Extreme Precision Pushing the boundaries of feasibility

High-precision gears that guarantee quieter-running transmissions are the reason ZF was founded. Over the last 100 years, the need for precision in development and production has grown exponentially.

One of the main criteria for ZF products has always been extreme precision. Since the company was first founded, employees in development and production have striven to be as precise as possible. In the early years, one tenth of a millimeter was the technical limit in bespoke manufacturing. But today’s production processes are capable of extraordinary precision – even when large and heavy components are involved.

A robot keeps the production machine supplied with transmission shafts.

ZF’s Passau plant, for example, produces shafts for the ERGOPOWER LII transmission, the heaviest version of this powershift transmission. It is designed for dump trucks with engines capable of producing nearly 540 horsepower (400 kilo-watts) and transporting loads of up to 50 tons. The automatic transmission weighs almost 2,650 pounds – well over a ton – with up to eight forward and four reverse gears. It has to be exceptionally robust, reliable and durable. Indeed, even under full load and over difficult terrain, gearshifts are totally smooth and do not interrupt tractive force.

The ERGOPOWER transmission uses eight shafts to transfer power. It takes eleven days to transform these shafts from rough, raw metal blanks to polished, high-precision components, in a 15-stage process involving state-of-the-art machine tools.

The heaviest shaft is nearly 14 inches long, has a diameter of around 11 inches and weighs 47 pounds. Despite these bulky dimensions, the smoothness of its bearing surfaces must fall within a manufacturing tolerance of just two micrometers, or two thousandths of a millimeter. By way of comparison: a human hair is between 50 and 70 micrometers thick.

Video: Ergopower sets precision benchmarks

Andreas Wiertz, Head of Shaft Production in Passau, explains why even large construction machines need high-precision components.

“The machines used to manufacture these shafts have extremely sensitive measuring systems so they can handle this kind of high-precision work. They even measure the temperature of each component and compensate for the corresponding thermal expansion of the metal,” explains Gerhard Rott, a Production Planner for Shafts. Finally, at the hard turning stage, a cutting tool with a tip made out of cubic boron nitride (CBN) – the second hardest material in the world after diamond – is used to ablate minute irregularities from the surface of the shaft. The excess material is shed in the form of tiny, glowing shavings. “We can’t use diamond for hard turning, because the heat generated would cause it to transfer carbon to the steel and eventually dissolve,” explains Rott.

The machines used to manufacture the shafts have extremely sensitive measuring systems that even measure the temperature of each component. For the quality control on a transmission shaft instruments were used that are capable of measuring to within one thousandth of a millimeter.

Once the workpiece has been completed, samples of the transmission shafts are taken to an air-conditioned room kept at a steady 20 degrees Celsius, where they are inspected using needle probes to make sure they comply exactly with the specified dimensions. “The more precisely these components are constructed, the less wear and tear there will be inside the transmission, and the quieter it will run,” comments Andreas Wiertz, Head of Shaft Production. “This in turn improves the lifespan and efficiency of the transmission, while reducing fuel consumption and emissions.” ZF customers reap the reward of this high precision: the ERGOPOWER transmission is up to 20 percent more fuel-efficient, and has a working life as long as that of the construction machines in which it is installed.

Pictures: Dominik Gigler

Further related articles