The Lad went to the National Space Centre at Leicester http://www.spacecentre.co.uk. It was a superb exhibition. Go there and take any of your juvenile connections and they will be entranced. Everything there is from the science and space engineering of the late Twentieth and early Twenty-First Centuries. There is hardware ranging in size from a full-size, Russian space-ship and Blue Streak rocket to a tiny lump of genuine Moon rock. The Planetarium at the Centre is a cinema unlike almost any other. Each of its many different programmes is a jaw-dropping cinematic experience that beats 3D any day. Games and hands-on and even ‘body-on’ experiences are everywhere. It is good for any weather too.
This post, though, is not about something in the Space Centre: not this time anyway. The Abbey Pumping Station Museum is next door to the Space Centre. http://www.leicester.gov.uk/your-council-services/lc/leicester-city-museums/museums/abbey-pumping-station
The Museum has much more engineering dating more from the Victorian era rather than the last half of the Twentieth Century. The Lad’s eye was caught this time by an outdoor exhibit. An old digger: it was a gigantic machine. Here it is.
The museum tells us that the sleeping giant is a Ruston-Bucyrus 52B Quarry Shovel built in 1935 as one of the last of this type of excavator built in Britain. It weighs 81 tons and was able to lift a load of 18 tons with its bucket while working until 1967 . (There is a better picture in www.Flicker by the way)
The picture, below left, shows it just before it was retired where it used to work in an Oxfordshire quarry. The standing figures give a scale to the size of the machine as does the lorry in the picture, below right, of another similar digger in its natural habitat, a quarry.
A building to envelope it would be at least as big as a good-size modern house. The cab and machinery enclosure, a large black shed, is the size of the largest room in that house. Extending from one side of the enclosure, pointing out and upwards, is the main boom. Part way along this main boom, and pivoting more or less at right angles is the dipper which is a bucket mounted on the end of its own smaller jib. Like a dinosaur heavily settled down, it is supported by large continuous (caterpillar) tracks.
As the drinking song of the young engineers has it about an entirely different type of machine.
“And the whole [expletive deleted] issue was driven by steam.”
This song, by the way, was already of ancient pedigree when The Lad, still nobbut a youth, roared it with his mates. Perhaps they still do. Anyway, let our attention not wander or get distracted by beer.
Being driven by steam, the ‘Steam Navvy’ needed not one but three engines. One drove the tracks to move the giant about the quarry and also to lift the main jib and the bucket. Another had the task of slewing the excavator through 360° on the tracks. The third was mounted at the junction of the jib and dipper to control the dipper bucket.
The image below shows the skeleton and heart of the beast
Now let’s look at one example of a modern successor, pictured below. Such a one is the JCB JS210 being built in England, in 2011, driven by a single, 4 cylinder, 172 hp, diesel engine. It weighs 21 tons and can lift 15 tons (15320 kg) maximum.
What are the fundamental, engineering differences between the dinosaur and a modern tracked shovel such as this JCB machine? The differences lie in the prime mover and the power transmission technology
Machines such as grabs and cranes are more effective and less costly to operate when they have fewer large parts and a simpler modular construction.
For the prime mover, those early steam engines are nests of parts; each part oscillating in different directions whilst, at the same time, sliding and spinning. All this while immersed in a dusty and dirty atmosphere. It must be inefficient with all those moving parts soaking up power quite independent of the power output. In addition all the seals and slides are sweeping up the quarry dust and grit that grinds metal and absorbs more energy in friction.
For the power transmission, only cable systems could be used for large forces over a long distance. With a cable you can only pull; you cannot push. So, where gravity cannot be persuaded to do half your job for you (see the dipper in this machine), the design of the cable system has to extend a long distance to the far side of the load and back to it again: then instead of push, you make do with a pull in both directions.
When we come to the modern excavator, for the prime mover we have the high pressure, high speed modern diesel engine developing great power. Driving only a hydraulic pump, the whole is more easily enclosed to protect against the harsh environment than is possible with the steam engine.
For the modern power transmission, the change is, in some ways, even greater with use of hydraulic rams. Their predecessor was the Bramah press patented in 1795 by Joseph Bramah. Since then, the press has been refined into the simplicity and sophistication of the hydraulic ram.
Their secret is that hydraulic cylinders have a relatively small number of parts and, compared to a steam engine, are simpler to make. Fundamentally, each is just an accurately ground cylinder and a similarly accurate shaft sliding through a single seal in the same directions as the power output. It is so simple and efficient: at least it became simple to make and operationally efficient when the design principles became fully understood. That happened only after engineers had expended extensive development effort, money and time
Its operational advantages stem from the rigidity of the ram. This rigidity ensures that, with a supply of fluid, as required, to both sides of the piston, a ram can both pull AND, without buckling, push. That simple phrase ‘a supply of fluid’, though, conceals a mass of engineering. The ram performs its simple but marvellous feat provided that it is under the influence of the of high pressure hydraulic fluid. That fluid has to be carefully controlled, metered, supplied through high tech flexible hoses or rigid tubing, monitored and guided by valves, driven by a relatively small internal combustion engine and pump both of which can easily be carefully shielded from the ravages of the site atmosphere.
These two major developments in prime mover and power transmission bring the design engineers and operators, finally, high power density, high power to weight ratio and, not least, high thermodynamic efficiency.
Sitting in the sunshine and looking at the old machine at the museum The Lad’s eyes narrowed and another picture emerged. The outline of the old machine faded and he saw a navvy with his shovel: the old machine’s even older predecessor. The dipper of the old machine is the shovel handle and blade of a navvy wielding his shovel. The jib is the man himself leaning into his shovelling task. That was, he imagined, the starting point for the original Victorian-era designer of the digger. Crucially, the navvy usually pushes the shovel AWAY from himself – as does the old machine.
This leads rapidly to another aspect of the design. The jib, dipper and the general support structure of the bucket itself is radically different in the modern machine from the old machine. The modern machine pulls the shovel TOWARDS itself. This, The Lad suspects is the way that the designers of the modern machine achieve its enormous reach arc (see the picture below).
The subject of linkage design is worthy of discussion as it is the meat and drink of many a mechanical engineer designer and of many books in itself. Some practising engineers use this as a resource, http://www.esdu.com/cgi-bin/ps.pl?sess=unlicensed_1110921102952zrc&t=doc&p=di_98023 . Worthy, yes; but too big a subject for this post.
But, just to get an idea, look at this diagram of the astonishing reach arc of the bucket movement of this modern engineering masterpiece.
You will find more detail at http://www.jcb.com/products/MachineProduct.aspx?PID=11&RID=2 .
Finally, The Lad indulged himself in just a little blue-sky musing. Maybe they used a coal fired excavator up until so late as 1967 because coal can be stacked at a quarry in a pile whereas oil fuel needs to be much more carefully handled. Besides, is it possible that the quarry master preferred to support his brother in arms – the coal miner? But, even before the old digger was de-commissioned, it was all changing. Even quarries and mines would never be the same. Far north of that quarry in the moorland and hills of the remotest part of Cumbria, thundered rockets engines on test. Then, high into the same sky arching over that quarry but half a world away in Australia, the rockets thrust the UK’s own Blue Streak ballistic missile.
One of those rockets has also retired standing in the Space Museum not two hundred metres away from the retired digger. Both were potent Engines in their time.
Engineering is one of the three drivers in the advancement of the human race. This blog aims to give to career seekers and also to the general public a taste of how this might be so. They are not well served by the current media. It is an engineer posting: not a ‘scientist’. It describes real professional engineering as it is in the real world usually in the present and occasionally as it was in the recent past.