The TME Bogie

Stephen Gosling

Bogie Side Elevation
The Taunton Model Engineers have, after many years of searching found a site suitable for a ground level 5" and 7¼" gauge railway. This will eventually take the form of a bent dog bone with a spur to the station and steaming areas and a raised 5" and 3½ track inside one loop. Phase 1, the station, steaming bays and first loop are now well on their way in the hands of the Site Works Gang and the Track Gang. Once this railway is complete, of course, the Club will need some stock and that is where I come in.
Whilst the TME is my club, I live in Leicestershire, nearly 200 miles away, and this seriously curtails my active involvement. In order to do my bit, I have therefore undertaken to run the Rolling Stock Gang.
During 2003, I designed and made a pair of 7¼" gauge bogies of the arch-bar type and the club very kindly made me an award at the annual Trophy Night. It was then suggested that I might like to draw them up properly in order that some could be made up for our new railway! After considerable further thought and discussion, I have now completed the design and detailing of an arch-bar type bogie specifically for the Taunton Model Engineers.

As I see it, the club bogies need to meet a number of criteria:

1. Cheap!

2. Easy to mass produce by a variety of people.

3. Parts to be of limited size for simple manufacture on modest equipment.

4. Parts to be as few and as simple as possible.

5. Vacuum braked.

6. Must ride well and comfortably, both laden and unladen.

7. Low maintenance (Tolerant of neglect!).

8. Must look like a "proper job" and not a hurried lash-up.

The solution I have identified is as shown on the drawing. The arch-bar arrangement avoids having to make special patterns and utilises the cheapest material available to us which is steel strip bought in larger quantities. It also has the advantage that it allows the axleboxes to be manufactured individually from stock bar on smaller equipment, such as my Myford. I have specified a sealed-for-life self-aligning ball race for each axlebox but the largest I can accommodate looks very small. In order to convince myself that they will be OK, I have worked out the L10 life (the length of life by which time it is expected that 10% of a large group will have failed) and came up with a number of rotations equivalent to 26,000 miles which has put my mind at rest. The axleboxes are machined from square aluminium bar and are probably the most complex part of the bogie. I have given them sloping faces purely for cosmetic reasons which I know is extra work but worth doing I feel. The bolster is plain rectangular steel with rubbing blocks bolted on and is pocketed to guide the springs.
Bogie Plan
The suspension is an interesting area in miniature rolling stock design as, typically, there is a huge difference between laden and unladen weights. An empty vehicle could weigh 100lbs but a fully loaded nearer to 700 so keeping the wheels on the track under all conditions does require a bit of thought. The TME bogie has four relatively light springs at each end of the bolster and rides on these with up to one adult on board. Once this load is exceeded, the springs compress enough to allow a fifth, central die spring to come into contact with the bolster and take a share of the load. This spring is very stiff and I do not anticipate that we will get too much movement of the bolster. However, should the vehicle be grossly overloaded there is, within each die spring, a dead stop.

This is the basic structure of the bogies. I have gone to some lengths to try to get the number of parts down. For example, on my original bogies, the springs were located by buttons riveted to the bolster giving the bolster assembly a parts count of 24. Now they are located in pockets which reduce the number to 8. The axleboxes were located by studs with nuts on both ends. That was eight studs and sixteen nuts on each bogie. Now we have standard bolts so there are eight nuts and eight bolts and I don't have to make the studs.

The brakes have been a bit of a challenge. I have seen an arrangement on bogies in use in the US which operates by a wedging action (see drawing). It is compact and simple and so I have adopted it. Again, reliability, simplicity of components and the ability to accommodate only a limited amount of maintenance have been the guiding factors. Initial thoughts were given to mounting a brake unit on the bolster in such a way that it could be simply removed for maintenance leaving the bogie in traffic. However, if the blocks are hung from the bolster, the distance that they must move before coming into contact with the tread is continually altering with the movement of the springs. As I planned to use a diaphragm type cylinder with very limited stroke, this is a major issue as the performance will change substantially depending on the load and the driver will have great difficulty in braking consistently. I have therefore, hung the brakes from the bolster on some simple pins turned from square bar. The blocks can now be set up just a few thou from the tread in the knowledge that they will not bind or fall back so far that they do not work.

With the completion of the design we have moved to the manufacturing phase and I am delighted to say that two of our members have stepped forward to make the bolsters and all the turned parts. Our Chairman was very fortunate to be able to acquire twelve pairs of wheels and axles from the Great Cockcrow railway where they had been deemed to have been worn out. These have been re-profiled and five new axles have been made making them ideal for our purposes. The axleboxes have been machined from square aluminium bar but I have downgraded the bearings from self-aligning ball races to self- aligning sintered bronze as a cost cutting exercise. The cost was reduced from £9 to £3 each by this measure. Unfortunately, we will have to make sure that they are oiled every running day now and I expect that their lives will be shorter as a result. However, the ball races could be fitted in the future by boring out the axleboxes and skimming the axles to suit without much difficulty.

When I made my original bogies, I bent the steel side frames in a jig in the vice and was successful. However, it took ages to get the bends to the right angle and all the same and I came to the conclusion that it would be worth making up some special tooling which would go in my 10 ton hydraulic press. This has been done and side frame strips can be pressed out quite quickly and repeatably now. I must admit though, that I was surprised to see that the press has to be taken right up to 10 tons to get a decent bend.

For the brake gear, patterns have been made for the blocks and another press tool has been made to produce the vacuum cylinders from stainless steel sheet. Stainless is a bit 'over-the-top' but I happened to have a sheet in stock! All of the rest of the brake gear is either turned from bar or pressed from strip. For the brake fittings, I have bought some aquarium aeration check valves and tees. These are very cheap but the stop valves intended for use as the vacuum release valves have proven too flimsy and I have ordered some plumbing stop-taps from Screwfix for £1.15 each. I find this an amazing price as I couldn't make them myself for that. Brake reservoirs are going to be empty disposable welding gas cylinders which, I must admit, will take a while to blow off but they came FoC which is a great point in their favour.

Six bogies have now been completed and the project has moved forward to the vehicle chassis and body stage. The vehicles themselves are to be finished ready for the official opening at Easter 2006.

Progress on the railway is steady but of course, expensive. To try to assist in the area of fund raising, I have given the bogie design to the Club and the drawings (37 A4 sheets) are now for sale at a price of £15 per set. If you are interested, either please give me a ring on 01455 823678 or Email.

Visitor will are most welcome to come and visit us and we look forward to seeing you and being able to demonstrate the TME Bogie!