OTEC Präzisionsfinish
Sf5streamfinishing 1540

Stream Finishing for Better-Performing Blades

May 20, 2020
Precision process improves airfoil finishes up to Ra < 0.2 µm in minutes, increasing blade efficiency.

A turbofan engine has many parts: at the front, a fan draws in air and directs it into the compressor, and consisting of numerous blades arranged in rows and decreasing in size toward the end of a narrowing tube. In a rotational movement, drawn air is compressed up to one-thirtieth of its volume, which then compresses and heats the gas. That air is fed into the combustion chamber, where it mixes with injected fuel and burned.

The resulting energy propels the high-pressure turbine, where turbine blades to drive the compressor are installed. The downstream, low-pressure turbine also is set in motion using this energy. The low-pressure turbine consists of longer turbine blades and is directly connected to the fan. The turbine ensures that the fan rotates, and the fan draws air past the compressor and the turbine. The cold air, which is fed past the interior, generates the greatest propulsive force. The process inside the engine merely ensures that the engine remains running. So, the exhaust gas-flow produces 20% of the propulsion and the fan produces 80%.

Both the turbines and compressor blades are subject to high temperatures and pressures, so manufacturers have implemented strict regulations for producing and processing them. Aerospace engine blades usually made of materials that are difficult to machine and have a low tolerance that must be met to obtain the ideal air-flow and maximum wear-resistance. These components are exposed to extreme temperatures, up to 1,000°C, meaning that the blade surfaces also have to be of the highest quality and optimally adapted to the conditions in the engine.

OTEC Präzisionsfinish GmbH has developed a process to improve the efficiency and safety of engine blades and produce fewer defects. Smoothing the surface of the airfoil (i.e., the blade body) has a positive effect, and depending on the required result the surface can be smoothed to values of up to Ra < 0.2 µm in a few minutes, increasing blade efficiency.

Industrial gas turbine blades can be processed by the same method.

The material is removed evenly and only a small amount of material is taken from the surface. Repairing the leading and trailing edges with precision rounding can reduce the quantity of rejected parts. Upstream machining, e.g. blasting, can damage these edges. OTEC’s method allows them to be rounded to a specified radius, and repaired. The rounding process is very precise and involves minimal material removal.

Deburring the root helps to improve safety by preventing the blade from becoming caught in the disc. Surface treatment not only prolongs the service life of the blades but increases their efficiency. This is also the ideal preparation for coating the components. Smoothing and rounding the engine blades is possible in a single step using OTEC’s stream finishing process.

In the stream-finishing process, the blades are clamped into a machine and lowered into a container of abrasive. Processing is carried out by both the rotation of the container and the movement of the workpiece in the media flow. The flow to the blades in the machine is clocked, i.e., the alignment angle of the workpiece changes at frequent intervals. This means processing can be precisely aligned to specific points on the workpiece, achieving a smooth surface and precise rounding without altering the shape of the blade.

An important benefit of stream-finishing is the ultrashort machining times, compared to conventional processes. Depending on the size and initial condition of the workpiece, the surface treatment of engine blades takes 2 to 20 minutes. As the blades are clamped individually, no damage will occur to the workpiece surface. All processing steps can be carried out in one machine.

The SF-5 stream finishing system can process up to five engine blades at once, ensuring high output and cost efficiency. Tests conducted after OTEC processing show positive results for residual stress, fatigue strength, and fluorescence control.