As the heart of a machine tool, the spindle is responsible for the type and quality of material removal. It allows enormous forces to act precisely. And this should happen absolutely reliably and with a very high degree of accuracy. One process that ensures the spindle runs smoothly is operational balancing, i.e. the balancing of a rotor in a machine. Imbalance can occur wherever parts rotate. Such imbalances cause vibrations that are too great at some point and lead to problems or the inability to meet specifications.

One example from everyday life is driving a car: Above a certain speed, the steering wheel starts to vibrate. The problem here is the imbalance of the wheels. In the worst case, these vibrations become so great that damage to the vehicle can occur. If these imbalances occur in metal-cutting machines, the dimensional accuracy is often no longer correct. The negative effect is even more extreme with grinding machines: If the vibrations here are too great, even the entire machine function is no longer fulfilled. This results in so-called "chatter marks" on the surface, which means that the workpiece can no longer be used.

Several steps to determine the original imbalance

Operational balancing needs to be well prepared. "With a machine tool spindle, you first have to find a suitable position to attach the vibration and rotation angle sensor to the machine," says Dr. Karl-Josef Hoffmann, Technical Consulting at Schenck RoTec. The first step is to carry out a measurement in the original state of the rotor. A suitable test weight is then estimated and attached to a balancing plane in order to carry out another vibration measurement. If the rotor is cylindrical (stretched shape), a second balancing plane is required. A further measurement run is carried out there with a second test unbalance. The first test unbalance can be removed if desired. The so-called "influence coefficients" can be determined from these measurement runs, which can be used to calculate the correction of the initial unbalance. Today, this can be done relatively easily with a mobile vibration measuring device.

Operational balancing can be carried out using mobile devices, which saves time and reduces costs accordingly. © Schenck RoTec

In addition to quality assurance, operational balancing offers the advantage that the spindle does not have to be removed from the machine tool and placed on a balancing machine. In particular, this eliminates time-consuming and laborious adjustment work. In addition, the unbalance condition in the original bearing is also taken into account. Balancing can often be carried out quickly and cost-effectively. Downtimes are significantly reduced.

Challenges with machine tool spindles

The SmartBalancer is a portable measuring device for operational balancing. It is based on the CAB measuring devices and offers their precision and convenience. © Schenck RoTec

"One challenge is finding suitable measuring points and getting to them," says Hoffmann. In a machine tool, the spindle is usually difficult to access due to other units and parts. Enclosures and supply lines make it difficult to reach the bearing points for the measurements. In order to be able to measure, protective housings have to be dismantled and chips and oil removed. Dirty systems are a common occurrence in everyday life. Cleanliness is essential when balancing, because although mobile vibration measuring devices are robust nowadays, they can still measure very accurately. If, for example, there are liquids or chips between the measurement object and the sensor, this leads to incorrect measurements. "Another important factor for successful vibration reduction is, of course, the balancer's experience and expertise," adds Hoffmann. This also includes an understanding of the design and machine dynamics. In addition, the balancer should recognize which vibration reduction is technically possible in the installed state. The more experienced the balancer is and the better the spindle is prepared for balancing, the less time is required for the work. In such a case, a machine tool spindle can be balanced within two hours.

Differentiation from balancing on a balancing machine

Balancing machines are generally used in the production of parts. Quality assurance is usually the main focus of operational balancing. If the motor, chuck or bearings of a machine tool spindle are changed or other repairs are carried out on rotating parts, unbalances are altered which lead to increased vibrations. Such work is often necessary when the spindle is subject to strong machining forces. Large workpieces and hard materials in particular lead to premature wear of the spindle and bearings.

One application in which both balancing methods are used is for companies that manufacture spindles and balance them on a balancing machine in their factory. The spindles are then delivered to the customer, installed on site and then the vibration condition is checked again using an operational balancing device. This is the only way to guarantee optimum running smoothness when installed.

A typical customer requirement for machine tool spindles is smooth running. This is essential for optimum production quality and also extends the service life of the tools. In summary, it can be said that balancing not only has a decisive effect on the quality of the workpiece, but also on the service life of the tool and the spindle.

Dr. Jens Zeyer, DIrector Marketing & PR at Schenck RoTec / am