A Mallet locomotive is a type of compound articulated steam locomotive, invented by the Swiss engineer Anatole Mallet (1837–1919).
The front of the locomotive is articulated on a bogie. The compound steam system fed steam at boiler pressure to high-pressure cylinders driving the rear set of driving wheels (rigidly connected to the boiler). The exhaust steam from these cylinders was fed into a low-pressure receiver and was then sent to low-pressure cylinders that powered the driving wheels on the swiveling bogie towards the front of locomotive.
Compounding
Steam under pressure is converted into mechanical energy more efficiently if it is used in a compound engine; in such an engine, steam from a boiler is used in high-pressure (HP) cylinders and then under reduced pressure in a second set of cylinders. The lower-pressure steam occupies a larger volume and the low-pressure (LP) cylinders are larger than the high-pressure cylinders.
A third stage (triple expansion) may be employed. Compounding was proposed by the British engineer Jonathan Hornblower in 1781.
The American engineer W. S. Hudson patented a system of compounding for railway locomotives in 1873US Patent 136729, Improvement in Locomotives, March 11, 1873 in which he proposed an intermediate receiver surrounded by hot gas from the fire, so that the low-pressure steam is partly superheated.
Mallet proposed cross-compounding in which a conventional steam locomotive configuration would have one high-pressure cylinder and one low-pressure cylinder.Mallet, A, On the Compounding of Locomotive Engines, Proceedings of the Institution of Mechanical Engineers, London, June 1879, page 328, quoted in The Steam Engine: A Treatise on Steam Engines and Boilers, Clark, D K, Blackie and Co, London He patented the system in 1874, and in 1876 the first locomotive to the patent was built, an for the Bayonne and Biarritz Railway. Several others followed for railways in mainland Europe.Loco Profile 6: The Mallets, Reed, Brian, Profile Publications Limited, Windsor, undated
The London and North Western Railway locomotive engineer F W Webb adopted the idea and converted some existing locomotives in 1879, followed by de Glehn and others in the 1880s and several American engineers in the 1890s which included some vertical boiler railcar applications.
Articulation
Mallet found typical main line railways were unwilling to adopt his ideas. In 1884, he proposed compounding combined with articulation; on lightly engineered secondary lines this could give greater power to locomotives whose axle load and size were limited. His patent 162876 in France specified four cylinders, two large and two small, with one pair of cylinders acting on two or three fixed axles, and the other pair acting on axles mounted in a swivelling truck.
The weight of the front part of the boiler was to be supported on an arc-shaped radial bearing. The truck could therefore turn into a curve and move to some extent laterally. Typically the support bearing was placed beneath the smokebox, hollowed and with a sliding seal to provide a route for exhaust steam from the low-pressure cylinders to discharge through a blastpipe within the smoke box.
Mallet considered that the major advantage of this arrangement was that it enabled the cylinders on the truck to be fed with low-pressure steam: the high-pressure cylinders were on the fixed main frame and only low-pressure steam needed to be carried through movable pipes to the swivelling truck.
The Mallet concept This came to be understood as a "Mallet" locomotive: an articulated locomotive in which the rear set of driving wheels were fixed in the main frame of the locomotive; an articulated truck carrying a second set of driving wheels; and compounding in which the high-pressure cylinders drove the axles on the main frame and the articulated axles were driven by low-pressure steam.
Mallet asserted that the advantages of his concept were: all the locomotive weight would be adhesive, yet there would be great flexibility of the locomotive as a vehicle; the difficulties with Meyer, Fairlie and other then-existing articulated systems would be eliminated as the moving pipes would be carrying low-pressure steam at only pressure, and would be easier to keep steam-tight; and A simple type of very powerful locomotive would be created.
The large-diameter pipe conveying the low-pressure steam from the high-pressure to the low-pressure cylinders acted also as a receiver, forming a buffer for the gas flow.
Independent cut-offs for the high-pressure and low-pressure cylinders were advocated by Mallet, but driving standards were inadequate and he later used combined cut-off control.
European versions
Large numbers of Mallet designs for narrow-gauge railways were built, but in 1889 the first six standard gauge examples were built by J A Maffei for the Swiss Central railways, and an banker (US: pusher) for the Gotthard Bahn, the last being the most powerful and heaviest locomotive in the world at the time. By 1892 110 Mallets were at work, of which 24 were standard gauge; by 1900 there were nearly 400, of which 218 were on standard gauge or Russian gauge ().
One of the examples in Germany was the class of s built by Maffei for Bavarian State Railways between 1913 and 1923. Mallet designs were popular in Hungary as well; 30 of were built between 1898 and 1902 (the last one served until 1958). was a (1B)B locomotive in service between 1905–1969 and until 1962. The strongest Mallet locomotives in Europe were the members of the MÁV 601 which were built for the Hungarian State Railways, a locomotive. With their length, weight and power output, the MÁV 601 was the longest, heaviest and most powerful steam locomotive built before and during World War I in Europe.(Béla Czére, Ákos Vaszkó): Nagyvasúti Vontatójármüvek Magyarországon, Közlekedési Můzeum, Közlekedési Dokumentációs Vállalat, Budapest, 1985,
North American versions
The first Mallet locomotive in the United States was Baltimore and Ohio Railroad number 2400, built by Alco in 1904. Nicknamed "Old Maude", it was an weighing and with axle loads of . Received negatively at first due to speed limitation arising from the short wheelbase and stiff suspension, it gained support during service, and it was soon followed by Baldwin examples, and then steadily heavier and more powerful successors.
In Canada, the Canadian Pacific Railway experimented with an unusual design of Mallet promoted by H.H. Vaughan, then Chief Mechanical Officer and Assistant to the CPR Vice-president. Their in-house compound design located both the high and low pressure cylinders adjacent to one another in the center of the locomotive driving opposite directions. Produced in the CPR's Angus shops, road numbers 1950 to 1954 were outshopped between 1909 and 1911. An additional "simple" (as opposed to compound) unit with road #1955 featuring the same arrangement was also produced. These were used in helper service in the Rockies and Selkirks. The units were unpopular with crews owing to frequent steam leakages and derailments resulting from the lack of pilot wheels. While not an outright failure these were considered an unsuccessful design, and by 1916-1917 these units had been converted to a conventional 2-10-0 arrangement. These six locomotives were ultimately the only articulated locomotives operated by a Canadian railway.
As weight and power and length increased, there were experiments with flexible boiler casings; from 1910 the Santa Fe road introduced jointed-boiler locomotives weighing , with a long boiler barrel, with a firetube reheater and a firetube feedwater section in front, each separated by a blank section, and variants of a telescopic or bellows type boiler casing."The Jointed-Boiler Locomotives," Trains magazine, February 1945 These were unsuccessful, and later engines used conventional boilers.
The largest compound Mallets were ten s built for the Virginian by Alco in 1918; in pairs they pushed coal trains headed by a . The AT&SF also had a number of compound s, assembled in their own shops from existing s using a kit, supplied by Baldwin, consisting of the front 10-wheel frame and a boiler extension.
Although compounds had been considered obsolescent since the 1920s, C&O thought them appropriate, in the late 1940s, for low-speed coal-mine pickup runs converging on the classification yard at Russell, Kentucky."Locomotive Notes," Trains magazine, August 1948 Only ten (of 25 originally ordered) were built before the order was cancelled, the last delivered in September 1949. The final loco, Chesapeake and Ohio 1309, is preserved on the Western Maryland Scenic Railroad. The 1309 was also the last steam locomotive that Baldwin built for the North American market.
The last compound Mallets to remain in use on a major North American railroad were the N&W class Y6b locomotives, retired in July 1959. Norfolk & Western 2156 is the sole surviving Y6a, preserved at the National Museum of Transportation in St. Louis.
Simple expansion versions in the US
By about 1920, the U.S. version of the Mallet as a huge slow-speed pusher had reached a plateau; the size of the low-pressure cylinders became a limiting factor even on the large loading gauge permitted in the U.S., and reciprocating masses posed serious dynamic problems above walking pace. Moreover, there were adhesion stability problems where the front engine tended to slip and then stall uncontrollably because of an imbalance of tractive effort and axle load, accentuated by the drawbar reaction, and inability of the intermediate steam receiver to accommodate the sudden pressure change. This was further worsened by dynamic instability of the front end in running.
The Chesapeake and Ohio Railway introduced 25 simple (non-compound expansion) locomotives in 1924 and 20 more in 1926. Although the simple-expansion concept diverged from Mallet's original patent, the locomotives were clearly a continuation of the concept and were still referred to as "Mallet" locomotives. As the front truck cylinders were now using boiler pressure steam, special arrangements were necessary to deliver it, through the truck pivot pin where only radial movement took place. These new locomotives took over service on a division; a single locomotive hauled in five hours.
Mallet development culminated in 1941 with the 4-8-8-4 Big Boy type on the Union Pacific railroad. They weighed with a tender; at long (including the tender), they could only be turned on a few of the system's turntables. They could develop on the drawbar at and were designed for a top speed of , though they rarely saw these speeds. Slightly shorter but even heavier and more powerful were s built by Lima for the C&O and the Virginian between 1941 and 1948, which weighed and could produce up to at .
The last Mallets
These U.S. locomotives were paralleled to some extent by heavy-haul versions in the USSR, though without any attempt at faster running. Two examples built in Russia in 1954–55 were probably the last Mallets built in Europe. Four locomotives numbered BB81 to BB84 were built by Nippon Sharyo in 1962 for the Indonesian State Railways' Aceh railway line. They were probably the only Mallets to be built in Asia and the last Mallets ever built.
Other continents
Although it had found early favor in Europe, especially on lightly engineered railways, the Mallet type was generally superseded by the Garratt locomotive by the mid-1920s.