Vehicle design develops with environmental pressure

Paul Boughton

The drive for more fuel efficient vehicles which retain a good level of performance has led to dramatic reductions in panel weight and a rethink of fastening techniques. Lawrence Large looks at some fabrication methods for lighter materials.

The growing need to protect the environment has led to a global drive to force down our dependence on fossil fuels and the emission they produce when consumed. The motor industry in particular is seen as a sector where substantial gains can be made in the battle to reduce the carbon footprint. Making vehicles lighter gives a relatively quick, a lighter vehicle uses less fuel and better economy means that less carbon is emitted to the atmosphere.

Environmentalists have agreed emission reduction programmes which in many countries have led to legislation to reduce energy usage. In the US, for example, industry is now expected to cut emissions across the board to achieve the targets set out in the Copenhagen Accord which recognised the need to reduce global emissions back in 2009.[Page Break]

Reduced vehicle weights

Designers around the world are busy engineering lighter models by using stronger materials to reduce vehicle weights. Replacing steel panels with aluminium alloys or composites can substantially cut vehicle weights without jeopardising the vehicle's strength and integrity. In the US, for example, Ford Motor Company is redesigning its most popular model, the F150 truck, utilising aluminium and magnesium instead of steel panels. The weight reduction achieved by this move, due in 2014, is expected to produce sufficient reduction in carbon emissions to meet the company's overall target.

The 2012 C-Class shows how far Mercedes-Benz has gone towards improving fuel efficiency of its entry-level saloon car. In European form, this year's model is 31 per cent more fuel efficient than its predecessor thanks mainly to stop/start engine technology and weight reductions. Cars in the US will also get lighter panels, in particular a new aluminium bonnet, highlighting the company's interest in weight reduction.[Page Break]

Fuel efficiency

Sustainability in vehicle design continues to be a driving force in the sector but to the car buyer who is searching for the best performing, yet most fuel-efficient vehicle, the areas where the greatest efficiency gains can be made may not always be that obvious.

According to hardware specialist, Southco, there is a continuing requirement in the design of new cars and trucks for the attachment of lightweight underbody protective shields. As well as contributing to weight reductions of the finished vehicle, these panels improve both aerodynamic efficiency beneath the vehicle and overall fuel economy by decreasing drag.

More cars and more underbody protective shields are good news for companies within the automotive OEM supply chain, as this increases the demand for associated components, such as robust fastening mechanisms. These fasteners must allow both rapid and easy installation on the assembly line, yet also enable subsequent fast access for post-assembly servicing.

Southco claims a leading position in the supply of equipment to tackle the access demands of this growing automotive sector with its range of Dzus quarter-turn fasteners, which offer an effective replacement to conventional threaded fastening devices and meet the underbody and protective shield requirements of today's cars and trucks.[Page Break]

Resistance to vibration

The key benefits of Dzus quarter-turn fasteners include resistance to vibration, fatigue and temperature changes, resistance to corrosion, and the choice of a variety of head style options. And Dzus fasteners can be simply secured to steel, aluminium or composite chassis underbody components through pre-punched holes.

Ulrike Sturman, industry marketing manager for transportation at Southco, explained: "There is a perpetual contrast between fasteners based on traditional screw thread systems and those based on quick-release mechanisms. It has long been perceived that, for a robust and durable fastening system, threaded fasteners offer the most cost-effective solution."

Although a threaded system might be cheap to buy, the time it takes to lock and unlock the system makes it expensive in terms of initial installation.

"The recovery gains of the Dzus quick access quarter-turn system, made from reducing the time it takes to install and then from future access needed for servicing more than outweighs the upfront investment," Sturman added.

More recently, the company has developed a range of new quarter-turns and accessories. These include a stud that has an orientated head feature to cam. This brings the added benefits of the head feature always engaging and locking in the same position. The lock position of panel fasteners can therefore be identified, and installation time is speeded up.[Page Break]

Self-piercing technology

Moving away from steel panels does present an assembly challenge for motor manufacturers as spot welding, possibly the most common joining technique used in car assembly, is at best problematical when assembling light alloy or composite materials.

An alternative system to replace welding of lighter materials is the self-piercing rivet (SPR). In its crudest forms, self-pierce fastening has been around for hundreds of years; compare it to the traditional hammer and nails. The system has developed over the past few decades and this versatile and efficient fastening process is certainly one to watch in the future.

The SPR technology originated in Australia and was invented at a Brisbane University in the mid-1970s. Keith Jones, who is the owner of fastener supplier Henrob, was out in Australia on a business trip and came across the technology. Jones saw the technology as a real business opportunity so purchased the intellectual property and brought it back to Europe for further development.

As a leader in this field, Henrob is committed to pushing the boundaries of SPR technology.

According to Henrob's Phil Halsall, there is an important difference in service life between a pre-drilled and riveted joint and one that is joined using SPR. The fatigue life of the SPR exhibits a tenfold improvement over the blind fastener and this is why SPR has been so readily adopted on many body-in-white vehicle applications.

Thanks to the technology, the company has a full order book and can plan with confidence. The market place has changed over the last three years and there has been complete swing in the way aluminium and high strength steel has been adopted in the automotive market place on a global basis and at every OEM.

Henrob is determined to keep one step ahead of its competition and continue its rapid expansion. Halsall believes that the company needs to manage this growth and deliver a level of profitability that will allow future investment in the technology development and high production volumes.[Page Break]

Self-clinching option

As materials are formulated to be lighter, panels are made from harder material to maintain their strength. Harder materials are harder to join and as well as being less suitable for welding, they are tougher to pierce. A fastener using a pre-drilled or punched hole overcomes the problem.

The PEM nut is an internally threaded fastener, that when pressed into ductile metal, displaces the host material around the mounting hole, causing it to cold flow into a specially designed annular recess in the shank of the fastener. A serrated clinching ring prevents the fastener from rotating in the host material once it has been properly inserted. Thus the nut becomes a permanent part of the host material offering strong, load bearing threads in relatively thin sheets of metal. The generic term for this type of fastener is self-clinching nut. A current day use of the technology illustrates its benefits over other joining methods.

In the German automotive sector, there is a company producing seat rails for three of the leading car manufacturers. In recent times, there has been a need to reduce component weight for environmental considerations while retaining strength for safety reasons. This has meant that the seat rail has become thinner but is now made from a harder material so self-piercing fasteners and even welding are no longer an option.

The result is that the majority of road cars made in Germany have their seats attached with self-clinching fasteners.

Although primarily intended for the aerospace industry, an interesting new development from Böllhoff Armstrong is suitable for joining thin plates or non-metallic panels uses a rivet nut to provide the depth of thread required and a threaded insert to provide the required strength and load distribution.

The new component was launched at this year's Farnborough International Air Show and includes a rivet nut made by Böllhoff in France, with a threaded insert from Böllhoff Armstrong fitted into it. The result is the Rivnut Aero which combines the benefits of both Rivnut and HeliCoil. The user is presented with a high resistance self-locking insert nut which provides a resistant thread on carbon steel and aluminium plates. The design prevents unscrewing and plate sliding and is very flexible in use because of its all-in-one installation kit.[Page Break]

Corrosion resistance

The shank is made of 316L stainless steel, which ensures the necessary corrosion resistance and mechanical characteristics. The HeliCoil Screw-lock helical thread insert prevents unscrewing and plays its part in providing the desired connection performance. The threads can be silver-plated to prevent binding.

The design has some real advantages combining the advantages of blind installation, reliable and reproducible results and possibility of checking the swage quality during the fitting operation with those of the helical thread insert, which include efficient and durable anti-unscrewing to ISO 2320, a high wear resistance of the thread as well as excellent resistance to vibrations and thermal shocks.

After the roller coaster ride of 2011, the European motor industry is seeing a marked upturn in fortunes and despite continuing economic uncertainties in the Eurozone, EU motor manufacturing is forecasting car sales of 14 million in 2012. The manufacturers' adoption of sustainable designs using lighter materials has certainly been a key element of this success. In their own way the fastener industry has made an equally valuable contribution by developing the joining techniques required for fabrication with lighter materials.[Page Break]

Self-piercing threaded stud

Aston Martin's designers and engineers teamed up with Henrob to develop a self-piercing threaded stud that would cope with the extreme demands of auto body applications.

Aston Martin's DB9 has a body structure fabricated from high-strength aluminium alloys and exotic aerospace composites, and it is every bit as impressive under the skin as it is on the surface.

When specifying a trim and earth attachment studs, the designers at Aston Martin were faced with serious challenges in finding a product that fulfilled all of their stringent design criteria.

The studs had to be fixed to anodised aluminium panels, which meant that welding was out, as the difficulty of welding through the anodic film would prevent the studs from attaching properly to the panel. The head generated during welding would have locally damaged the anodic film, thus compromising the anti-corrosion performance of the body structure.

The studs also required a high degree of electrical continuity, so they could be used to earth the many electrical components attached to the body structure. This meant that adhesively bonded studs could not be used, as little current would pass through the adhesive layer between the stud and the car body. As heavy items of in-car equipment would be attached to the body using the studs, the studs required a high degree of mechanical strength, torque and vibration resistance.

Over 200 Henrob self-piercing studs are inserted in each DB9 body in a simple one-shot process that does not require pre-drilled holes, is quiet, clean and repeatable.