Precision at the grinding wheel

Greater process reliability in the grinding of landing gear components

Landing gears are subject to enormous stresses. The main landing gear, which is situated by the center of gravity of the aircraft, bears the main load of the aircraft during ground movements. Especially during landing, the landing gears of modern widebody aircraft have to absorb high shock loadings and deflect them away from the airframe. For this reason the extremely complex structures consist largely of heavy, forged steel components which even today normally account for four to five percent of an aircraft´s maximum take-off weight.

To satisfy the demands of aeronautical standards, landing gears have to be  comprehensively overhauled at regular intervals. This entails examining all the components at predefined intervals for signs of wear or material defects and testing their functional performance. The work carried out during a landing gear overhaul includes high-precision machining of the components. Some of this entails grinding, during which the component surfaces are machined with the aid of abrasives and bonded cutting grains and individual pieces of material have to be separated from each other.

Lufthansa Technik has set itself the goal of increasing process reliability during the grinding of large, coated landing gear components and thus of further improving the quality of the work. The technological groundwork for this was laid by the development of a grinding process with integrated quality assurance that could be monitored online.Thus under the "Grinding" project, which was supported by the Federal Ministry of Education and Research (BMBF), new measures were developed in order to systematically eliminate damage to individual parts during the grinding process.

The first step was to develop a temperature sensor that triggers an alarm and suspends the grinding process as soon as a critical temperature is reached. In this way damage to the landing gear component can be avoided. A specially developed fiber optic cable sensor enables the surface temperature of the component to be accurately measured online even though the component is constantly covered by the grinding wheel and the cooling lubricant during the grinding process. As a result it is possible to avoid grinding burn on the expansive parts, which could lead to uncontrolled weakening of the components. To avoid damaging vibrations in the grinding process, which can cause surface blemishes, new parameters were used as the basis for the machining, as a result of which a stable and virtually vibration-free grinding process can be guaranteed. A further outcome of the project is a laser-based measurement system that measures the component throughout machining. This system prevents the grinding wheel from colliding with the component if the operator has made an error in entering the machining parameters, thus enhancing process reliability significantly.

As a result of the "Grinding" research project, which run from January 2011 to November 2013, manufacturer-independent data has been obtained. In addition, new grinding parameters have been identified which optimize machining, process reliability has been increased and despite the improved quality of the work, a lot of time has been saved. As the results obtained constitute unique characteristics, the rights to these have been protected through several patent filings. In this way in collaboration with the Institute of Production Management and Technology of the Technical University of Hamburg-Harburg Lufthansa Technik has succeeded in increasing its experience in the grinding of landing gear components and in extending its technical edge in the area of landing gears.