Technical

At many UK railwayana auctions, an example of the familiar "SC" plate is available. These plates fitted behind the shedcode plate on the smokebox door of locos fitted with self-cleaning smokeboxes. But what is a "self-cleaning smokebox"?

Like much steam loco technology e.g. bar frames, steam reverse, adjustable wedges etc, the self-cleaning smokebox was pretty standard across the world apart from the UK. The self-cleaning box did exactly what the name suggests - the ash and char that accumulated in a non-fitted box was expelled up the chimney in a self-cleaning box. There are two main features of the system:

	a system of plates which require the gases from the flue tubes to be sucked forward along the lower part of the box before reversing direction to mix with the exhaust steam being ejected from the blastpipe, and which help shatter the ash into smaller pieces; and
	a spark arresting screen which restricts larger particles from being exhausted, and which remain in the agitated gas flow until they are small enough to pass through.

I attach two diagrams which hopefully illustrate how the self-cleaning smokebox works:

	Sketch of a Chinese JF smokebox. The vertical screens with handles to facilitate removal, marked 1, present a blank wall to gases passing through the flue tubes. This has the effect of a. shattering the ash particles and b. forcing them to pass under the horizontal plate surrounding the blastpipe. The short sloping plate 3 prevents the gases taking a direct route into the base of the chimney and give a further shattering opportunity. The gas flow thus scours the lower part of the box immediately behind the door. Particles too big to pass through the spark arrestor screen fall back and one can imagine that when working, the whole lower part of the box is seething with suspended particles which eventually become small enough to pass through the mesh,
	Elevation of a Chinese JS smokebox, showing the same plates. The device ahead of the chimney is a feedwater heater and that behind is the regulator which is fed by an external steam pipe from the dome. The Chinese term for the self-cleaning system is "spark pulverizing device".
	The smokebox front open on SY 1470 at Baiyin, Gansu province, China, March 2008. The standard American style smokebox has a small central door, which is presumably not designed to be opened very often as it is fastened by numerous dogs around the circumference. The superheater tubes, the baffle plates behind the blast pipe and the LHS panel of the self cleaning screen show up well.

An obvious drawback of a self cleaning box is the necessity to dismantle the screens to access the tubeplate, but presumably this is outweighed by the elimination of the need to clean the smokebox at the end of each trip.

Valve Gears

This is not intended to be a comprehensive treatise nor unduly technical, just a few observations based on a few photos.

Southern valve gear

This is a bit recherche. Southern is uncommon to say the least, and NSWGR K1353 class 2-8-0 5595 must be one of a very small number of extant locos with this.

Southern valve gear on 5595 at the railway museum at Thirlmere, NSW.

Southern valve gear was invented by William Sherman Brown of the Southern Railway in the USA, and it was first used in 1913.

Conjugated drive for three cylinders

The most famous of the designs to derive a drive to the valve of a third cylinder is the Gresley gear as used on A3s, A4s and V2s. However, it was also used elsewhere, for example on the Baldwin Pacifics in Thailand and the D57 4-8-2s in NSW. The problem with deriving a drive to the third cylinder is that it is 120^oout of synch, compared with the derived drive for, say, a four cylinder LMS Coronation or GWR King, wherein the drive for respectively the inside and outside cylinders was 180^o out of synch and thus could be easily transmitted.

D57 5711 at Thirlmere NSW. No modesty panel as for an A3 and what appears to be an unnecessarily massive rocking lever or sway beam. No wonder the less agriculturally minded engineers turned to rotary cam gear.

"Heavy Harry", the solitary H Class 4-8-4 of the Victorian Railways had a more elegant solution, taking the drive from the combination lever one each side back to two transverse shafts whose motion was combined.

	RHS of Heavy Harry showing the drive to the long "conjugate shaft".
	LHS. The drive to the short conjugate shaft. The combined motion of the two shafts gives the correct phasing.

Walschaerts

Walschaerts needs no introduction but it is worth noting the difference between inside and outside admission, and the positions of each gear when the loco is going forwards or backwards. It is not uncommon to see excited captions to photos querying headlamps or some such, when a brief examination of the photo shows the loco is travelling backwards. Most piston valve locos with Walschaerts gear have inside admission, i.e. the steam is admitted between the valve heads.

Close-up of NSWGR Pacific 3820 at Thirlmere. Notice that the radius rod joins the combination lever above the valve spindle. For an outside admission loco such as a BR Southern Region "Merchant Navy", the radius rod connects below the valve spindle. The Class 38s had a "stirrup" hanging down from the rear of the slide bars, which would contain the connecting rod if the little end pin broke. This was also a common feature of South African locos.

Chinese QJ. This is in full forward gear - notice the radius rod is in the bottom of the expansion link. Also note that the return crank follows the crank by 90^o. For an outside admission loco, the return crank leads.