch is always helpful especially if the information collected may be shared with another analyst. Figure 1 shows a machine drawing with the correct view direction indicated on the drawing.

Using this view direction the 2nd item we need to document is the direction of rotation for each of the shafts. As with analysis of shaft dynamic Orbit information we will see that direction of rotation plays an important roll in determining what is an expected vs. unexpected center line response. Direction of rotation in our example above will also provide us with important information concerning the gearing by allowing us to determine if the gear is on the up or down mesh side. Gear forces along with other internal design loads will play a roll in determining whether a calculated center line is normal for a particular machine.

The 3rd item required is the orientation of the proximity probes with respect to a machine reference. Again a sketch is very helpful.”