Engineering Abstracts 1954
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Engineering Abstracts 1954
Large Nine-cylinder Diesel Engine
A nine-cylinder engine designed to develop 11,200 b.h.p. at 115 r.p.m. under normal conditions at sea has recendy been completed by Burmeister and Wain. It has cylinders 740 mm. in diameter with a piston stroke of 1,600 mm., and is to be installed in a tanker under construction at the Nakskov Skibsvaerft for A.P. Moller, Copenhagen. It is the highest-powered marine engine constructed by Burmeister and Wain.
The new engine is equipped with three turbochargers of the Brown-Boveri VTR-630 type, these blowers being similar to those employed in the main engines of A. P. Moller’s M.S. Dorthe Maersk and the Songkhla, Samoa and Sibonga of the East Asiatic Co. The output of the 11,200 b.h.p. engine is slightly higher than that of a corresponding 12-cylinder non-turbo-charged unit, which is about 10 feet 10 inches longer and 35 per cent heavier than the nine-cylinder turbo-charged engine. Ten nine-cylinder engines of this type are being built by Burmeister and Wain and one with 10 cylinders, for a cargo ship, at Eriksbergs Mek. Verk. developing 12,500 b.h.p. In Japan an engine has been installed in the recently completed Hanmasan Maru, a cargo ship of 10,200 tons with a service speed of 17} knots. The fuel consumption of these engines, based upon the results of similar but smaller units installed in ships now in service is expected to be 0 3341b. per b.h.p. hr., the mechanical efficiency being about 88 per cent. In normal service the mean indicated pressure is approximately 1021b. per sq. in.
—The Motor Ship, March 1954; Vol. 34, p. 518.
Cas Turbine Progress
Marine auxiliary and industrial gas turbines ranging in power from 140 to 900 kW are being developed at Bedford by W. H. Allen, Sons and Co., Ltd. Of the four basic designs involved, the smallest is an all-radial-flow single-shaft unit with a rating of 200 b.h.p. (140 kW) at an air inlet temperature of 60 deg. F. (15 deg. C .); the prototype is now undergoing extensive testing and development. Designed originally at the request of the Admiralty, this small engine is intended for any duty where low fuel consumption is of less importance than such advantages as light weight, compactness, quick starting, rapid acceptance of load, and absence of cooling water. An ingenious feature is the use of a one-piece turbo-compressor rotor, with the centrifugal impeller vanes machined from one face of a forged steel disc and the centripetal turbine vanes from the other. By thus taking the familiar back-to-back arrangement of radial-flow components to its logical conclusion, the opportunity is presented of cooling the turbine by direct heat transfer across the rotor disc. The Allen designers have applied this principle so successfully that the ferritic steel disc of the prototype engine shows every sign of lasting for many thousands of running hours despite the use of inlet gas temperatures up to 800 deg. C. (1,470 deg. F.). A larger gas turbine of familiar type to the 200-b.h.p. unit has been designed, and manufacture will commence shortly. Six d.c. generator-driving versions of it have been ordered by the Alfred H olt shipping concern for auxiliary use aboard Blue Funnel liners, the rating being 350 kW at an air inlet temperature of 85 deg. F. (30 deg. C.). These sets are too large to make efficient use of a radial-flow turbine, so the single-stage centrifugal impeller is driven in each case by a two-stage axial turbine. On behalf of the Admiralty, the company is designing and constructing two interesting gas-turbo-alternators rated at 500 kW in tropical conditions. No details have yet been released for publication.
—The Oil Engine and Gas Turbine, March 1954; Vol. 21, p. 436.
Centripetal Turbine tor High Specific Outputs
In 1939 the U.S. Navy Department requested bids on two 2,500-h.p. propulsion turbines for the Navy submarine chaser PC-452. The author’s Company was awarded the contract on the basis of the evaluation made by the U.S. Navy Department, which took into consideration the weight of the turbines, their overall dimensions, and their efficiency. The Company’s design study resulted in the choice of a compound turbine arrangement—this is, a high-pressure and a separate low-pressure turbine for each of the two propelling units. It also indicated that it was necessary to operate these turbines at no less than 15,000 r.p.m. if the design target (with regard to bulk, weight, and efficiency) was to be met. This high speed ruled out the axial-flow wheel for the last stage of the low-pressure turbine, because this type is incapable of passing the large volume flows at such high r.p.m., without prohibitively low efficiency. The high-specific-speed centripetal turbine came to the rescue, and was used not only in the last stage but also in the next-to-last stage of the low-pressure turbine,
a sectional view of which is shown in Fig. 10. This particular application of the high-specific-speed centripetal turbine made possible the attainment of a specific weight, for the complete unit, of only slightly over 31b.-per-shaft horsepower, including the double reduction gear, turning gear, and other accessories. This specific weight is by far the lowest that has ever been achieved for a marine propelling steam turbine. The units fully met the efficiency guarantees, and their small dimensions made it possible to accommodate them easily in the confined space of the slender hull. Several different sizes of turbosuperchargers for Diesel engines have been constructed in the past four years by the author’s Company. In the course of the development of these units it has been clearly demonstrated that the high-specific-speed centripetal turbine wheel is the ideal answer for this application for the following reasons: 1. This turbine is capable of an r.p.m. sufficiently high to bring the compressor driven by it into a specific speed range where maximum compressor efficiency can be obtained. An axial-flow turbine designed for the same flow and stress conditions would have to operate at lower r.p.m., which would result in a correspondingly lower specific speed of the compressor, thus preventing the attainment of maximum possible compressor efficiency. 2. Under the particular flow and operating conditions encountered in the service of exhaust turbines for Diesel engines, distinctly higher turbine efficiency can be obtained with the centripetal turbine than is possible with the axial-flow type. 3. The efficiency of the centripetal turbine, which in itself is high, can be further increased by the recovery of kinetic energy in the turbine exhaust through the use of an exhaust diffuser in the form of a simple conical duct. 4. The centripetal turbine, owing to its low number of husky blades, is far less delicate than the axial-flow wheel and better able to cope with the severe service of Diesel-engine exhaust gas operation. In addition thereto, it is cheaper to manufacture. 5. The turbine blades and turbine rotor hub can be cooled in a simple manner, similar to the one described for the aircraft exhaust gas turbines. This reduces the metal temperatures even under the extreme conditions of Diesel-engine preturbine temperature encountered during engine overload conditions, to values which permit designing for virtually unlimited time to rupture. 6. The inertia of the centripetal wheel is lower than that of an equivalent axial-flow wheel, which results in a more rapid change of turbo-supercharger speed with a change in engine load. 7. The stationary turbine nozzles, directing the flow into the centripetal wheel, can be arranged between parallel radial walls, which makes it practicable to provide for pivoting the guide vanes to adjust the nozzle areas and angles for matching the turbo-supercharger to the engine.
—R. Birmunn, Transactions of the A.S.M.E., February 1954; Vol. 76, pp. 173-187.
Motorships for South America
Canadian Vickers, Ltd., of Montreal is again delivering ships to The Flota Mercante Grancolombiana. First two of a series of four additional motorships, the Ciudad de Valencia and Ciudad de Cali, have been in service several months. The other two vessels, the Ciudad de Ibague and Ciudad de Cumana will be delivered in the forthcoming months. In accord with their building programme, Flota Mercante Grancolombiana, S.A., placed orders with Canadian Vickers, Ltd. for the construction of these four new vessels to augment their very fast cargo carrying service between the major ports of Venezuela, Colombia, Ecuador, New Orleans, New York and Montreal. Each ship is propelled by a single Nordberg six-cylinder Diesel engine, rated 4,275 b.h.p. at 160 r.p.m., direct connected to
the propeller shaft. The engine has a 29in. bore by a 40in. stroke and is of the two-cycle, single-acting type with port scavenging and port exhaust. The propulsion Diesel is independently scavenged by two motor-driven blowers each of 200 h.p. rating, taking air from the engine room through a Maxim silencer and each discharging 10,000 cfm. at about 2 -41b. per sq. in. discharge pressure to a common intake manifold. This method of obtaining scavenge air results in a shorter engine, and in case of failure of one of the blowers, the engine is still capable of maintaining 70 per cent ship speed with the remaining blower. The main engine control platform is at the after end of the engine on the port side and the log desk, engine telegraph, instrument panel, alarm panel, telephone booth are all disposed conveniently around the operator and are actuated by automatic controls. They have an actual air delivery of 68 cfm. at 870 r.p.m. and are of the two-stage type, water cooled and fitted with inter and after cooler. Auxiliary power on each of the vessels is supplied by three Nordberg four-cycle, eight-cylinder inter-cooled-super-charged Diesel engines. These engines are of the single-acting, trunk piston, mechanical injection type with cylinders of 9in. bore and ll^in . stroke rated 580 h.p. at 600 r.p.m. Each drives a 400 kW. 120/240 volt, 3 phase Westinghouse generator arranged for parallel operation and capable of carrying a 25 per cent out of balance current.
—D. Shearing, Diesel Progress, December 1953; Vol. 19, pp. 42-43.
Diesel-electric Tanker
The accompanying illustration shows one of the two Diesel-electric propelling motors of 1,750 h.p. at 128 r.p.m. in the 9,300-tons Soviet tanker General Asi Aslanow. The motors are supplied with direct current at 700 volts from four 700 kW. D.C. generators, each driven by a supercharged six-cylinder four-stroke cycle Diesel engine at 900 b.h.p. at 720 r.p.m. Cylinder diameter is 308 mm. and stroke is 330 mm. The pistons are of light alloy and are not liquid cooled.
—S. Antonow, Schiffbautechnik, December 1953; Vol. 3, pp. 359-362.
Cas Turbined Coaster
The first merchant vessel to be equipped with propelling machinery consisting of gas turbines fed by free-piston gas generators has recently completed her sea trials. This coaster, the Cantenac, is one of two vessels of the same type ordered by the French Ministry of Merchant Marine on behalf of Worms & Cie., from the Chantiers et Ateliers Augustin Norm and at Le Havre, as war loss replacement.
The Cantenac is a vessel of 850 tons with three holds and has her propelling machinery arranged aft. She is 185 feet in length b.p., 30 feet 6 inches moulded breadth, 14 feet 5 inches moulded depth to main deck, and has a mean draught of 13 feet 6 inches.
The propelling machinery consists of two Pescara-SIGMA type GS-34 free-piston gas generators; two gas turbines connected to a reduction gear, each fed by one gas generator; and a double reduction gear connecting the turbines to a single shaft and reducing the number of revolutions to 220 r.p.m. The complete design of this vessel, both hull and engines, has been carried out by the shipbuilders. Saving in Weight. The free-piston gas generator operates on the two-stroke cycle and consists of a horizontal cylinder with two opposed pistons, each of which is directly connected to a compressor piston. The outer part of the compressor cylinder constitutes a compensating cushion. The mixture of exhaust gas and scavenge air forms the gas which is used to drive the turbine. A comparison of the weight of a gas generator installation compared with that of a corresponding Diesel installation is in favour of the former scheme. A typical four-stroke Diesel engine, developing 1,800 h.p. at 220 r.p.m., and with 40 per cent supercharge, weighs about 1261b. per h.p., whereas a gas generator installation consisting of two GS-34 generators, one turbine and reducion gear of the same power and speed, is about 891b. per h.p. The saving in weight and bulk is only one advantage.
There is also flexibility of operation, avoidance of vibration, ease of maintenance and low starting air consumption. It is claimed that in all probability the free-piston gas generator unit will become the engine with the highest thermal efficiency, with a specific fuel consumption referred to the turbine shaft of less than 0-331b. per s.h.p. per hr. At the moment it is in the region of 0 391b. per s.h.p. per hr., a figure not far different from the consumption of 0-331b. per b.h.p. per hr. obtained from a supercharged two-stroke Diesel engine. Trials were run on 19th January on fuel No. 1, which corresponds to a Redwood viscosity of about 950 secs, at 100 deg. F. The unit is designed to run on both distillator and light residual fuel.
The engine was run with two gas generators developing 1,200 h.p. on one trial and with one only developing 700 h.p. on another. It is understood that the tests were highly satisfactory and it was found possible to establish, in particular, the ease and speed of manoeuvrability of the propelling machinery, as well as the ease with which one fuel was substituted for another.
—The Shipping World, 17th February 1954; Vol. 130, p. 213.
Free Piston Cas Generators
The Baldwin-Lima-Hamilton free piston generator development was conducted under a (U.S.) Navy Department contract to obtain a unit suitable for naval-combatant requirements. Consequently, it was designed with a view toward high specific output, reasonably low weight, and compactness, and to provide high thermal efficiency and reliability. In other words, to fulfil its purpose, it had to compete favourably with the thermal efficiency of modern Diesel engines and give the added advantages of simplicity, low initial cost, and smooth vibrationless operation. 'I'he power plant constructed was a twin unit with two gas generators supplying gas to a single turbine and reduction gear. It is outward-compression with both direct-bounce and reverse-bounce cylinders for control. The two gas generators are synchronized to reduce pulsations of the gas to the turbine and they can be operated together or singly, as the load requirements necessitate. The principal specifications are given as follows: —-
Table 2.—B-L-H Model B Gas Generator Specifications
Power-cylinder bore, inches ... ... 8 ¼
Compressor-cylinder bore, inches ... 23
Direct-bounce-cylinder bore, inches ... 8 ¼
Reverse-bounce-cylinder bore, inches... 23
Piston stroke (full load), inches ... 11
Cyclic frequency (maximum), cycles per min. ... ... ... ... 1,035
Exhaust pressure to turbine, lb. per sq. in. gauge ... ... ... ... 90
Exhaust temperature to turbine, deg. F. 1,295
Gas horsepower (maximum) ... ... 885
The complete power plant, after completion of its acceptance trials at the builders’ plant, was sent to the U.S. Naval Engineering
Experiment Station at Annapolis, Md., in 1950, for further test and evaluation. The principal data obtained during 700 hours of test operation are presented. Fig. 11 shows the gas-generator output, based on adiabatic expansion of the gas. The output pressure to the turbine was carried to 901b. per sq. in. gauge as contrasted with 501b. per sq. in. gauge for the French SIGM A Model GS-34. Of course, the relation of horsepower output to exhaust pressure is mainly a function of the size and characteristics of the turbine used in these tests. The curve shows outputs up to 1,770 gas-horsepower with the 901b. per sq. in. gauge maximum pressure to the turbine. On later accelerated tests to determine piston-ring and cylinder-liner suitability, where more severe operating conditions were imposed without a turbine, the same output was reached with only 701b. per sq. in. gauge exhaust pressure and the same exhaust temperature. This was a result of increasing the effective orifice area because the turbine used has a smaller equivalent orifice. The gas-horsepower curve in Fig. 11 shows that the output is increasing without any decrease in rate up to the limiting exhaust temperature and pressure set by this particular design. The shaft-horsepower curve in Fig. 11 is based on the desired 85 per cent efficiency for turbine and reduction gear, as it was in the SIGMA data. T hat efficiency
was not reached with the turbine used on the actual tests. However, turbines are available with peak efficiency of 85 per cent or higher. The shaft-horsepower curve is corrected for the power requirement of the auxiliary equipment which cannot be driven directly from the turbine. Thus about 1,420 s.h.p. is available from an installation with a weight and space no greater than that of current Diesel installations and less than that of other types of power plants having comparable thermal efficiency. The model now under development at B-L-H is much smaller and lighter. Fig. 12 shows the measured fuel consumption on a gas-horsepower basis and those calculated for a possible turbine efficiency of 85 per cent. The maximum thermal efficiency of 40 3 per cent at the gasifier discharge is very satisfactory and compares favourably with the SIGMA maximum of 3 8 2 per cent. This increase in thermal efficiency
is to be expected in view of higher operating conditions. The correction to a shaft-horsepower basis shows a greater reduction than was made for the SIGM A design because exact information was available on the B-L-H power plant and deduction was made for all losses, including supply of auxiliary control air, cooling-water pumping, and similar power expenditures.
The only deduction made for the SIGM A unit was based on turbine efficiency of 85 per cent. It is not known if the previously published data include corrections for power to auxiliary equipment. After making these corrections to a minimum shaft-horsepower basis, the thermal efficiency is still 32’3 per cent. The turbine was rated at a maximum inlet temperature of 1,350 deg. F. However, the peak pressure of 901b. per sq. in. gauge was reached with less than 1,300 deg. F. As stated before, without the turbine and with a variation in the gas pressure-to-orifice relationship, the same temperature gave as much load with only 701b. per sq. in. gauge exhaust-gas pressure.
—J. J. McMullen and W. G. Payne, Transactions of the A.S.M.E., January 1954; Vol. 76, pp. 1-14.
German Medium Speed Four-stroke Engine
A feature of German post-war shipping has been the increasing number of ships propelled by either direct-coupled or geared M.A.N. four-stroke Diesel engines. These are m anufactured with exhaust-gas pressure-charging or as normally-aspirated units, the supercharged units having the relatively conservative mean effective pressure of 9 1 kg. per sq. cm. (129 per sq. in.). It is known, of course, that the Maschinenfabrik Augsburg Nurnberg A.G. has been actively pursuing the development of the highly supercharged four-stroke engine, the results of which have been previously published.
Such engines, designed for, and operating on, higher pressures, have not yet been put into commercial service, although some are now on order, but there is, nevertheless, much of interest in the current design. This is an engine with a cylinder diameter of 400 mm. and a piston stroke of 600 mm., which, as a six-cylinder unit without supercharging, will develop 785 b.h.p. at 275 r.p.m., the corresponding piston speed being 5 5 m. per sec. The engine, which is also designed for industrial duties, is built with from five to ten cylinders—with and without pressure charging. Normal supercharging amounts to about 60 per cent, corresponding to an m.e.p. of 91 kg. per sq. cm., so that a 10-cylinder engine supercharged to this extent develops about 2,100 h.p. This is the continuous rating and does not include a 10 per cent overload allowance. A feature of the design is that arrangements have been made whereby a drive can also be taken from the forward end, if desired; this is particularly advantageous in such vessels as ferries, or for fishing boats where a drive is generally required for the trawl winch or winch generator.
The exhaust-gas turbo-blower is of the standard M.A.N. design with self-aligning bearings of the multi-surface type between the turbine rotor and the blower, to ensure the minimum of friction. The construction of the engine is generally simple. The welded bedplate supports the columns and the cylinder block which, for the larger engines, comprises a number of units bolted together. Cast integral with the cylinder block is the air suction manifold which supports the camshaft seating and the fuel pumps. It will be seen that the arrangement of the camshaft is such that the need for push rods is obviated.
The drive for the camshaft is by means of a 2J-in. simplex chain. W ith four-stroke engines of similar size, reversing is generally effected in three stages, the push rods being lifted from and replaced on the cams in the first and third stages; the camshaft is moved to its requisite position during the second phase. W ith this engine, however, all cams have oblique surfaces, so that it is not necessary to lift the rollers from the cams, an arrangement which enables a simplification of the reversing gear and ensures a quicker response. Owing to the excellent heat transmission properties of the aluminium alloy of which the pistons are composed, the pistons are uncooled.
With 60 per cent supercharging, the temperature at the centre of the piston crown, measured by thermo-elements 6 mm. below its surface, remains below 210 deg. C. The temperature within the vicinity of the topmost piston ring is about 20 deg. lower—about 190 deg. C. at the same load—at which temperature sticking of piston rings should not be encountered. An interesting point about the pistons is that they are ground spherically from the bottom to the top, i.e. with a smaller diameter at the bottom and top than in the middle. Furthermore, the pistons are ground slightly oval in their cross-section, the degree of ovalness increasing from the bottom upwards to about the top piston ring, where the cross section gradually reverts to round form.
— The Motor Ship, February 1954; Vol. 34, pp. 486-487
Diesel Engine Synchronization
The purpose of synchronizing is to obtain a fixed relative position of two rotating shafts. In a marine Diesel propelled installation this can be of value by removing, or at least reducing, disturbing vibrations which may occur in consequence of unbalanced inertia moments of the crankshafts, or resulting from a disadvantageous relative position of the rotating propeller blades. The De Schelde synchronizer, which has been installed in several ships with various kinds of propulsion, permits the angle of synchronization of the shafts to be set at any desired position from 0 deg. to 360 deg., even during running, by simply turning a small handwheel on the “adjusting differential”. It will also keep this angle constant by means of sensitive automatic control of the fuel pumps on both engines (or groups of engines), without changing the position of the fuel-control levers.
The different applications of the De Schelde synchronizer shown in Fig. 1 are: (i) Synchronizing of propellers only, as installed in the motor liner Willem Ruys; (ii) synchronizing of crankshafts only, as installed in the single-screw motorships Carbet, Carimare and Caraibe of the French Line. A similar installation was provided for a motorship built by Kaldnes Mek. Verksted, Tønsberg, for Wilh. Wilhelmsen; and (iii) synchronizing of propellers and crankshafts.
This arrangement is used in the three motorships which were built for the Argentine Government, and in the new Swedish motor liner Kutigsholm. The engines in the Kungsholm consist of two Burmeister & Wain eight-cylinder two-stroke single-acting direct-coupled crosshead units, type 74 V TF 160, each developing 7,000 b.h.p. at 115 r.p.m. The synchronizer installed in this vessel comprises two main parts, as illustrated in Fig. 2. The first is the hydraulic differential (A) and its driving parts; and the second, the hydraulic coupling rod (B), mounted in the fuel pump control system and operated by the hydraulic differential. The hydraulic differential is an arrangement of two slides rotating one within the other. The outer slide is driven by the port engine and the inner by the starboard engine, each with a transmission ratio of 1:1. Both slides rotate in the same direction. If the starboard engine runs with the same r.p.m. as the port engine there will thus be no relative rotation between the two slides, and any relative rotation velocity of the slides is equal to the difference in r.p.m. between the two engines. Ports are milled in both inner and outer slides and, in addition two chambers are drilled in the centre of the inner slide. This divides the hydraulic differential into two parts (Fig. 3). Chamber “A” with ports I, II and III forms the oil supply side; and chamber “B” with ports IV and V forms the oil drain side.
There is always an open connexion through port I between chamber “A” and the oil supply line, taken from the gear pump. Similarly, chamber “B” is always connected with the oil drain pipe through the synchronizing gearbox. Ports II and V provide the oil supply and oil outlet to and from the hydraulic coupling rod on the starboard engine; ports III and IV provide these for the port engine. The hydraulic coupling rod is connected in the fuel pump control system in place of an ordinary solid rod, and consists of a cylinder, a spring-loaded piston and piston rod. The action of the oil under pressure from the hydraulic differential underneath the piston is counteracted by the spring. When the synchronizer is not working, there will be no oil pressure in the hydraulic coupling rod. The spring holds the piston against the bottom of the cylinder and the coupling rod will then act as a solid connexion. If, however, oil pressure is directed from the hydraulic differential to the coupling rod, the cylinder will be moved downwards, the position of the piston being fixed by the fuel-oil hand-control lever, reducing the capacity of the fuel pumps and slowing down the engine. The stroke of the piston in the cylinders can be adjusted from 0 to 8 mm.
—The Shipping World, 20th January 1954; Vol. 130, pp. 116-118.
Welding Marine Diesel Engine Structures
In ships’ machinery the materials employed have to withstand a variety of conditions. For instance, cylinder liners have to resist corrosion and wear at elevated temperatures, so has the piston, while other parts transmit or carry varying mechanical loads. For the sliding parts and cylinder liners some form of cast iron will give best service. The liners may be of alloy iron proportioned as a good compromise between low friction loss and high wear resistance. In the past all engine frames were made in cast iron, but welded steel construction is now the most popular material for the structural parts of the engine. Fewer limitations than are imposed by foundry practice enable material sections to be proportioned more economically, and due to the better distribution of material, greater accessibility to the working parts and more streamlined flow of both scavenge air and exhaust gases are made possible. In general, considerable weight saving can be achieved due to the better properties of rolled steel. Economies
in machining can also be expected in view of the possibility of making beds and the like in fewer sections, thus saving in the number of machine joints. Modern fabrication can now be made with a small machining allowance, so achieving a saving in time and weight of metal removed. Pre-machining of detail parts on smaller machines saves time occupied on large machines later. In the design of successful fabrications the designer must have an intimate knowledge of the shop processes and must at the outset decide which of the two assembly methods suit the shop facilities best. As all engine beds are virtually a series of broad flange or deep web girders with transverse members and flanges to take the engine columns, they can be assembled from plates and sections to the shape required; the facings, which must finally be accurately machined, can be fitted last and adjusted so that they are well within the desired machining allowance. The adjustment of these facings can be made by the amount they overlap the supporting webs or by trimming the webs if they butt up on the underside. In this method of construction the main body need only be held to within reasonable limits of accuracy, and distortion due to welding will be less important, as the facings are fitted last. This method, described in its simplest form, may include a number of sub-assemblies designed to facilitate welding and reduce the risk of major error on final assembly.
—J. A. Dorrat, The Welder, January-March 1954; Vol. 23, pp. 126-130.