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  • Writer's pictureChris Hoskin

Eccentric activity

The latest bulletin from Didcot is in and work is progressing on machining the motion components.

Left to right in these images are:

The eccentric straps which provide fore and aft motion from the eccentrics on the driven axle;

The eccentric rods which connect the eccentric straps to the expansion link, using that fore and aft motion to make the expansion link rock backwards and forwards; and

The expansion link which controls the movement of the valve, and by how much. A "die block" fits in the slot in the link, and its position in the link determines how much the valve moves and therefore how much steam that goes to the cylinders,

The outer lugs that are on the top of the expansion link in the photo fit between the forks of the eccentric rods. The other end of the eccentric rods are bolted onto the eccentric straps using the bolts on the left hand side of the straps in the photo (the second bolt on each strap is hidden behind the cloth).

The protuberance pointing towards the camera on each of the eccentric straps is the oil pot to provide lubrication to the interface between the strap and the eccentric.

For a simple explanation of Stehenson's valvegear and how it all fits together on a GWR 2 cylinder locomotive, a good starting point is this page on the GWR Modelling website.

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