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Locking bushes enable reconfigurable tooling

06 October 2010

More commonly used for connecting hubs, sprockets, cams and gears to rotating shafts without having to cut a keyway, novel locking bushes that are able to transmit torque both radially and axially are playing their part in the assembly of Europe’s most ambitious new military aircraft

Amid all the resources that the European aerospace industry can bring to bear, even the humble mechanical locking bush has an important role to play in the Neuron project to build an unmanned combat aerial vehicle demonstrator, due to make its first test flight next year. Neuron is currently one of the most ambitious and high profile aerospace projects in Europe and is led by Dassault of France with partners in Sweden, Italy, France and Switzerland.

During the build phase of this futuristic aircraft, reconfigurable fixtures in the form of hexapods support the fuselage during assembly. As additional parts to the fuselage are built in different assembly stages, new fixtures are added or moved to new locations on the structure, thus giving maximum support and also enabling easy accessibility for the assembly workers and inspection personnel.

This reconfigurable tooling arrangement relies on the holding properties of ETP-Express locking bushes, which are available in the UK from Lenze’s Techdrives division.

From robot to work holder
The six leg hexapod structure is well known in robotics and flight simulators for the ability to achieve true movement with six degrees of freedom - that is X, Y and Z axes plus yaw, pitch and roll. Hexapods are more commonly found with powered actuators, but a project between the University of Linkoping, ETP and SAAB sought to develop it for work holding. Here the additional advantages of high rigidity, ability to accept side loads, very precise adjustment and small footprint become relevant.

Each hexapod leg is fitted an ETP-Express locking bush that allows sliding movement until tightened, at which point it becomes rigid without backlash. Hexapod holding fixtures can be laser aligned and locked in position. When the time comes to reconfigure the fixture, the bushes are simply loosened, giving immediate free release of the legs.

ETP-Express shaft locking bushes are more commonly used for connecting hubs, sprockets, cams and gears to rotating shafts without having to cut a keyway. They work by pressurisation of a cylindrical chamber that sits between thin inner and outer walls, causing them to expand and grip the hub and shaft. Pressurisation that sets the rated performance is easily achieved by tightening a single radially-mounted screw.

Useful features
These bushes have features that are particularly useful for this hexapod application. As well as being able to transmit torque radially, ETP-Express is equally good at transmitting the axial forces present in the hexapod. Partial tightening of the pressurisation screw achieves a light resistance force against movement which is ideal for setting the position. Once set, the full tightening torque is applied without any resulting axial movement that could disturb the set position. Loosening the single screw gives immediate and free release.

The Neuron project will produce an unmanned aerial vehicle to serve as a technology demonstrator with the first flight planned for 2011. Its 6,700kg weight, 12.5m wingspan and ‘Star Wars’ appearance is based on a high degree of build precision – an achievement that in no small part can be attributed to the novel adaptable fixtures used in its assembly.

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