The centrepiece of Sheffield's Kelham Island Industrial Museum, the 400-ton, 3-cylinder, single-expansion prototype is spectacular in action, running at over 70 RPM, stopping, reversing, and accelerating back to full speed in two seconds flat.
It originally drove an 80-ton heavy plate mill which rolled armour plate for warships through 2 world wars. The rapid reversing enabled each red-hot steel billet to be passed between the mill rollers several times before cooling. This was made possible by having an (unavoidably inefficient) single-expansion configuration with special valve gear, and limiting crankshaft momentum by driving an independently-mounted flywheel via reduction gearing.
A splitter unit between the flywheel shaft and the rolling mill drove the top and bottom main rollers. The top roller could be raised and lowered, hence the universally-jointed driveshaft from the splitter unit. The feed rollers on the mill race were also driven (by means unknown) allowing them to carry the billet beyond the main rollers' reach.
On
the model, the flywheel is formed from 2 hub disks sandwiching a 5½" gear ring, which
meshes with a wide-face 57 tooth gear on the crankshaft. ►
2" sprocket wheels are used as gears inside the splitter unit.
◄ Inside the base, a motor/gearbox module drives the flywheel and mill, and the engine from underneath via 50T gears concealed in the crankshaft bedplates. A long-throw cam and follower reverse the direction of drive every 10 seconds (in which time the red "billet" travels the length of the mill race) and swings over the reversing arms of the valve gear (above, left). The feed rollers are driven via a train of 38T gears and "idlers".
The top roller journal blocks slide vertically, adjusted by screw-jacks inside the mill stands.
Builder's comment: Not a popular model with other exhibitors. The power needed to back-drive the engine and mill at prototypical speed (despite using a silent copier motor and nylon gears where possible) makes the model rival the prototype for noise!