No one really wants the
complication of articulated locomotives! By far the easiest engines to
maintain are those which are rigid: meaning that the driving wheels remain
parallel to the boiler at all times. As traffic increases on a railway,
the company is faced with new problems. More trains can be operated with
existing equipment but this will be limited to the signalling capability of
the line and the number of locomotives available. In addition, more trains
mean more staff who have to be paid and managed.
The alternative is to run
longer trains and this means more powerful locomotives are needed. Here one
will very quickly reach a limit for a number of reasons.
The size of boilers are
limited by the loading gauge of the line.
Track and civil engineering
structures all have a maximum axle loading so the heavier locomotive will
have to spread the weight over more wheels.
More driving wheels limit
the radius of curves that the locomotive will negotiate.
This is when everybody and
their dog gets out paper and start to design their own articulated
locomotive. In fact some designs have been so bizarre that they must have
been designed by the dog! Here we shall restrict our copy to common solutions
found on narrow gauge lines.
The first narrow gauge
railway to need articulated locomotives was the venerable Festiniog Railway.
The tiny 0-4-0 tender locos built by England were quickly inadequate for much
of the traffic and a new design by Robert Fairlie was built called 'Little
Wonder'. He had supplied double Fairlie locomotives elsewhere which had been
hugely unsuccessful but the new Festiniog engine became a resounding success.
The design consisted of two
pivoting power bogies mounted under a double boiler. This removed firebox
size restrictions but the fireman was presented with twice the work. As with
all articulated locomotives, one weakness has always been maintaining the
integrity of the flexible steam joints but Fairlie did manage to solve many
of the problems.
It is true to say that most
double Fairlie locomotives were unsuccessful and only the Festiniog continue
to use the configuration.
Single Fairlies were
also built and ran on a number of lines.
'Taliesin' an 0-4-4 single Fairlie built new by the Festiniog Railway
were built in France for military use and are nothing more than a developed
Fairlie. Over 70, 0-4-0 0-4-0 locomotives were built and operated with
mixed success. Larger examples were also built.
The original design by Jean
Meyer of France became the most common of articulated locomotives despite
suffering from a problem of not being able to include large fire boxes.
Kitson solved the problem by moving the bogies apart. These engines were
exported around the world and became larger and larger in size.
'Monarch', which moulders at
the Welshpool and Llanfair Railway was originally built for Bowaters Railway
Kent. This is an original Meyer design built by Bagnall. A circular marine
firebox was fitted which did not contribute to its success and 'Monarch' has
become the locomotive that no one wants to operate.
The Swiss, Anatole Mallet,
developed his design as a flexible engine. The rear power truck forms part of
the rigid structure and the front bogie hinges from the rear. A true Mallet
is a compound. That is, the high pressure steam exhausted from the rear
cylinders is transferred to the larger cylinders of the front bogie and used
for a second time. The advantage of the Mallet is that the flexible joints
are kept to a minimum and those that are used have minimal movement.
The disadvantage is that the
front of the boiler overhangs while negotiating curves which reduces
stability and restricts the locomotive's speed.
Mallets were always very
popular in Europe but never really caught on in the UK.
The United States developed
the Mallet into behemoths of staggering proportions. One design even involved a
flexible joint in the boiler (designed by a dog).
The Garratt is the most
famous of all the articulated locomotives and its advantages are well known.
However it does have one very bad characteristic: its length... This leads to
very long steam pipes and other things having to be stretched to vast
distances. There are only two known examples of compounded Garratts, these
being K1 and the Burma Railways loco.
the World's first Garratt - K1 now at the Welsh Highland Railway
newly rebuilt Garratt No 87 at Boston Lodge,
Some really huge Garratts
have been built and the Garratt is certainly one of the most successful
articulated designs. Needless to say, there are a bunch of Garratt
look-alikes which appeared in order to circumnavigate the patent.
A number of designs were also
developed which allow the addition of an extra driving wheel which possessed
some sideways movement. One such system can be found on the NG15 Mikado
(2-8-2). In this case, the Krauss-Helmholtz bogie.
The requirements of logging
railways have spawned a whole range of unique articulated designs.
Locomotives were required to run on terrible track, sometimes, even, just on
logs, climb very stiff grades and be very serviceable. Speed was certainly
never an issue.
Shay locomotives had regular
fire-tube boilers offset to the left to provide space for a two or three
cylinder "motor," mounted vertically on the right with longitudinal drive
shafts extending fore and aft from the crankshaft at wheel axle height. These
shafts had universal joints and square sliding slip joints to accommodate
motion of the swivelling trucks. Each axle was driven by a separate bevel
gear, and used no side rods.
Driving all wheels, including those of the tender, together with small
diameter wheels were the strength of these engines, their entire weight
developing tractive effort. A high ratio of piston strokes to wheel
revolutions allowed them to run at partial slip, where a conventional rod
engine would spin its drive wheels and burn rails, losing all traction.
Shay locomotives were often known as sidewinders for their side-mounted drive
shafts. Most were built for use in the United States, while many found their
way to 30 additional countries, territories or provinces.
Although the Shay was the
most common geared locomotive, it had a significant flaw that was not
corrected. Because the drive shaft lies outside the trucks, instead of along
the centreline, truck rotation when following track curvature causes
substantial drive line length change, unlike the central drive shafts of
Heisler locomotives and Climax locomotives. In modern drive shafts, this
effect is accommodated by roller splines instead of bronze slip joints (shown
between "Sonora's universal joints") that lose their ability to slide under
Wreck photographs show Shay locomotives, before or after uphill curves, where
they failed to respond to change in track curvature, thereby running off the
track "for no apparent reason." Some texts refer to these locomotives as
"rail spreaders" and "flange hounds," both characteristics of trucks that do
not steer freely under heavy load.
Other disadvantages include the noise of the gearing, and the very low top
The Climax locomotive has two
steam cylinders were attached to a transmission located under the centre of
the locomotive frame. This transmission drives drive shafts running forward
and rearward to gearboxes in each driving truck.
Unlike the somewhat similar Heisler design, there were no side rods on the
trucks. The gearboxes drove both axles on each truck.
Many loggers considered the Climax superior to the Shay in hauling capability
and stability, particularly in a smaller locomotive, although the ride was
characteristically rough for the crew.
The Heisler locomotive was
the last variant of the three major types of geared steam locomotive, Charles
L. Heisler receiving a patent for the design in 1892 following the
construction of a prototype in 1891. Somewhat similar to a Climax locomotive,
Heisler's design featured two cylinders canted inwards at a 45 degree angle
to form a 'vee-twin' arrangement. Power then went to a longitudinal
driveshaft that drove the outboard axle on each powered truck. The inboard
axle on each truck was then driven from the outboard one by external side
(coupling) rods. The Heisler was the fastest of the geared steam locomotive
designs, and yet was still claimed by its manufacturer to have the same low
speed hauling ability.