Miniature Traction Engine

This is a work in progress around the research, design and building of a miniature traction engine, in particular a Burrell 6NHP traction engine.

Cherry Hill's model engines a book by David Carpenter

My son bought me this book for my birthday as I’ve seen a number of Cherry Hill’s models and they are inspirational.

This book has made me think about my approach, my research and as I’ve got older how I can more calmly make parts and remake parts.

The main image shows the elevation view of this traction engine printed on a sheet of A4 paper. This is meant to be a small traction engine that works and make to enchant.

Basic dimensions:

  • Length: 263mm
  • Width: 122mm
  • Height: 176mm
  • Bore / Stroke: 12/17mm
  • Rear wheels:
    • diameter: 97mm
    • width: 22mm
  • Front wheels:
    • diameter: 70mm
    • width: 12.5mm

I wanted to work out the scale of this miniature traction engine. So, I’ve looked at a number of drawings, images and specifications. All information that I can find online. My conclusion is it is 1/20th scale.

Axles

front axle

The front axle was quite simple to design and make. A tapered 3/16″ square mild steel with machined ends to accept the wheels directly.

This axle then rocks and rotates on the axle pivot that is located under the smokebox.

temporary aluminium axle in Burrell

The rear axle bearings are phosphor bronze. These fit into an 8mm hole in the tender and each is bolted into place with 3 off 12BA bolts.

A brass tube sits over the bearing as it protrudes through the brass sideplate and bridges the space inside the tender. Thus protecting the rotating shaft.

Boiler

boiler endplates

The boiler design is to heat partly from underneath and with through tubes from the firebox. This means I can make a simple copper boiler and fix this into a steel firebox. Firstly I need to learn how to make the boiler endplates.

What is the right angle for the boiler to sit relative to the road surface?

I calculated the efficiency of the engine and boiler so I could work out the water and fuel consumption.

The materials used in boiler construction is important, brass fittings need to be avoided.

Chimney

tapered chimney barrell

The chimney was produced in a number of sections. The lower section has some compound curves and was machined on the lathe and milling machine.

The mid-section of the chimney was made from 3/4″ round mild steel and machined to a thin 1.2mm thick wall at 2.5°, this was quite tricky but rewarding.

Crankshaft

One of the initial design questions was around the clearance between the crankshaft and the top of the firebox, in particular the clearance to the conrod bigend. However, making the crankshaft as two parts that splits at the pin allows the conrod big-end to be smaller. This then allowed me to increase the stroke to 14mm and still have clearance.

The double eccentric was made from one piece of mild steel. This will give me forward and reverse.

Crosshead Guide

crosshead guide

This is based on the Stuart 10V, a great steam engine and a great runner. I machined the crosshead guide from cast iron.

I then worried that it looked a tad heavy and so bolted it to the cylinder to put it into context.

Cylinder

burrell cylinder drawing

The cylinder design has taken some thinking to come up with an idea that I think will work. The basic principle is to make a central saddle that gets soldered to the boiler.

Firebox

firebox silver soldered

The firebox on this small model has been designed to be separate to the boiler. Manufactured from mild steel and silver soldered together.

The boiler is going to be clamped to the firebox.

The firebox door is a simple single hinge and latch design.

Flywheel

flywheel rim detail for the Burrell

The flywheel is just 72mm in diameter. My first attempt to make the flywheel and it came out too heavy on the rim.

Some help from a model engineering colleague and I re-visited the flywheel making the rim lighter. Also, used a chamfer and curved outer surface to created visually a much lighter look.

Mamod vs the Burrell Model

Burrell and Mamod traction engines

I just had to compare this model that I’m making with the Mamod engine. OK, it’s a steam wagon, but still comparable in terms of the front end design.

Nameplate

Burrell Nameplate

The brass plaque on the valve chest cover could have had my name etched onto it.

In the end though I decided on the makers nameplate: Chas Burrell & Sons.

The only small issue is the size of this plate and the writing is just 1.4mm high. A miniature traction engine needs a miniature nameplate.

Plans and Kit of Parts

miniature traction engine drawing

The parts made and bought laying on top of the engine plans.

Could this be a set of plans for sale or a kit of parts?

There are some models that at some point during the build take shape and I end up just staring at them. Pondering the design and dreaming how to solve certain issues with the design. This 1/20th scale Burrell is one of those models. Wonderful, engaging, frustrating and intriguing.

Nigel Taylor [email protected]

Smokebox

smokebox and lower chimney assembled

The smokebox is gradually taking shape with some basic dimensions being sketched out and then I’ve machined the main ring out of mild steel.

The lower chimney is curved and some of the turning was done freehand, with this you need to keep tool overhangs to an absolute minimum. Plus take your time and use very fine cuts. I wear a pair of heavy duty gardening gloves when I’m turning like this to minimise damage to my hands.

brass etching

For the lettering around the door I bought a Genmitsu 3018pro CNC engraver, but after a few upgrades I still reverted to a tried and tested etching technique using Ferric Chloride.

I then found a way to simply do brass etching. Silver soldering the hinge and ensuring it was all aligned was quite scary.

I then started again with the door lettering and etching.

Steam Valve

Their are a number of ideas for the steam valve. Once I had settled on the location I then had to look at the specific design for this valve, for that I needed some inspiration.

Burrell steam valve schematic title

After lots of soul searching as it’s not a scale solution I opted for a remote valve for the Burrell. The space for a scale steam valve machined into the saddle was just too tight.

Steering

I started to make a mould for the steering wheel, this worked out reasonable as a first attempt. Then I thought this would be better just machined from solid brass.

The steering wheel and handle with an 8BA threaded central boss. I can just about operate the wheel with my rather large hands.

Tender

Tender with Edge Beading

The tender and hornplates have been constructed together.

The beading was challenging, but also rewarding as the design moved on a step.

Towbar

towbar on rear of traction engine tender

Made from a length of 1/4 inch square brass with a machined groove. This was then riveted to the tender.

The multiple towing points show that this engine really was for use on the road and maneuvering trailers in the yard. A real workhorse.

Valve Chest

Burrell valve chest

I’ve roughed the Burrell valve chest out of brass. The valve pushrod bearings were made as one part, silvered soldered into the valve chest. The centre part was then removed on the milling machine. This part is quite small as the Burrell is just 1/20th scale. The main valve chest is 22 x 20 x 7mm.

Wheels

The front wheels are just 70mm OD and the rear wheels are 97mm OD. A total of 64 parts and 60 rivets go into making the four wheels.

traction engine wheels
burrell traction engine

The miniature traction engine has not quite made it onto the wheels, but I do now have a Burrell on “Matchboxes”.

This have given me the ability to visualise it more clearly and think carefully about the next parts.

8 thoughts on “Miniature Traction Engine”

    • Hello David, I have some hand drawn plans of the main layout and parts. However, in the meantime I have learnt how to use FreeCAD 0.19 and so I will start pulling together a model of this engine. Thanks for the interest, best regards, Nigel

      Reply
  1. Nice work. I like small models – among other things, negotiations with the domestic manager for workshop space are easier!

    Reply

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