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.[1]
Etymology
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.[2] The OED also cites the obviously derivative gab-lever from 1839.[3]
Origins
One of the first self-acting valve gears used for steam engines was the
Gab valve gear
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.
Stationary engines
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
Locomotives
With the development of the first steam locomotives, reversing was an obvious necessity.
The first locomotive gab gears used two 'open' gabs, side by side, each potentially (when engaged) hooking over the same pin. If both gabs were ever engaged simultaneously these would jam, probably damaging the valve rod. 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.
A better solution was to use a single double-sided gab. These were initially X-shaped and sat between the two connecting pins.
Another mechanism, used on Stephenson's locomotives in the 1830s, was the 'coupled gab'.
The final form of the gab valve gear was the 'closed' gab. Like the X-gab, this was a coupled pair of gabs, although in this case they faced inwards and there was a single pin between them. Once again, the gabs were driven by the eccentrics and the pin drove the valve spindle.
Developments
The use of
Notes
References
- ^ Snell, J.B. (1971). Railways: Mechanical Engineering. Longman.
- ^ Specif. Kellys Patent (1792) No. 1879. 8 'Clear of the notch or gabb of the catch lever'
- ^ Robinson, R.S. (1839). Nautical Steam Engineering. p. 97.
- ^ a b Semmens & Goldfinch (2003), p. 111.
- ^ ISBN 978-0-19-860782-3.
- ^ Snell, J.B. (1964). Early Railways. Weidenfeld & Nicolson.
- ^ Wood, Nicholas (1838). A Practical Treatise on Rail-Roads.