While we have previously identified that alignment is the primary cause of premature coupling failures (see post here), a further explanation of static versus dynamic alignment is in order.
Static alignment is the condition of the machinery at rest (think of the alignment we perform when the equipment is first installed). Static alignment gives us the opportunity to correct issues such as soft foot, gross misalignment and to bring the system to within specifications.
Dynamic alignment is the condition of the machinery during sustained operation. Think of an electric motor moving from its mechanical center to the electrical center, the thermal growth experienced by an internal combustion engine or a shaft moving axially in response to forces in the machine train. It is common to perform a “hot alignment check” on equipment. A hot alignment check is when the machinery is allowed to achieve its operational steady state condition (i.e. after a compressor train has operated for a minimum of 24 hours) and is then shut down with the express intention of quickly performing an alignment verification.
Depending in the machinery, dynamic alignment is preferred as the equipment will experience measurable changes and can result in the equipment operating beyond alignment specifications from the initial static condition.
Remember to always align equipment to the tighter of either the coupling or equipment specifications!