LMS Route: Nuneaton to Birmingham New Street

LMS Route: Birmingham New Street to Tamworth

Saltley Shed: mrsalt1236

Ex-LMS 8F 2-8-0 No 48376 can be seen being turned inside of Saltley shed's No 3 roundhouse. Built by Horwich works in November 1944, No 48376 was to remain in service until July 1967 when it was withdrawn from 9L Buxton shed. This photograph illustrates clearly the layout of the 9F's cylinder and Walschaerts valve gear. The Walschaerts valve gear is a type of steam engine valve gear invented by Belgian railway mechanical engineer Egide Walschaerts in 1844. The gear is sometimes named without the final "s", since it was incorrectly patented under that name. It was extensively used in steam locomotives from the late 19th century until the end of the steam era.

The Walschaerts valve gear was slow to gain popularity. The Stephenson valve gear remained the most popularly used valve gear on 19th century locomotives. The Walschaerts gear had the advantage that it could be mounted entirely on the outside of the locomotives, leaving the space between the frames clear; this caused adoption first among some articulated locomotives. The Walschaerts valve gear is an improvement on the earlier Stephenson valve gear in that it enables the engineer to operate the steam engine in a continuous range of settings from maximum economy to maximum power. At any setting, the valve gear satisfies the following two conditions: At the instant when the space on one side of the piston starts to expand, i.e. at the very start of a stroke, the valve opens to admit steam from the boiler into that space. The pressure of this steam provides the driving force. At the instant when the space on one side of the piston starts to contract, the valve starts to release steam from that space to the atmosphere, so as not to impede the movement of the piston.

In an economical setting, steam is admitted to the expanding space for only part of the stroke; at a point set by the engineer, the intake is cut off. Since the exhaust is also shut, during the rest of the stroke the steam that has entered the cylinder expands in isolation, and so its pressure decreases. For maximum economy, the engineer carefully sets the cutoff point so that, when the exhaust valve opens, the steam is down to exactly atmospheric pressure. Thus, all the mechanical energy available from the steam (in the absence of a condenser) is used. The Walschaerts valve gear enables the engineer to change the cutoff point without changing the points at which intake starts and at which exhaust starts. With the Stephenson valve gear, in contrast, if the engineer shortens the intake period, the openings of the intake and exhaust are delayed, occurring after a stroke has begun. Economy also requires that the throttle is wide open, so that no energy is wasted pushing steam through a constriction, and that the boiler pressure is at the maximum safe level to maximize thermal efficiency. For economy, a steam engine is used of a size such that the most economical settings yield the right amount of power most of the time, such as when a train is running at steady speed on level track.

When greater power is necessary, e.g. when gaining speed when pulling out of a station and when ascending a gradient, the Walschaerts valve gear enables the engineer to set the cutoff point near the end of the stroke, so that the full pressure of the boiler is exerted on the piston for almost the entire stroke. With such a setting, when the exhaust opens, the steam in the cylinder is near full boiler pressure. The pressure in the steam at that moment serves no useful purpose; it is wasted driving a sudden pulse of pressure into the atmosphere, but this waste is more than compensated by maximized economy at other times. This sudden pulse of pressure causes the loud choo sound that members of the public associate with steam engines, because they mostly encounter engines at stations, where efficiency is sacrificed as trains pull away. A steam engine well adjusted for efficiency makes a soft hhHHhh sound that lasts throughout the exhaust stroke, with the sounds from the two cylinders overlapping to produce a nearly constant sound.

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