A few years ago, I inherited responsibility for the upkeep of the family field near Corfe Castle, in Dorset. Dorset Wildlife Trust advised us that the most wildlife friendly way of cutting the grass was to mow a few narrow strips every year. Implementing this scheme required a tractor, which we had, my late father’s 1970s vintage David Brown 885, in a corner of the field, under the collapsed remains of a shed built by my grandfather. It hadn’t been used for many years, and although I was able persuade the engine - a three cylinder diesel – to burst into smoky life, the hydraulics which drive the three point linkage to raise and lower the topper (a mower with vertical axis blades) would not operate.
I had only driven the tractor a couple of times before, and I wondered if I just didn’t know how to operate it. I bought a manual, but however I manipulated the controls, the topper remained firmly planted on the ground. I noticed that the hydraulic oil, which is shared with the gearbox, was emulsified with water, so I changed the oil and filter, to no effect. I became fed up with trying to diagnose the problem with the tractor in the brambles under the collapsed shed, and so with the aid of a colleague from Siemens, Dave Edbrooke, and his trailer, the tractor was taken the twelve or so miles to my home and parked at the side of the house, where it stayed for several months while Sally and I went sailing in the Mediterranean.
The three point linkage is one of those apparently simple inventions which are ground breaking, in this case literally so. Before Harry Ferguson’s linkage, ploughs were towed behind the tractor, as they had been for millennia behind animals, and relied principally on their on their own weight and shape, and the operator’s skill to penetrate the ground. The three point linkage uses the weight of the tractor to force the plough to the correct depth, to quickly raise it when the end of the furrow is reached, and to control the downforce on the tractor’s drive wheels.
I couldn’t spend too much time working on the tractor, and had no desire to restore it, only to make it work well enough to mow the field. I downloaded a workshop manual, and began to understand the tractor’s “Selectamatic” linkage actuation system with its three modes, height, constant depth, and traction control, which uses a complex valve arrangement to direct the hydraulic oil according to the dictates of the controls and feedback from the linkage. Luckily, David Brown tractors were very popular, and many are still in existence, leading to the ready availably of spares.
The problem was corrosion in the delicate spool valve, caused by water in the oil which had leaked into the gearbox through perished gear lever gaiters. Removing the valve required considerable dismantling of the mechanical system, but eventually I was able to remove the valve completely. After cleaning and the replacement of corroded parts, the linkage, to my great relief, worked properly, and the proving run to the end of the track at my house was a very good moment.
There was one interesting incident when the repairs were nearly complete. I realised that the rear wheels were installed incorrectly, and I had to remove the rusty wheel nuts, which I did by heating them with the gas until they were cherry red. I was working from home, and I went into the house to do my morning’s work. When I came out, the tractor was gone! After several minutes of wondering what had happened to it, I noticed it at the end of the garden, almost completely hidden in the vegetation.
I hadn’t put it into gear, or applied the handbrake, and the tractor had rolled down the slope of the garden, passing through a partially opened gate, and uncannily missing all the trees and other obstructions in its way.
At first sight, the David Brown 885 is well, agricultural - crude and lacking in sophistication, but as I worked on the machine, I realised that this impression was entirely mistaken. It is heavy and robust, yes, because that is required of a tractor, but everything about the design is tremendously clever, and in my view, it is beautiful in its simplicity. Nothing is there that need not be there, and everything that is there has a use. Machines are “magazines for storing the thoughts of men”, and this machine is a warehouse for the thoughts of engineers, now dead, who must have been very fine thinkers and who produced an ingenious and finely detailed machine. The main frame casting is excellent, the engine is robust, and the hydraulic system for controlling the three point linkage masterful.
I spend all of my working life designing things, using sophisticated tools – 3D CAD, finite element analysis, spreadsheets, and I can look up almost anything I want, immediately. I can even, and frequently do, make 3D prints of concepts. The people who designed the 885 had none of these advantages. They produced 2D drawings with paper and pencil, performed hand or slide rule calculations, and looked up material properties in reference books. Although I can sometimes see how they could have done things more efficiently if they had had modern tools at their disposal, the David Brown 885 is an astonishingly good piece of work, and I am glad that I had the opportunity to appreciate it.
Σχόλια