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An 1899 Buckley and Taylor cross-compound steam engine from the Paleão cotton mill (Soure) is preserved in the smart NORTESHOPPING mall in Porto.
It has been lovingly restored to the highest standard, and rotates by the action of a hydraulic motor driving the flywheel using the gear teeth originally provided for hand barring. It provides an excellent opportunity to closely study the working of a steam engine with complex and fascinating valve gear, and sets a standard which puts many museum exhibits to shame.
The engine was designed to produce 240-249 HP, but could provide as much as 320 HP. It drove cotton spinning and weaving machinery until the late 1940s, when it was relegated as standby after electric power took over.
The engine had followed an earlier example from Buckley & Taylor, installed in 1891. The excellent information boards even record the name of the erector - James Hattersley of Buckley & Taylor.
The shopping centre is a few km from Porto city centre, and is easily reached using the frequent Metro rail service.
The Norteshopping website has an interactive map.
Address: Norteshopping, Rua Sara Afonso, 4460-841 Senhora da Hora, Portugal.
A photo of the engine in its original location may be seen here.
Thanks to its compact size, accessibility, excellent lighting, and flat, light colour scheme, there is an excellent opportunity to study the workings of a mill engine.
There is of course much that cannot be seen. In an effort to shed some light on the engine's workings, the basics and some of the more mysterious elements will be explored here.
Steam was provided by Lancashire boilers, the firing end of one being preserved near the engine. Steam passed to the smaller (high pressure) cylinder via a stop valve (seen in Photo 12, with the maker's name on the hand wheel). After doing its work in the high pressure cylinder, it passed to the larger low pressure cylinder, from where it was exhausted to the condenser. The combined condenser and air pump are below the engine, visible from the underground car park.
Now for the complicated part. Steam is admitted to the cylinders and exhausted via semi-rotary valves. The inlet valves are at the top of the cylinder, at each end, and the exhaust valves at the bottom.
The valves are worked by a system of rods. A pair of eccentrics can be seen at the left side of Photo 7, one of which works a rod to oscillate a 'wrist plate'. This is rocked to and fro to move a pair of rods to operate the inlet valves. The other eccentric opens the exhaust valves.
The operation of the exhaust valves is straightforward, but things are different with the inlet valves. Instead of having a simple rod linking the wrist plate to the inlet valve, the rod is effectively 'telescopic'. In Photo 8, note the rod coming up at about 45 degrees to work the inlet valve. It may be apparent that the square section of the rod slides within the box section. If we compare Photos 10 and 11, taken during a different part of the cycle, we can see that the square section has moved further into the box. When it has gone in a certain distance, a latch engages with a click and holds it there. The presence of the latch is shown by the raised portion on the box, the latch itself being the small square bar. The bar is pressed down by a spring. Note, too, the pair of cranked levers which serve to lift the latch to disengage it and thereby release the square shaft.
An attempt will now be made to explain the reason for this arrangement.
In the interests of efficient working, it is desirable that an engine's steam admission valves open widely and quickly, and stay open just long enough to admit the required amount of steam, and then close quickly. If the engine is lightly loaded, it is desirable to cut off the steam supply early and allow it to do its work on the piston by expanding.
This cut-off is made to happen in the following way: the 'telescopic' rod starts to pull the valve open. When it has moved a certain distance, the small cranked lever lifts the latch to release it, and spring force then takes over, pulling out the now-freed square rod and closing the steam inlet valve. The spring is located in the cylindrical housing seen at the top of Photo 9. The grey part of that housing is the dashpot - it contains a piston. On tripping the valve, the piston compresses the trapped air, thereby cushioning the movement at the end of travel, so as to avoid damage due to impact forces. The rate of damping can be controlled by the small bleed screws whose brass thumbscrews can be seen.
The point at which the steam inlet valves are tripped is determined by the governor.
If we go back to the latch, and the small cranked levers which cause unlatching, we can see that they are worked by rods linked to a pair of bronze gear sectors. The sector furthest from the camera is rotated by a crank, hidden from view, which in turn is moved by a slender rod coming down from the governor. This sloping rod can be seen in Photo 12, and its connection to the governor can be seen in Photo 13.
If the load on the engine decreases, due, for example, to some of the textile machinery being shut down, its speed will increase, and the governor balls will move further outwards. The various bell cranks will transmit the movement of the balls to the slender rod. This in turn will alter the position of the bronze gear sectors, and thence alter the precise point at which the steam inlet valves will be tripped closed.
Study of Photos 12 and 13 might suggest that nothing of the sort would happen, because there is a brass or bronze latch arrangement which would not tolerate any pushing. However, the latched parts are normally held together by tension in the rod, because the lower part of the slender rod is pulled by a counterweighted lever, just visible in Photo 9. It will be apparent that there is an adjustable striker which would cause the latch to be tripped. This would evidently serve to trip the engine to prevent it overspeeding on loss of load. The lever could also be used to manually trip the engine.
This system of operating the steam valves owes its origins to George Corliss, and this type of engine is broadly categorised as a Corliss engine. Buckley & Taylor, like most engine makers, applied their own ingenuity in applying the Corliss principles.
Note that there are many opportunities to study a variety of Corliss-type engines in operation in UK museums.