Stephenson valve gear
Encyclopedia
The Stephenson valve gear or Stephenson link or shifting link is a simple design of valve gear
that was widely used throughout the world for all kinds of steam engine
. It is named after Robert Stephenson
but was actually invented by his employees.
was known as gab motion in the U.K. and V-hook motion in the U.S.A. The gab motion incorporated two sets of eccentrics and rods for each cylinder; one eccentric was set to give forward and the other backwards motion to the engine and one or the other could accordingly engage with a pin driving the distribution valve by means of the gabs: - vee-shaped ends to the eccentric rods supposed to catch the rocker driving the valve rod whatever its position. It was a clumsy mechanism difficult to operate that moreover only gave fixed valve events.
In 1841 two employees in Stephenson’s locomotive works
, draughtsman William Howe and pattern-maker William Williams, suggested the simple expedient of replacing the gabs with a vertical slotted link, pivoted at both ends to the tips of the eccentric rods. To change direction, the link and rod ends were bodily raised or lowered by means of a counterbalanced bell crank
worked by a reach rod that connected it to the reversing lever. This not only simplified reversing but it was realized that the gear could be raised or lowered in small increments, and thus the combined motion from the “forward” and “back” eccentrics in differing proportions would impart shorter travel to the valve, cutting off
admission steam earlier in the stroke and using a smaller amount steam expansively in the cylinder, using its own energy rather than continuing to draw from the boiler. It became the practice to start the engine or climb gradients at long cutoff, usually about 70-80% maximum of the power stroke and to shorten the cutoff as momentum was gained in order to benefit from the economy of expansive working and the effect of increased lead and higher compression at the end of each strike. This process was popularly known as "linking up" or “notching up”, the latter because the reversing lever could be held in a precise positions by means of a catch on the lever engaging notches in a quadrant; the term stuck even after the introduction of the screw reverser. A further intrinsic advantage of the Stephenson gear not found in most other types, was variable lead. Depending on how the gear was laid out, it was possible to considerably reduce compression and back pressure at the end of each piston stroke when working at low speed in full gear; once again as momentum was gained and cutoff shortened, so lead was automatically advanced and compression increased, cushioning the piston at the end of each stroke and heating the remaining trapped steam in order to avoid temperature drop in the fresh charge of incoming admission steam.
American locomotives universally employed inside Stephenson valve gear placed between the frames until around 1900 when it quickly gave way to outside Walschaerts motion. In Europe, Stephenson gear could be placed either outside the driving wheels and driven by either eccentrics
or return cranks or else between the frames driven from the axle through eccentrics, as was mostly the case in Great Britain.
considered Stephenson valve gear: "(...) the most universally suitable valve gear of all, for it can be worked out for a long engine structure or a short one. It can be a very simple valve gear and still be very accurate, but its great advantage is that its accuracy is self-contained, for the exact relationship between its points of support (eccentrics on shaft, valve crosshead, and link hanger arm) have but little effect on the motion of the valve. Its use on engines in which all the cylinders lie in one plane, represents, in the belief of the writer, the best choice." Another benefit of the Stephenson gear, intrinsic to the system, is variable lead: usually zero in full gear and increasing as cutoff is shortened.
Stephenson valve gear is a convenient arrangement for any engine that needs to reverse and was widely applied to railway locomotives, traction engine
s, steam car
engines and to stationary engines that needed to reverse, such as rolling-mill engines. It was used on the overwhelming majority of marine engines.
Details of the gear differ principally in the arrangement of the expansion link. In early locomotive practice, the eccentric rod ends were pivoted at the ends of the link while, in marine engines, the eccentric rod pivots were set behind the link slot (or below on a vertical engine). These became known respectively as the 'locomotive link' and the 'launch link'. The launch link superseded the locomotive type as it allows more direct linear drive to the piston rod in full gear and permits a longer valve travel within a given space by reducing the size of eccentric required for a given travel. Launch-type links were pretty well universal for American locomotives right from the 1850s but, in Europe, although occurring as early as 1846, they did not become widespread until around 1900. Larger marine engines generally used the bulkier and more expensive marine double-bar link, which has greater wearing surfaces and which improved valve events by minimising geometric compromises inherent in the launch link.
In the Gooch valve gear the reversing and cut-off functions were achieved by raising or lowering a radius rod which connected the valve-rod to a "stationary" link pivoting around a fixed point. The advantages sought were reduced height for the gear and lighter action as the reversing lever was only required to lift the weight of the radius rod. This meant that the link was convex (in relation to the eccentrics) instead of concave. Gooch valve gear had the disadvantage of angularity between the valve spindle and the eccentric rod in full gear, whereas the best forms of the Stephenson gear, the thrust was in a straight line. The Gooch gear gave constant lead at whatever cutoff. This was observed to be a disadvantage when similar locomotives fitted with either Gooch or Stephenson gear were compared in service Gooch gear was never popular in Britain except with one or two engineers down to the 1860s, but it was quite common in France.
in 1855) combined the features of the Stephenson and Gooch gears. The reversing and cut-off functions were achieved by simultaneously raising the radius rod and lowering the link or vice-versa. As with the Gooch gear, this saved space but the Allen gear gave performance closer to that of the Stephenson. Moreover the straight expansion link simplified manufacture. Once again, the Allan gear was not often used in the UK but fairly common on the Continent. Notable UK examples are narrow gauge locomotives produced by George England (eg "Prince" & "Palmerston" in preservation on the Ffestiniog Railway) and Fletcher-Jennings (No 1 "Tallyllyn" & No 2 "Dolgoch" in preservation on the Talyllyn Railway).
Valve gear
The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle...
that was widely used throughout the world for all kinds of steam engine
Steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...
. It is named after Robert Stephenson
Robert Stephenson
Robert Stephenson FRS was an English civil engineer. He was the only son of George Stephenson, the famed locomotive builder and railway engineer; many of the achievements popularly credited to his father were actually the joint efforts of father and son.-Early life :He was born on the 16th of...
but was actually invented by his employees.
Historical background
During the 1830s the most popular valve drive for locomotivesLocomotive
A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...
was known as gab motion in the U.K. and V-hook motion in the U.S.A. The gab motion incorporated two sets of eccentrics and rods for each cylinder; one eccentric was set to give forward and the other backwards motion to the engine and one or the other could accordingly engage with a pin driving the distribution valve by means of the gabs: - vee-shaped ends to the eccentric rods supposed to catch the rocker driving the valve rod whatever its position. It was a clumsy mechanism difficult to operate that moreover only gave fixed valve events.
Robert Stephenson and Company
Robert Stephenson and Company was a locomotive manufacturing company founded in 1823. It was the first company set up specifically to build railway engines.- Foundation and early success :...
, draughtsman William Howe and pattern-maker William Williams, suggested the simple expedient of replacing the gabs with a vertical slotted link, pivoted at both ends to the tips of the eccentric rods. To change direction, the link and rod ends were bodily raised or lowered by means of a counterbalanced bell crank
Bell crank
A bell crank is a type of crank that changes motion through an angle. The angle can be any angle from 0 to 360 degrees, although 90 degrees and 180 degrees are common....
worked by a reach rod that connected it to the reversing lever. This not only simplified reversing but it was realized that the gear could be raised or lowered in small increments, and thus the combined motion from the “forward” and “back” eccentrics in differing proportions would impart shorter travel to the valve, cutting off
Cutoff (steam engine)
In a steam engine, cutoff is the point in the piston stroke at which the inlet valve is closed. On a steam locomotive, the cutoff is controlled by the reverser....
admission steam earlier in the stroke and using a smaller amount steam expansively in the cylinder, using its own energy rather than continuing to draw from the boiler. It became the practice to start the engine or climb gradients at long cutoff, usually about 70-80% maximum of the power stroke and to shorten the cutoff as momentum was gained in order to benefit from the economy of expansive working and the effect of increased lead and higher compression at the end of each strike. This process was popularly known as "linking up" or “notching up”, the latter because the reversing lever could be held in a precise positions by means of a catch on the lever engaging notches in a quadrant; the term stuck even after the introduction of the screw reverser. A further intrinsic advantage of the Stephenson gear not found in most other types, was variable lead. Depending on how the gear was laid out, it was possible to considerably reduce compression and back pressure at the end of each piston stroke when working at low speed in full gear; once again as momentum was gained and cutoff shortened, so lead was automatically advanced and compression increased, cushioning the piston at the end of each stroke and heating the remaining trapped steam in order to avoid temperature drop in the fresh charge of incoming admission steam.
American locomotives universally employed inside Stephenson valve gear placed between the frames until around 1900 when it quickly gave way to outside Walschaerts motion. In Europe, Stephenson gear could be placed either outside the driving wheels and driven by either eccentrics
Eccentric (mechanism)
In mechanical engineering, an eccentric is a circular disk solidly fixed to a rotating axle with its centre offset from that of the axle ....
or return cranks or else between the frames driven from the axle through eccentrics, as was mostly the case in Great Britain.
Applications
Abner DobleAbner Doble
Abner Doble , was an American mechanical engineer who built and sold steam-powered automobiles. His father was William Ashton Doble, inventor of the Doble water wheel, and his grandfather and namesake was the founder of the Abner Doble Company.Abner Doble began apprenticing at his family's factory...
considered Stephenson valve gear: "(...) the most universally suitable valve gear of all, for it can be worked out for a long engine structure or a short one. It can be a very simple valve gear and still be very accurate, but its great advantage is that its accuracy is self-contained, for the exact relationship between its points of support (eccentrics on shaft, valve crosshead, and link hanger arm) have but little effect on the motion of the valve. Its use on engines in which all the cylinders lie in one plane, represents, in the belief of the writer, the best choice." Another benefit of the Stephenson gear, intrinsic to the system, is variable lead: usually zero in full gear and increasing as cutoff is shortened.
Stephenson valve gear is a convenient arrangement for any engine that needs to reverse and was widely applied to railway locomotives, traction engine
Traction engine
A traction engine is a self-propelled steam engine used to move heavy loads on roads, plough ground or to provide power at a chosen location. The name derives from the Latin tractus, meaning 'drawn', since the prime function of any traction engine is to draw a load behind it...
s, steam car
Steam car
A steam car is a light car powered by a steam engine.Steam locomotives, steam engines capable of propelling themselves along either road or rails, developed around one hundred years earlier than internal combustion engine cars although their weight restricted them to agricultural and heavy haulage...
engines and to stationary engines that needed to reverse, such as rolling-mill engines. It was used on the overwhelming majority of marine engines.
Details of the gear differ principally in the arrangement of the expansion link. In early locomotive practice, the eccentric rod ends were pivoted at the ends of the link while, in marine engines, the eccentric rod pivots were set behind the link slot (or below on a vertical engine). These became known respectively as the 'locomotive link' and the 'launch link'. The launch link superseded the locomotive type as it allows more direct linear drive to the piston rod in full gear and permits a longer valve travel within a given space by reducing the size of eccentric required for a given travel. Launch-type links were pretty well universal for American locomotives right from the 1850s but, in Europe, although occurring as early as 1846, they did not become widespread until around 1900. Larger marine engines generally used the bulkier and more expensive marine double-bar link, which has greater wearing surfaces and which improved valve events by minimising geometric compromises inherent in the launch link.
Derivatives
As a harmonic valve gear, the Stephenson arrangement may be considered as optimum. Nevertheless the fact the link needed to be bodily displaced in order to reverse meant that it required considerable vertical clearance. At the time of its introduction, it was deemed important in the locomotive world to keep the centre of gravity, and therefore the boiler centre line as low as possible. Because valve gears in Britain were generally placed between the frames beneath the boiler, the extremely cramped conditions made the valve gear inaccessible for servicing. Also reversing could be a strenuous occupation as it entailed lifting the weight of the link plus eccentric rod ends. In order to address these problems two main variants were developed :Gooch valve gear
Allan straight link valve gear
The Allan straight link valve gear (invented by Alexander AllanAlexander Allan (locomotive engineer)
Alexander Allan was a Scottish mechanical engineer. He was born at Montrose, Angus, in 1809 and died on 2 June 1891.From 1843 to 1853 he was Works Manager at the Crewe Works of the Grand Junction Railway, later London and North Western Railway, under Francis Trevithick. Here he was responsible for...
in 1855) combined the features of the Stephenson and Gooch gears. The reversing and cut-off functions were achieved by simultaneously raising the radius rod and lowering the link or vice-versa. As with the Gooch gear, this saved space but the Allen gear gave performance closer to that of the Stephenson. Moreover the straight expansion link simplified manufacture. Once again, the Allan gear was not often used in the UK but fairly common on the Continent. Notable UK examples are narrow gauge locomotives produced by George England (eg "Prince" & "Palmerston" in preservation on the Ffestiniog Railway) and Fletcher-Jennings (No 1 "Tallyllyn" & No 2 "Dolgoch" in preservation on the Talyllyn Railway).
See also
- Walschaerts valve gear invented by Belgian railway mechanical engineer Egide Walschaerts in 1844 becoming the most widley used valve gear in Europe and North America.
- Baker valve gearBaker valve gearBaker valve gear was the main competitor to Walschaerts valve gear for steam locomotives in the United States. Strictly speaking it was not a valve gear but a variable expansion mechanism adapted to the Walschaerts layout replacing the expansion link and sliding die block...
invented by American engineers in 1903 and widely used in North America. - Caprotti valve gearCaprotti valve gearThe Caprotti valve gear is a type of steam engine valve gear invented in the early 1920's by Italian architect and engineer Arturo Caprotti. It uses camshafts and poppet valves rather than the piston valves used in other valve gear...
invented in the early 1920's by Italian architect and engineer Arturo Caprotti based on automotive valves it uses camshafts and poppet valves. Considered more efficient than any other method.
External links
- Diagram of standard Stephenson valve gear.
- Trouble shooting Stephenson valve gear, with diagrams.
