From tossing and clearing to super skiving

Mechanism of existing hob peeling processes. © Mitsubishi Heavy Industries

Increasing demands on the component spectrum of planetary gear systems has prompted manufacturers to partially replace the conventional ring gear manufacturing techniques of shaping and broaching with the alternative process of gear skiving. This process, in which material is continuously removed from the gear to produce its teeth, allows for greater productivity and flexibility (Figure 1). However, due to the intensity of the process, its industrialization on a large scale has so far been hampered by a shortened tool life for a typical skiving cutter (PSC).

Improving the gear skiving process

Illustration of a conventional skiving cutter (Pinion Skiving Cutter; PSC; left in the picture) and the super skiving tool. © Mitsubishi Heavy Industries

The new Super Skiving Tool (SSC), developed by Mitsubishi Heavy Industries Machine Tool (MAT), aims to improve cutting conditions while increasing tool life.

Cutting blade cross-section and use in the skiving process. Pinion Skiving Process is shown on the left, Super Skiving Process on the right. © Mitsubishi Heavy Industries

In contrast to a conventional hobbing cutter, the SSC tool consists of several cutting planes (Fig. 2 and 3). Each cutting plane represents a separate cutter with a specific number of cutting teeth. The cutting volume can therefore be distributed more widely and the cutting load per tooth can be reduced - thus reducing wear.

A comparative test conducted by the WZL machine tool laboratory at RWTH Aachen University between the super-skiving cutter and a conventional skiving tool showed that the super-skiving technology was able to improve the cutting of harder materials, which is used for ring gears. It was also shown that the cutting tool itself has a longer service life, making the skiving process more cost-effective. It was also demonstrated that by choosing the right combination of materials, tools and machining parameters, the super skiving workpiece can have an improved tool life compared to the single-stage skiving tool, even on harder workpieces above HRC 35. The process also reduces residual stresses, which can lead to distortion during the final heat treatment of ring gears.

Continuous internal gear grinding process (below) and comparison between processes, quality and costs. © Mitsubishi Heavy Industries

After this heat treatment, the gears are finished by a grinding process which, like gear skiving, was difficult to scale up for mass production until recently. Grinding the gears refines them and significantly reduces the distortion that occurs during heat treatment. To make the grinding process more efficient and more suitable for mass production, MAT has developed a gear grinding machine that features the world's first continuous internal gear grinding process (Fig. 4).

Next steps

If the challenge of the last ten years has been to make vehicles more efficient and use less fossil fuel, the challenge for the next decade is to make similar progress for electric vehicles.

Transmissions are just a small part of a much larger environment that the industry needs to explore and invest in to achieve the quality and efficiency required for the expected widespread adoption of electric vehicles. Automakers and their supply chains must innovate when it comes to designing and manufacturing each component of an electric vehicle to ensure that the vehicles and their production achieve the range and price needed to drive mass adoption.

Alexander Spatzig, Head of Machine Tool Business Development Europe, Mitsubishi Heavy Industries Europe / ag