Proceedings of the Institution of Mechanical Engineers

Evidence has been obtained that, in extremely severe conditions of friction, fusion occurs at the surface of friction between cemented carbides and steel. It is shown that steel will fuse with tungsten carbide at a temperature between 1,300 and 1,325 deg. C. (2,372 and 2,417 deg. F.). A theory is put forward to account for the “cratering” wear on cemented carbide tools containing free tungsten carbide. According to this theory, a fused layer of an alloy between tungsten carbide and steel is formed at the friction surface and is carried away by the chip. Titanium-tungsten carbide does not fuse with steel so readily, and hence is worn away much less rapidly than tungsten carbide in steel-cutting at high speeds. Micro-examination of worn tool surfaces shows that the type of wear is consistent with this theory. The conditions existing at the tool-work surface are discussed and the type of temperature distribution in the tool near the cutting edge is estimated. It is shown that the temperature distribution near the cutting edge can control the extent of cratering, the “built-up” edge, and deformation of the tool. In the light of the theory, some of the basic properties required in a tool material are discussed.

The problem of longitudinal gauge variations in hot- and cold-rolled sheet and strip metal is considered. It is suggested that they are caused mainly by elastic distortions of the mill in response to variations in the separating force between the rolls. A unique relation is shown to exist between the roll force and the thickness of the strip leaving the rolls, and this forms the basis of two new methods of continuous automatic control of gauge. These methods are independent of direct measurement of the thickness of the sheet or strip during rolling. They employ an electrical roll-force measuring element the output of which, after suitable modification, continuously adjusts either the position of the rolls or the tension applied to the strip, so that a substantially constant thickness is maintained in the rolled material. Some preliminary experiments on a 10 inches by 10 inches two-high mill have demonstrated that these methods of control are correct in principle.

In this paper are discussed those characteristics of boiler design and performance which have favoured the parallel development of automatic control of boiler auxiliaries.

Consideration is given to those temporary conditions which arise during periods of load change, and which, if not accurately estimated and followed by the necessary corrective action, may lead to over-regulation of firing rate or feed-water supply. Instruments are provided to determine and inform the operator of the necessary action to be taken, and manual remote-control eliminates the laborious duties of routine readjustments necessary to maintain efficiency. In this field automatic control has many advantages and is quicker and more accurate in response than the human operator.

The theory of elementary forms of automatic control devices is dealt with simply, and the mechanical means, employing pneumatically operated devices, by which these theoretical requirements are met are described. Examples are given of the use of standard apparatus in building up a composite control system, and the more complicated installations of boilers fired by spreader stokers, pulverized fuel and cyclone furnaces are dealt with.

The paper describes tests on a 0–28 per cent carbon steel in tension and compression, and in flexure of beams of rectangular cross-section, to a maximum strain about three times that at the initial yield. The object of these tests-was to investigate the shape of the stress-strain curve immediately after the initial yielding, and to determine whether in a case such as flexure the upper yield stress could be relied upon as a criterion of design. The results from this material indicate that the stress-strain curve falls rapidly but not immediately from the upper to the lower yield value, and that a beam is capable of withstanding a slightly greater bending moment than would be predicted by calculations based on the direct stress tests including the upper yield stress.

In order to achieve a thermal efficiency in the gas turbine comparable with that realized in steam practice, a higher turbine-entry temperature is necessary.

Limiting discussion to the combustion and expansion sections of the gas turbine, the paper first indicates that the use of metals in high temperature and stress conditions necessitates a new approach to component design. The phenomena of creep and fatigue assume major importance whilst pure temperature effects, such as expansion and thermal shock, are additional problems. These properties are defined and an indication is given of their significance in the design of combustion chambers, turbine wheels, and blades.

Susceptibility to heat treatment, intended to induce the desired high-temperature properties, is another important factor to be considered in material selection, as are fabrication characteristics. Since the latter govern both detail and general design, the merits of forging, casting, and welding are outlined when applied to high-temperature alloys.

The paper concludes with a survey of the problems likely to be encountered with future materials, particularly ceramics, whilst suggesting that increased efficiency may be obtained by further research on established alloys or by design innovations such as cooling.

The objective of this lecture is to outline the various characteristics of the cavitation phenomenon and to point out in general how this knowledge may be used to alleviate and, in some cases, to eliminate the losses due to damage of materials and decrease in performance of hydraulic equipment. It is shown that difficulty with cavitation is encountered over the entire field of hydrodynamics but that, although the manifestations may appear to differ widely, they can usually be understood on the basis of present concepts of the mechanics of cavitation. It is found that such understanding usually brings with it suggestions of methods for lessening or eliminating the difficulty.

In this paper standardization in design is considered. The argument is developed tat the detailed design of the parts of a mechanism demands a logical approach. The designer should state on the drawing, in the simplest possible terms, what he really wants. The necessary range of basic (or abstract) engineering standards, both for him to achieve this aim and to provide for economic production, are given.

The basic standards cover definitions, geometric analysis, ultimate standards of size, basic sizes, limits and fits, measurement, surface texture, drawing practice, standard forms (for example, screw threads), and standards of design. Their nature and scope are examined in general terms and some of the main compromises necessary are discussed.

The necessity is shown for a sound framework, wide enough to cover the normal needs of all engineers. The dangers of a parochial outlook are stressed and the needs of both small and large producers are emphasized. These standards are shown to be the essential preliminary to sound standardization of articles, which should be a natural by-product of sound design. Their main economic value, however, is that they affect every article which the engineer designs.

At the present time there is only a partial framework and much disagreement on details. To standardize articles before agreement has been reached on the approach to detailed design of the articles, leads to difficulties in standardization and sometimes to bad standards. Several good solutions can be propounded for most problems, and many different ones are in local use. A single, wide, and all-embracing plan which can be used by everyone is needed. Education and explanation can then lead to general use, to the lasting advantage of the engineering industry.

A simplified theory is presented for the flow of air through a single orifice or constriction. This theory is then extended to cover the flow of air through a labyrinth gland having several constrictions of equal area. Experiments are made over a wide range of pressure ratios to determine the coefficient of discharge for a single annular constriction having a small clearance. It is found that the curve has the same general characteristics as that for a circular sharp-edged orifice. The discharge coefficients are then used to calculate the critical ratio of inlet pressure to outlet pressure for a labyrinth gland. The results obtained agree well with experiments on a stationary labyrinth gland of the staggered type in which the number of constrictions may be varied from one to fourteen.

The theoretical pressure distribution is derived and compared with experimental values under conditions where the pressure drop across the last constriction is either less or greater than the critical value. Finally, the coefficients of discharge are obtained experimentally for the final constriction and for the preceding n-1 constrictions in a gland having n constrictions. It is concluded that the carry-over effect in a staggered gland is small.

The state of knowledge about stress conditions in gear teeth is briefly reviewed, and directions in which further research is required are indicated. The assumptions underlying methods of stress-comparison, and the chosen load-points, are considered in relation to profile-modification, leading to revised conceptions of “precision” and “commercial” classes of gear. The present (British standard) basis for the design of “commercial” teeth is stated and a new basis for the design of precision, high-duty gears is offered.

Criteria of surface stress are re-examined and current British standard rating formulae for wear are shown to be equivalent to the use of an unmodified criterion S_c = F_c/R_r in conjunction with a speed factor which is a function of pitch circle velocity and gear ratio. New values for zone factors and strength factors for spur gears have been determined on the revised assumptions, and it is shown how the factors for helical teeth may be derived from those for the virtual spur gears. The assumptions underlying rating formulae for variable load-cycles are also examined, and a revised formula is suggested whereby present inconsistencies are removed.

An examination is made of the dynamic characteristics of wheel sets and bogies, and of the various forces which act upon a bogie under service conditions. The fundamentals of bogie design are considered, and particular mention is made of recent developments in methods of body suspension. Problems of frame construction, braking, and power transmission are also considered. The paper concludes with a survey of the development of bogie design on the railways of London Transport Executive and elsewhere, and a restatement of the basic problems in the relation of bogie to track.

In view of the intensive development of gas turbines for aeronautical purposes, the author advocates the adoption of aero-design techniques for the production of gas turbines for marine and stationary purposes. A review is made of the more obvious features of various typical aircraft turbines, and the conclusions are drawn that multiple combustion systems and single- or two-stage turbines should be employed with a considerable reduction in the mass of metal in the assembly as compared with extant stationary designs, in order to reduce transient temperature stressing.

A description is given of an experimental heat exchanger embodying the principles of light scantlings, symmetry of design, and purity of suspension.

A brief excursion into the performance calculations for a marine turbine leads to a recommendation to use a compound cycle of 8/1 overall compression ratio, with centrifugal compressors for robustness, and with a high combustion-temperature involving the use of air-cooled blading. A life of 10,000 hours appears to be within reach, which figure is considered to be adequate for merchant ship service. A compact arrangement is suggested for a turbine of this type giving 4,000 s.h.p. at an overall thermal efficiency of 30 per cent. The manoeuvring and reversing problems are to be overcome by the fitting of a variable-pitch propeller, but development work on this device is required to reduce its alleged tendency to greater losses as compared with the fixed-pitch type.

A general outline of the mechanical engineering activities of Bristol and the surrounding areas is used to illustrate the achievements resulting from local enterprise and development. Details of industrial techniques are not described.

The production of aircraft engines of world renown and the work of paper conversion, for which Bristol is one of the most important centres in Britain, are given major emphasis and reference is made to the manufacture of mechanical handling equipment, and brewing and paint-making machinery.

Problems are discussed associated with the burning of heavy fuels in medium-speed, two-cycle, trunk-piston types of engine. Tests with a stationary engine are described, and performances, rates of cylinder-bore wear, and degrees of fouling are compared, when five classes of fuel are burnt.

Pre-treatment of fuels in the centrifuge is considered, and the results from practical experience show how certain fuels with high ash content, probably in an oil-soluble form, promote rapid cylinder-bore wear, even after being subjected to thorough treatment.

Comparative rates of deterioration of the lubricating oil are recorded when different classes of fuel are burnt, and figures are given which show how fuels with a high sulphur content promote the formation of sulphuric acid in the crank chamber.

Findings with the stationary engine are correlated to marine requirements, and the economy aspect is presented of bunkering a lower grade of fuel in a coastwise vessel of 2,500 tons dead weight. The conclusion is reached that a marine Diesel fuel, preferably a distillate, instead of gas oil could be bunkered with financial advantage, but that in the light of present knowledge the use of a boiler-grade fuel could not be recommended.

The paper elucidates the manner in which impeller blades, turbine and axial compressor blades, aircraft and marine propeller blades or similar bodies, vibrate in their higher modes. It is demonstrated that complex nodal patterns derive from consistent series of simpler modes and that the frequencies of the latter may be plotted in families of curves.

Classical work of Chladni, together with more recent investigations on free square and circular plates, is first reviewed. The similarity of the manner of vibration of the cantilevered plate is then described with a note on the relation of the torsional series of modes to the simple flexural “bar” type modes. The vibration nodal patterns of impeller blades are discussed and diagrams illustrate how “complex” modes were resolved into their constituent “basic” modes, each having an ordered place in the families of modes and frequencies. Similar analysis, with examples, is shown to be applicable to turbine blades, and aircraft and marine propeller blades.

In this paper is treated the important problem of crankcase explosion prevention in practice. After dealing with the cause of such explosions the author concludes that, whilst much can be done to minimize explosions, to prevent them altogether is not practicable. The merits of ventilation of crankcases are discussed and ways are suggested to prevent bearings running hot, while means to detect a temperature rise in inaccessible bearings and pistons are described.

Tests carried out with the object of preventing injury to personnel and damage to property in the event of an explosion are described. A relief valve which opens and closes instantly is described and illustrated. The danger of burning gases issuing from relief valves is discussed, and the experiments are described which resulted in the evolution of a simple but effective flame trap.

Relevant particulars are given of a Diesel engine in which two crankcase explosions took place, one before and one after a relief valve was fitted. The gas pressure in the crankcase when damage resulted is estimated and views are expressed regarding the relation of crankcase cubic contents to the area of relief devices.

Jacketing by a transfer medium of the condensing vapour kind is a suitable method for heating many process units to temperatures of about 300 deg. C. (572 deg. F.) when fine control is necessary. The mixture of diphenyl and diphenyl oxide known as Dowtherm “A” has a good relation between vapour pressure and temperature for such a purpose. In the paper the other relevant properties are discussed.

The principle of operation of a circulating system and the design of an installation of vaporizing and auxiliary equipment having a rated output of 11,000,000 B.Th.U. per hr. are described with reference to the special problems presented by Dowtherm owing to its inflammability, high freezing point, searching nature, scouring properties, and possibility of degradation.

Preliminary work on two possible alternative materials with lower freezing points and less propensity to leakage is briefly mentioned.

The exceptional hardness of the diamond and its resistance to abrasion are becoming more widely appreciated throughout the industrial world and resulting in its increasing use in the manufacture of drills, drill crowns, crowning wheels, hardness indenters, lathe tools, nozzles, saws, wheel dressers, wire-drawing dies, and many other products.

The industrial diamond as distinct from the gem diamond is, broadly speaking, one that—on account of imperfections such as inclusions, spots, cracks, cuts, inferior crystal form, or dark colour—cannot be economically cut and utilized as a gem stone. The unit of weight for industrial diamonds, like that for the gem stone, is the metric carat; which is equivalent to 0.2 gramme. Approximately tons of diamonds a year, an amount corresponding to about 80 per cent by weight of the world's present annual output, is used in industry.

The increasing demand for industrial diamonds was responsible for the reopening of the mine in the Transvaal in which the modern mechanical plant described in this paper was installed.

The most striking feature of the deformation of metals is the formation of slip lines. Recent investigations suggest that, when formed at low temperatures, each slip line is the result of a displacement of the material along a single lattice plane through a distance of about a thousand atomic diameters. Moreover, there is much evidence that the steps on the surface which appear as slip bands attain their full height in a small fraction of a second, though their length may thereafter increase slowly. At higher temperatures and at slow rates of strain the slip bands appear, under the electron microscope, as clusters of lines about a hundred atomic diameters apart. The origin of slip lines, the reason for this clustering and the cause of work-hardening are discussed. The two conceptions used in the discussion are the dislocation line and the vacant lattice site.

Slip lines are believed to have their origin in a certain arrangement of dislocation lines of frequent occurrence in the interior of the crystal. These are known as Frank-Read sources; their relation to recent work on the growth of crystals is shown. Where a slip line terminates dislocations must remain in the crystal; to the internal strains round these is ascribed work-hardening, much as in Taylor's theory of 1934†. It is now, however, possible to explain what it is that stabilizes the dislocations and prevents them from moving back when the stress is released.

Finally, vacant lattice sites are shown to be formed in a cold-worked material. If the temperature is high enough for them to diffuse, they soften the material round the slip band and allow the observed clusters to form. They also play a part in producing the observed “fragmentation” of cold-worked material.

The results are presented in this paper of investigations on the pressure distributions around aerofoil section blades while rotating in a wind tunnel. The experimental difficulty of obtaining pressures during rotation was overcome by the use of a special gauge attached to the hubs and illuminated by “Strobo-flood” for direct observation. Pressure diagrams and lift coefficients were obtained along the radius of a double-blade set, arranged as a windmill, for various angles of incidence.

Static tests were conducted also in all cases to find values for comparison. As the purpose of the work was to contrast rotating and static conditions, two main facts are emphasized: the similarity in the pressure diagram shapes and the significance of the spin factor. It therefore becomes possible to correct ordinary test results obtained when using stationary aerofoil blades in wind tunnels by adding the effect of a vortex of rotation of uniform strength.

A method is given for calculating the radial distribution of pressure in a steam turbine with long blades, the conditions for radial equilibrium, for continuity, and for energy balance being satisfied simultaneously. An example, illustrating the application of the method, shows that if there is pure impulse at the root of the blade, an appreciable degree of reaction may be reached at the outer end of the blade. It is shown that if the steam flow is to be parallel to the axis of the turbine then the variation of nozzle outlet angle with change of radius depends on the steam velocity at the inner radius. An alternative theoretical analysis of the problem is given which yields quickly the radial distribution of pressure where the nozzle outlet angle is uniform. This analysis is extended to give the degree of reaction at the blade tip in terms of that at the blade root.

The paper concludes with a brief note showing that in geometrically similar turbines operating with the same blade speed, but different speeds of rotation, the radial distribution of pressure is the same.

In this paper is described an attempt to increase the output of a spinning-disk sprayer without sacrificing the homogeneous drop-size characteristics found by previous investigators. A disk sprayer inches in diameter was designed, and the characteristics were determined by feeding various quantities of water centrally on to the disk, and noting the resulting drop sizes and their distribution over a range of disk speeds from 600 to 2,000 r.p.m.

The critical flow rate for the production of uniform-size drops is in the range 6–22 cu. cm. per sec., and the size of drops produced is inversely proportional to the speed of the disk. The product of drop diameter and peripheral velocity of the disk is approximately constant and varies with the smoothness of the surface of the disk rather than with the rate of liquid flow.

Above the critical flow rate the homogeneity of the drops disappears and a wider range of sizes is observed. The distribution and sizes of the drops are independent of the rate of liquid feed over a range of from 22 to 110 cu. cm. per sec, the size of the majority of drops being dependent on the peripheral speed of the disk. Above that range the liquid comes off the disk in a sheet, in which little or no break-up of the liquid takes place.

It is concluded that spinning disks of large diameter will produce drops of uniform size even at relatively low rotational speeds.

The author believes that simplification, while it must not be regarded as the panacea for all the ills of our industry, can if correctly applied be of immense benefit and that, though much has been said and written on the subject, few practical examples of its application in design and manufacture have been made available.

The early history of machine tools is reviewed in this paper, and as the horizontal boring machine incorporates the general features of most machine tools it is used as a typical example.

Some suggestions are made on the subject of simplification from the aspect of design, and practical examples are given of its application to patterns, castings, machining, lubrication, controls, measuring systems, and electrical equipment. Single-purpose machines, tool equipment, and accessories are also examined.

It is proposed that the maximum advantage cannot be gained from a process of simplification unless a high degree of standardization is accepted, and therefore in reviewing the design of a complete range of horizontal boring machines a new principle of unit construction is explored in theory and practice. Examples are given of the application of this unit system to all sizes of horizontal boring machines, including the universal type. The paper concludes with a description of a new slideway grinding machine generally constructed from standard units.