Southern Railway - Bulleid's Q1
elevation shows the weighshaft bracket, frame rivets for the
motion plate and cylinder casting.
As Hornby's shrewd foresight amply proved, beauty is not
necessarily the guide to popularity: Bulleid's austere Q1, the
most powerful British 0-6-0 ever designed, created huge demand
in '00' gauge, and a good deal of interest in larger gauges as
From such a fertile mind as Bulleid's we have come to expect
originality to supplant convention, both visually and
1938 Bulleid was just too late to cancel the order for more of
Maunsell's pedestrian Q Class, but the coming war produced the
necessity for a much more powerful 0-6-0 than the Southern
possessed - what was a predominantly passenger railway was
soon to be inundated with heavy war-time freight duties.
has been written about practicality and oddity of these
engines but little seems to focus on the unconventional
cylinders and valve gear. Even a glance at the drawing
reproduced here may require the clues to be spelled out.
A seemingly ordinary British set of Stephenson's gear
drives through a rocker (spot the ratio?) to achieve the
height of the valve spindle above the
Notice, however, that the crank is on front dead centre, so
the valve needs to open the front port. The piston valves are
therefore of the outside admission type. The GWR 4-4-0 Classes
from around the turn of the 20th century were rebuilt with
outside admission piston valves so that the existing
'slide-valve' gear could remain, but the Q1 was deliberately
designed with outside admission valves in spite of the much
higher steam pressures on the glands that this entails.
rocker, already spotted as a travel-increasing one, is always
a point of interest in a valve gear design. I have had much to
say about the introduction of a 180° rocker in the driveline,
and how it never produces a mirror of valve events - the new
drive direction sets against the crank and eccentric
angularities. This occurs in 4-cylinder engines driven by two
gear sets, adds complexity to the design of conjugated gears
and becomes a nightmare in scissors arrangements.
should never feature purely as a convenient means of lifting
the drive onto a different plane (LBSC's frequent trick). Most
British Stephenson's layouts correctly employed locomotive
links to drive directly to outside admission valves and those
designs with slide valves 'on top' via a rocker had the
correct launch type links for an indirect drive.
here we see Bulleid deliberately breaking the rules and the
result is clear from the large trunnion pin offset, in this
case correctly in front of the die. Because the angularity
errors of both connecting rod and eccentric rods tend to
summate on the same side in this arrangement, so correction
has to be greater and is incomplete. Although the size of the
eccentrics for locomotive links is larger than the launch link
equivalent of smaller throw, Bulleid keeps things within
bounds by using the odd-armed rocker. Clearly this feature can
be used either to increase valve travel or to miniaturise the
gear. Perhaps here we see a compromise of both.
is worth mentioning here a point which may not be clear to all
readers. If the drive features a rocker ratio that increases
the die-to-valve movement, as on the Q1 and many of the GWR
outside cylinder designs, one particular item needs careful
attention. The advance setting of the eccentrics in
Stephenson's gear is directly responsible for the amount of
lead steam - the smallest critical dimension on the whole
locomotive. Therefore it should be clear that where a rocker
produces magnified valve travel it will also give greater lead
unless the eccentrics are backed off from the norm to
accommodate this. There is no question that the design team
knew exactly what they were doing - all the details pointed
out here have been attended to in the Q1 gear. So much of the
Q1 design would be unconventional to the drawing office staff
that one has the distinct impression that Oliver Bulleid must
have spent all his time dominating the scene.
class member C1; view from right side under the boiler,
showing top of valve rockers and short links to the two valve
spindles above the cylinders. Photo: Rob Speare.
view of survivor C1, this time from the left. Left of photo
centre, the motion stretcher and on it's right the suspension
point of the swing links supporting the radius rods. The
massive wayshaft is to right of the photo, with the two lifting
arms supporting two pairs of lifting links (as photo below).
Photo: Rob Speare
taken from the left, the lifting links are centre, and you can
see the expansion link is pivoted ahead of it's centre line.
Photo: Rob Speare
The boiler, with a rather short barrel, had a large firebox
constructed using the formers for the 'Lord Nelson' engines
and featured a large-volume smokebox with a Lemaitre 5-jet
exhaust, so just how well was the steam distributed?
These days it is a relatively straightforward matter to
analyse the gear by computer in order to ascertain the
distributive qualities of the design.
is perhaps apposite to state the full parameters by which we
might judge performance. Firstly we require sufficient
full gear cut off for reliable starting with a load; secondly,
an even distribution of steam to both front and back ports so
that each end of the double-acting cylinder produces even
power. This is apparent from the cut off and release
differences throughout the working range. A larger
tolerance of inequality can be suffered in full gear than at
running cut offs, where smooth running is hampered by poor
event equality. Except for engines likely to work
equally often in reverse as forwards the shorter cut offs in
back gear are relatively unimportant, though the Q1 manages
usual practice in the simulator is to set the valves for lead
equality, which was the valve setters' primary
operation. Lead is a function of the valve gear geometry
and not a physical quantity that can be machined, so the valve
can only respond to the ability of the gear to produce the
necessary lead at each dead centre crank position. If it
cannot support this, the act of setting the valve on its
spindle to equalise leads will simply destroy all the other
events. In such cases it is clear that some compromise
must take place.
a perusal of the Q1 motion drawing I should have been
surprised to find immaculate valve events. Whatever
regularity appears at the dieblock, the swings of the hangers
must give a rocking action to the dieblock end of the link to
the rocker. These small disturbances are then multiplied
by the rocker ratio before reaching the valve. In mid
gear the lifting arm is some 12° out of square with the gear,
so maybe the forward gear performance differs somewhat from
back gear. The table of cut offs shows how well the
designers have achieved the aims.
most engines the Q1 distribution is not perfect but a
difference of less than 3% in full gear is quite
acceptable. The offset of the expansion link trunnion
can be manipulated within fine limits to fix the point of the
change from front port bias to rear port bias such that any
inequality of distribution in the shorter cut offs does not
affect smooth running. The cut off table (see the
differences column) shows that the whole range in forward gear
is a practical solution. These parameters naturally
affect the back gear results and as the 0-6-0 is predominantly
to run forwards we can detect from the back gear column that
the compromise judgements are sound.
leads are slightly unequal, amounting to 1/32nd", yet it is
important to note that full restoration of equality in the
leads would greatly impair all the other events in the
cycle. It really is quite amazing how much upset this
small amount makes and it is salutary to ponder that this
dimension constitutes only 0.003" in 5" gauge. LBSC,
bless his soul, had not the means of knowing that his 'tram
ticket' valve setting methods and abhorrence of 1/64ths" were
not the best means of securing the power available, even on a
19th century invention.
The greatest culprit preventing the admittedly fairly small
distribution differences from equating is the lifting arm, and
is the greatest reason for back gear's failure to mirror
forward events. Its arc of operation is far from symmetrical.
Had this arm been set in line with the gear in its mid gear
position the action throughout the full range would have been
better. Illustrative of the effect, even with a short lifting
arm, is the Great Western arrangement that portrays in back
gear an almost perfect mimic of foregear. Good suspension is
an integral part of the gear as a whole.
As history reports, Oliver Bulleid continued in his inimitable
style right into retirement, producing locomotives capable of
prodigious feats. The price of experimentation and
unconventionality had to be paid in teething troubles, but he
should justly be credited with projecting the steam engine
almost single handed into the 20th century. The 'Charlies', an
endearment awarded to his first and successful Q1 Class
0-6-0s, apparently caused Stanier to enquire where one put in
the key to wind them up; but Sir William doubtless meant only
a slight jest as his engineering pre-eminence would clearly
recognise the work of a master. 40 healthy goods locomotives
had appeared in war time austerity and saved materials
equivalent to 9 engines and tenders !
This analysis was first published in Model Engineer magazine of
March 13th 2010. Photographs and captions © Rob Speare.
This article was first posted on the web as
part of an appraisal of O.V.S. Bulleid designs on the site www.bulleidlocos.org.uk .