Balancing Machines: Types, Functions, and How to Choose the Right Machine

The right balancing machine reduces vibration and wear, ensures product quality, and can be tailored to the rotor, production volume, and manufacturing process. Today, customized balancing solutions are available for virtually every industry, from individual part production to fully automated high-volume production.
Which balancing machine is right for you depends on three key questions: What is the shape and design of your rotor? What balancing quality is required? What is your production volume?

• Cylindrical rotors with their own shaft journals: horizontal balancing machines
• Disc-shaped rotors without a shaft: vertical or static balancing machines
• Individual parts, prototypes, and small production runs: manually operated universal balancing machines
• High-volume production: semi-automatic and fully automatic balancing systems
• Rotors installed in the machine: portable balancing equipment for field balancing

A balancing machine supports the rotor, measures its unbalance, and calculates the correction required to compensate for it. Depending on the machine type and application, measurement and evaluation are performed in different ways, for example based on the measuring principle, the number of correction planes, and the rotor support system. The objective is always the same: precise correction of the unbalance.

Measuring unbalance: How does a balancing machine determine the location and amount of unbalance?

The rotor is set in rotation. Sensors detect the vibrations caused by unbalance as well as the resulting centrifugal forces or vibration displacement. Based on these measurements, the measuring system calculates how much mass must be added or removed at a specific angular position in each correction plane.

On manually operated balancing machines, each correction is followed by a verification run until the required balancing quality is achieved. The measuring system is at the heart of this process. At Schenck, this includes the measuring systems of the CAB family and the Schenck ONE software platform. They digitally process the measured values, provide a visual representation of the unbalance, and guide the operator through the individual process steps required to correct the unbalance.

Static or dynamic balancing: Balancing in one or two planes?

Whether one or two correction planes are required depends on the type of unbalance.

• Static unbalance: Static unbalance occurs when two unbalances have the same direction and angular position. The principal axis of inertia is parallel to and offset from the axis of rotation. Even when stationary, the rotor tends to settle with its heaviest point at the bottom. Correction is performed in one plane. In practice, this is often the case with flat or disk-shaped rotors.
• Dynamic unbalance: This type of unbalance is particularly common with longer, cylindrical rotors. The principal axis of inertia is inclined and offset relative to the axis of rotation. When the rotor rotates, this results in a wobbling motion. Correction is performed in at least two planes.

Force-measuring or displacement-measuring: What is the difference between hard-bearing and soft-bearing balancing machines?

Balancing machines measure unbalance using two physical principles that differ primarily in how the rotor is supported.

Unbalance correction: How is unbalance corrected?

Unbalance is corrected by adding or removing mass at the calculated position. Possible correction methods include:

- Drilling,
- Milling,
- Grinding,
- Welding,
- Adding screws or threaded weights,
- Bonding.

The appropriate correction method depends on the rotor. Axle drives, for example, can be corrected by drilling and, optionally, by welding at the drive flanges. For crankshafts, the measuring system calculates the correction in a way that minimizes the number of holes required. In many cases, a single correction step is sufficient. On manual balancing machines, the operator performs the correction, while in automatic balancing systems, the correction process is integrated directly into the machine cycle.

The required balancing quality depends on the rotor type, rotor mass, and maximum operating speed. 
For rotors with rigid behavior, DIN ISO 21940-11 defines the procedures and permissible unbalance tolerances. 
The standard uses balance quality grades G, expressed in mm/s. The lower the G value, the lower the permissible residual unbalance and the tighter the tolerance. The permissible residual unbalance for your rotor is determined based on the balance quality grade, rotor mass, and rotational speed.

For guidance, the standard provides reference values:

• G 2.5: for example, gas and steam turbines, machine tool drives, and medium-sized and large electric motors
• G 6.3: for example, fans, pumps, pulleys, and flywheels

Important: In practice, zero residual unbalance cannot be achieved. The tolerance should therefore be defined at an economically reasonable level, balancing the requirements for quality and cost. 
For flexible rotors, such as large turbines and generators, this approach alone is not sufficient. Unlike rigid rotors, flexible rotors can change shape due to deflection across the operating speed range, which can also cause the measured unbalance values to change.

The starting point is typically the specific balancing task. Rotor geometry, weight, speed, production volume, and manufacturing environment determine which balancing solution is most suitable. The following overview matches typical rotor types with the appropriate balancing solution.

Aerospace applications are subject to specific requirements. Schenck horizontal balancing machines for these applications comply with SAE ARP 4048. Moment weighing systems complement the portfolio and help optimize blade distribution on the rotor.