Harmonic drives improve machine tool precision

Innovative gear systems have replaced traditional gearbox and motor assemblies and have enabled a machine tool builder to reduce the size and weight of tool heads and minimise backlash.

The Winbro Group, one of the world's leading designers and manufacturers of high performance laser and EDM machining centres for producing specialised parts for gas turbines, is integrating the latest Harmonic Drive gear technology into its machine tools to improve levels of precision and reliability.

Supplied by Harmonic Drive UK , the innovative gear systems have replaced traditional gearbox and motor assemblies and have enabled Winbro to reduce the size and weight of tool heads, minimise the effects of backlash and improve positional accuracy and repeatability.

Based in Coalville, Leicestershire, Winbro produces custom built machining centres for many of the world's major manufacturers of gas turbines, which are used in the aerospace and power generating industries.

In each case, turbines require extremely high operating temperatures to function efficiently; and can be in excess of the melting point of even the most advanced materials used for the turbine blades.

To overcome this problem, turbine blades, nozzle guide vanes, combustors and other components use complex air cooling techniques, with each part having many tens or hundreds of cooling holes drilled through its outer surfaces, through which cool air from the turbine compressor is drawn.

The shape, size and angle of these holes are critical to ensure that a boundary film of cooling air is formed along the surface of the component, to separate it from the hot turbine gasses without affecting the operating temperature and thus the efficiency of the turbine.

The Winbro machining centres have been specially developed for drilling precise patterns of cooling holes in turbine parts.

However, this is rarely a simple process, as project manager, Chris Clements, explains, "Parts such as blades and vanes often have complex shapes and the holes have to be drilled at precise locations on the surface of the component.

In addition, the geometry of each hole has to be accurately machined to produce the correct flow pattern." Depending on the application, Winbro manufactures machining centres that use either Nd:YAG lasers or EDM technology for precision drilling applications.

Lasers generally have the advantage of faster drilling speeds, while EDM generally offers the advantage of achieving improved metallurgical results and also can drill holes where direct line of sight is not possible.

In each case, the company has largely standardised on the use of Harmonic Drive gear technology to move and position tool heads and work pieces.

Harmonic Drive gear units comprise three basic component parts: an elliptical Wave Generator, Flexspline and Circular Spline, which fit one within the other to form an integrated unit that is easy to install and configure.

The overall construction is extremely simple and reliable and allows each component to be used as the input, output or fixed part, enabling the gear unit to be used for reduction gearing, gearing for increasing speed, or differential gearing.

For Winbro, this technology offers a number of benefits, as Clements points out.

"The Harmonic Drive system offers a number of important advantages over conventional gearbox and motor units.

For example, it is considerably simpler, lighter and more compact,.

Used on our latest laser systems, Harmonic drive units used in conjunction with directly coupled encoders allow us to achieve rotary accuracies of 20 arc seconds, with repeatability being 10 arc seconds." More recently, Winbro has incorporated the new Harmonic Drive FHA hollow shaft servo actuator into both the work piece manipulator and laser head on its machines.

This actuator has a large central hollow shaft which allows the laser beam to be directed through the gear unit or other services such as pneumatics, hydraulics and electrics to be passed through, it is available with an absolute position output shaft encoder providing direct measurement of output shaft position and eliminating the need for referencing and its feedback systems are compatible with most major controller manufacturers.

"This has saved us valuable development time and reduced both the space and number of components that are needed in the tooling head", explains Clements.

"The FHA actuator is supplied as a complete system, with gearbox, motor and encoder, and is quick and simple for us to integrate into our machining centres.

Just as importantly, Harmonic Drive has provided an exceptional level of technical support, both from its factory in Germany and through its engineers in the UK, our project teams to develop innovative and reliable motion control solutions quickly and cost effectively.".

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