page_title: Steam motor
section_title: Sentinel
hierarchical_section_title: Steam motor / Development / Sentinel
caption_attribution_description: Teesside Bridge and Engineering Company Sentinel 4wVBTG locomotive number Teesside Number 5 FRANK GALBRAITH (works number 9629 of 1957) on display in the great hall at the National Railway Museum, York. Monday 1st June 2009
context_page_description: A steam motor is a form of steam engine used for light locomotives. They represented one of the final developments of the steam locomotive, in the final decades of the widespread use of steam power.
context_section_description: 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 waggon 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 Willans suggested that a worn-out conventional locomotive, a Manning-Wardle named Ancoats, could be rebuilt with the boiler and engine of a Sentinel steam waggon 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 waggon, placed horizontally between the frames. This was of 750 mm (2 ft 5 ¹⁄₂ in) 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.
  Production Sentinels achieved a more accessible layout by mounting their engines vertically. The smaller 'Balanced Engine' design had the boiler and engine at opposite ends of the frames, with low bodywork over the water tank, a raised cab and a small cylindrical casing over the engine. The more familiar type was the later 'Central Engine' that placed the engine centrally within a large box-shaped cab. These CE engines were later increased in power by slightly enlarging the boiler and raising boiler pressure to 275 psi. This increased their power from the original 80 bhp of the BE locomotives to 100 bhp. All of these locomotives were of 0-4-0T wheel arrangement, with both axles driven by chains from the same engine. To give greater power, Sentinel patented the concept of using multiple, identical steam motors, each driving one axle. These were the 'Double Engine' locomotives of 200 bhp. The boiler was enlarged again and was fitted unusually with two chimneys, one for the blastpipe of each engine.
  The gear ratio of the final drive depended on the market and the work expected of the locomotive. They varied between a 6:1 and a 1:1 drive. The original steam waggon, using an engine of the same size, might carry a load of around 15 tons. The railway locomotive might achieve a tractive effort of 123,000 lbf and haul a load for slow shunting work of 650 tons. Typical shunting loads were around 350 tons at 5 mph in a level marshalling yard, the weight of a typical goods train.
  By 1925 the vertical arrangement of the engine had been taken advantage of to improve steam porting and access for maintenance. The Super-Sentinel engine used two camshafts: inlet and exhaust, placed near the crankshaft in the crankcase and operating the poppet valves through long pushrods. In the original waggon engine, all four valves were mounted at the far end of the cylinder from the crankshaft, requiring long narrow ports to the other end of the cylinder. With the locomotives the easily accessed vertical engine allowed a pair of these valves to be moved to the lower end of the cylinder, allowing for a short, efficient port with less 'dead space'. The crankcase of the engine, together with the crosshead trunk guide, was enclosed and provided with oil splash lubrication. A small plenum was left exposed between the cylinder's piston rod stuffing box and another gland seal into the crankcase. This allowed any steam that did leak past the seals to be vented to the atmosphere, rather than condensing within the crankcase.
caption: Sentinel chain-drive shunter of 1957
exif: {"Image Make": "Canon", "Image Model": "Canon EOS 50D", "Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop CS2 Windows", "Image DateTime": "2009:07:16 16:16:18", "Image YCbCrPositioning": "Co-sited", "Image XPComment": "[84, 0, 101, 0, 101, 0, 115, 0, 115, 0, 105, 0, 100, 0, 101, 0, 32, 0, 66, 0, ... ]", "Image CustomRendered": "Normal", "Image ExposureMode": "Auto Exposure", "Image WhiteBalance": "Auto", "Image SceneCaptureType": "Standard", "Image ExifOffset": "716", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "1486", "Thumbnail JPEGInterchangeFormatLength": "7691", "EXIF ExposureTime": "1/100", "EXIF FNumber": "10", "EXIF ExposureProgram": "Shutter Priority", "EXIF ISOSpeedRatings": "800", "EXIF ExifVersion": "0221", "EXIF DateTimeOriginal": "2008:06:01 11:39:40", "EXIF DateTimeDigitized": "2008:06:01 11:39:40", "EXIF ComponentsConfiguration": "YCbCr", "EXIF ShutterSpeedValue": "53/8", "EXIF ApertureValue": "53/8", "EXIF ExposureBiasValue": "0", "EXIF MeteringMode": "CenterWeightedAverage", "EXIF Flash": "Flash did not fire, compulsory flash mode", "EXIF FocalLength": "50", "EXIF SubSecTime": "33", "EXIF SubSecTimeOriginal": "33", "EXIF SubSecTimeDigitized": "33", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "1800", "EXIF ExifImageLength": "1200", "EXIF FocalPlaneXResolution": "792000/149", "EXIF FocalPlaneYResolution": "1056000/199", "EXIF FocalPlaneResolutionUnit": "2"}