Gresley Valve Gear.

The Garratt locomotives are fitted with single top slide bar on the outside and inside engines, and the piston valves of the outside cylinders of these engines (on both power bogies) are operated by the Walschaert valve gear. To operate the valve for the inside cylinder, the Gresley combining gear has been adopted. (Illustration No. 1.)

The valve spindles of the left and right outside engines are extended through the valve chest covers. One of these valve spindles is connected by a short link (8in. long) to the end (e) of a two-to-one lever carried on a ball-bearing pivot (d), near the centre of the engine. The other valve spindle is connected by a similar link to a short forked lever attached at (b) to the other end of the main lever; this short lever having a one-to-one ratio. This one-to-one lever is attached by a link at (c) to the valve spindle operating the piston valve of the inside engine.

In this simple manner the combined motion of the two outside valves is modified to give the correct motion to the inside cylinder valve.