A steam motor is a form of steam engine used for light locomotives and light self-propelled motor cars used on railways. The origins of steam motor cars for railways go back to at least the 1850s, if not earlier, as experimental economizations for railways or railroads with marginal budgets. These first examples, at least in North America, appear to have been fitted with light reciprocating engines, and either direct or geared drives, or geared-endless chain drives. Most incorporated a passenger carrying coach attached (usually rigidly) to the engine and its boiler. Boiler types varied in these earlier examples, with vertical boilers dominant in the first decade (as a space saver) and then with very small diameter horizontal boilers. Other examples of steam motor cars incorporated an express-baggage or luggage type car body, with coupling apparatus provided to allow the steam motor car to draw a light passenger coach.
An early example with the all-in-one (coach, baggage, mail and/or express matter compartments) was photographed working on the Nashville, Chattanooga & St. Louis Railroad during the American Civil War, in Tennessee, circa 1863-64. One American firm, Grice & Long, devised various versions in the mid-1860s for use on suburban and city street railways, using their proprietary mechanical patents.Grice & Long trade advertisement; The Scientific American, 26 October 1861. In the 1930s, some highly evolved steam motors represented one of the final developments of the steam locomotive. The concurrent development of internal combustion-powered or electric-motored railway motor cars proved most popular circa 1900-1950s and those obviating the need for steam-powered cars.
Principles All steam motor cars apparently depended on economical motive equipment and steam source. Ease of maintenance was apparently another important consideration, at least as much as initial manufacturing costs. The principle of the latest versions of the steam motor is to use the developments of the high-speed steam engine, and to apply them to light locomotives. Rather than a large conventional locomotive having only two cylinders, moving at the speed of the driving wheels, the steam motor uses several small cylinders geared to run at high speeds. With all other factors remaining the same, doubling the speed of a piston engine doubles its power. The steam motor allowed small, light engines to be used. As many of the engine's performance losses remain constant, or are also related to the engine size, these small engines could also be more efficient overall.
Essential features
All steam motors had the following characteristics: Small size Late model motors were of a standard size, according to the manufacturer's product line. Where greater power was required, multiple motors were used, one per axle or bogie. Enclosed crankshaft lubrication In the most modern form of steam motors, the crankshaft and often the valve gear, was enclosed within a crankcase that contained an oil sump. This provided a generous supply of lubrication and also excluded dirt. Much earlier examples, e.g. mid-19th century, employed all manner of mechanisms with manual lubrication through small drip cups. Geared drive This allows a high crankshaft speed, encouraging efficiency. Although not all geared steam locomotives made use of this, particularly the US designs such as the Shay and the Climax, it was an essential part of the most modern steam motor concept.
The final drive of early Sentinel-patent locomotives was by chain. Later designs, particularly those by Abner Doble, preferred spur gears. Gear drives required the steam motor to be mounted low-down, alongside the axle. Geared drives appear in some of the earliest steam motor cars used in North America, although more conventional reciprocating drives with either a single power axle or doubled power axles, linked by connecting rods, were quite common. Most of these used conventional Stephenson motion valve gear.
Additional features Fuel and water consumption savings was one more important consideration in most steam motor cars used around the world. The less overall cost in operation was of great importance to buyers. Fuel could be coal, and in some fewer cases, oil of various types. Advanced valve gear Late-manufacture steam motors were usually fitted with poppet valves driven by camshafts. In the steam motors built by Sentinel, the motor was derived from their already advanced steam wagon design. Small driving wheels In a conventional steam locomotive, the 'gear ratio' is set by the size of the driving wheels. In steam motors using a geared drive, the wheel size can be reduced. This makes for a lighter and more compact chassis, particularly by reducing the unsprung weight of large wheels. Small wheels also allow the motor to be mounted on a bogie within a passenger coach to form a railcar, rather than the large wheels being the size of a locomotive.
Advantages These features give advantages: Higher efficiency This is mostly owing to the high speed of the latter-day modern engines and the reduction gearing, but also their other advanced design features. Reduced servicing The use of oil-bath lubrication reduces the time spent in daily oiling. Reduced maintenance The use of oil-bath lubrication reduces the rate of wear, thus reducing the need for periodic maintenance. This is mostly due to the exclusion of dirt, as well as the generous and reliable lubrication. Although other oil-bath systems on steam locomotives, such as the Bulleid chain-driven valve gear, gained a poor reputation for reliability, this was mostly due to the difficulty of sealing such a large container. With the steam motor, only the motor's relatively small crankcase was a sealed box. Simpler maintenance Maintenance, when required, involves smaller components. These are easier to work on, requiring less specialised lifting gear. The motor may also easily be removed in one piece for maintenance, either on-shed, or by return to the manufacturer. This also allows a vehicle to be returned to service more quickly by swapping motor units. Low manufacturing cost Components are manufactured in greater volume, as many designs of locomotive may be built around standardised motor designs. The machinery required to manufacture steam motors is also smaller, thus less specialised and cheaper. Reduced hammer blow The smoother drive of the geared motor, and its multiple cylinders, reduced the dynamic effect of individual cylinder strokes. Articulation With multiple motors, there is no need for a single large frame to carry all of the driven wheels. This was an attractive feature for the Colombian locomotives, where a powerful locomotive was provided on a flexible chassis.
As the driving wheels were small and articulated, there was no need for separate carrying axles. All wheels could be powered, so that the locomotive's entire weight could be used for adhesion.
Development
Ganz Works, Hungary
Between 1901 and 1908, Ganz Works of Budapest and de Dion-Bouton of Paris collaborated to build a number of railcars for the Hungarian State Railways together with units with de Dion-Bouton boilers, Ganz steam motors and equipments, and Raba carriages built by the Raba Hungarian Wagon and Machine Factory in Győr. In 1908, the Borzsavölgyi Gazdasági Vasút (BGV), a narrow-gauge railway in Carpathian Ruthenia (today's Ukraine), purchased five railcars from Ganz and four railcars from the Hungarian Royal State Railway Machine Factory with de Dion-Bouton boilers. The Ganz company started to export steam motor railcars to the United Kingdom, Italy, Canada, Japan, Russia and Bulgaria.Railroad Gazette - Volume 37 - Page 296 (printed in 1904)Modern Machinery - Volumes 19-20 - Page 206 (Printed in 1906)John Robertson Dunlap, Arthur Van Vlissingen, John Michael Carmody: Factory and Industrial Management - Volume 33 - Page 1003 (printed in 1907
Paget locomotive
The first multiple-cylinder locomotive to demonstrate some of the principles of the steam motor was the Midland Railway's Paget locomotive of 1907. This was one of many attempts to build a balanced locomotive, so avoiding the problems of hammer blow. It also followed contemporary advanced stationary engine practice in using single-acting cylinders. The locomotive has been variously described as either inspired by, or actually using, the design of the Willans engine that represented the peak of steam engine design at this time. In fact, the rotary valves used for the locomotive were entirely different from the Willans' characteristic central valve spindle, even though they did both use single-acting trunk pistons. Eight cylinders were used, two driving the front axle of the three driving axles, four the middle axle and two the rear axle. All three axles were coupled by external coupling rods. The central axle was a complicated forging as a four-throw internal crank axle, although following traditional Stephenson practice. To provide additional space for the cylinders, the locomotive was outside framed.
A significant difference between the Paget and a steam motor locomotive is that the Paget valvegear was driven centrally, every cylinder's valve being driven by geared shafts from a single jackshaft that was in-turn driven by the coupling rods.
Although the locomotive achieved its goal of avoiding vibration and hammer blow, and its novel boiler and firebrick firebox was also simple and reliable, it was not considered an overall success. The rotary valvegear absorbed more power to drive it than a conventional design and suffered from problems with thermal expansion. After a mechanical failure of the valvegear seizing and blocked a main line for several hours, the locomotive was withdrawn and later scrapped.
Sentinel The first locomotives designed around the full steam motor concept were by Sentinel. In 1922 two separate Sentinel-based steam motor projects were put forward. The first, in May 1922, was Sentinel's own concept for a light steam railmotor based on their steam wagon boiler and engine. No railmotors were built to this pattern, although a pair of similar 'rail lorries' were later built for export to India.
In the same year, Kyrle WillansLetter K Willans to H Hilton, 18 September 1949, in possession of the Crofton Branch of the Kennet and Avon Canal Trust suggested that a worn-out conventional locomotive, a Manning-Wardle named Ancoats, could be rebuilt with the boiler and engine of a Sentinel steam wagon in the locomotive's frames, connected by a roller chain drive. This was the first steam locomotive, and the first example of the steam motor for railway use, to be constructed by Sentinel. Although Sentinel were to become known for gear-driven locomotives, other examples of this conversion of an older coupling rod locomotive continued to be produced; two examples survive, although derelict, at the Coalbrookdale Museum of Iron.
Sentinel's first new-construction locomotive, Nº 5156, was built in July 1923. Like the railmotor design, this used the newly developed engine of the Super-Sentinel wagon, placed horizontally between the frames. This was of gauge, establishing a precedent for Sentinel of building locomotives across a range of gauges by simply changing the axles and bearings under a standard chassis. The boiler used was also the new Super-Sentinel pattern at 230 psi, with the 'spiral' firebox.