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Main Cylinders

For most of the photos on this page, you can click the image for a larger picture.

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A Clan locomotive, like the Britannia pictured above, has 2 outside cylinders. Each cylinder assembly consists of 2 cylinders as you can see in the photo. The 'motion' cylinder is the lower one and provides the motive force for the locomotive. It has dimensions of 19.5 inch (diameter) X 28 inch (length) (495 mm X 711 mm). It contains a single piston which is driven backwards and forwards by the force of the steam - the maximum piston thrust is 67,196 lbs which is just under 30 tons. Steam must be applied to the left hand side of the piston to move it to the right and then to the right hand side of the piston to move it to the left. The timing of this is obviously critical and is the function of the valve cylinder in conjunction with the valve gear.

The valve cylinder sits above the motion cylinder. It's longer than the motion cylinder but has a smaller, 11 inch diameter. Inside are 2 pistons connected by a rod and also externally to the valve gear.


The diagram shows what's going on inside. Steam is admitted from the boiler at the top centre of the valve cylinder (in this example and in our engine although this can vary). Steam is directed to the appropriate end of the motion cylinder by the positioning of the valve pistons. Again, steam is exhausted from the motion cylinder to the blastpipe and chimney through the uncovering of the correct port. The graph inside the cylinder shows the steam pressure within the cylinder throughout the complete cycle. For the purposes of this page, we'll go no further as this is a complex and interesting subject.

On a Clan, the combined motion/valve cylinder is a casting and is one of the largest on the locomotive. To make this cylinder requires a lot of ingenuity!

In simple terms, the standard casting process involves the following

"Carefully withdraw" is the challenge as there is no way that the cylinder pattern could be withdrawn from the mould in one piece. Therefore the pattern has to be designed so that it can be inserted in and removed from the casting boxes. If you want to watch a video on how a simple part is cast, then this video is a good start. Note that the wheel pattern is made in 2 halves.


Copyright National Railway Museum

The starting point is that we're lucky enough to be able to access the original British Railways drawings which are held by the National Railway Museum.

Our first job is to create a 3D CAD model from the original 2D BR drawing. In CAD terms, the CAD model is the first thing to be created followed by the CAD drawing that contains manufacturing details.In our case somebody's done all the hard work and we have access to the original 2D BR drawing for our CAD expert, Keith Greenhow, to create the 3D CAD model. Keith will prepare the drawings so that our pattern maker, Tony Dance, can make the pattern.

Highslide JS This is one I prepared earlier! This is what we want to end up with or something very similar. This is a newly cast cylinder for a Standard Class 5. Hengist's cylinders will be very similar in appearance.
You can see the exchange port between the motion cylinder and the valve cylinder.
Highslide JS Keith's starting point for the Outside Cylinders CAD model is the flange that connects to the frame, a sketch is created in the CAD software using details from the BR drawing. The BR drawing is in imperial units (feet/inches) with fractions but Keith will need to work in inches and decimal places. This is the right hand half looking at the drawing. IN some places, the origin of a radius on the the drawing can be difficult to deduce.
Highslide JS The profile of the sketch is then given a thickness and the 3D modelling begins.
Highslide JS The flange is symmetrical so only half is drawn it's then mirrored to complete the full flange shape.
Highslide JS Here the motion cylinder is sketched and revolved, this is also symmetrical and mirrored.
Highslide JS The steam chest/valve cylinder is sketched and revolved.
Highslide JS Completed cylinders and flange plate.
Highslide JS Connecting these three main parts together now begins with the steam inlet cavities.
Highslide JS The exhaust outlet ports are added between the steam chest and flange.
Highslide JS The steam inlet flange and pipe is added to the steam chest to complete the main structure.
Highslide JS All the small bosses and radii are added and the 3D model is complete.
Highslide JS From the model, details can be extracted to help Tony create the wooden pattern, this shows the profile of the motion cylinder. Tony prints the drawing at full size and measures directly from the print. You will see this drawing in use on page 2.
Keith's drawing of the motion cylinder is important for Tony as it's now a 'pattern drawing' i.e. scaled up for shrinkage and that's how Tony can take off measurements and apply them to the pattern. This is the whole reason why Keith is doing 'pattern drawings' which changes the 'works drawing' into something the pattern maker uses to make life much easier and to all but remove a chance of error. When Tony places the half cylinder pattern on Keith's drawing which is on his bench, it will show him if errors are creeping in. Without Keith's drawings, Tony would have to painstakingly draw out these sections, adding shrinkage allowance, on white faced hardboard - this would be a real minefield for errors creeping in.
Highslide JS This is the outside view of the cylinder assembly from the CAD model. The valve cylinder/steam chest is on top and the motion cylinder is below. CAD allows you to rotate the assembly in all directions for a specific view. For the cylinders we are using Spheroidal Graphite or SG Iron with a shrink rate of 1/80. The pattern will be 1/80th larger than the finished casting to allow for this shrinkage.
Highslide JS This is the frame side view of the cylinder assembly. The steam inlet is to the top and the exhaust steam ports can be seen coming from the outside ends of the valve cylinder onto the flange plate. These connect with the exhaust steam spider and then onto the blastpipe in the smokebox.
Highslide JS

In order to produce a complex casting like this, the pattern must be broken up into sections so that the patterns can be lifted out of the sand mould without disturbing the mould. Tony and Keith working with Phil Yates from the Advanced Manufacturing Research Centre in Sheffield, decide how best to split the pattern. They decided that the pattern will be made in 4 sections, split horizontally as can be seen. This will allow the various pattern sections to be removed from their section of the casting mould before the complete mould is assembled. If you look carefully, you can see a small step in the plane between sections 2 and 3. This makes the pattern making just a little more complex.

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Put the sections together and you get a complete outside cylinder.

Highslide JS Phil then used a 3D printer to create this model of the patterns. This is very useful for Tony in helping visualise how the pattern will be built.
Normally, you don't see this as the cylinder is enclosed within sheet metal covers. The valve cylinder is on the top with the steam inlet at the top centre. The connecting ports between the valve cylinder and the motion cylinder can be seen clearly

Highslide JS This is the 3D model broken into its component parts. In the bottom left is the bottom of the motion cylinder looking down from the top. The flange for bolting to the main frames is at the bottom. In the top left is the top half of the motion cylinder, again looking down from the top. Here you can see the exchange ports with reinforcing bars in the walls. The frame flange is at the top.
The middle component is the bottom half of the valve cylinder. The diagonal arms are the exhaust steam passages which will connect with the exhaust steam spider located between the frames. The view is looking down from the top of the assembly.
The final component is the top of the valve cylinder and this is upside down. The main steam inlet is in the middle. This goes off at an angle which is going to pose a tricky problem for Tony!
Tony says that "The plastic print is a really useful addition to the pattern making process and allows me to see in my hand where I have to add all sorts of removable bits and pieces to allow the Moulder at the foundry to release the pattern from the sand. When I make my first visit to the foundry, I will take Phil's plastic print and this will allow the moulder to more easily see in solid 3D what he has to do and then he will advise me as to what I need to do in pattern terms".
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