Gab valve gear

Gab valve gear was an early form of valve gear used on steam engines. Its simplest form allowed an engine to be stopped and started. A double form, mostly used on steam locomotives, allowed easy reversing.

The word gab or gabb may derive from a word for mouth, recorded by the Oxford English Dictionary from 1724, and probably medieval in origin from other forms related to gossip or idle chatter. The OED also gives the steam engine sense of gab as a notch in the valvegear as possibly being of Flemish origin, from the word gabbe. This is cited in the OED from 1792. The OED also cites the obviously derivative gab-lever from 1839.

One of the first self-acting valve gears used for steam engines was the eccentric valve gear. This placed an eccentric on the engine's crankshaft, that in turn drove a strap and a long rod to the valve's actuating spindle. This was a simple valve gear but worked well for rotative engines that ran continuously for long periods, and in only one direction. For early mill engines this was acceptable.

The simplest form of gab valve gear or 'gab clutch' was a simple notch in the valve rod, where it hooked over the valve spindle. A hand lever allowed this notch to be lifted, thus disengaging the valve drive and promptly stopping the engine.

Where an engine had to be stopped and started precisely, such as for a winding engine, it was useful to do this by means of the valve gear. This allows the engine to be stopped within a fraction of a revolution, where using a throttle or stop valve in the steam supply slowed the engine gradually and so would be far less precise.

Winding engines for mineshafts were required to be reversed, for hoisting and lowering of the shaft cage. These engines began by using the slip-eccentric valve gear, with a gab clutch. The eccentric is loose on the crankshaft and can rotate freely between two stops. These stops represent the positions for the eccentric to run the engine in each direction. When the gab was disengaged and the crankshaft stopped, the manual lever was used to drive the engine valves in reverse, which also re-set the eccentric to the opposing position. The gab could now be re-engaged and the engine restarted in the opposing direction.

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