Class 08 Project

Cheltenham Society
of Model Engineers

Next Public Afternoon
(1330 - 1600hrs)

Class 08 Project

This was my first project in 7.25"G. Until then I had always been an 'elevated track' buff. It was only after running my 'Western' diesel on the ground (so to speak) at a Hereford SME rally that I really appreciated what fun could be had with 'real railroading'. I was hooked and wanted to have my own 7.25"G locomotive as soon as possible.

At the 2002 Southern Federation Rally at Hereford, I had seen a Class 08 Diesel that was owned by a member of the Bridgend SME, and this had really taken my fancy. It could be moved around in the back of a car, plenty of power and should not take too long to construct. It was based on the model sold by Compass House Engineering with modifications to its basic appearance. I decided to follow a similar path.

'Compass House' primarily sell the Class 08 as a 'ready to run' model but they will sell the individual castings/parts (at least they did when I bought mine). However, they do not supply any drawings.

I looked at their list of parts and decided to buy from them the following:

Everything else was to be made from raw material with dimensions taken from photographs and/or the real thing. The 7.25"Gauge Society handbook was also consulted and this gave me basic gauge dimensions and buffer height etc. Drawings would be produced of all the parts made (that was the intention anyway).

I collected all the purchased parts from 'Compass House' and at the same time took the opportunity to see a finished model and measure the wheels for diameter/thickness etc. It was obvious from the finished model that it is made as a production 'ready-to-run' model. The frames were 'spot welded' to the underside of the top plate and the wheels drilled and pinned to the axle with a pin through the wheel boss and axle. All of this made for quick production but poor maintainability.

The purchased axles had a wheel 'back-to-back' dimension of 6.750" whereas the 7.25"Gauge Society recommend a dimension of 6.8125" for standard gauge prototypes. Also, I felt the finish on the part of the axle that was to run in the axle box was very rough. I decided to make new axles with the recommended back-to-back dimension, proper keyways for keying the wheels and a polished finish for running in the axle boxes.

Photographs showed that standard BR Buffer Stocks could be used and that these should be mounted towards the bottom of the buffer beam (this was measured as 3" wide plate from the size at the end of the frames). The 7.25"Gauge Society handbook gave the dimension between the buffer heads and the height of the buffers above the rail height. These dimensions allowed the position of the axle box in the horns to be calculated.

The original axles seemed to be a little long. With the dimensions used between the frames, the supplied axlebox material and using 0.5" plate for the cranks, there seemed to be about a 0.375" gap between the outer axlebox face and the cranks. This did not look prototypical, so the length of the axles was reduced by 0.3125" on each end. This gave a gap of about 0.0625" between the outside face of the axlebox and the crank and this looked about right.

Adding a crank pin onto the cranks plus a retainer gave a clearance of about 0.125" from the outside of the motion to the outside of the running board. Again, this looked and seemed about right.

The required suspension springs were deduced from the weight of the locomotive and the required riding height, which was taken from the 7.25"G standards and which stipulates a buffer centre height of 5.25" above the rail. Each part of the locomotive was weighed and listed on a spreadsheet as sprung weight or un-sprung weight. Suitable 85AH batteries were found and the specified weight added into the spreadsheet. Allowances were made for any parts still to be made. The overall 'sprung' weight was estimated as 181lbs (including batteries). This weight was divided by six to give the weight on each spring. Parkside Electronics catalogue defines the lbs force per inch of compression for each of the springs they sell - just the information needed. The nearest springs available were their No 89 and these were set into the dummy spring castings so as to give the loco the correct riding height when the batteries were in place. Rated at 158 lbs/inch each spring would compress 0.191" and still have 0.209" of travel remaining. However, the total travel of the spring was such that if the spring was compressed too much the locomotive wheels would strike the underside of the top plate. Steel 'end caps' were made to protect the aluminium castings and the plastic axle boxes from the ends of the springs. These 'end caps' were also designed to limit the amount of spring compression so that the top and bottom 'end caps' came together and limited movement. The movement was such that the spring compression was stopped when the wheels were about 0.125" from the top plate.