GCR Mechanical Engineering

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Technical Information

Great Cockcrow Railway
Mechanical Engineering Section..

	In the early days of the GCR most of the mechanical engineering was done either at people's home workshops, or at the railway using the very limited facilities that existed. These were basically a bench with a vice, an electric hand drill and a small number of hand tools. As the railway, and the fleet of locomotives and rolling stock grew it became apparent that something more was required. As a result the railway now has access to a proper workshop. The work carried out there mainly centres around locomotives and rolling stock, but assistance is given to other departments of the railway, with manufacture and modification of components and assemblies. The railway has a small number of skilled mechanical engineers that staff the workshop. They either assist locomotive owners who require help, or carry out work for owners who can not do their own work. The work carried out on locomotive in the early days was mostly very basic, consisting mainly of fitting new bushes, pins etc, which were made in people's home workshops. As the age of locomotives, and the work that they were required to do increased, the work needed to keep them running also grew. In addition some of the engines that had come to the railway were found to be lacking in terms design or build quality, so quite a lot of work has gone into improving these engines. During the running season the main objective of the workshop staff is to keep engines in traffic, so any repairs are mainly confined to those required to keep the engines running. In the winter the larger jobs and major overhauls are started, with the objective of finishing the work by the start of the running season. However sometimes the larger overhauls run on into the summer. As a result some engines may not be seen working for some considerable time. To illustrate the sort of work that goes into a heavy overhaul, the rebuild of one of our locomotives is documented below. The objective is to return everything to an as new condition.
The first job to be done before the engine is moved to the workshop is to clean it using paraffin hot water, rags and elbow grease. Once this has been done the engine is moved to the workshop to be stripped down. The engine is stripped down, to a bare set of frames, and once again cleaned to remove dirt and oil. At this stage all the parts are examined and measured to decide what work is required. The larger items like frames, wheels, smoke box etc may then be sent away for sand blasting, to remove dirt, paint and rust, ready for painting. Once the sand blasted parts have been returned from blasting they are cleaned to remove sand and then examined for any defects that may have been masked by the accumulated layers of paint. The horn blocks are also checked for alignment and ware. After any problems have been dealt with the parts are painted.		A set of tender frames ready for painting after sand blasting
Attention would normally then turn to the driving wheel sets. If the crank pins are not scored and are still very close to their original size then they will not be touched. However if they are badly scored or worn they will be replaced with new ones. Over the years the wheel treads wear and are turned to recreate the correct profile. After a number of turnings the wheels reach a point where they can't be turned any more, as it would weaken them beyond an acceptable level. Once they reach this stage, they are generally fitted with steel tyres, as per full size practice. Steel tyres are turned from blanks, flame cut from EN8 plate. The bore of the tyre is finished, whilst the OD is only rough turned, the OD being finished to the correct profile once the tyre has been shrunk onto the wheel casting. Once the tyres have been machined the bore is carefully measured and the wheels are turned so that they are larger than the bore of the tyre. The fit used is 0.001" per 1" of diameter of the wheel. This means that an 8" wheel will be 0.008" larger than the inside diameter of the tyre, which is going to be fitted to it.					Boring the inside diameter of a steel tyre on a CNC lathe
Once the tyre and wheel have been machined the tyre is heated so that it expands to a larger size than the wheel that it is being fitted to. It is then dropped over the wheel and left to cool. If the bores of the axle boxes are worn badly or the axle journals have been machined to restore their finish, t hen the axle boxes are bored out and fitted with shell bearings. If the horn blocks have been machined or the axle boxes rubbing faces are worn then slippers are fitted to them to restore the fit between the two components. Once the axle boxes and wheels have been finished they are refitted to the frames. At this point the centerlines of the axles and centers of the crank pin holes in the coupling rods would be accurately established. Should the figures differ then the coupling rods would be bored out to the correct centers.				Turning a wheel ready for a tyre to be fitted. The other wheel of the set has already had a tyre and new crank pin fitted
Once the wheels and coupling rods have been finished attention would then turn to the cylinders. The first job is to measure and assess the cylinder bores. If these are very close to original size and have no major scoring then they will be lightly honed to remove glaze and restore the surface finish. If however the bores are badly scored or worn then they will be bored out and honed to the next suitable size up. Should the cylinders have to be rebored then new pistons would be made, and the rear cylinder covers modified to provide accurate location in the new cylinder bore. If the engine has piston valves then the condition of the valve liners would be assessed. If required the liners would be replaced, the design of the liners has been changed on some loco's to make valve fitting easier or allow fitting of piston rings. Once the work on the liners has been completed careful measurements of the ports and cylinder blocks are taken. These and some measurements of valve gear parts are entered into an excel spreadsheet. This then gives a dimensioned drawing of the valve bobbin. The bobbins are then compared to the spreadsheet dimensions. In the passed errors of up to 1mm have been found in bobbin dimensions, whilst this sounds a very small amount, the effect on engine performance can be very significant. Should the valve dimensions be different from the spreadsheet then new bobbins would be made. Once all the work on the cylinders has been finished they would be refitted to the frames.
Once the cylinders have been refitted, the slide bars would be surface ground to restore the flatness and surface finish. If the crossheads are worn the sliding surfaces are built up with bronze and then machines back to the original dimensions. Attention would then turn to the valve gear. If the engine was known to have good valve events then the work would mainly consist of replacing worn bushes and pins. If however the valve events were not satisfactory then the objective would be to improve the events. The first thing that would be done would be to clean and accurately measure all the components of the valve gear. Once this had been done the design and manufacture of the valve gear would be checked by entering the dimensions of the components into a computer valve gear simulator. Once the simulated valve gear has been optimised the real parts would be modified to the dimensions given by the simulator. Once this has been done the valve gear would be reassembled. At this stage the valves would be set using similar techniques to those used on full size engines. In addition the engine would be run on compressed air to prove the valve timing and assembly. Once ancillary items like lubricators, brake gear, ash pan and general pipe work have been refitted attention would then turn to the boiler. In the main this would generally consist of de-scaling and cleaning. However in the past new regulators have been fitted, stays replaced, leaks repaired and fire tubes cleared of build ups that reduce their inside diameter.			An old (left) and new piston valve liner. The ports have been changed from 4 slots to a number of square holes to allow the fitting of piston rings. In addition the ends of the bores have large chamfers to make fitting of the vale bobbin easier.
Once the boiler work and painting has been finished the boiler would be reunited with the frames and piped up. The engine would then be ready for test steaming and running. This is usually done when the railway is closed, so that it can be done without the pressures of a running railway.			Southern Railway West Country Class being test steamed after having larger cylinders and a new design of valve gear fitted. The casing over the boiler has not been fitted at this stage to allow access to steam joints and allow the valve gear to be seen from above

Glossary of Terms

Bobbins	The name given to a piston valve assembly. On a full size engine this would normally be two separate piston valves mounted on a rod, however most miniature engines they are combined into one part.
Bush	Normally a ring of a bearing material pressed into another component with a steel pin running through the center of it.
Center line	An imaginary line running through the center of something.
Connecting rod	The rod which connects the cross head to the driving crank pin.
Coupling rod	The rods, which join the driving wheels together.
Crank pins	Steel pins that are pressed into the driving wheels. The coupling rods and connecting rods are mounted on them.
Cross head	A person with a hangover or an assembly which slides along the slide bars, allowing the backward and forward movement of the pistons to be transmitted to the wheels.
Frames	Two steel plates, which make up the chassis of an engine.
Mechanical Lubricator	Normally small pump driven off a convenient part of the engine, which pumps oil into the cylinders for lubrication.
Piston valve	See Bobbin.
Ports	Holes in the valve liners or cylinder blocks, which allow steam to pass from the valves to the main cylinder bores.
Sand blasting	A way of removing dirt, paint and rust by propelling sand with high pressure air at the item to be cleaned.
Shell bearing	A replaceable semicircular bearing.
Slide bars	Parallel steel bars, which the cross heads, slide along.
Slipper	A metal plate screwed onto a flat surface to remove ware.
Stay	A metal rod, which connects two flat surfaces of the boiler together to provide support.
Valve events	The timing of the opening and closing of the valves, which control the admission of steam to the cylinders
Valve gear	The system of levers, which operate the valves