WHAT GERMANY IS DOING 0 LIGHTEN ROAD VEHICLES
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ALTHOUGH the advantages of weight reduction in road vehicles are generally recognized, the substitution of light alloys for steel in the construction of certain components has been delayed, partly by the conservative tendencies of the designers, but also
by the higher initial cost of the lighter materials. This has been the experience in Germany, as elsewhere, but a number of exhibits on view at the International Motor Exhibition in Berlin in February, 1939, dealing with the use of magnesium alloys in road vehicles, indicates that, in that country at least, the possibilities and,the limitations of such a change-over are being systematically explored.
It is pointed out by German experts that if the changeover is to be both successful and economic, three principles will have to be observed. The substitution of light metals for steel must be limited to parts where weight can be saved without sacrifice of strength. The light-metal parts must be so designed as to achieve maximum reduction of weight. The design must exploit the special fabricating advantages of light alloys and so compensate for the greater initial cost of the material.
It will be appreciated, therefore, that light metals are not suitable for all parts of road-vehicle bodies. These should incorporate both steel and light-alloy members, the latter being most effectively used for thin-walled girders and other members made from sheet. Thus, light metals can well be used for stressed-skin (monocoque) bodywork.
With mechanical properties approaching those of aluminium alloys, and with a yield point for extruded sections which compares favourably with that of unheat A32 treated aluminium alloys, magnesium alloys have already proved eminently suitable for light constructions. For vehicles, they are used mainly in the form of sheet and extruded sections, the excellent welding qualities of Elektron alloy AM.503 being, in this respect, a great advantage.
Parts constructed of magnesium sheet can be more easiiy beaten by hand than those of steel or soft aluminium alloys. Ease of working and welding is also important for repair work. When assembled with other materials, such as heavy metals or wood, magnesium alloys must be insulated. All joints exposed to the atmosphere should be coated with lacquer before assembly. If there be a risk of fretting at the joint, an elastic insulating material should be inserted. Steel parts in contact with magnesium should be plated with zinc or cadmium. With the usual chromatecl finish, magnesium-alloy parts are proof against atmospheric corrosion, and stress corrosion is not observed.
In general, and taking into account the principles already outlined, magnesium alloys can be used for all parts which do not transmit main loads. Even if exposed to attack as a result of injury to the surface layer, the corrosion of such parts will be attended by no more
serious consequences than in the case of steel. Components such as doors can readily be made of magnesium alloys.
Thin-walled light-alloy members, although especially suitable for stressed parts, are more easily damaged by surface corrosion than thick-walled parts, and only experience will show whether they can be used in the salty atmosphere of coastal regions. -Upon the solution of this problem, together With that of the question of the fatigue behaviour of magnesium components, depends the delimitation of the respective spheres of magnesium and aluminium alloys in their application to road vehicles.
Magnesium castings can also he used for both stressed and unstressed parts. Even where a saving in weight is not considerable, the economy in production costs (e.g., Opel oil pump made as a pressure die-casting in Elektron) will provide an incentive to effect a changeover.
As previously mentioned, a few vehicles incorporating magnesium alloys have already been produced and operated in Germany by way of experiment. For instance, a Post Office van made of Elektron alloys is still giving good results after six years' service, whilst a large propaganda van went into service towards the end of 1938. In spite of much condensation of moisture on the inner walls of this vehicle, no corrosion has been observed, and repairs have been carried out with ease.
In collaboration with the I.G. Farbenindustrie, Bitterfeld, the Waggonfabrik A.G. of Uerdingen (builder of the propaganda van), has also constructed a number, of light-metal experimental vehicles, particulars of which are given below.
The passenger trailer lends itself readily to light construction. The stressed-skin structure has three frames in the plane of the window columns. The cross-bearers which connect them beneath the floor are combined structurally with two chassis members, which, in their turn, support the seats and the gangways down the centre of the car and transmit the loads to the two transverse laminated springs.
For various reasons 'these members are made of welded steel, the framework being composed mainly of 11 S.W.G. sheets. The loading in the side supports of the seats is taken by the body, which, except for a few members, is made entirely of magnesium alloys, and which also serves to stiffen the car against torsion. All uprights and stays are made of Elektron alloy AM.503, the 17 S.W.G. sheets of the walls and roof being made of Elektron alloy AM.537.
In order to economize assembly time the extruded sections were specially designed. The side walls and roof have been strengthened against buckling. The floor plate, which transmits to the body all transverse forces emanating from the coupling and the running gear, is made of aluminium-magnesium (A1-Mg3) alloy.
The ground clearance is reduced to 23i ins. The 5-ft. 3-in. opening for double sliding doors has been stiffened top and bottom by sheet channels made of Al-Mg5 alloy, the lower one serving both as step and door frame.
In this form the body, with a length of 27 ft. 6 ins., weighs 1,650 lb., exclusive of internal fittings. The sliding door, which is a double-walled hollow metal structure, is made of Elektron sheet and sections. With fittings, it weighs 35i lb. The following weights are given for the bus trailer:— Sprung weight : body, including fittings, about 5,000 lb. Unsprung weight: running gear, with drawbar and springs, wheels (tyres 900-20), etc., about 2,100 lb. As the maximum pay-load is about 8,150 lb., the total weight is approximately 15,250 lb.
The large percentage of the total weight accounted for by unsprung components, and the ratio between the payload and the weight of the sprung portion of the empty vehicle, points to the need for careful redesigning of the springs, brakes and running gear. Both axles of the trailer were equipped with springs of equal strength for receiving an equal load. Owing to the low ground clearance, as well as for reasons of price, the
laminated type of spring was chosen. Fitted transversely, the springs remained independent of steering and braking loads.
Wheel design was adapted to the special needs of the service, the brake drum being detachable without removal of the hub. Brake shoes and back plate were made of aluminium alloy Al-Mg5.. An important factor in unsprung weight is the tyres (900-20) and the steel disc wheels. A reduction of the wheel weight being essential, the use of light alloys for this purpose was clearly indicated. Accordingly, an Elektron wheel was designed, and this is to serve as a basis for further developments. The detachable Elektron wheel body is mounted on a light steel hub, the mounting and brake design being, in other respects, the same as for steel wheels. The saving in weight with these experimental wheels was 26 kilos. per vehicle.
Elektron superstructures on German post office vans afford a good example of increased disposable load resulting from the use of these light alloys. In some cases the lower weight of the superstructure would justify the choice of a lighter chassis; in any case, it reduces running costs. The Elektron superstructures in question were mounted' both on Daimler-Benz chassis, operating on bad cross-country roads, and on Bergmann 15-cwt. battery-electric chassis engaged on parcelde:ivery service in the town.
Cheapness, lightness and modern appearance were essential. The last requirement shows that the hot forming of magnesium alloys for road-vehicle application is possible. In fact, the surprising discovery was made that in some cases Elektron sheets could be shaped more cheaply than the usual steel sheets, even where it was a question of only single parts.
In the superstructure of the Bergmann electric chassis, with the exception of mudguards, wheel casing,s, front bonnet covering the battery, and transverse bearers beneath the floor, which are made of aluminiummagnesium alloy, only Elektron was used. The design resembles that of the Daimler-Benz superstructure. Pressings in Al-Mg3 provided a cheap method of effecting strong joints between intricate extruded sections at spots where, normally, there would have been no room for rivets. Pressings were also used at points where a sharp bending of deep sections would have been attended by difficulties, as in the curve of the roof illustrated.
The pressings were made in simple wooden dies, and, owing to the uninterrupted flange, they are very strong. Further 'examples are illustrated. Welded stressed Elektron joints were avoided for reasons of strength. Assembly by welding was also avoided in the interest of economy. Riveting by hand or compressed air was found quite satisfactory. The weights of the super'structures are given in the following table. Cross-country Parcelvan on van on DaimlerBergmann Benz chassis chassis Chassis ... 3,180 lb. 3,000 lb. Previous steel superstruc ture 1,580 lb. 1,100 lb.
Elektron superstructure ... 990 lb. 675 lb.
Payload 1,870 lb. 1,650 lb.
Increase in payload of Elektron superstructure 31% 26% It should also be mentioned that with the Elektron superstructure on the Bergmann chassis the length of the freight compartment was increased by 19 ins, as compared with the steel superstructure. Two points needed special investigation in respect of the application of magnesium alloys to heavy road vehicles :— (1) How do riveted connections behave on the thickwalled members under high dynamic load with relatively high stress? (2) How does magnesium react to rough conditions of service involving temporary injury to the lacquer coatings, with risk of corrosion by atmosphere and dirt?
It was decided to examine these questions by constructing in Elektron an experimental lorry trailer with a payload capacity of 8 tons. The chassis consists of longitudinal members, on each side of the vehicle, made of T sections in Elektron alloy AM.503 and intermediary webplates in alloy AM.537. The cross-bearers are also extruded sections, except for the box-shaped bearers in front of the rear axle, which are made of hot-flanged sheets. Practically all riveting was done by means of a pneumatic tool—a recommended procedure with aluminium alloys.
The behaviour of these riveted connections under the alternating stresses resulting from the torsional action of the frame is all the more interesting by reason of the rigidity of the deep bearers. At the forward end the
chassis frame rests on a riveted ring-shaped magnesium casting (Elektron alloy AZF). on which is placed the steering crown with ball-joint (40 per cent, saving in weight). The cross stiffening of the chassis is effected by means of, steel fenders riveted to the ring-shaped casting and the longitudinal members.
The steel drawbar, spring shackles and brake cylinders are screwed to the drawbar frame, which is also a magnesium casting. The two magnesium castings are shown in one of the illustrations.
The drawbar frame has been designed from the point of view of high capacity for elastic energy to absorb impact stresses. Magnesium castings possess surprising qualities of plastic deformation, whilst their ability to withstand impact stresses is demonstrated by the use of cast magnesium for German artillery wheels.
Not until the experimental applications described have been thoroughly tried out will it be possible to decide whether the parts in question are suitable for incorporation in ordinary vehicles, but the results to date would appear to show that magnesiumalloys, which have hitherto been successfully used in aircraft, and which, in this instance, have yielded much data capable of extended application to ultra-light-alloy designs in other spheres, will shortly find a wide field of application in road transport, where, as has been shown, such application not only confers the advantage of increased pay-load, but simultaneously decrease operating and maintenance costs