Vibration of the journal bearings along the length of the engine frame relative to the original crankshaft centerline is referred to as dynamic misalignment. Dynamic misalignment is a result of excessive flexibility in the engine frame or support structure. Most stationary engine frames are not designed with sufficient strength to restrain the bearing loads without the additional strength of a skid or foundation. Realizing this, the engineer who designs the foundation or skid must provide a strong support for the engine frame. When the compressor is mounted directly to a concrete block and mat type foundation, the physical dimensions of the foundation generally provide sufficient additional stiffness to restrain the bearing forces.
Skid-mounted units are more likely to have a dynamic misalignment problem because the skids are an additional element between the engine frame and foundation and introduce additional attachment point or set of anchor bolts. One of the primary functions of the skid is to provide mounting for the compressor, piping, and auxiliary equipment. The skid provides strength during transportation and lifting to minimize static deflections or sagging. However, the skid must also be designed for the dynamic forces and moments of the engine and compressor. A finite element analysis can be used to calculate the mode shapes and pinpoint the most flexible element.
