ASHCROFT STEAM TABOR INDICATOR No 6028 IN BEAUTIFULLY CONSTRUCTED BOX WITH STAINESS STEEL, PRE-WORLD WAR I LOCKS AND HANDLE. Dated 1894
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What is an indicator?
An indicator is a small, originally mechanically-operated instrument that gives an insight into the operation of a range of pressure-operated machines — steam engines, gas and oil engines, compressors, condensers, even guns — by comparing the rise and fall of pressure during the operating cycle. The use of an oscillating drum allows variations in pressure to be recorded on both the outward stroke and the return journey. Excepting some of the continuously-recording instruments and virtually all maximum-pressure recorders, indicators usually give a trace in the form of a closed loop.
A steam engine works by allowing steam to enter a cylinder behind a piston, then pushing the piston along to the opposite (far) end of the cylinder. The piston is connected to a crank, and the crank is often connected to a heavy flywheel. If the engine is single acting, the steam would be exhausted at the end of the piston stroke and the momentum of the flywheel, once the engine was running, would return the piston passively to its original position. Steam would be introduced, and the cycle would recommence. Most engines are double-acting, however. When the steam is exhausted from the head side of the piston at the end of the outward stroke, another charge of steam is introduced on the crank side of the piston to actively push the piston back again. This results in continual motion as steam is admitted and exhausted on both sides of the piston alternately.
The sides of this near-rectangle illustrate the steam cycle in graphical form. The steam valve opens atA, allowing the pressure to rise instantaneously to its maximum level, shown atB. The piston starts to move forward, propelled by steam that is admitted until the piston reaches the far end of the cylinder atC. The exhaust valve then opens, releasing all the steam, and the pressure drops toD. At this point, the admission of steam to the other side of the piston (or the momentum in the flywheel) pushes the piston back to its original positionA.
The engine indicator is a fascinating instrument that records how the pressures in the cylinders of steam and internal-combustion engines change during the operating cycle. Used properly, they can identify problems ranging from bad valve settings to constricted steam pipes.
One of the major catalysts in development was the increasing realisation that the steam engine, useful tool though it had proved to be, was inefficient. Many reasons have been advanced to account for this process, but it is probable that — once the Watt-type engine began to replace the old Newcomen 'atmospheric' designs — it could be seen that too much coal was being consumed in relation to output.The first attempts to register pressure generated in the cylinders of steam engines were undertaken with columns of mercury, the levels being judged by the displacement of the metal in small-bore tubes.
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The advent of the Watt-type steam engine inspired the pursuit of efficiency. The first indicators required careful watching, as the readings could only be taken manually, but it was not long before the automated moving-tablet instrument had appeared.
This method was unsatisfactory, as the mercury often oscillated too greatly for satisfactory recordings to be taken and the glass tubes were prone to break. A better solution was proposed by James Watt (1736-1819), who is customarily accorded credit for defining the 'horse power' and was also the first (as far as we know) to produce a 'stand alone' method of indicating steam pressure within a cylinder. About 1790, Watt produced an indicator in which a small piston, travelling within a brass cylinder, moved a pointer; the greater the pressure, the greater the deflection, which, as the engines of the day were slow running, could be seen by an observer. A skilled man could note the progress of the pressure during the steam phase and also the vacuum produced by the condenser.
The first major improvement in the design of the Watt Indicator was made by an employee of Boulton & Watt, John Southern (1758-1815), who designed the first method of recording the operating cycle of the steam engine automatically. In the summer of 1796, Southern suggested adapting the 'recording indicator' by adding a recording-board or tablet that slid within a supporting frame. A cord attached to the beam pulled the tablet sideways as, simultaneously, the pencil-pointer recorded the rise of pressure in the cylinder on a sheet of paper. As the beam returned, a weight attached to the free end of the operating cord reversed the movement of the tablet. The pencil recorded the cylinder pressure as it dropped to nothing, and in so doing closed the diagram of pressure against time.The shape of this diagram remained characteristic of indicators made into the present century, the earliest datable survivor being made in January 1803. However, indicators were in perpetually short supply until the 1820s. This was partly becauseJames Watt was a secretive man, obsessed with keeping his ideas from others, and the development of a recording indicator was kept from prying eyes. However, details of one such instrument were published in an 'Account of a Steam Engine Indicator', a letter submitted by 'H.H. junr.' toThe Quarterly Journal of Sciencein 1822. This credits knowledge of the indicator to Joshua Field of Maudslay, Son & Field, and manufacture to a foundry owned by 'Mr Hutton of Anderston', Glasgow.Indicators of this type were made into the 1840s, and the idea of a moving tablet reappeared several times in the late nineteenth century. The culmination was a Wayne design, patented in Britain in 1894.
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Left:drawings of the original McNaught indicator, published in a French textbook in 1844, the year of John McNaught's death. Note that the recording drum is co-axial with the piston.Right:the later (and ultimately more successful) design had the drum offset from the centre line of the piston. The example pictured was made in Glasgow in the 1850s by McNaught's successors.Museum of Making collection.
The first major advance to be made after the reciprocating tablet was the oscillating drum. The credit for this is customarily given to the Scotsman John McNaught—largely on his own testimony!—but circumstantial evidence suggests that the idea may have occurred first to Henry Maudslay. The original McNaught indicator had the recording drum concentric with the piston cylinder, but the perfected version had the drum on a platform that protruded from the cylinder laterally. A swivelling pulley, known as the fairlead, allowed the cord attached to a suitable part of the engine to approach the indicator at an angle.
The McNaught indicator relied on a comparatively steam-tight piston sliding in a tube beneath a large coil spring. When the outward stroke of the engine piston began, the pull on the cord turned the drum through a half-revolution. The admission of steam to the cylinder raised the pencil to make its trace. When the inward stroke of the piston began, a helical spring in the recording drum rotated it back through the half-circle to its starting position. This allowed the pencil to complete its loop.
The McNaught indicator was popular, as it was comparatively simple, easily copied, and acceptable efficient. Many attempts were made to improve it, notably by Duvergier in France and Joseph Hopkinson in England, but many of these experiments sought methods of obtaining continuous diagrams instead of improvements to the basic instrument. From this period, too, came the first attempts to develop the so-called 'lining indicator', which constructed an average diagram from a large number of strips taken from successive engine cycles, and the first integrating 'totalisers', which deduced the cumulative or average values numerically.
Once the value of the perfected McNaught rotating-drum pattern had been universally admitted, steam-engine indicators were made in great quantity by many manufacturers. But they were expensive. Regarded as precision tools, they still cost more than four times the average English weekly wage in 1914.
The next great advance was made in the USA, where the first high-speed steam engines had been developed in the 1850s. The advent of the Allen engine, promoted enthusiastically by Charles Talbot Porter (1824–1910), was the catalyst. Porter realised that the McNaught-type indicators being made by the Novelty Iron Works were too prone to vibrate when used at high speed, giving tremulous diagrams that were impossible to interpret, and sought something better. The project was given to consulting engineer Charles B. Richards (1833–1919), who within a very short time had produced a workable design by combining the offset recording drum and the internal coil spring of the McNaught with a system of levers, inspired by Watt parallel motion, that amplified the movement of the piston four-fold. This kept the movement of the piston to a minimum, allowed a short stiff spring to be used to damp vibrations, and kept the instrument as compact as possible.
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The Richards indicator was the first of the 'amplifying designs', capable of multiplying the comparatively small movement of the piston to give a trace at least four times the size. This particular instrument was probably made by Hannan & Buchanan of Glasgow, about 1880.Museum of Making collection.
The Porter-Allen engine and the prototype Richards indicator (made by the Novelty Iron Works) were exhibited at the International Exhibition in London in 1862. There the engine attracted much adverse comment, but proved to work smoothly and in perfect safety-confounding the doom-laden predictions of its many detractors. The indicator was used successfully to test a variety of engines on display, though one or two well-known British engineers refused to have anything to do with it. A trial of a railway locomotive was arranged, and the existence of the indicator came to the attention of Elliott Brothers, renowned as makers of optical equipment. A licence was negotiated, and the first Elliott-Richards indicators appeared in 1863. More than ten thousand of them had been made by 1876, and work continued until the end of the nineteenth century.
The Richards indicator proved to be sturdy and reliable, and was still being used in quantity when Europe went to war in 1914. However, the success of the Porter-Allen engine had begun a quest for ever-greater pressures and ever-increasing speed. Above 250 rpm, even the Richards indicator struggled to provide reliable diagrams. This was largely due to the inertia of the parts in the amplifying mechanism, which were long and comparatively heavy.
The first real successor was the work of the American engineer, Joseph W. Thompson, whose indicator was patented with the backing of the Buckeye Engine Company in 1875. The Thompson amplifier (which embodied a more mathematically correct straight-line approximator than its predecessor) was much lighter than the Richards equivalent, taking the form of the letter 'M', and soon proved to give good diagrams at 350 rpm or more. Consequently, Thompson-type indicators were made by many companies after Buckeye withdrew. The best known are the American Steam Gauge Company, Schaeffer & Budenberg, James Robertson & Sons and Dobbie McInnes. Dobbie-patent instruments were still being made in the 1960s substantially in their original 1898-type external-spring form.
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The three principal U.S.-made indicators of the late nineteenth century: an early American Steam Gauge-made Thompson (left), the perfected Ashcroft-made Tabor (centre), and an 1895-type Crosby (right).Bruce Babcock and Museum of Making collections.
The indicator designed by Harris Tabor, patented in the USA in 1878, offered an alternative to the Richards and Thompson patterns. Made by the Ashcroft Manufacturing Company from the early 1880s, the Tabor relied on a slotted vertical standard to direct the pointer in a straight line. The earliest Tabor was too weak to succeed, but the modified design of 1886 proved to be amply strong enough to compete on level terms with the Thompsons.
When the Thompson patent lapsed, many other designers tried their hand. This is particularly evident in the USA, where instruments such as the Straight Line (1890), the Calkins (1890) and the Lippincott (1900) all briefly prospered. There was even a place for aberrant designs such as the British Kenyon of 1878 and its US equivalent, the Rae, which used Bourdon-type pressure tubes instead of piston springs; the 1887-patent Bachelder, with its adjustable leaf spring placed horizontally; and the British Simplex of 1894, briefly promoted by Elliott Brothers, which had a tong-like spring.
If the Thompson system was the most popular prior to 1910, then it was eclipsed by the perfected Crosby indicator thereafter. Made in Boston, Massachusetts, the 1882 Crosby and its strengthened 1895-patent successor were made in large quantities. They had the merit of exceptionally light amplifying gear, much lighter than even the Tabor, and a comparative absence of inertia effects.
By 1900, the first of the external-spring indicators were being seen. The earliest is usually acknowledged as the British McKinnell & Buchanan, patented in 1893 on the basis of the Richards mechanism, but the original Maudslay & Field indicator —sometimes said to date as early as the1820s — may have taken a broadly comparable form.
Many of the earliest designs were poor compromises, including the external-spring Tabor, the first Maihak and the 1902-type Dreyer, Rosenkranz & Droop pattern. The problem was simply that spring-support standards or rods had to be accommodated on the cylinder cap or platform of an otherwise conventional enclosed-spring design. The ideas made sense financially, but were less acceptable where efficiency was concerned. The external-spring Tabor, patented in 1900 by William Houghtaling, even had the amplifying rod spindle in a separate chamber.
The 'second generation' of external-spring designs had the springs beneath the platform, above an abbreviated piston cylinder, and were also often fitted with vulcanite or similar sheathing to allow springs to be changed when the metalwork was hot. The most successful of these indicators were patented by John Dobbie in Britain and by William Trill in the USA.
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Typical external-spring indicators: a British Dobbie McInnes Design No. 1A 'Large' (left), with the spring exposed beneath the platform; an American Davidson-patent Crosby (centre), with the spring above the bridge; and a post-1918 German Lehmann & Michels instrument (right), with the spring above a Crosby-like amplifying mechanism hiiden within a protective casing.John Walter and Museum of Making collections.
'Third generation' external spring indicators returned to springs that were mounted on top of the platform, but often relied on extended piston rods or bifurcated or duplicated amplifying links of Thompson or Crosby type to allow the spring to be concentric with the piston rod. Indicators of this type were made in the U.S.A. in Boston, Massachusetts, by the Star Brass Manufacturing Company (Webster patent), the American Steam Gauge Company (Jerrauld patent), and the Crosby Steam Gauge & Valve Company (Davidson patent); and in Pittsburgh, Pennsylvania, by the Bacharach Industrial Instrument Company (Maihak copies). Both Maihak and Lehmann & Michels made Crosby-type indicators in Hamburg, Germany, in accordance with Wilhelm Lehmann's patent of 1909.
Another idea to see success in the 1920s was the cantilever-spring indicator, a descendant of the Bachelder of 1887 patented in Germany in 1924 by Alfred Adolf von Gehlen of Hamburg and made by Maihak AG. This substituted a robust spring-steel bar for the spring, the comparatively minimal deflection under load suiting the instrument to high speed/high pressure recording. The amplifying mechanism and the pointer were essentially the same as the standard Maihaks, however.
Mechanical indicators such as the Maihak Typ 30 and Typ 50 and the Dobbie McInnes Design No. 4 were still popular in the 1960s. Though their distribution declined as first electric and then electronic analysers became available, they were regularly used to indicate marine diesel engines. Indeed, the two German manufactures, Lehmag (formerly Lehmann & Michels) and Leutert (successors to Maihak) still offer external-spring and bar-spring instruments for this particular purpose.
Indicators and internal combustion
The gas engine patented in France in January 1860 by Jean-Joseph-Étienne Lenoir, the first of its type to be exploited commercially (though the underlying ideas had originated in the eighteenth century), finally provided the steam engine with an effectual rival. Lenoir's engine looked like a small horizontal steam engine, but a mixture of gas and air was drawn into the cylinder to be ignited at the half-way point. The near-instantaneous combustion of the charge then thrust the piston to the limit of its travel. Aided by the energy stored in a large flywheel, the motion was then reversed. As the piston returned, a charge was drawn in behind it and fired again. The process was continuous, with two power strokes for each revolution of the crank.
Several hundred Lenoir engines had been made by 1865, though experience had shown them to be prone to overheat and run erratically if the cylinder-wall temperature rose to a point where the charge was ignited prematurely. The overall efficiency was merely four per cent: twice as good as the best steam engine of the day, but still a notably poor return. The Lenoir was doomed to be the plaything of the rich and the few businessmen who saw promise in its technology, yet it was also the catalyst for the development of better designs. These included the odd-looking Otto & Langen gas engine of 1867, with a vertical cylinder and a rack-and-pinion to convert the reciprocation of the piston-fired upward, returned by gravity-into rotary motion. Overall efficiency of fourteen per cent was more that four times that of the Lenoir, and more than 4500 Otto & Langen engines were made in the 1870s and 1880s. Gas-Motoren-Fabrik Deutz had made more than forty thousand of the 1876-type horizontal-cylinder successor by 1895.
By the publication in 1897 of Frederick Grover's bookA Practical Treatise on Modern Gas and Oil Engines, internal combustion was so well established that 'Acme', 'Premier' and 'Simplex' — among many others — had become household names. The general design had settled on the classical layout of the horizontal steam engine, with a sturdy bed-frame and a large flywheel, though cylinders cpould be set side-by-side, in tandem, or at opposite ends of the bed.
Excepting the vehicle and aero engines, which were almost always petrol, gas engines were favoured (at least in Britain) for domestic and light industrial applications. Initial reliance on supplies of 'town gas', drawn from mains piping, was gradually eroded by self-contained 'producer gas' units after 1900. The smallest producer-gas units offered commensurately limited power, but were cheaper and easier to run than steam plant. Most relied on a mix of air and steam passed through anthracite to produce combustible gas that could be fed into the engine indicators were customarily supplied in sturdy boxes made of beech, mahogany, oak or walnut, though Bacharach of Pittsburgh preferred galvanised sheet-steel. These boxes were all carefully fitted to accept additional springs, scale rules, steam cocks, screwdrivers, wipers and oil bottles. It is widely believed that empty spring chambers or forlorn posts signify that something is missing, but most of the manufacturers had standard box patterns that would hold at least four springs. However, only a single spring was offered with each indicator: additional springs brought in more money. Purchasers who were interested only in the performance of one single-cylinder engine needed no more than the basic spring, but, at the other end of the scale, consulting engineers and the inspectors employed by insurance companies could require a dozen.
Jointed pantographs simplified interpretation of the diagrams by splitting them into narrow vertical strips, and spare pistons allowed an indicator to be used with low-pressure steam or high-pressure internal combustion engines at will.
Some boxes were fitted for reducing wheels, enabling indicators to be used with engines of widely differing strokes simply by altering the diameter of a particular pulley. It seems likely that the earliest designs emanated from central Europe — a Bohemian inventor, Ladislav Stanek of Prague, filed a patent in Germany as early as 1878. The greatest distribution of reducing wheels was ultimately to be found in the U.S.A., where the Lippincott-patent 'Victor' (1897) promoted by Robertson & Sons and the 'Faultless' by Trill (1903) were just two of many. Ironically, reducing wheels were uncommon in twentieth-century Europe, though German manufacturers offered increasingly sophisticated designs in the 1920s and 1930s before briefly reviving them after the Second World War. In Britain, wheels of this type are exceptionally rare — Dobbie McInnes made a few in accordance with a 1901-vintage British Patent granted to Wade, but most Britons preferred to use levers and bars attached to the reciprocating parts of the engine.
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Two examples of reducing gear.Left, the Crosby design.Right, a typical European-type reducing wheel, based on a Stanek design of the 1880s. This particular example was sold with a Lehmann & Michels indicator of the early 1920s, fitted in the same box. The graded exchangeable bushes allow the reducer to be adapted to a range of cylinder strokes.Museum of Making collection.
Analysing pressure/time diagrams was greatly helped by the use of a polar planimeter, a mathematical instrument patented in Switzerland in 1853 by Jacob Amsler. This allowed any area bounded by a single continuous line to be quickly and accurately computed, and was an ideal, if expensive adjunct to the engine indicator. A few planimeters were even boxed with indicators and reducing gear. Among the best-known of the pre-1914 designs are the Swiss-made Amsler and Corradi, which can be found with the names of distributors or manufacturers such as Elliott Brothers or the Crosby Steam Gauge & Valve Company. Keuffel & Esser planimeters were popular in the U.S.A., though they were originally made in Germany prior to the First World War; far less common were the distinctive Lippincott, Trill and Willis patterns, and the earlier 'Coffin Averager'.
Tabor, 1878
The indicator patented in the USA by Harris Tabor in 1878 was the result of a prototype shown to Charles Porter that could run efficiently at 450rpm. This had interested the Ashcroft Mfg Co., which was soon making Tabor indicators in quantity. The first few hundred of these, despite the statements that have been made to the contrary, were made in close accord with the patent drawings. The essence of these Tabors — which were being made in quantity by 1883—is a small cam-plate elevator attached directly to the piston-rod extension. In 1886, however, after at least 413 of the first-type indicators had been made, Ashcroft replaced the flimsy elevator with the more familiar amplifying mechanism: a sturdy standard, attached to the cylinder cap, containing a curved track to ensure that the pencil arm moved vertically. Links pivoted at both ends joined the pointer bar to the piston-rod extension and the cylinder body. Series production of the new-pattern Tabor indicator began in 1887, and continued for many years.
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A boxed Tabor indicator, no. 1951 dating from 1889 or 1890. Note the design of the guide pulley for the drum cord; later examples replaced the knurled fairlead retainer with a large hexagonal-headed nut. This particular set is lacking the steam cocks.John Walter collection.
Tests conducted by the U.S. Navy in Brooklyn Navy Yard, in 1886, with Richards, Thompson, Tabor and other indicators, revealed that the moving parts of the Tabor instrument were thirty per cent lighter than the nearest rival and had 24·3 per cent less friction.One of the most popular systems of indication in the 1890s, the basic Tabor pattern was extensively marketed in Europe. Among the British distributors were the Globe Engineering Works and John Musgrave & Sons Ltd of Bolton, who began work in 1889. Engravings accompanying a short descriptive article published inEngineeringin June 1889 depict an Ashcroft-made indicator no. 915, but the printing blocks may have been several years old.
