Metal Progress and Its Bearing on Commercial-motor Design and Production
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WAR and its horrors must not be V V allowed to distract attention too strongly froth the developments taking place in metallurgy and engineering, and their influence on the design and prcduction of commercial motors. As an instance of progress, a new fuelinjection pump may be mentioned, this being of the unit type and operated on the principle of port control. The production of this type of pump has been possible only by the aid of modern alloys. For example, the pump body enclosing the precision mechanism is a nickel-iron-alloy casting of excellent machineability, with the power to assume a high surface finish whilst having unusual physical properties and a fine-grained and dense structure.
A nickel-manganese-molybdenum iron has also been adopted for crankshafts of alloy cast iron. This material has the following composition: Carbon 2.85 per cent., silicon 2.0 per cent., manganese 0.95 per cent., nickel 1.6 per cent., molybdenum 0.6 per
cent. The tensile strength given is approximately 26 tons per ?q, in., with a Brinell hardness of approximately 270.
Steel that Reduces Breakages
Of recent years lorry capacities have been much increased by the manufac ture of third-axle units. The axle shafts of these are from 34 ins, in diameter and are of nickel-chromium steel, which is heat treated to give a Brinell hardness number of 300, or an equivalent tensile strength of about 67 tons per sq. in. By the use of this steel, breakages have been entirely eliminated. For the U bolts by which the axle housings, are attached to the spring, 3.5 per cent.nickel steel is used, and the spring clips are also of nickel-chromium steel.
It is well known that the ordinary universal joint does not transmit uniform velocity from driving to driven • shaft. There is an acceleration and deceleration of speed twice for each revolution, the extent of which depends on the angularity between the shafts. In a new constant-speed-type universal joint, constant speed is obtained, by maintaining the place of the driving engagement between the two yokes at half the shaft angle. Steel balls are inserted in non-concentric intersecting races cut in the yokes. The balls are made to roll angularly half the distance traversed by the yoke.
In this type of joint, pressures are naturally rather heavy per unit area, and high shock load is inevitable. Hence, an alloy steel had to be sought that would maintain a big safety factor and cut down dead weight to a mini mum. Eventually, two case-carburized and hardened nickel-molybdenum steels were adopted. The first of these has the following analysis : Nickel 1.65-2 per cent., molybdenum 0:2-0.3 per cent., manganese 0.4-0.7 per cent., carbon. 0.15-0.25 per cent. The composition of the second steel is: Nickel 3.25-3.75 per cent., molybdenum 0.2-02 per cent., manganese 0.4-0.6 per cent., carbon 0.15-0.25 per cent.
Nickel steel is also being used for the chasstt frames of six-wheeled singledeck buses, and for the casings for the rear axles. An interesting use of alloy steel is for the worms and pinions of the lift assembly of lifting trucks, which pick up, transport and deposit industrial loads from two to six tons in weight. These vital parts have to be strong enough to withstand severe service, particularly shock, The steel used has the following analysis: Carbon 0.35-0.45 per cent., manganese 0.6-0.9 per cent., chromium 0.8-1.1 per cent., vanadium 0.1.5-0,18 per cent.
Powder Metallurgy for Magnets Permanent magnets for magnetos, etc. have, hitherto, been cast from one or other of the new nickel-ironaluminium or nickel-iron-aluminiumcobalt alloys. Now developing is the manufacture of these magnets by
means of powder metallurgy. It is claimed that, in this way, magnets too small for economical and satisfactory casting can be produced. The magnets are formed by sintering. The powder is first preformed in tabletmaking machines, and the tablets loaded into boats and passed through the hot zone of the sintering furnace into the cooling extension.
The magnets made by this method range from 1/224 oz. to / oz. in weight. They cost rather more to make than cast magnets, but against this there is no scrap. Better permanent-magnet qualities are claimed, and a higher mechanical strength.
For cylinder blocks for commercial vehicles, a nickel-alloy iron containing 2 per cent, nickel and 0.25 per cent. aluminium is being used, and it is claimed to give hardness and wear resistance combined with excellent machineability, being drillable at 130 ft. per minute.
A new alloy for dies has been put on the market. This contains approximately 50 per cent, of nicker, 25 per cent, cobalt, with additions of chromium, titanium and iron. It is said to keep its strength at tempera
tures above 1,090 degrees C. A new case-hardening process for timing gears for oil engines has been invented. The steel used has 0.1-0,2 per cent. carbon, 1.65-2 per cent. nicket, and • 0.2-0.3 per cent, molybdenum, and the method is known as the Five Point Deephard process. It is claimed to give a hard and wear-resistant surface, high toughness, and a gradual transition from the hardened face to a ductile core.
A novel type of commercial" vehicle is the armoured car for transpOrting
cash for wages, etc. It is used by banks, large institutions, factories, etc., chiefly in the United States. This type of vehicle is furnished with an armour plating lig in. thick of 3.5 per cent. nickel steel. Usually, plate for armoured cars of this type is approximately / in. thick, so that a considerable saving in weight is achieved, whilst greater bullet resistance is claimed.
Success of Flame Hardening A successful and economical use of flame hardening is in connection with the cam tracks of clutch actuator cams. Normally, parts of this type would be oil quenched in batches, but the necessity of high hardness on the track part only, and the fact that heating above the critical point and quenching all over caused too much expansion to secure a snug fit in the threaded portion, led to the adoption of selective flame-hardening of the track. This reduced distortion virtually to nil, and enabled the threads to be cut after the track portion had been hardened.
A recent series of tests on the flame hardening of plain and alloy irons has shown that an average surface hardness of 500-550 Brinell can be attained on cast iron. Less heat is required to produce a desired depth of surface hardness on nickel-chrome-molybdenum irons. Distortion stresses increase with the depth of the hard layer in the same iron, so that the hard layer should be as thin as is consistent with the service required.
Of the stresses set up, 25-40 per cent, Of them can be relieved by annealing at 150 degrees C. with only a slight decrease in hardness. Carbon should be reduced to a maximum of 3.2 per cent., otherwise the response to the treatment will be less.
A low-temperature silver-brazing alloy has been developed and is lower in price than alloys of this type previously used. It has high electrical conductivity and can be used on copper and copper-rich alloys, on which it gives extremely strong joints resistant to corrosion. Melting at 694 degrees C., it flows readily and efficiently, and is self-fluxing on copper. It can be lobtained in the form of strips.
Silicon-copper alloys are being increasingly used for constructional work, e.g., in frames, on account of their relatively high strength combined with excellent corrosion resistance. Silicon-chromium steel is finding extended employment for engine valves of commercial-motor engines. The introduction of a lead-bearing openhearth steel, with much improved machineability, whilst retaining all the high strength and responsiveness to heat treatment of a carbon steel, has definitely enabled machined parts for vehicles to be produced at lower cost.
New case-hardening mixtures are the liquid carburizers or activated cyanides for the production of light cases. Another new development in this field is the introduction of neutral salt baths as the heating media in hardening and heat treatment.
A recent advance is the molybdenumnickel-cobalt-copper steel. This is a deep-hardening metal having the same strength and hardness throughout, even in sections 16 mm. thick. The advantage of uniform strength throughout the section is great when parts of engines are concerned.
Two free-machining steels of non. alloy type deserve mention. The first of these is a low-carbon open-hearth case-hardening steel which can be machined 50-100 per cent, faster, than other low-carbon open-hearth Steels. The second is a medium high-carbon open-hearth steel, which machines from 35-75 per cent. faster than the equivalent medium steels of this class. These 'two steels are also claimed to be 10-25 per cent, better in physical properties than similar steels.
Case-hardening Improvement Case-hardening in gaseous atmospheres has been considerably improved. The speed of the operation is increased by saving the time necessary to bring the container and carburizer to heat. Satisfactory gases are obtained by mixing natural gas or volatile hydrocarbon with a flue gas, or by using a partly burned fuel gas. Good temperature uniformity and constant circulation of gases over the work are obtained in this way.
The electrolytic chromium plating of rnotor cylinders is another interesting development. Hitherto, this has been applied only to large cylinder castings, which are given a porous coating of chromium. The advantages of this coating are that it holds oil, and can easily be honed. The life of cylinders
so treated is said to be much greater.
Chromium-molybdenum steels for case-hardening purposes are being replaced in Germany by manganese steels containing small amounts of silicon, chromium or vanadium. An electro-induction method of hardening inserts for valve tappets of engines has also been introduced. The inserts are placed in holes in the face of an intermittently revolving drum which carries them around between two inductor blocks. Heating to 815 degrees C. is virtually instantaneous, and, with further movement of the drum, the inserts pass through a spray of water and, finally, drop off on to a conveyor belt at the bottom of the Machine. In this way 6,000 parts an hour can be hardened.
On the production side a new cutting alloy should be mentioned. This consists of a basis of tungsten-titanium carbide, and it is said to differ from ordinary tungsten carbide in having greater strength, lower frictional resistance and lower heat conductivity. It will machine hard steels up to 682 Brinell at 20-30 ft. per minute, but is more generally applicable to steels with 350 Brinell, which can be cut at 100 ft. per minute.