johnsmachines

Reader Jenny asked about original mounting methods for the dredger engine.

This picture appeared in the Rees Cyclopedia of 1819, and as far as I know, is the only picture exisiting of the engine in its use as a dredger engine.

Gun boats had no method of propulsion, so they were towed into position.  They were very strongly constructed to withstand the enormous forces of firing cannons, mortars and siege guns.  Often they carried only one very large blackpowder gun.  Trevithick chose one of these vessels to mount his steam engine to dredge the Thames.  Somewhere I have seen a drawing of the construction of one of these vessels, showing the massive oak beams and thick deck.  At this time I cannot relocate the drawing, but when I find it I will post it.

Many more of Trevithick’s “strong steam” engines were used to pump out mines, and operate factories and mills.  We can assume that those engines would have been mounted on a base of bricks, stone blocks, or large wooden beams.  The engines weighed about 4 tonnes, so it is possible that they just sat on the strongly constructed factory floor, with a slot for the flywheel and driving gear.

And in writing this I have come across another drawing from the Trevithick era of one of his engines mounted for use in a factory.  This layout drawing was by John Rastrick, an engineer at Hazeldines, who made the engine which is in the London Science Museum.  It shows a 4 hp Trevithick puffer engine, driving 2 “scouring barrels” and “a set of polishing lathes”.

This is very interesting to me.  First, the engine mounts are likely to be masonry, possibly topped with wooden beams.  The flywheel is mounted not on the crankshaft, but on a separate, extra shaft.  The connecting rods are straight, and rather too thin to be wood;  more likely steel or cast iron.  The safety valve weight is suspended on a long shaft, well away from the front of the boiler.  The front boiler support has angled ends, not squared. The crosshead has tapered edges, not parallel.  And the firebox door is approximately as I had guessed it, although rather smaller proportionately than the firebox than I have made it.

I am sure that this is not intended to be an accurate representation of the details of the Trevithick engine, but I suspect the engineer who drew it would have got the proportions approximately correct.

Wonderful!

Now I have to decide whether I have made a model dredger engine, or a model factory engine.