Thursday, 27 October 2016

HR Loch Valve Gear

HR Loch chassis progress so far.

The Loch class employed Allan's Straight Link valve gear and the parts and castings to build this feature are readily available from LGM.

The valve gear is suspended from the links that depend from the reversing shaft levers which are situated above the radius link on the reversing shaft.

The forked ends of the eccentric rods attach to the straight radius link in which the jointed valve rod pivots, these parts were assembled to form two separate units. This part of the valve gear is fairly easy to fathom from available drawings and is intended to work, the eccentrics will motivate the radius link and the attached valve rod will move horizontally.

More obscure is the manner in which the valve gear is supported from the reversing shaft lever above and this is where the model and prototype are going to have to go different ways as I don't think it feasible, at least not without huge effort, to model this feature of the valve gear according to prototype.

The lifting links depending from the reversing shaft levers will not be attached to the valve rod and radius link prototypically, they will remain cosmetic. They don't move anyway, at least not much, other than when the valve gear is reversed.

Allan's straight link valve gear rear view from above.

The complexity of the arrangement of links depending from the reversing shaft levers is evident in this view, the rear lifting link attaches to the forked end of the rear valve rod. The front lifting links attach to the radius link.  All the parts are now in place though the lifting links are not attached to the valve gear, they remain cosmetic.

The reversing shaft levers are mounted on the transverse reversing shaft just behind the motion plate.

Loco body in place showing the gap under the boiler through which the valve gear can be seen.

When your eyes accustom themselves to the murk under the boiler and you get in close there's still not a lot to be seen through the gap between boiler and frames. Most of the movement will be in the gloom behind the motion plate...and Yes, it has occurred to me that inside motion in this case might not be worth the time spent on it.