During introductory courses in balancing, the concept of “phase lag” is typically given a cursory mention without much discussion of what it is or why it happens. In this paper, the concept of phase lag in the context of rotordynamics is explored.
In order to make the subject more approachable, the action of the input (rotor’s unbalance) and the output (vibration of the rotor) is likened to a simple child’s toy. Through this analogy, the procedure of balancing a rotor is better understood.
The paper concludes with an appendix of 5 cases with differing operating speeds relative to the system’s critical speed. In the said Appendix, a visual representation of the characteristics of the rotor, including its operation relative to the system’s critical speed, is given. This is communicated via Bode plots, time domain visualizations of the rotor, and the analogous operation of the said child’s toy.

Mike is a licensed Professional Engineer in the State of New York and holds a Bachelor of Science in Mechanical Engineering from the Rochester Institute of Technology. In addition, he has a MBA from the State University of New York at Buffalo, and holds the certifications of Category III Vibration Analyst and Category I Balancing Technician, per ISO standards. Mike has 17 years of experience in working with rotating equipment, performing duties such as product & tooling design, controls (PLC), product testing, and field service troubleshooting. He began his career Cameron Compression’s Centrifugal Division (now Ingersoll Rand). From there he moved on to become a Reliability Engineer for Praxair’s air separation, liquid hydrogen, and liquid CO2 market segments. Currently Mike is a design and rotor balance engineer for jet engines with Parametric Solutions, Inc. in Jupiter FL. Mike particularly enjoys developing intuitive visual, and whenever possible “hands on,” explanations for machinery phenomena, as opposed to rigorous mathematical ones. He has found such approaches beneficial to himself as well as others during training and problem solving. Through the combination of his work and studies, Mike continuously strives to build “The Business Case for Reliability.”