Reversing rod

A Johnson Bar is a control lever on a steam locomotive, used to control the timing of the admission of steam into the locomotive's pistons. By controlling this timing, the amount of power delivered to the wheels is regulated, as is the direction that the wheels rotate, giving the lever the alternate name of the reversing lever.

Historians have not identified the reasons why engineers called the reversing lever a Johnson Bar, but the reversing lever is described in both British, and American railway journals of the time. The best accreditation of the invention is documented under the Walschaerts valve gear.

The reversing lever in locomotive history is documented from about 1842. The American Railway Master Mechanic's Association had members that published illustrations. Parts for the reversing lever are drawn in the "Locomotive Dictionary", 1st edition (1906), by George Little Fowler. The lever is linked to the valve gear piston inside the high pressure steam port. The steam pressure faucet is closed to cut off power, and after coming to a halt, the valve gear piston could safely be moved a short distance using the lever as a fulcrum linked to the piston inside. Although the linkage was designed to give leverage, the Johnson Bar would take a firm throw to move and lock in position. Some locomotives had a screw reversing gear instead.

The other use of the lever is to set the point of steam cut-off. This is measured as the percentage of the piston stroke during which live boiler steam is still being admitted to the cylinder. For starting the locomotive (when maximum torque is required), "full" cut-off may be as high as 85%, meaning that the piston is subject to almost full boiler pressure throughout most of its stroke. Conversely, when running at high speed and not accelerating or climbing a gradient, cut-off can be set to as low as 10%. This reduces torque but greatly economises on fuel, since far less steam is supplied to the cylinder and it is more efficiently used, being more greatly expanded and thus exhausted at a much lower temperature than when working at full cut-off. For steady-state running of most locomotives it is most efficient in terms of fuel and water to run with the throttle wide open and a low cut-off.

Greenly, H., "Walschaerts Valve Gear", Marshall
Danbury Railway Museum - Danbury, CT
George L. Fowler, "Locomotive Dictionary", 1st edition (1906), University of Wisconsin Library
reversing lever

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