My earliest recollection of reading a rail accident investigation report concerned a cow and an engine called James. The overspeed had been attributed to wooden brakes, which apparently (as the crew claimed) had been known to be defective for some time[1].
It was with this in mind that I embarked on a new project to build a brake van in 5″ gauge with working clasp brakes and mechanical force balancing.
Armed with a far more academic work on the construction of 5″ wagons[2], and with a set of lazer cut parts [3] the project started after the 2017 show season.
Planking in real wood was to be an essential feature and; as the van was to be funded by the sale of a landrover I had converted to uncomfortably sleep in, the body was to mimic the draughty cold or stifling heat the originals were said to have been[4]. A band saw was therefore acquired and the shed built up over several years of sawing and rough carpentry. For the doors and windows, halving joints and rebates were cut using a miniature CNC / engraving machine. The windows were glazed with microscope slides.
Chassis
Wheel castings had become available again in this, the third decade of the 21st century so following the recipe of baking them in a coal fire, turning to rough size and pressing onto axles machined to within a fraction of a thousandth of an inch, they were finish turned between centres and trimmed to length with a fixed steady[5].
Rather nice “Timkin” axle box covers were on sale as 1″ to the foot jewelry and artistic licence justified putting them on.
Leaf springs were considered at length, but in the end coil springs were used, with wooden spring seats, CNC machined from solid.
Brakes
In order to provide a reasonable amount of brake force, the wagon would have to be heavy. So a side task of making packing cases filled with lead was put in place. [6]
The eight shoes themselves were a complicated shape so there was no real option other than to CNC them from solid wood. The prospect of seeing smoke billowing from them, James’ style, would be worth it anyway.
Following the prototype portrayed in Hewson’s book[2], they are hung from the chassis, each on a pair of straps with turned pins held in place with split pins. Each pair across an axle is pulled with an A frame, so that the force on each is the same. The two A frames per axle are balanced by a torsion arm pulled by a central brake rod. The torsion arm itself being suspended from the body on two more hanger straps. In this way the force pulling the brake rod is (almost) equally distributed onto each of the 4 shoes on an axle. Unfortunately, at this point, the suspension movement kicks in and as the body heaves, the brake rod displacement will change if the shoes are to remain in contact with the wheels. One problem would be, for example, that with the brakes hard on, either loading or unloading the wagon would upset the clamp force. The answer is compliance. The brake ‘rod’ is made as a long tension spring and wire rope so that a specific displacement at one end relates to an almost constant force at the other.
At the centre of the wagon, the two brake ‘rods’ (springs) are joined, again with a torsion lever. Unfortunately there is no force balancing, or ‘front-back brake force distribution’ to use automotive parlance. Correctly implemented, this would of course depend on the acceleration; to prevent the rear axle locking under heavy braking.
It is the intention that the brakes could eventually be under remote control, so provision has been left for a vertical pull rod into the cabin. This could be by pneumatic or even vacuum cylinder, or a cop out of a radio control servo. Some sort of antilock braking (ABS) would be needed though, to prevent flat spotting of my nicely turned wheels.
The central torsion lever also has the drive from the handwheel on the veranda. This arm is dog clutched to the brake rods’ lever in the mechanical equivalent to the electronic OR gate we are now all familiar with.
The handwheel itself (a nice brass casting) drives a lifting lever through a Whitworth thread. This arm is located on, but not connected to, the hanger pivot for the rear brake compensator, just since it happened to be convenient.
Track Testing
The ballast was too much, with two boxes (each 7lb) the suspension was compressed halfway, but the bump stops were hit more than the wheels leaving the track. Coil springs meant that there was little damping so heave and roll resonances at 1-3Hz were evident. Well lubricated leaf springs would perhaps have been better.
The brakes operated nicely, either gently compensating for a gradient or smoothly bringing the train (two open wagons with the rest of the ballast) to a gliding stop.
Now all I need is a 5″ gauge model of Gordon’s hill.
References:
[1] “Thomas and the breakdown train” Rev. W. Audry, (1946) … “Never mind James, it wasn’t your fault, it was those wooden brakes they gave you. We always said they were no good.”
[2] ‘Constructing 5″ Gauge Wagons’, D.Hewson, HMRS 2017.
[3] 17D Limited, Units 12 & 13 Via Gellia Mill, Bonsall, Matlock DE4 2AJ.
[4] “As Safe as Yesterday”; Steve Shiels, No reference available (private publication?). See the story of Consett Steel Works Railway.
[5] “Heat Shrink Wagon Wheels” wordpress post.
[6] “Lead Bricks” wordpress post.
philbarber2 