My last posting, about continuous springy beam chassis raised my friendly disagreement with Will, and simply shows that we each have, and should have, our own approach to all things, but it caused me to think back on my own development which has led to the way I do things.

My industrial apprenticeship years had a great influence on the person, or at least the engineer, that I have become. I served as an apprentice electrician in a large, electrical maintenance team, but unusually for that type of work, I was directed to a professional electrical engineers education, qualifying as long ago as 1959. In those days electrical engineers were offered two qualifying paths: "heavy current" or "light current". The latter was essentially radio and television which in those days worked by thermionic valves; the electronic revolution was just about to start!

So my experience is quite definitely in maintenance and repair of heavy current devises rather than electronic devises. But what matters in my current thesis is that I have a back-ground of keeping things running i.e. maintenance. Because of that, anything that I may design from scratch will always be influenced by my need to make maintenance and repair easy. Also, throughout these informative years I was continually advised to keep things as simple as possible - the old KISS principle.

Another unofficial principle of engineering is Sod's Law, which basically means that if it can go wrong, it will go wrong. Making sure that it can't go wrong may be difficult and may also prove to be too expensive; therefore make sure that your design makes it easy to put right again.

My apprentice ship to railway modelling started even earlier than my electrical one, but it has been continuous and was brushed-up by mentors of proven ability when I joined Manchester MRC in the early 1980s. This is when I came to know that split-axle chassis simply do not have electrical pick-up problems, and that never again would I build a scraper-collector chassis. (I was rubbish at it anyway!!). Split-axle is a little more complex initially but removes a massive maintenance problem for the chassis life time.

I keep notes of my modelling in order to record how I built something, (saves re-invention) and I'm adding below a copy of a page from my "My Locos" file. This was written in the mid 1990s and based on my experiences and experiments over the then last three to four years. It has been up-dated more recently.

Simple perusal shows the influence of my experiences during employment, and careful reading will highlight those points that lead to easier maintenance, but the majority of it comes from my mentors who had developed their ideas over many years starting around the end of the second World War.

1. Split axle construction
2. Outside axles to fix the ride height, with one being the driven axle and not allowed upward or downward movement, and the other being sprung downwards but normally riding at the top of its horn way.
3. Any other driving axles allowed to ride 0.020” higher than the outer drivers but sprung downwards
4. The ability to drop any or all axles without the need to remove the wheels from the axle.
5. Similar rules for the tender chassis; but with weight transfer from tender to loco; and electrical plug and socket connection between loco and tender, to aid pick-up.
6. Easily removable brake rigging.
7. Ability to remove motor and gearbox from chassis but leave all axles in place with normal ability to roll chassis on its wheels.
8. Driving wheel crank pins to be ‘push-fit’ to wheel and normally to be ¹/₁₆” diameter, tapped for a 14BA screw.
9. All driving wheels mounted on their axles using the lathe as an aid to true squareness, ‘quartering’ and pushing to gauge.
10. For all locos with outside valve gear; draw a ‘space’ diagram which shows the dimensional relationships of side frames, cylinder assembly, motion bracket, axle side play, coupling rod, connecting rod and crank pins.

11. Quarter by eye using fine-pen lines on tyre flanges, one side marked in line with crank pin, second side marked in line with a spoke about 90 degrees lagging the crank pin.
12. Springs acting on top of 'top-hat' axle boxes to be approx 20mm cantilever of 27swg phosphor-bronze wire.

For what its worth, some of you may find those to be sensible guidelines for scratch building.

Regular readers who study my posts may have noticed that I treat my own list with flexibility. For example my Q4 (see http://www.rmweb.co....71-q4-progress/)has the drive-chain mounted on a middle axle which is sprung; so my para.2 above is not fully met. The reason is that the motor/gearbox unit could not be housed inside the body if mounted on either outer axles.

Likewise,para.7 is not fully met if one is wordily pedantic! The motor, with the worm gear is easy to remove but the gearbox stays on the axle, however the required end result of roll ability is achieved. The whole unit - motor / gearbox / axle / wheels, - will drop out of the chassis. OK the likelihood of that need may be very low, but Sod's Law will ensure that need if you fail to plan for it!

Dave.