US patent 5435693 describes an invention that provides a simplified system for securing gas turbine blades or vanes to a supporting structure to form a gas turbine blade or vane ring for a gas turbine.

Granted to Siemens, the new patent describes a simplified system that is composed of a supporting structure in which there are blade or vane holding grooves shaped in a corresponding manner to gas turbine blade or vane roots.

The invention is based on the discovery that the platform and the blade or vane root of the gas turbine blade or vane, as well as the holding grooves of the supporting structure, have to be shaped in the same way in the longitudinal direction or axial direction, in order to achieve particularly simple, even individual, assembly of the gas turbine blades or vanes.

Traditionally, these shapes are different: the platform longitudinal edges are curved in the axial direction, whereas the blade or vane root is rectilinear in the axial direction. This means that the platform elements located in-between the roots have to be introduced radially into their operating position in order then to be secured to the rotor disk by means of an additional holding which is fitted in a rectilinear direction of movement.

By contrast, the invention proposes that the blade or vane root be shaped in such a manner that the blade or vane root surface which respectively faces the suction-side profile wall and pressure-side profile wall be respectively convexly or concavely curved, in accordance with the platform longitudinal edges. All the geometric surfaces which influence assembly are then curved in the same direction, so that all the components which form the gas turbine blade or vane ring can be fitted together individually in a direction of movement corresponding to their curvature.

Complying with this geometric condition also makes it possible to provide gas turbine blades or vanes having a pressure-side platform and a suction-side platform, which each have an approximately equal platform width as platform overhang along the profile wall. The platform width is the distance from the pressure-side or suction-side profile wall to the closest platform longitudinal edge.

The approximately constant platform width allows significantly simpler and more efficient cooling of the platform. During use in a gas turbine, this leads to a more uniform temperature distribution, which in turn lengthens the service life of the gas turbine, on account of the reduced material stresses.

The design is subject to the condition that the gas turbine blades or vanes and, if necessary, platform elements, have to be suitable for displacement – assembly – into the supporting structure in the axial direction, based on their installation position in the gas turbine.

For this situation, the platform width which is constant along the blade profile is only possible with a blade or vane root which is curved in the same way as the blade profile. In the case of gas turbine blades or vanes with a rectilinear blade or vane root and a curved platform longitudinal edge – the traditional situation – the transition region between the straight blade or vane root and the curved platform must perform a certain geometric adjustment. In operation, the occurring forces and the mechanical and thermal loads have to be dissipated. This is not the case with the gas turbine blade or vane referred to in the patent. Moreover, it is possible to avoid the presence of platform overhangs in sections, as in the case of gas turbine blades or vanes with a rectilinear blade or vane root and a rectilinear platform.

On account of the curvatures of the suction-side and pressure-side platform longitudinal edges running in the same direction, and the corresponding blade or vane root surfaces, it is no longer imperative that a platform element be fitted in between, as is currently required with existing technologies. This also therefore eliminates the need for the holding element for the platform element.

Moreover, more gas turbine blades or vanes than has hitherto been the case can be provided in one ring.

The patent particularly highlights a number of important advantages to the new configuration.

For example, a gas turbine blade or vane of this generic type has a blade or vane root surface on which all the lines of curvature running in the longitudinal direction run on an arc of a circle parallel to the bent platform longitudinal edges. As a result, it is possible for the gas turbine blade or vane to be produced particularly easily and inexpensively.

Alternatively, it would also be possible for the suction-side blade or vane root surface and the pressure-side blade or vane root surface to be curved with respect to one another in such a manner that the blade or vane root becomes more pointed – with a wedge-shaped reduction in its cross section in the longitudinal direction, tapering from a leading edge end to a trailing edge end. In operation, a gas turbine blade or vane of this type would be pressed into a correspondingly shaped holding groove in a supporting structure as a result of the shear forces occurring in the hot gas and thereby axially fixed in place.

This new configuration is described in the patent as ‘particularly advantageous’ for a suction-side or pressure-side platform protuberance, the platform width of which is approximately constant over 30 per cent of its length running in the longitudinal direction, to project from the suction-side profile wall to the suction-side platform longitudinal edge and/or from the pressure-side profile wall to the pressure-side platform longitudinal edge. On account of the approximately constant platform width, the transition from platform to blade profile is exposed to more uniform thermal and mechanical stresses in operation. A platform configured in this manner can be cooled particularly well and uniformly and avoids platform protuberances which are uneven on account of having significantly different widths along the blade profile. In addition, fatigue strength can be increased on account of the stresses now being more even.

A particularly small platform can be achieved if the suction-side and/or pressure-side platform protuberance is designed as a platform stub with a relatively short platform width. In this case, the gas turbine blade or vane is almost platform-free, which significantly simplifies its structural design. This simplification leads to a reduction in costs when designing the gas turbine blade or vane and producing it.

Finally, notes the patent, the material stresses which occur in the transition region between blade profile and platform and which are responsible for the premature fatigue are eliminated.