Planetary gears date back to the beginning of the 18th century when Scottish engineer William Murdoch invented them to improve the power gear ratio of a steam engine used to lower the groundwater level in a tin mine. Today’s power transmission engineering is still relying greatly on planetary gear sets, whether on bicycles, in cars or commercial vehicles with their broad range of powertrains or in electric motors for mechanical engineering. Their compact build and the option of creating a positive locking link between various drives and generators in the same vehicle make them the ideal device for transmitting the torque in hybrid and all-electric vehicles. Manufacturers however face the particular challenge of making the internal and external geared wheel whose quality standards have increased massively over the past couple of years. Under a tier 1 order for the mass production of internal geared wheels, PITTLER T&S used its SkiveLine machines to supply a trend-setting end-to-end solution for machining before hardening.

Both ring gears and planetary gear sets should be as compact, lightweight, powerful and accurate as possible. These requirements can be met only, if the sensitive component is made with highest precision and economic efficiency. Providing both attributes is particularly challenging, since the component walls are very thin and many processing steps are needed to produce grooves, oil bores or lubricant cavities on the inner and outer radius. A leading commercial vehicle manufacturer who contacted the DVS TECHNOLOGY GROUP member PITTLER T&S with the task of making his existing production line much leaner and economically more efficient also confirmed this finding. In particular, the large number of six different process steps and set-ups had to be reduced significantly.

Six set-ups reduced to two

After a couple of successful trial runs on-site at the premises of the experienced manufacturer of turning and gear cutting machines based in Dietzenbach, Hesse, the combined process design had fully convinced the prospective customer. Based on the PITTLER SkiveLine series of machines, an end-to-end production process was developed to cover everything from turning before hardening to gear cutting by means of the PITTLER SKIVING technology.

Process OP10 combines turning, gear cutting and drilling on the outside diameter. Use of the PITTLER SKIVING technology for internal gear cutting and all upstream lathing steps was integrated into process OP20. Thus, a component ready for installation was available after downstream nitrogen hardening. Having two SkiveLine machines plus automation unit take care of the entire machining process resulted in optimised cycle rates compared to the process being handled by separate machines and in a higher total productivity involving just two steps and set-ups. Another plus was that rigging and charging times could be kept very short although the actual machining times differed greatly.

Clamping system solution

An appropriate design of the clamping tools was essential to high-precision machining. This problem was solved in a joint effort between group business unit DVS SPANNTECHNIK and PITTLER. “The first trial runs already suggested that the component walls were of greatest significance because they were very thin after all material had been removed”, says Omar Sharif, PITTLER’s skiving technology product manager. “Conventional clamping would have deformed the component excessively in the machining process. A radial run-out of this magnitude would have made it almost impossible to achieve the specified final quality”, he concludes.

Instead, a fixing spindle is used to clamp the component in the first operation (OP10). The spindle just centres the component from the inside by three fingers holding the workpiece axially at a force of 10,000 N. This special tool helped to reduce the deformation attributable to clamping to just 2 µm. The second operation yielded a similarly successful result. A six-web diaphragm chuck also designed by DVS SPANNTECHNIK picks up the component at its outer diameter after the automation cell has fed the component into the workspace in the correct orientation. Introducing the force through just two points of the chuck reduced the deformation to 4 µm plus tension forces released by the workpiece and imperfections from the actual machining process. The innovative clamping tools reduced the total radial run-out of the internal gearing to just 40 µm.

William Murdoch probably could have only dreamed of such a precise and efficient solution for the production of planetary gear set components. The commercial vehicle manufacturer enjoys numerous benefits from the new production line. Fewer machines, fewer set-ups and fewer persons are needed to produce larger quantities of more accurately machined internal geared wheels of improved concentricity.