Keeping the weight down has long been a recipe for success for Lotus. On two new models the company is taking advantage of bonded aluminium space frames

Lotus, the legendary sports car manufacturer, is building on the successful Elise model. Three new models incorporating a bonded aluminium space frame chassis are headed for road and track.

This summer, Lotus is to begin building a new sports car for GM subsidiaries Vauxhall and Opel called the VX 220 and the Speedster. Both take advantage of space frames made by Hydro Automotive Structures that weigh in at just 68kg.

In fact, the VX 220/Speedster tips the scales at only 850kg, paired up with a 2.2 litre mid-mounted engine. The bonded aluminium chassis offers not only light weight, but also high torsional rigidity, absolutely necessary for precise control at high speeds.

The bonded frame allows an even lighter chassis than with other methods. Adhesive bonding allows thinner extrusions because the aluminium is not subjected to the strength-sapping welding process, according to Lotus.

In all, there are more than 60 parts in the Elise space frame. The individual extrusions come from Hydro Aluminium Extrusion in Tonder, Denmark. Machining is performed at Hydro Automotive Structures in Bromyard, England, anodising at Hydro Aluminium Extrusion in Bedwas, Wales and assembly is at Hydro Automotive Structures’ plant in Worcester, close to Lotus.

The bonding adhesive is precisely applied by robotic equipment. The assembled chassis is placed in an oven and heated to 185degC to cure the adhesive.

Lotus and Hydro developed the space frame for the Elise – a first for a production road car. The high strength-to-weight ratio and the light components meant that Lotus could use a smaller engine – a 1.8 litre in the original Elise – saving money and even more weight. The Elise weighs just 699kg and even the beefed-up 143hp Elise 111S tips the scales at just 714kg.

Also in production now with space frames from Hydro is the 340R, an exclusive sports car based on the Elise. The stripped-down 340R will be limited to a production of just 340, the company says. The concept car was originally designed with no doors, no side windows, no roof and minimal bodywork.

Next in line is the M250 coupe. Light weight, combined with a powerful V6 mid-placed engine, mean that the M250 will perform well on acceleration. It is expected to go into production in 2002.

• Hydro Aluminium Structures
• Hydro Aluminium Extrusions

June 2000

Alan Quinn visited Thermex in Redditch and discovered a completely new approach to heat exchanger design

Most proprietary shell-and-tube heat exchangers and oil coolers feature an outer cast or extruded aluminium shell, with oil inlet and outlet ports, and a tubestack comprising copper or cupronickel tubes.

Cooling is provided by water flowing through those tubes. Hot oil passes over the tubes, where its flow is turbulated (churned up!) by aluminium baffle plates, which act to increase the heat transfer.

There is a relationship of flow to heat dissipation and temperature: for a given amount of energy (kW) dissipated as heat from the oil, the temperature drop will decrease as the oil flow increases through the cooler. Accordingly, laminar flow – a straight, unmixed run of oil – fails to bring the bulk of the oil into direct contact with the heat transfer surface.

Thermex has recently introduced the hydroforming process (high pressure forming by water compaction) into fuel cooling applications. Hydroforming is a technique which uses a high pressure water-based emulsion to expand metal into the shape of a die cavity – a little like blowing up a balloon inside a milk bottle. The technique is preferable to conventional forming techniques because of the absence of welds and the lack of distortion. Also, because no metal is cut out to form the shape, there is no material wastage. Here, the process is used for forming single copper tubes with integrated baffles, to create the necessary turbulation without any baffle plates at all.

The outer tube sleeve is brazed at its ends onto the hydroformed component. Importantly, this reduces both the number of components and the man-hours expended in manually building up tubestacks and painstakingly brazing on curved baffles. What is more, the use of hydroforming is effective in overcoming the damage (fretting) problems caused by resonance.

The hydroforming process being used by Thermex was originally developed for Yanmar, the large manufacturer of marine diesel engines. The process has the additional benefit that it eliminates the need for worm-drive hose clips (or even bandclamps?) to secure tube fuel lines to the cooler unit, thereby providing much greater safety-in-use, in accord with international marine safety standards.

Hydroforming has been explored in the automotive industry as a means of being able to consolidate the number of parts required for suspension parts. The long-term potential exists that a component supplier will take the process on board to supply components on a production line basis. But the process is not fast, since the machine has to regain pressure from the ring main after each operation before carrying out the next.

• Thermex

October 1999

Even if you’ve devoted your career to studying sheet steel, you’ll find a new handbook from Swedish Steel an invaluable reference. We certainly loved it here in the office.

The extraordinary Sheet Steel Handbook produced by Swedish Steel has become one of the ‘must have’ references for designers over the last few years. The massive, detailed technical guide to designing with sheet steel has recently been re-issued as ‘Edition II’, but it’s a spin-off publication which we think is as likely to generate demand.

The new Sheet Steel Forming Handbook is based on the ‘Fabrication’ chapter from its larger parent. This was easily the most interesting section to designers, and Swedish Steel took the decision to reprint that section on its own some time ago – many of you may already own a copy. The new Handbook goes much further however: not only does it seem to cover every angle, but it tackles a lot of theory which seems way beyond the call of duty. That said, it’s so well presented that no subject seems the slightest bit esoteric.

Chapters cover material properties (including plastic forming, effect of strain rate and temperature on yield stress), size shearing (angles, clearances, deviations) and plastic forming (bending, spinning, flexforming). Other properties covered are formability and material behaviour, tooling materials and surface treatments. These are followed by an extensive ‘Materials Data’ section, designed to be updated as materials change – this gives all the data so crucial for setting up finite element (FE ) analyses.

Many phenomena which occur in metal forming operations are covered in depth in the handbook, though mainly from the point of view of how they may be tackled on the press, rather than at the design stage. Nonetheless, all the advice remains extremely valuable to designers, in that it may instill greater understanding of and, therefore, confidence in the metal forming process itself.

For example, springback, which is caused by the stresses arising during forming and their release after the load is removed, is more complicated in press operations than in bending, because stretching, drawing and bending are often involved. Anti-springback measures possible in drawing are to reduce punch edge radii to ensure coining and to optimise blankholder forces.

Compressive stresses arising in deep drawing can cause wrinkling, especially if the workpiece is not properly restrained relative to the tooling. Wrinkling and buckling can occur in two different positions, sometimes on flat sections and sometimes in areas not in contact with the tool. Reduced sheet thickness and increased strength usually mean a greater risk of creasing. There are several ways in which the risk of wrinkling can be reduced. One is to reduce blank holder force. However, the increase must not be too great, otherwise there is a risk of fracture. Another way to minimise the risk of creasing in large, flat parts is to use larger edge radii, which makes it easier to stretch the material over the edge onto the punchface area. A third method is to provide drawing beads to retain and control material flow in sensitive areas.

• Swedish Steel

September 1999

Alan Quinn explains how MCP Equipment’s advanced computerised vacuum casting machines can save time and money alongside rapid prototyping facilities.

Those who use rapid prototyping techniques such as stereolithography face a cost dilemma when they need to produce the original prototype in numbers for technical evaluation and market testing.

It seldom makes economic sense to use the rapid prototyping system itself for replication. Most are far too expensive to run for anything other than a master prototype. Some systems also produce prototypes that are too brittle to withstand practical trials.

One way around the problem is to operate a vacuum casting machine alongside the rapid prototyper. Costing about a tenth of the CAD system the vac-caster employs a mould made from the original to cast several facsimiles faithfully replicating the details in a strong resin with a finish as good as that of injection moulding.

MCP Equipment supplies vacuum casting systems for shots up to 14 0 The company says that current generation of machines are much more suitable for this prototyping role than were their predecessors. One serious drawback with earlier system was inconsistent control of resin mixing processes and mixing times. This resulted in unacceptable reject rates.

The new systems overcome such problems by automatic sequencing and monitoring of all operations under microprocessor control. Apart from supervision there is no significant manual element in the process. They are also self-contained and fully integrated as systems . Meaning that the casting resins and mouldmaking materials are formulated for compatibility. In MCP’s view this contributes significantly to the quality and economy of the finished casting.

There are also important advantages in the use of silicone rubber as the mouldmaking medium. The material is inexpensive and reproduces detail faithfully. Moulds can also be made very quickly much faster than hand tooling for an injection moulding.

How vacuum casting works

A vac-caster of medium capacity will typically have two chambers. The two-part polyurethane resin is mixed in the upper chamber and the mix is cast into the mould in the lower.

A split two-part mould is made by casting silicon rubber around the master prototype in stages. This is then placed in the casting chamber which can be provided with an electric jack for fine-tuning the mould position.

A robotic unit controls the mixing of the resin and hardener and the pouring of the mix into the mould.

Both operations are carried out with both chambers evacuated. This ensures that all air – the usual cause of imperfections – is extracted during mixing. Evacuation also draws the mix into every part of the mould during casting no matter how intricate the mould may be.

MCP says that with this technique there is nothing to prevent the casting being an exact facsimile of the master clear of both internal and external defects and with a finish identical to that of the mould.

The vacuum in the machine is continuously maintained at a pre-set value by a semiconductor sensor. A digital display of the operational parameters is updated by a PLC throughout the process.

The casting will attain initial hardness in the casting chamber. Some applications require rather more than this and employ a hot-air drying chamber to speed hardening and improve molecular bonding and heat resistance.

The two-part mould is then split and the casting removed. Several castings can be produced from the same mould before wear and tear necessitates a new set. One telephone R&D centre has used the same mould for over 60 complex castings.

• MCP Equipment
• Tel: 01785 815651

May 1999

Problems in hydraulic systems arising from abrasive and adhesive wear can be solved by PVD hard coatings, says Dr ANDREW BLOYCE of Balzers

Providing high power density, compact dimensions and outstanding controllability, hydraulic systems support a wide range of industrial applications. But the performance of hydraulic systems is often compromised by the tribological load bearing capacity of the conventional materials pairs used in hydraulics, such as nitrided steels and bronze.

Problems can also arise if fluids are contaminated with solid particles – these can cause abrasion and erosion, resulting in damaging leaks. Optimum filter configurations and regular maintenance cannot completely eradicate this problem. In addition to abrasive wear, the performance of hydraulic systems is also compromised by adhesive wear/seizure, which can cause failure, even in high performance pumps using clean oil.

These problems can now be solved by protecting hydraulic system components with physical vapour deposition (PVD) coatings. Significantly increasing the load bearing capacities and wear resistance of components, PVD coatings promote exceptional surface hardness and sliding characteristics. They have been used successfully in tooling applications for many years and have improved the reliability and cost-effectiveness of many machining applications.

The coatings are applied under high vacuum conditions by evaporation (TiN, TiCN, CrN) or by sputtering (WC/C). During these processes, the coating material is evaporated in plasma or sputtered from targets by ion bombardment and then deposited on the parts to be coated. This high vacuum process offers significant advantages over atmospheric thermal spraying techniques and gas/chemical bath nitriding and galvanising. The benefits include:

• Compound material composition, including amorphous carbon coatings (WC/C) which provide excellent friction behaviour combined with outstanding hardness

• High accuracy – PVD coatings are only a few thousandths of a millimetre thick. This ensures uniform profile coverage to eliminate finishing requirements

• Increased load capacity – high vacuum deposition prevents contamination and ensures a metallurgical bond with excellent layer adhesion.

Screw spindle pumps are exposed to both abrasive and adhesive wear when used to convey contaminated oil or grinding system coolants. The life of these pumps, however, can be significantly extended by applying a TiN coating to the spindles. Moreover, in applications involving water and oil, WC/C coatings provide effective protection against seizure between spindles and between the spindle and the casing.

Similarly, the life of a rotary vane pump with additive-free hydraulic fluid can be greatly increased if the vanes are PVD-coated. Tests have shown how WC/C coating is the most effective way of preventing seizure in the contact zones between the vanes and the stator ring. Nitrided pistons in axial piston pumps can often fail under very high pressure and speed conditions owing to seizure between piston and barrel. WC/C coatings dramatically increase the performance range and reliability of such pumps. In fact, identical tests involving coated and uncoated nitrided pistons have shown that the PVD coating process eliminates seizure problems which compromise the reliability of the uncoated pistons.

WC/C coatings also improve the performance of sliding show/cam plate systems. Contamination can cause abrasion of bronze sliding shoes, leading to the pitting and destruction of the pump. Offering significant cost savings by extending component life, WC/C coated steel sliding shoes are much less sensitive than the bronze components. Even after an exhaustive 1000 hour test, the shoes remained virtually unmarked.

• Balzers

July 2000

David Milborn of British Steel Engineering Steels explains the latest developments in air-cooled forging steels

Air-cooled carbon and micro-alloyed steels are steels which instead of being heat treated after forging, are cooled in controlled conditions, either in still air or in an air-stream created by a fan. In Europe many forgers now have the equipment and know-how to enable them to carrying out cooling accurately and reliably.

British Steel Engineering Steels produces a range of air-cooled carbon and micro-alloyed steels offering tensile strengths from 700 to 1000 N/mm2. Carbon steels are used for lower tensile strength applications in the 750-850 N/mm2 range while micro-alloyed grades offer UTS levels up to 1000 N/mm2.

For the component manufacturer, the primary advantage of adopting an air-cooled grade rather than a conventional heat-treated version are the significant cost savings that can be achieved. The use of an air-cooled micro-alloyed grade (38MnSiVS5) for hub flanges has resulted in savings of (UK pounds)0.52 per part. However, there are further benefits in terms of the properties and performance of the finished component, including:

• reduced variation in mechanical properties
• uniformity of properties across the section
• reduced distortion.

In most cases improved machinability is also obtained. An air-cooled forging steel developed by British Steel for Rover’s re-launched MGF is also treated with calcium in order to improve machinability.

Engineering Steels has developed its own process for adding calcium during secondary steelmaking in a way which prevents this highly reactive element from being contaminated by contact with the atmosphere or with the slag lying on the surface of the molten steel. The calcium is injected into the ladle using a steel-sheathed wire to ensure that it reacts only with the oxides in the depths of the ladle.

The Air-Cooling Process

The enhanced properties of air-cooled steels can only be reliably achieved if cooling is carried out using conveyor equipment offering optimum control. The crucial criteria are the speed of the conveyor, the control of temperature (via fans or shrouds to accelerate or retard cooling) and the length of the conveyor which should be sufficient to ensure that the individually spaced components cool to less than 600degC before being removed. If the conveyor is to be used for a range of components, a variable speed and moveable fans/shrouds may be required.

The degree to which cooling rate can be controlled also depends upon the section size of the component. Still-air cooling is successful with smaller components, while fan cooling is often needed for larger section sizes. A fast cooling rate leads to the formation of a higher proportion of perlite in the steel which in turn yields higher strength and ductility. In micro-alloyed steels, accelerated cooling gives rise to finer precipitates and higher strengths. Very slow cooling rates produce softer components with a higher proportion of ferrite and a coarser perlite inter-lamellar spacing.

Properties and Applications of Air-cooled Steels

Air-cooled steels have a successful track record for certain automotive applications such as crankshafts and conrods. However, until recently their use was restricted because of concerns about their impact resistance; their inherently coarse grain structure meant that they could not be used for components subject to high impact loads.

Enhanced alloy design, coupled with a greater understanding of in-service conditions such as fatigue, has now resulted in the development of a new generation of forging steels which can be used in a wide range of components and applications.

Further applications for air-cooled grades are the subject of current research and development initiatives at British Steel. The company is currently working to develop new steels which will be used for the manufacture of fracture split conrods in which the large end cap is separated from the rod by fracture splitting. Fracture split conrods have been specified for a number of new passenger car engines, particularly in the US.

A collaborative project between Engineering Steels and British Steel Forgings is focusing on the need to develop steels which can be easily fracture split but which will also offer excellent machinability.

• British Steel Engineering Steels
• Tel: 0114 288 2361
• Fax: 0114 288 5033

Insulated bearings ensure trains go further without stops. Tommy Miller reports

Research has identifed the train as being an mode of travel for journeys up to 1000km. This assumes however that we have a new generation of trains able to go faster than ever before and to travel much longer distances without maintenance. These twin demands place added stress on traction motors and the rolling bearing which are fundamental to reliable operation.

To achieve these goals, designers are turning to the use of AC motors. Since an AC motor has neither a commutator nor brushes, high speed and maintenance-free running are possible. And as the adoption of AC traction motors has gathered pace, so has the demand for maintenance-free traction motor bearings.

NSK calculates that on conventional trains of the future, maintenance-free running intervals of 2,000,000km will be required. This compares with intervals of 900,000km for current generation trains equipped with AC traction motors.

The biggest obstacle to achieving the higher performance is electric erosion of traction bearings. Electric erosion occurs when electric current is allowed to flow through motor bearings to earth. This causes damage to the bearing, the extent of which depends upon the magnitude of the current and the duration of the conditions. The damage is usually in the form of craters on the bearings and rolling element surfaces. Often, it results in premature bearing failure.

With the new performance requirements being placed on trains, the importance of preventing this problem has increased. NSK has responded by developing a range of insulated bearings for traction motors.

Plasma spraying technology is employed to coat the outside and end faces of the outer ring of the bearing with a ceramic material consisting mainly of alumina (Al2O3). A thin metal layer is applied prior to the application of the ceramic coating to improve its adhesion.

Although the thickness of the ceramic coating may vary from 0.05 to 0.5mm, the range and outside dimensions of the coated outer rings are the same as conventional bearings without the coating. Therefore, it is not necessary to adjust bearing housings to accommodate special dimensions.

Following the plasma spraying process, the bearings are coated with a special acrylic resin. The resin impregnates the pores of the ceramic coating and seals in the surface to eliminate the harmful effect of humidity.

• NSK