January 2021

36 n GEARS AND GEARBOXES January 2021 www.drivesncontrols.com Rack-and-pinion drives help to build aircraft parts precisely T he complex process of producing aluminium wing ribs for aircraft involves milling the structures out of solid material using high-speed machining centres. For some components, more than 95% of the material needs to be removed. For this precision task, a Chinese supplier to the aeronautic industry is using several Ecospeed 25100 type machining centres from German-based Starrag Technology. These centres are equipped with a parallel kinematic machining head that enables highly dynamic machining simultaneously on five axes. The spindles run with a nominal power of 120kW at 30,000 rpm in S1 mode. With an acceleration of 1 g on all five axes, and jerk values of up to 200m/s3, the linear axes also need to achieve extremely high dynamics. The Chinese company is using these machines to produce wing ribs and other structural components for a variety of aircraft. They can achieve a maximum cutting volume of up to 10,000 cm3/min and can, for example, transform a 4-tonne aluminium block into a complex 120kg structural component in less than 20 hours. The machines can perform horizontal machining on vertically positioned aluminium blocks. One advantage is that gravity removes the chips automatically. To make this possible, the aluminium blocks, which can be up to 10m long, are fitted horizontally onto a carrier pallet before being swivelled into a vertical position. For one of the latest Ecospeed 25100 installations, the carrier system and swivel table for large workpieces were designed and built for Starrag by another German engineering business, MSB. The 10m-long and 2.5m-wide workpiece carriers each weigh around 60 tonnes (including the workpiece). The pallet is clamped hydraulically onto the carrier before being rotated through 90 degrees for the milling operation to begin. This rotary motion has to be performed with high precision, because even slight irregularities in the movement could introduce unacceptable tension into the final machined component. A high level of synchronicity is needed between the four actuators mounted along the swivel table.“We have already developed many similar systems,”explains MSB’s managing director, Jürgen Enders.“We have often implemented the swivelling movement with hydraulic cylinders. But this was out of the question for this project because the hydraulics take up a lot of space and require a foundation with a sub-structure, when the user wanted to remain flexible. “That's why the designers opted for an electromechanical system with a four-gear segment driven by servomotors, in association with compact three-stage planetary gearboxes.” This mechatronic design not only needs much less space, but has other advantages. Because there is no need for a hydraulic system, there is no risk of leakage. Also, the power and cooling requirements for the electromechanical drives are much lower than those for an hydraulic system. The drive technology is not only compact and precise, but also clean, safe and energy-efficient. The swivel drive consists of four individual gear rim segments. The rotary motion provided by the servomotors is transmitted to an output pinion via a three-stage Redex KRPX planetary gear connected to the motor with a 310 ratio and a maximum output torque of 10,050Nm. In total, a maximum torque of around 600kNm can be applied to the rotary axis. Redex also supplied the pinion that engages in a gear rim segment with a segment angle of about 95 degrees at a pitch circle diameter of 4,496mm. The gear rim teeth are hardened and ground. The installation’s planetary gearboxes were developed specifically for use in precision rack-and-pinion machine tool drives. They are characterised by high positioning accuracy, which is achieved, for example, by ensuring precise tooth engagement on the gear rim segment. The gears (braced electrically against each other) can also be used as twin drives for precision positioning drives. Two further advantages for the Starrag application are the enclosed design of the housing, which prevents chips frompenetrating, and the compact size of the gearbox, which is mounted onto the base frame of the swivel table without needing extra mounting brackets. Since being installed at the Chinese site, the high-speed machining centre, including the rotary table, have been producing wing ribs with high productivity and precision. n A Chinese manufacturer of aircraft components is using an extremely large workpiece carrier system and swivel table to feed aluminium blocks up to 10m long to a machining centre. The table’s four synchronous drives incorporate gear rim segments and precision planetary gearboxes. Four slewing rings on the Starrag machining centre turn a 10m-long workpiece carrier with the component to be machined through 90 degrees. The synchronous rotary motion is controlled by servomotors and three- stage planetary gears that apply a torque of up to 600kNm to the rotary axis.