Gresley's Conjugated Valve Gear
Although the LNER used a variety of different valve gears, it will forever be associated with
Gresley's conjugated valve gear.
Against a background of increasing train sizes in the early 20th Century, it was necessary to build
locomotives that were more powerful and with a larger cylinder area.
A move from two cylinders to three or four cylinders would enable this increase in power with
greater efficiency, smoother running, reduced sway and hammer blow.
Gresley preferred a three cylinder solution as this would
aid maintenance and reduce fewer compromises in the steam passages. However, unlike a four cylinder arrangement,
the motion for a central cylinder could not be trivially derived from the outside cylinders.
One solution (as used in the Peppercorn A1 Pacific)
was to fit a complete set of Walschaert valve gear on the central cylinder.
Gresley considered this a compromise that added 'unnecessary' weight
and could be difficult to maintain. His solution was a '2 to 1' lever that derived the motion for the central
valve from the motion of the two external Walschaert valve gears. This became known as a 'Conjugated Valve Gear'.
Click for animation (626KB)
(Shockwave Flash required)
Gresley first describes the new valve gear in 1915, in
his Patent No. 15,769 "Improvements in Valve Gear for Locomotive and other Steam Engines".
Holcroft had filed a patent for a similar arrangement in 1909. After the expiration of this patent had been
confirmed, construction of the first three cylinder locomotive was started.
At this time, Holcroft also helped Gresley perfect the
design.
GNR/LNER Class O2 2-8-0 No. 461 was completed in May 1918.
This was followed by a new K3 2-6-0 fast freight design in 1920.
Early trials of the K3 found that the central valve tended to
over-travel due to rigidity of the cross member that carried the main pivot of the mechanism.
This rigidity also resulted in the main pivot bearing bolts fracturing.
The problems were fixed by reducing the valve travel, and fitting a cast steel box type cross stay.
Gresley was now ready to use the conjugated valve gear on a new
express Pacific that he had been planning. The result was the famous
A1 No. 1470 Great Northern, unveiled in March 1922.
Although the A1 Pacifics proved to be capable locomotives,
a set of trials against the Great Western (GWR) 'Pendennis Castle' 4-6-0 in 1925, showed them to have a hungry
appetite for coal. This high coal consumption was due to the limited travel and short lap of the valves - implemented
after the problems with the K3s.
The solution was to fit GWR-style long travel valves and the cast steel cross stay. Initially
Gresley was reluctant to accept that the steel cross stay
would be sufficient, but quickly accepted the solution after a trial with two Pacifics demonstrated a 20%
reduction in coal consumption.
After these initial problems were solved, Gresley would use his
conjugated valve gear on all of his large production locomotives, including the
A4 Pacific,
V2 2-6-2, and
D49 4-4-0 classes.
Gresley's conjugated valve gear was not without
criticism. The main problem was that it was sensitive to good maintenance.
The excellent daily performances given by the A1s, A3s,
A4s, and
V2s during the 1920s and 1930s were dependent on
good lubrication and a timely replacement of worn bearings.
However, maintenance suffered during World War 2. Skilled labour became scarce, and the valve gear's roller bearings
often had to be replaced by plain brushes which wore quickly. These problems led to poor valve timings and complaints
from enginemen, although actual mechanical failures were very rare.
Gresley died in 1941, and was succeeded by
Edward Thompson.
Thompson ceased production of locomotives with the conjugated
valve gear, and rebuilt a number of three cylinder Gresley types.
Although it is usually considered that his criticism of
Gresley's valve gear was over-stated, his switch to simpler
designs (e.g. the Thompson B1) were much more suitable for
wartime conditions.
Further Reading
The following pages within the LNER Encyclopedia provide more detail about related subjects:
Acknowledgements
Thank you to Charles Dockstader for permission to use his
Windows Valve Gear Program for the above animation.
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