SKF black design

SKF black design

Saving weight is currently a huge challenge for the aerospace industry. Enhancing performance or achieving lower emissions can offer manufacturers major advantages in a competitive market. The use of composite materials offers opportunities in all these areas.

When it comes to interface parts, saving weight by shifting to a composite is a challenge because of out-of-plane loading. To avoid increasing costs, function integration is a necessity.

SKF Solutions helps to achieve these goals, thanks to two major futuristic technologies – SKF Black Design and SKF bearing integration.

1.SKF Black Design is the capability of making high-performance, out-of-plane loaded parts from fiber-reinforced materials. Once the way of using the composite technology for an interface part is fully accomplished, it will open up a new field of opportunity, brought about by composite technology through function integration of such a bearing.

2.SKF bearing integration efficiently embeds a bearing into a composite part, by providing a strong interface to the outer ring of a rolling bearing or integrating a spherical plain bearing into a composite housing.

These composite technologies are able to address a very wide range of issues:

Reducing the weight of existing metallic parts by consolidating them into one composite solution

Enabling function integration of new features such as sensor solutions

Solving fatigue problems on cyclically loaded parts

Avoiding corrosion by using a material insensitive to a corrosive environment

Designing out noise and vibration by adding stiffness, shifting weight or embedding 

Focus 1: Development of composite structural parts with SKF Black Design

SKF Black Design challenges gravity

This technology provides the capability to design and build parts for structural interfaces in a composite material. Within the aircraft industry, the use of composite materials has soared, yet traditional design techniques provide only limited improvements in their structural performance. This has restricted the applications that composite solutions could be applied to. SKF has succeeded in meeting the technical challenge of turning metal structural interface parts into lightweight and high-performing composite parts. The result of years of research and development is called SKF Black Design. SKF achieved this exceptional result by changing the paradigm of composite-part engineering. Deploying conventional part designs in composite rather than metal (“Black Metal” approach) yields only modest improvements in performance, and the parts remain prone to unfolding and delamination. However, in SKF Black Design, the matrix (resin) material is used where it performs best: in compression. To make this happen, SKF engineers developed new forms, shapes and part geometries, designed to make sure that in areas subject to out-of-plane loading, the resin remains in compression through the laminate thickness. This leads to higher shear stress resistance and eliminates the unfolding effect. The use of hemispherical washers to avoid any tendency to punch through and, in addition, provide compression in the composite, is an example of a clever solution that is proven to significantly increase the strength of the part.

The designs also incorporate sophisticated corrugations and the carefully balanced use of bulk and continuous reinforcing materials – all knowledge that the SKF Composite Centre uses to help its partners.

The goal of weight saving in aircraft is to reduce both fuel consumption and the impact on the environment. This has led to a 50 % increase in the use of composite materials in the latest generation of commercial aircraft. Increased use of composite materials in aircraft, however, is limited by the structural performance obtained with the traditional design of composite parts. SKF Black Design provides the structural performance required by combining the use of existing carbon-fibre-reinforced polymer (CFRP) materials with innovative design techniques. SKF Black Design extends the scope of application of composite solutions to structural parts with a competitive mass-versus-cost ratio, compared with the current metallic solutions.

The composite materials most commonly used in the aerospace industry are made by stacking pre-impregnated plies of carbon fibre. The resin enables transmission of the load between fibres and ensures the cohesion between the plies. This technology fits well with fuselage, wing, frame and stringers as the laminate is primarily subjected to in-plane loads. For these geometries, the carbon fibres provide high strength and stiffness in the orientation of the fibres; however, in the direction normal to the plane of the fibres, the mechanical properties of the material are dominated by the resin. The resin is the bonding agent and has a poor strength compared with carbon fibre (about 50 times less). Therefore, use of composite materials is limited for structural parts with out-of-plane loading, such as structural interface parts such as T-shaped fittings and cleats (fig. 1). With traditional geometry, metallic fittings and cleats are subjected to folding/unfolding phenomena. In composite fittings of the same shape, the same loading pattern leads to the separation of the CFRP plies in the 90° corner due to interlaminar stresses in the resin. This phenomenon is called delamination and occurs at a very low level of loading, which results in a non-feasible part.

Fig. 1: Out-of-plane loading, example.

The composites industry has investigated new technologies, such as 3D weaving and high-performance resins, in combination with traditional part geometry (“Black Metal” approach) to solve this issue. This approach, however, has shown significant limitations in terms of mechanical performance and cost competitiveness.

Another approach is possible with SKF Black Design

SKF Black Design is based on concepts that solve the challenge of out-of-plane loading by adapting the geometry of the part.

The first concept consists of designing the part geometry and ply layup to maintain the resin in compression when the part is loaded in its application. The resin withstands far more stress in compression than in tension, and compression also improves the resin’s shear stress capability. This design philosophy has commonly been used for structures throughout civil engineering history, such as arches and bridges, where the design has required keeping the structural components in compression.

Fig. 2 shows how weak the resin is in tension and the effect of compression on the shear stress capability of the resin.