100 YEARS OF BALANCING TECHNOLOGY

For more than 100 years, balancing technology has been closely related to the name Carl Schenck: In 1908 the Darmstadt-based company built its first balancing machine. This does not mean, however, that balancing had not been on the agenda before: The fact that even the rotors of the earliest steam engines had to run smoothly made balancing a point of interest even in those early days. Look here. for many important milestones for the history of balancing.

BALANCING IN ITS EARLY STAGES

Nowadays we find it hard to believe that balancing of a steam turbine rotor took three to four weeks of hard manual labour in the early days of industrialisation. Around 150 years ago, the available technology was comparatively simple, and the result of the balancing process rather inaccurate. Boilers exploding, or flywheels disintegrating at high speeds, constituted a serious hazard. Inadequate balance quality also caused bearings to wear down quickly. Experienced engineers recognised these dangers and started looking for solutions.

The Canadian engineer H. MARTINSON was one of the first to look into the subject of balancing from a theoretical point of view. In 1870 he was granted what was probably the first patent for a balancing machine. The rotor was mounted isotropically on soft coil springs, driven by a universal-joint shaft. By gradually moving a piece of chalk towards the rotating rotor, you were able to determine the position of the unbalance with some degree of accuracy. However, there is no record as to whether this machine actually worked, or whether it was ever built in major quantities.

... FROM ROLL-BALANCING FACILITIES TO BALANCING MACHINES – SCHENCK APPEARS ON THE SCENE

As technical development advanced rapidly, the problems caused by rotor vibrations became more and more obvious. Workers required a lot of skill, experience and approximately three to four weeks of labour to statically balance rotors on knife-edges, using the "roll-off " method. A workable solution was described in 1907 by Dr. Ing. FRANZ LAWACZECK in his paper "Zur Theorie und Konstruktion der Balanziermaschine" (Theory and design of a balancing machine).

In 1908 CARL SCHENCK, who had also started looking into the subject of "roll-off" balancing at that time, concluded a licence agreement with Lawaczeck . The "Lawaczeck principle" remained valid right up to the forties: It consisted of a pendulum-mounted fixed bearing on the one side of the rotor and a radially flexible bearing on the other side. After initial correction in one plane, the rotor was re-installed. In 1915 Schenck took over the sole worldwide licence for this machine.

During this period, a number of new optical and mechanical measuring methods were developed, whose measuring accuracy was quite remarkable. The "Lawaczeck model" was capable of achieving a balance quality equivalent to a centre of gravity displacement of 0.001 mm - a balance quality which would even today be perfectly adequate for many applications.

FROM MECHANICAL TO "ELECTRICAL MACHINES"

In 1935 a machine patented in the USA, featuring electrodynamic vibration sensors and stroboscopic determination of the unbalance angle pioneered a change-over to a new design.

In 1942 Schenck was granted a patent for a "Method and facility for dynamic balancing by determination of the angular position of unbalance by means of a periodic curve displayed on the screen of an oscillocope". This was the first balancing system suitable for large-.volume production. Due to its high accuracy the system was used right through the second world war for balancing gyroscopic stabilizers for naval vessels.

The wattmeter method, the next step in the development, suppressed undesirable parasitic vibrations. With the basic components known at this time, i.e. wattmeter, vibration sensor and angle reference generator, it was possible to determine the position and magnitude of the unbalance in one measuring run. Unbalance values were displayed on two pointer instruments.

In 1953 the illuminated-spot vectormeter brought a further advance. The combination of both values in a single display unit and the "storage" of the measured values in the form of a light spot on a screen significantly simplified the balancing process - the unbalance was now visible. Even today, the vectormeter is an indispensable part of modern measuring instruments - a practice-proven method of showing the position and magnitude of the unbalance on modern measuring instruments with display screen.

In parallel to the development of what was then called "Workshop Machines" - nowadays referred to as Universal Balancing Machines - automation of the balancing process moved forward with great strides. "Balancing lines" for crankshafts determined the unbalance of crankshafts and the required drilling depth for its correction. A simple transport system transferred the crankshafts from the measuring station to the drilling unit and back. Altogether, unbalance measurement, correction and check run for a crankshaft took around 2 minutes.

THE DAWNING OF A NEW AGE IN BALANCING

The rapid economic and technical development in the post war period also left its marks on balancing technology. Until the present time, the automotive industry, aeronautical and aerospace technology, the energy generating and electrical industries and mechanical engineering with their constantly rising requirements are the driving forces for its continuous further development.

In the early fifties a completely new method was developed for large- volume production of crankshafts: Mass centering of crankshaft forgings or castings. This was a new process which enabled the actual axis of inertia of a crankshaft forging or casting to be determined and marked with centering holes.

Also in the early fifties, rapidly increasing demand for electrical power resulted in larger power stations being built with ever increasing power generation capability. The development of Series RI and DI balancing and over-speed test rigs for turbines and generators made it possible for the first time to systematically correct the unbalance of rotors with a total weight of up to around 80t.

For the first time, mass correction was performed by a drill unit integrated into the balancing machine. These systems were suitable for many tasks in large-volume production and took into account the economical aspect of balancing. As a result , the cost of balancing decreased significantly, as the time-consuming task of removing the rotor from the balancing machine and re-installing it on the drilling machine was no longer necessary.

The aeronautical and aerospace industries presented their own challenges for balancing technology. A number of pioneering new solutions for demanding tasks were developed, often venturing on to new terrain. The emergence of jet engines brought a new advance in balancing technology: A series of horizontal and vertical machines was built, tailored exactly to the requirements of the jet engine manufacturers.

During this period, the aerospace industry emerged as a new partner. The first balancing machines for satellites and rockets were developed, along with moment of inertia measuring tables, or centre-of-gravity weighing systems.

In contrast, the problems of motorists were much more down-to-earth in the early sixties: As cars got faster and faster, and new types of tyres emerged, unbalance became a real challenge. Motor car manufacturers started balancing tyres during the production process. In addition, special workshop machines were required for retrofitting.

Towards the end of the sixties, the product range was extended with a series of MAN spin test rigs manufactured under a licence. Apart from turbine and compressor discs, their main areas of application were in fatigue testing of fast-moving tools such as polishing and grinding discs.

After 1968 hard-bearing machines became more and more common in the industry. Although early models did not achieve the same accuracies, they had a practical advantage in everyday operation in as much as they were significantly faster. There was no need for a rotor-specific calibration - all the machine operator had to do was to enter some basic geometrical dimensions and start the balancing run. It only took one balancing run for the machine to display the magnitude and angular position of unbalance. This was a significant advantage, which was revolutionary at the time. Today, most balancing machines, with the exception of machines intended for specific purposes, operate according to this principle.

FROM ELECTRONIC TO MICROPROCESSOR-BASED MEASURING UNITS - THE START OF THE DIGITAL AGE

In the seventies, the mechanical foundations for balancing machines had basically established themselves. Electronics made their appearance in balancing and diagnostic technology. In 1971 the electronic wattmeter measuring principle was introduced, the first computer-controlled balancing systems were introduced in 1974. The next major change came with the emergence of digital technology: At the beginning of the eighties, microprocessors started appearing in measuring systems.

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