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Inspecting Bulleid's Q1 Valve Gear - by Don Ashton

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 well.

From such a fertile mind as Bulleid’s we have come to expect originality to supplant convention, both visually and functionally.

In 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.

Much 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 cylinder bore. 

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.

The 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.

It 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.

Yet 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.

It 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.

BULLEID Q1                      
Cut offs Release  
DIFF % Front
DIFF % in
DIFF % Front
76.24 73.56 2.68 74.32 70.61 3.71 17 93.78 92.37 1.41 93.15 91.43 1.72
72.60 70.13 2.47 69.37 65.68 3.69 92.62 91.09 1.53 91.59 89.50 2.09
68.17 66.05 2.12 63.26 59.81 3.45 91.20 89.41 1.79 89.63 87.65 1.98
62.77 61.19 1.58 55.77 52.86 2.91 89.45 87.36 2.09 87.16 84.93 2.23
56.22 55.39 0.83 46.83 44.77 2.06 87.27 84.84 2.43 84.03 81.59 2.44
48.41 48.49 -0.08 36.83 35.72 1.11 84.53 82.50 2.03 80.05 77.48 2.57
39.47 40.42 -0.95 26.76 26.39 0.37 81.11 79.02 2.09 75.04 72.49 2.55
30.01 31.43 -1.42 17.87 17.93 -0.06 76.82 74.75 2.07 68.88 66.52 2.36
21.04 22.36 -1.32 11.07 11.31 -0.24 71.51 69.55 1.96 61.63 59.60 2.03
13.59 14.50 -0.91 6.46 6.77 -0.31 65.12 63.33 1.79 53.70 51.97 1.73
8.17 8.74 -0.57 3.66 3.95 -0.29 Mid 57.80 56.13 1.67 45.76 44.12 1.64


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.

It 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 very well.

My 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.

After 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.

Like 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.

The 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 !

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Note: Don's article was first published in Model Engineer magazine of March 13th 2010, and has been reproduced with Don's consent.  The late Don Ashton produced a web-site devoted to model valve gears.  To access, please follow this link.