The Use of Steel Long Products in Future Vehicle Construction

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The Use of Steel Long Products in Future Vehicle Construction

Introduction

General industry trends and designs point toward steel long products continued and growing use in automotive applications because of the material's capability to reduce mass and improve performance, without sacrifice to cost.

The motor vehicle of the future will undergo a number of developments and changes without essentially altering the process and materials used. Most major components will remain the same, but some will be replaced. Without doubt, steel will continue as the material of choice especially as it pertains to issues such as safety, durability, and affordability. Although development of other materials will continue, it is by no means at a fast pace.

This is not to say that novel and innovative steel components will not need to be developed. Any major breakthrough in another material will most likely threaten the use of steel. Steel, therefore, will require a continual development cycle in the areas of composition and cost.

Although not as efficient as steel, many new materials could suddenly emerge as competitors, entering the market at lower costs, simply to obtain market share and acceptance. While this is unlikely in the near future, it could be a possibility further down the road. In addition, a reduction in the use of flat stock is likely, but on the upshot, this will be compensated for by an increase in bar stock.

Steel will continue as the material of choice and while it would be almost impossible to predict, the trend toward increase usage of rod and bar of approximately six percent over the next 10 years is highly likely.

To attain this figure will require extensive research into coatings for durability and wear. High modulus and alloy steels will need to be adapted for further applications, as will continual development into fatigue resistance. Corrosion is well-covered by the steel industry, but as the components are reduced in size and mass, corrosion resistance will need continual development.

With the constant increase in engine power and reduction in size, component wear characteristics will also require continual development. Although reduced in size, and increased in quantity, gears, shafts and bearings will be expected to accept greater loads. This should help offset material and coating development costs that would be borne by the steel companies.

The amount of steel used in future engines, within the next 10 years, will in all likelihood, increase. This will happen because the internal combustion engine will continue to develop into a more efficient package in terms of weight and size. With these efficiency gains, a number of components will be under greater loads and stresses.

Crankshafts will most certainly become steel and smaller in size. The flywheel and clutch systems will also be reduced. Valves will remain in steel, although reduced in size, and the number used per engine will increase. Diesel engines will start to make inroads into the market and will also benefit from development. Essentially, the future engine will have as its base a steel crankshaft, camshafts and connecting rods.

Let's review the situation component by component:

Powertrain Systems

The powertrain system offers the greatest opportunity for overall vehicle mass and cost improvement. With a more efficient powertrain, achieving additional secondary mass and cost savings is possible. For example, if the powertrain system is 10 percent more efficient, the weight of the vehicle can be reduced dramatically, with reductions in fuel tank volume and braking and suspension systems, while decreasing the amount of stress on the components of the vehicle. The use of steel bars and rods in powertrain applications, particularly the engine and transmission, can reduce inefficiencies that exist in current and proposed alternative engines.

Powertrain general developments will focus on the internal combustion engine, with additional power and better fuel economy from a lighter and compact engine at the forefront of this development.

Ironically, one of the most important developments for the internal combustion engine has been the continual refinement of oil and fuel. Fuel is considerably cleaner burning than ever and synthetic oils provide much better protection and reduced engine wear. These factors alone have allowed engine designers to reduce the mass of engine components and assemblies. Even with this reduction in mass, warranty claims have been reduced and maintenance schedules extended.

High volume petroleum-powered engines currently consist of pistons, a block, a head and a crankcase all constructed of an aluminum alloy. This is by far the largest application for aluminum (specifically in cast form) used in present vehicles.

Connecting Rods

The connecting rod is subjected to very high tensile, compression and bending loads. These loads are transmitted to the crank and piston via the "gudgeon pin" (piston wrist pin). Due to the previously described load conditions, this pin in a production engine is always chromed steel. This trend will continue, as at present, because it is the most efficient method of construction for both cost and strength.

The connecting rod itself, used in a high performance engine, is usually a drop forged steel component. Motor manufacturers in the United States, however, have a tendency to use a powder metal forged connecting rod. They can do this because most U.S. engines are designed for torque, and a long piston stroke is preferred to reduce the loads on the connecting rod.

European manufacturers, however, prefer a shorter piston stroke. With global vehicles becoming more common from all manufacturers, a compromise on stroke is inevitable. This will probably result in a compromise of both systems, likely to be an equal stroke/bore engine.

Although other types of exotic materials such as titanium have been tried, steel is by far the strongest and most cost-effective material, and will continue to be so for the foreseeable future. Additionally, it offers the best compromise in creating equal stroke/bore engines.

Crankshaft

The crankshaft on a high volume, low-load, production vehicle is generally constructed from spheroidal graphite cast iron. Although this is advantageous in terms of reducing initial costs, this method of construction does carry some weight penalty.

A cast iron crankshaft is effective, but it is not as efficient as a drop forged steel crankshaft. In highly stressed engines, such as those used in high load conditions such as racing, crankshafts are typically drop forged. These crankshafts have typically lower reciprocating mass and are considerably lighter.

While there is a cost premium for this type of crankshaft, the costs are offset by further mass and size reductions in the flywheel and clutch. As an additional benefit, the bending vibrations for a small cylinder engine are greatly reduced. The rigidity of a drop forged crankshaft creates a more efficient block and crankcase design, allowing for a smaller, lighter power plant. A drop forged steel crankshaft would also provide improved fuel economy.

Although already constructed in steel, the flywheel and clutch system could be reduced in mass, decreasing the rotational masses in the powertrain and, in turn, reducing vibrations. Additionally, the same speed could be achieved for lower engine revs, thus significantly improving the fuel efficiency and range of the vehicle.

The use of steel crankshafts in U.S. vehicles has been studied over a number of years, and in most cases has been rejected due to cost. However, this has not been the case in Europe, where many manufacturers utilize a steel crankshaft in their base engines.

With the higher power-to-displacement ratio found in European engines, the benefits of the steel crankshaft help provide a more fuel-efficient engine. This trend is changing with the purchase of many of the European vehicle manufacturers by the U.S. car companies. Technology that is in use within Europe is being developed for U.S. vehicles.

Cylinder

In most cases, today's engines have an Overhead Cam (OHC) cylinder head. This is either a single or double cam system. The number of valves in the head also has increased over the years, and with the advent of the OHC engine, the average valves per cylinder is now four. As the greater number of valves improves the breathing efficiency of an engine, it is safe to say that this trend will continue, and should reach an average of five valves per cylinder by the year 2005.

These valves, whether standard or sodium filled for additional cooling, will remain a steel component. The heat dissipation and loads that are required from these components leave very few alternatives to this cost-effective solution.

Although manufacturers such as DaimlerChrysler use titanium valves in some of their high-revving, two-valve engines, it is not the preferred solution. Titanium has inferior wear resistance characteristics compared to steel, and can suffer from "fretting" even when sufficiently lubricated. Additional coatings have been tried in an effort to reduce the wear, but these coatings have had only marginal effect. The increased cost of these units is also well above its steel counterparts.

The auto industry now offers greater servicing life on its new engines, and will continue to use steel valves for mainstream applications.

Camshaft

The camshafts vary depending upon the type of engine configuration. Currently, U.S. manufacturers continue to use a number of "push-rod" engines. These engines use push-rods to operate the valves and place the camshaft under a higher load than the OHC type of engine. In these cases, the camshafts are constructed from steel rod with cast lobes.

Due to the reduced loading, the cams for the OHC engines are constructed from cast iron. These operate cam buckets and are under a reduced load compared with the push-rod engine. These buckets, as in all cases, are chromed steel. The buckets operate the valves, and move in a cast or sintered sleeve fitted in the cylinder head. This method of operating the valves is very efficient, with minimum frictional losses and positive valve location. It would be highly unlikely that this method would be superseded in the near future. The proliferation of OHC engine designs will render the push-rod system obsolete for passenger vehicles within the next few years, with the OHC engine being the preferred option.

Diesel Engine

The diesel engine, although not popular in the U.S., is very common in Europe. This is due in part to the lower prices for diesel fuel, coupled with better fuel efficiency from the diesel engine. The trend for the use of diesel engines has been growing in the U.S., in part, due to the efforts of the Partnership for a New Generation of Vehicles (PNGV) government initiative.

A long-range highly fuel-efficient and affordable solution cannot currently be achieved with an all-electric powered vehicle. Therefore, a hybrid solution is being suggested industry-wide. These hybrids will likely encompass the diesel engine as part of the solution.

With better fuel efficiency over its gasoline counterparts, a hybrid diesel engine would provide the ideal solution. This type of engine relies on a cast iron block, although Metal Matrix Composites (MMCs) are being. However, as the diesel principle remains, the use of steel components within the engine is essential. The higher compression used in the diesel cycle demands that a steel crankshaft be used, plus connecting rods and pins, as in the gasoline engine. As the engine is direct injection, the need for a complicated valve train is diminished.

Manual Transmissions

The manual transmission can have four to six forward gears. The gears are generally constructed from steel, although there is a trend toward sintered metals. However, it is proving to be very difficult to control the wear characteristics found in a gear set environment with this type of manufacture.

A great deal of development will be required to provide an effective solution to this problem. The benefits of sintered metal gears do not provide major advantages over the standard steel gear sets; therefore, the cost of an extensive development program could easily outweigh the minimal gains for this type of material substitution.

Major advancements continue on steel precision, net shape cold forging. This method of manufacture provides many advantages. As the process provides an accurate shape, machining time is greatly reduced. The material flow is also consistent, allowing for a reduction in size and mass for the same strength and fatigue resistance. In addition, as the material remains steel, with only a change in process, long-term cycle testing would not be required.

Automatic Transmissions

Although automatic transmissions are becoming more efficient, there are still frictional losses associated with this type of system. Despite the differences within the clutch/torque converter systems, the automatic utilizes gears in a similar manner to the manual transmission.

Automatic systems that emulate the manual transmission, minus the clutch system associated with a manual box, are being developed. These systems, sometimes referred to as Tiptronic/Steptronic, are more efficient than the standard automatic transmission and run through a torque converter, yet are not as efficient as the manual system. They do, however, still have as many as five forward gears, set up in the same manner as the manual transmission. These gears are made of steel and are identical to the manual gear sets.

The CVT transmission offers many advantages over its automatic counterparts in that it is lighter and more efficient. In addition, it also provides a "stepless" system, providing a smooth transition of power to the wheels. One of the early problems with this system was the thrust belt drive. This was originally a rubber/fiber belt that would have a tendency to slip and stretch under severe load conditions. The belt problem restricted the use of this type of transmission to smaller vehicles with limited power outputs.

When Ford adopted this type of transmission for their smaller cars, they developed a new belt, which contained over 300 steel blocks, retained by two loops of spring steel that replaced the problem-prone rubber/fiber belt. These blocks are machined from cold forged bar stock.

The continual development of this transmission by Ford and other manufacturers could provide an interesting addition to the vehicle powertrain. With its stepless delivery of power and efficient combination of engine and gearbox torque, a more efficient power curve could be achieved. This could be a first step in providing a more fuel-efficient vehicle.

The gearbox principle still relies on steel gears, however the number of gears are greatly reduced by more than 50 percent, as the belt provides the variation in the power delivery. Needless to say, the minimal number of gears will be constructed from steel.

Differentials

As with the gearbox, the rear wheel drive (RWD) case is constructed in aluminum. The differentials are similar, with the crown wheel and pinions constructed in steel. As long as there is a gearbox on the vehicle, a differential is required. This number increases with 4-wheel drive (4WD), as an additional center and rear differential is required to transmit the power to all four wheels.

Drive Shafts

Although FWD vehicles represent the mainstream type of production vehicle, a large number of AWD or 4WD vehicles remain. A "prop" shaft to the rear wheels is required in AWD and 4WD vehicles. A number of variations have been attempted to manufacture a lighter "prop" shaft. As the loads are not as severe as the drive shafts on a
FWD vehicle, this unit is a candidate for a lighter approach.

Carbon fiber wound over a thin wall aluminum tube has been developed and used, but difficulty in joining of the ends and the substantial cost penalty associated with this type of approach has seriously limited its viability for use in production vehicles. The use of a steel tube is still the normal approach for this type of system, with the connecting end and center joints made of steel. The connecting joints are also manufactured from a steel forging.

To increase the benefits of a RWD platform, it is likely that the transaxle system could be developed into a suitable system for a production vehicle. The transaxle system consists of a gearbox and differential system, combined in a cast aluminum case, mounted at the rear of the vehicle. This system is attached to the engine via a "torque tube," consisting of a tube with an inner prop shaft and location bearings. Because the shaft is running in a tube supported by a central bearing, the diameter of the shaft is greatly reduced, and in all present cases, is solid bar stock. This trend will continue in the future, as it is the most cost-effective solution.

In a FWD configuration, the drive shafts are of solid stock. The shafts are "spline" on either end for insertion into the "constant velocity" (CV) joints used to transmit drive from the gearbox to the wheel. Due to the torque requirements, and the one-to-one correlation of wheel speed to shaft speed, these shafts have to accept various load conditions. The shafts not only accept acceleration loads, but also severe deceleration loads when the vehicle is under a braking condition. For these reasons, the shafts must be strong and flexible. Steel rod stock provides the perfect solution, and will continue to be the material of choice in the foreseeable future. The shafts can be manufactured as a one-piece unit, and no other material has the required properties for this application.

Constant Velocity Joints

The CV joints are now common on all FWD vehicles and although there are a number of different manufacturers of the joint, the basic concept remains the same.

The joint attaching to the wheel consists of a forged bell shaped casing with a spline end. Inside the "bell" is a bearing cage and a number of balls that act as bearings usually allowing 30 degree of swivel of the joint. The spline end inserts into the suspension upright, through a bearing, and is attached to the wheel and brake rotor via a nut. Like the drive shaft, the loads placed on this component are quite considerable.

For this reason, the component will continue to be constructed in steel. With the demand for higher output engines continuing, this unit will be under considerably more stress. No other substitute material is available, or is adaptable to the various load conditions a CV joint must withstand during the driving and braking cycles that a vehicle exerts on this component.

Steering

Steering is an integral part of the motor vehicle and has developed into a very effective system. In most cases, the method of steering a passenger vehicle or truck is by a "rack and pinion" system. Whether this is power-operated or not, the system remains almost identical.

The housing for the rack is an aluminum component, although the rack, ball joints and steering arms are in steel. The steering system may develop into an electronic system, but the positive operation of the steering mechanism will remain unchanged.

From a safety aspect, next to the brake system, steering is one of the most important features of any vehicle. The evolution of the steering system in this area is deemed to be satisfactory. Even in the realms of racing, where the lightest component is a requirement, a steel rack and pinion is still used. The construction of this system will remain in steel for the foreseeable future.

Suspension

Suspension components greatly vary depending on whether the vehicle is a FWD, RWD or AWD. Manufacturer preference also plays a key role in the type of system used.

In most cases, the suspension medium for passenger vehicles is a coil spring. Leaf type springs are used in some small passenger vehicles, but essentially they are found in the rear suspension of trucks.

Coil Springs

Coil springs provide a very good suspension medium with progressive rates and providing a good trade off between ride and handling. Air and liquid suspension systems have never attained the acceptability of the coil spring and are therefore limited to a few specialized vehicles. Even the most advanced active ride suspension systems rely on the coil spring.

The continuing trend to lightweight vehicles will eventually have an effect on coil springs, reducing the size and diameter as the vehicle loses weight. This mass reduction will create increased opportunities to use coil springs in vehicles that currently use different springing mediums.

The popularity of the SUV has forced manufacturers to provide a passenger car-like ride for these vehicles, hence coil springs have replaced the leaf springs that these type of vehicles initially used.

Another development in suspensions has been the reintroduction of torsion bars as a springing medium. Torsion bars were popular in the late 1940s and early 1950s, but declined in popularity with the introduction of the coil spring and the McPherson strut.

Torsion bars are becoming popular with the SUV and truck manufacturers as an alternative to both the coil and leaf springs. The additional mass of the truck and SUV requires a large coil and package diameter to compensate for the extra weight of these vehicles. With the additional ground clearance and the underbody package space, the torsion bar could effectively provide more than adequate springing at reduced mass.

It is possible to produce a lightweight steel forged suspension arm, or wishbone. It is highly likely that a steel forging could be lighter and stronger than the present forged aluminum units in production today. There are many advantages for this type of unit in that the ball joint could be an integral part and that additional bushing could be reduced. As a singular component, the arm would offer some advantages over the aluminum unit, but as a system the unit would be substantially less expensive, with less assembly and additional strength.

Uprights are now mainly aluminum construction, with the occasional cast iron unit used in some heavy-duty applications. Forged steel could be beneficial in some instances, but the possible gains over aluminum units would be negligible. For a lightweight application, a unit constructed from steel plate would be more efficient. However, as outlined in the previous paragraph, a similar integrated upright system, including the hub bearing, could provide cost, mass and manufacturing efficiencies, and should certainly be considered.

Ride and handling has become one of the major selling points of the vehicle with an emphasis on the reduction of unsprung weight. Suspension components have been reduced in mass to try and achieve this aim. The introduction of aluminum components in the suspension system has proved to be inefficient in both cost and reliability. To withstand the input of excessive suspension, driving and breaking loads, the aluminum components require substantial reinforcement. In nearly all cases, this increases the component mass above the mass of a steel system.

For increased ride and handling characteristics, most vehicle manufacturers provide a sport model of their base vehicle, with the addition of anti-roll bars increasing quite dramatically. The reduction of package area in most passenger vehicles has also reduced the physical size of the bar. Although the size has been reduced, the operation remains unchanged. The trend has now moved to a solid, smaller diameter bar, rather than a larger diameter tube. This trend will continue and, with ever greater package restraints, more bars will be used for this application. Trucks and SUVs are not as constrained, and with their higher mass, will still depend on a tubular solution for this application.

Bearings

The bearings on the vehicle are an essential part of the operation. The necessity for bearings in the engine, gearbox, wheels and ancillary applications will continue.

Bearings will be reduced in size as other components are reduced. However, the number required will increase slightly. Greater stressed components will require a better bearing surface and greater range than previously. The use of both roller and ball bearings will continue well into the future and it is possible that either roller or ball bearings will replace a number of shell type bearings.

Rose-type bearing applications are also increasing. Most locking and latching systems in vehicles now have at least two rose joints in the system. Many mechanical operations are now utilizing these types of bearings, from accelerator operation to heating vent operation. It is highly probable that in the next few years every mechanical mechanism on the vehicle will have these joints.

Fasteners

Steel fasteners are used extensively in the construction of vehicles. Bolts, nuts and washers are mainstays of the vehicle fastening system, and will continue to be made of steel. Steel clips hold a number of components in position. All critical components on the vehicle such as the suspension, steering, brakes and powertrain components are held in place by steel fasteners. Continual development of materials and coatings for these fasteners has allowed the nut, bolt, washer and screw to withstand the rigors of an open environment, with a minimal reduction in strength.

Rose-type bearing applications are also increasing. Most locking Ironically, one of the factors that will increase the use of steel fasteners is the proliferation of aluminum body structures. The most