Newer Isn't Always Better
Written: 2000.07.10
Last Revised: 2001.04.30
Magnetic bearings vs contact bearings
If you've heard of magnetic bearings, you'll know how cool they are. You'll also know that they're a perfect example for the SW vs ST debate. Unlike conventional ball bearings or fluid bearings, a magnetic bearing uses forcefields to suspend and control the rotor inside the stator. Since the system is inherently unstable (a common failing of forcefield-dependent technologies), a computerized control system must constantly monitor the shaft position and adjust it thousands of times per second. It does this by adjusting the current flowing through a series of electromagnets arrayed around the shaft. It's far more advanced than primitive contact bearings such as ball or roller bearings, so contact bearings will become obsolete, right? Wrong. It all depends on the application:
Magnetic bearings have ultra-low friction, which permits very high rotational speeds.
Magnetic bearings have ultra-low wear, which permits long service lifetimes.
Magnetic bearings don't need lubrication, so they can be used in low-contamination environments.
Magnetic bearings have built-in health monitoring, thanks to the computerized control system.
Magnetic bearings improve thermal isolation of the rotor, because of the lack of direct contact.
But ...
Magnetic bearings have inferior load bearing density, so they're always bulkier than contact bearings for a given load.
Magnetic bearings are terrible at handling transient loads, so they're useless for dynamically loaded systems.
Magnetic bearings are expensive to purchase and install. Normal bearings don't require dedicated computer control systems, power amplifier networks, or emergency crash-landing systems in the event of power failures.
Magnetic bearings are less robust. While a contact bearing can be damaged but still marginally functional, a magnetic bearing either works, or it doesn't. There is no gradual degradation or robustness; it's simply alive one second, and dead the next. In something like a military vehicle, there would have to be a very good reason to adopt it in spite of this drawback.
So they're newer, they're more advanced, and they're definitely reminiscent of the sort of thing you would see on Star Trek. But while a Star Trek writer will always replace contact equipment with forcefield equipment, real engineers aren't so foolish. The vast majority of the bearing systems in the world are still contact bearings, even though magnetic bearing technology is already quite mature. Newer isn't necessarily better. Real engineers tend not to throw out old technologies in favour of new ones, but rather, they tend to add new technologies to their growing inventory of tools.
Aluminum vs Steel
Steel has been in use far longer as an engineering material than aluminum, due to the relative ease with which it can be mined, refined, and processed. Aluminum was developed for industrial use more recently than steel. It's used heavily in high-tech applications such as aircraft and race cars. This means aluminum is simply better than steel, right? Wrong. It all depends on the application:
Aluminum alloys resist corrosion better than steels.
Aluminum alloys have a higher strength to weight ratio than steels.
But ...
Aluminum has no "lower fatigue cycle limit", unlike steel. The lower fatigue cycle limit is the lower limit beneath which cyclic loading won't cause fatigue cracks growth. So what does that mean? It means that you can't design an aluminum structure to prevent fatigue cracks; you can only estimate the rate at which those cracks will form and grow. In other words, an aluminum aircraft fuselage is bound to suffer a catastrophic failure sooner or later; it's only a matter of time. But before you throw away those frequent-flyer points, I should point out that aircraft designers deal with this by specifying a maximum lifetime for their planes. Sure, fatigue cracks are inevitable in aircraft, but if you take the plane out of service soon enough, disasters are unlikely. However, some cut-rate airlines operate planes beyond their service lifetimes, and speaking as one who once specialized in the study of fracture mechanics, I would avoid such airlines like the plague. Ideally, you want your plane to be so new that it still has that "new plane smell" :)
Aluminum may be stronger pound for pound, but not inch for inch. For any given size of structural beam, a steel beam will be much stronger than an aluminum beam.
Aluminum has a high thermal expansion coefficient, meaning that its density changes dramatically with temperature. You can't always build functional machinery or safe bridges out of materials which shrink or expand too much with changing temperatures.
Aluminum has low surface hardness. This means it will suffer rapid erosion in high-wear conditions.
Aluminum is much more flammable than steel, as the US and British navies both learned the hard way. A 1977 fire in the British Amazon and a 1975 collision between the American John F. Kennedy and Belknap demonstrated the problem: aluminum catches fire easily, burns hot, and is difficult to extinguish. Massive damage to the Belknap and the Amazon (which would not have occurred with steel) led to the widespread abandonment of aluminum in warships.
Other non-ferrous alloys such as magnesium and titanium-based alloys also have a combination of strengths and weaknesses, making them good for some applications and bad for others. As with the previous example about bearings, the history of materials science has been that new materials supplement existing ones in our engineering inventory, rather than replacing them. I have often been criticized for being too hard on people who don't know these things, but you don't need to be a materials science expert to know this. You just need to think!
We still use cast iron heavily in spite of all of the more "advanced" materials available to us; don't people ever wonder why? "They only use it because it's cheaper", some might object, but leaving aside the fact that the situation isn't quite that simple, I would point out that cost efficiency is a legitimate engineering parameter, just as legitimate as any other parameter such as strength or weight. Cost concerns won't disappear as productivity increases with new technology; no matter how advanced you are, some things will always be easier to make and process than others, so you will always face the choice.
There are myriad parameters influencing an engineer's choice of materials, so it's not as simple as a Star Trek writer or a wet-behind-the-ears high school kid might think. There is no universal "best" material, and newer isn't always better.
Microwave heating vs convection heating
Sure, microwave technology is newer. It's more efficient. But if you think a microwaved T-bone steak tastes better than a barbecued T-bone steak, you must have lost all of your taste buds in a childhood accident. OK, I admit, this example is a bit facetious. But I do love barbecued food, so I couldn't help it.
In any case, there is no universal "best" way to heat things. Microwaves are quite good at what they do. Barbecues are quite good at what they do. Yet again, we didn't obsolete an older technology in favour of a new one; we simply augmented our technology library.
Nuclear weapons vs chemical explosives
Obviously, nuclear weapons are superior to chemical explosives, right? They're far more powerful. But you don't have to be a nuclear physicist to know that they're not exactly ideal for every situation. Only an idiot would replace every conventional explosive in the world with nuclear weapons, because nukes tend to make things radioactive. They also can't be built in arbitrary yields; there is a minimum yield defined by the minimum critical mass required to produce a nuclear fission chain reaction. This makes them extremely difficult to use in most situations.
Newer isn't always better. Yet again, we didn't replace an old technology with a new one; we added the new technology to our inventory.
Aircraft vs naval vessels and ground vehicles
The first major vehicle was the wagon. Second came the boat. Both were repeatedly improved and refined over the millenia, until they were both superseded by the newest vehicle: the aircraft. Wait a minute ... that's not right. We still use cars, trucks, and ships, don't we? Is this because we're too primitive to wholeheartedly embrace the airplane? Surely not even the most devoted technology-caste advocate would try to sell that one. Boats and cars have some obvious advantages over aircraft, which also have advantages of their own. It all depends on the application. You design to suit your purpose, not to deliberately use a particular type of technology.
Yet again, we didn't replace land and sea vehicles with aircraft; we added aircraft to our inventory of vehicle types. Newer isn't always better.
Polyester vs cotton
'Nuff said :)
Acknowledgements
Lee Atkins and Simon Addison, for pointing out that an earlier connection between the HMS Sheffield and aluminum fires was actually a popular misconception.
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