Reversing gear
often called a reverser, controls the valves on a steam engine
Reversing gear is a mechanism used to both control the direction of travel of a steam locomotive and adjust its engine's steam cutoff.
Reversing lever
The most common form of reversing gear uses a lever to engage (known as a Johnson bar in the United States) mounted parallel to the direction of travel on the driver’s side of the cab. It is controlled by a handle and sprung trigger at the top, and pivots at the bottom to pass between two notched sector plates. The reversing rod, which connects to the valve gear, is attached to the lever either above or below the pivot in an alignment that gives good leverage. A square pin is arranged to engage with notches cut in the plates and holds the lever (and valve gear) in the desired position when the trigger is released.
The advantages of this design are that change between forward and reverse gear can be made very quickly (as is needed in, for example, a shunting engine).
Limitations and drawbacks
The reversing lever has a catch mechanism which engages with a series of notches to hold the lever at the desired cut-off position. This means that the operator does not have a full choice of cut-off positions between maximum and mid-gear, but only those which correspond with the notches. The position of the notches is chosen by the locomotive designer or constructor with a view to the locomotive's intended purpose. In general engines designed for freight will have fewer notches with a 'longer' minimum cut-off (providing high tractive effort at low speeds but poor efficiency at high speeds) while a passenger locomotive will have more notches and a shorter minimum cut-off (allowing efficiency at high speeds at the expense of tractive effort).
“Reversing gear” enters the record as often called a reverser, controls the valves on a steam engine. Crown Archives preserves that source wording while asking what Reversing, gear and often can confirm, complicate or overturn.
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