How do we make... teeth?

In an engine, gearwheels connect the camshaft and crankshaft to control the valves and transmit the torque for various units, such as fuel and oil pumps, or the alternator. The teeth on a gear mechanism rotate together with the shafts to which they are connected, or rotate on axes. "In other words, the gearwheel is the heart of the engine. It is indispensable as a drive and control element," says Markus Bucher, Head of the Camshaft and Gearing Competence Center at MTU Friedrichshafen, when explaining the role of the gearwheel in the engine. The teeth interlock and as a result, the rotational movement from one gearwheel is transmitted to another. "The ensemble works like the movement in a clock," explains Bucher. The company's own gearwheel production department manufactures more than 700 different gearwheels – they are also delivered to production locations in Aiken (USA) and Suzhou (China). But how do the teeth reach the wheel evenly?

Blanks arrive "toothless"

Gearwheels required in low quantities are manufactured on single machines, whereas gearwheels required in high quantities (such as for engines in the 2000 and 4000 series) are manufactured in series. The blanks are roughly machined on a lathe to begin with, and then the soft machining starts along a 12-meter-long conveyor belt. From this moment on, everything is automated: The gearwheels are only on the belt for a few seconds until the teeth milling machine automatically grips each gearwheel one after the other using a ring-shaped loader and mills the teeth into the steel that is still soft. This involves the wheel in the gear hobbing machine rotates so quickly that it cannot be seen by the human eye. During milling, oil is sprayed into the machine and material is removed from the blank. The hob looks like a snail: Using the hob, it is possible to manufacture teeth numbers, profile offsets and any necessary helix angles by varying the machine settings. The number and dimensions of the milled holes depend on the series. A medium-sized gearwheel has 89 teeth that are 1 cm deep. According to Bucher, "these are more like holes than teeth." Within three minutes, a blank is transformed into a wheel with holes. "A rotary cutter then removes any sharp burrs that remain after having milled the teeth," explains Bucher. The soft machining of the gearwheels is then completed in a continuous washing machine: Here, the gearwheels disappear under a hood, and the cooling and lubricating oil is removed. The gearwheels are grasped by a robot hand, arranged in weight and size order, then stacked in a container and passed on to the heat treatment section.

Within a short period of time, a blank is transformed into a wheel with teeth that a layperson may even believe to be finished. However, in this state, the teeth would only last a few hours when in operation as they are subject to enormous strain in the engine. The gearwheels are not made really robust until they are exposed to temperatures over 900 degrees, purposefully enhancing the edges using carbon and quenching the steel in an oil bath. The soft material – which may be altered as a result – is hardened.  

Following heat treatment, the precision work is completed on the hard machining line. "The requirements for the gearwheel only allow tolerances of up to a few thousandths of a millimeter. Everything must fit together," says the shift supervisor, Andreas Drach. The gearwheels are then once more placed on a conveyor belt that is approximately 12 meters long. In turn, the gearwheels are processed on a hard turning machine, a gear grinder and a continuous washing machine. After being checked for cracks and finished off with a bearing bushing if necessary, the wheels are ready for packaging. They are then stored in the "supermarket" that directly supplies the consumer.