Gab valve gear

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 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.

A hand lever allowed this notch to be lifted, thus disengaging the valve drive and promptly stopping the engine.

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.

On the footplate of a rattling locomotive with no suspension and a poor trackbed, this is known to have been the cause of breakdowns, whether by driver error or by a loose gab slipping into accidental engagement.

As the gabs, unlike in the stationary engine, were remote from the driver they were provided with wide V-shaped jaws to help them engage with the pins.

The driver's reversing lever[note 1] moved the centre of this bar, thus the pins, up and down to engage one at a time with opposite faces of the X-gab.

[note 2] In 1844, William Williams, a pattern-maker for Stephenson, made the remarkable invention of realising that if a closed gab was made into a curved link, so that it fitted the pin closely throughout its travel, then the valve gear could also be set into an intermediate position, and that this would also have the effect of giving expansive working.