Unbalanced Spinning Equipment

Mike Sondalini, PWW EAM System Consultant

with permission of BIN95 Business Industrial Network

Posted 12/11/2025

Unbalanced spinning equipment. Vibration from out-of-balance rotating equipment can be frightening. The ground shakes, machines jump about, hold down bolts come loose and parts break. An unbalanced rotating body will produce forces on its bearings and transmit them throughout its structure and into the foundations.

Causes of Unbalanced Spinning Equipment

Common causes of unbalance:

Bent or bowed between support bearings
Overhung weight bends shaft under gravity
Unevenly distributed solid or liquid inside rotor
Loose parts on the rotor
Eccentrically manufactured diameters on the rotor
Misalignment of the drive train to the rotor axis
Loose drive couplings flop about
Loose tolerances between assembled parts on rotor
Shoulders on rotor manufactured out-of-square to axis
Voids or cavities within the rotor
Misalignment of bearings force shaft to bow

The drawings below provide examples of some of the problems listed in the table above.

Reducing Problems Caused by Unbalanced Spinning Equipment

Minimizing vibration involves minimizing out-of-balance forces. The following table indicates simple actions to take to reduce the problems of unbalanced spinning equipment.

Make the rotor with all diameters on the same axis
Machine the rotor from one piece of material
Machine the rotor complete without altering the initial machining setup
Finish machine multiple part rotors when fully assembled on the shaft
Reduce lengths of unsupported sections and overhangs
Keep tolerances tight on parts assembled on the rotor
Align the drive train to the axis of the rotor carefully
Ensure the bearing supports are aligned

Balancing Rotors

Unbalanced spinning equipment can be made to run smoothly by balancing the rotating parts to within acceptable limits. Balancing is the process of attempting to improve the distribution of mass in a body so that it rotates in its bearings without unbalanced centrifugal forces. It can only be attained to a certain degree.

When a part is to be balanced it is necessary to specify the quality of balance required. The more precise the balance required the higher the cost to attain it. Relevant standards have been set for various types of components. The list below is taken from BS 6861 and shows typical grades of balance for common components.

Attaching or removing weight at relevant points on the rotor permits balance corrections. For example, specifically weighted tags can be welded to the rotor, or metal can be removed by drilling holes into the rotor at relevant positions. To minimize the possibility of vibration problems, specify to the machine shop the degree of balance quality required of the part. To improve the accuracy of balancing send all the attached assemblies such as couplings, bolts, keys and pulleys with the rotor so the effect of the components can be allowed for in the balance corrections.

Mike Sondalini

Mike Sondalini is a Senior Consultant at PWWEAM System-of-Reliability. BEng(Hons), MBA, CPEng. As a consultant and trainer, Mike was able to present his insights to his clients, suggesting innovative approaches to plant and equipment reliability. Their feedback was resoundingly positive. Efforts which earned him an international reputation for articulate, out-of-the-box articles on plant and equipment reliability, life-cycle EAM, maintenance management, work quality assurance, and team building. After decades of dedicated research, Mike authored “Industrial Manufacturing Wellness: The Complete Guide to Successful Enterprise Asset Management” a revolutionary approach on how maintenance and physical asset management systems should be run, the book detailed who, what, where, when, why, and how outstanding reliability could be achieved. Each step based in scientific and mathematical understanding to ensure repeatability of results and optimal outcomes.

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Unbalanced Spinning Equipment