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Computer fan bearings

When I first got into messing with computer hardware, the received wisdom as regards fan bearings, for cooling fans on computers, was that there were two types, ball bearings and sleeve bearings, and that the tradeoffs were that ball bearings were noisier, but that sleeve bearings tended were less reliable, and tended to fail silently, likely letting the device they were cooling overheat and fail. Ball bearings get a lot noisier before they fail, and were thus the recommended solution for most purposes.

But these days, there are a variety of names for fan bearings. In Newegg’s list, today, of 120mm fans for sale, the various bearing types are described as follows (with each bearing type followed by the number of fan models that contain it):

  • Sleeve (43)
  • Ball (15)
  • 2 Ball (19)
  • 1 Ball, 1 Sleeve (2)
  • Fluid Dynamic (17)
  • Hydraulic (1)
  • Hydro Wave (7)
  • Nanoflux Bearing (NFB) (4)
  • Ever Lubricate (11)
  • EverLasting Quiet (1)
  • Rifle (2)
  • SSO (2)
  • Z-AXIS (1)

Besides ball and sleeve, the principal alternative in that list is “fluid dynamic”. To computer people, fluid dynamic bearings have a high reputation, as being the thing that replaced ball bearings in hard drive spindles, making them a lot quieter. Hard drives no longer make an annoying whine just from spinning, like they did prior to about five to ten years ago (depending on manufacturer).

I disassembled a fluid dynamic bearing from a failed Seagate drive, to see how it worked. (The drive had failed with a head crash; the bearing was still fine.) Disassembling it required grinding, because it appeared to have been welded together (with a tiny, exquisite weld). Revealed was the following (click on the image to see a 4x larger version):

Photo of Seagate fluid dynamic bearing

The main shaft of this bearing is an ordinary plain bearing (aka sleeve bearing): a cylindrical shaft rotating inside a cylindrical enclosure, separated by oil. Nothing special needs to be done to get the oil evenly-enough distributed to separate the two parts, since the shaft naturally drags the oil around with it. The trickery comes at the end of the shaft, where there is a bronze ring shrink-fit on to the shaft, to handle thrust (that is, loads coming from one end of the shaft or the other). This thrust bearing would, in the normal course of things, not have any sort of principle that would restore fluid to the interface; so the bronze ring would touch the steel enclosure. Although bronze and steel are a good combination for bearings, which gives relatively low friction and wear, still, spinning 24 hours a day, they’d wear out quickly if touching. To prevent this, the designers of this bearing have added a special pattern of grooves to the steel surfaces that would contact the bronze, as is visible in the photo; these re-direct fluid that would slip off an edge of the interface back into the middle of it. That way, the thrust surfaces touch each other only on startup of the hard drive spindle, a rare occasion and one during which it is not spinning particularly fast.

But the chances that anyone will ship such a beautiful piece of machinery inside an ordinary computer fan are pretty slim. Indeed, the computer fan bearings which I’ve taken apart, and which have been described as “fluid dynamic bearings”, operate on an entirely different principle. The shaft is the same sort of thing: a sleeve bearing. But the thrust is taken up differently. The following diagram, from a Scythe brand fan (which Scythe describes as having a fluid dynamic bearing made by Sony), is a good example:

Diagram of Scythe fan

Most of those parts are about the same as they would be on a sleeve bearing fan. The fan is held in by a plastic split washer that fits into a groove on the bottom of the fan spindle, as in an ordinary sleeve bearing fan. The porous bronze sleeve, filled with oil, is also usual in sleeve bearing fans. The difference is the “rotor suction magnet”, which takes the thrust load off the plastic split washer. The way computer fans are arranged, the force produced by the wind from the fan is trying to lift off the top of the bearing, on which the fan blades (not shown) are mounted. The magnet overcomes this force, replacing it with a force in the opposite direction, which gets taken on the bottom end of the shaft.

I can think of a couple of reasons why this might be better. One is that the bottom end of the shaft has a larger surface area than the groove which holds the plastic split ring, and so can handle the thrust force better. The flimsy plastic split ring also will bend a bit, likely making the surface area on which the thrust is taken even smaller. Another reason is that the magnet’s strength might be chosen so as to exactly counterbalance the wind force — although the wind force depends on a lot of things, including supply voltage and air pressures, and thus could never be exactly counterbalanced. In any case, the reason isn’t that the bottom end of the shaft sports any particular cleverness; when I took one of these bearings apart, there was nothing like the sort of oil flow channeling that the Seagate bearing had.

But whatever the reason, a lot of companies make such fans, using different names. Of the above fan bearing names, besides “Fluid Dynamic”, the “Nanoflux Bearing” and likely the “Ever Lubricate” bearings use this principle of having a magnet to take up the thrust force. In some designs, the magnet is put below the bottom of the shaft, to magnetically attract the steel end of the shaft. It is thus also sometimes called a “magnetic bearing”, a term which suggests the sort of ultra-expensive magnetic levitation bearing that Iraq was once trying to get hold of for their gas centrifuges for uranium. Such is marketing. As for what the generic name for such devices should be, I suggest “thrust magnet bearing”; it’s reasonably terse, and sort of conveys what the thing is. It won’t wildly excite marketing people, but I don’t think it’ll make them wince, either.

In other fans, ordinary sleeve bearings are described as “fluid dynamic bearings” — which in a sense they are, since sleeve bearings do involve fluid dynamics. The “Hydro Wave” bearing that I took apart was an ordinary sleeve bearing. This seems misleading, but not necessarily in any serious way: on the forum at silentpcreview.com, there seems to be a consensus that sleeve bearings are better than was traditionally thought. My guess is that this is because the denizens of that forum tend to operate their fans at low speeds, where there isn’t much thrust force. Also, even without any additional magnets, the magnetic field loop that is used to turn the fan provides a restoring force against thrust. In some sleeve bearing fans, the fan hub can be pulled out a few millimeters against that force before one hits the split washer that retains it. In those fans, especially in low-speed ones, adding thrust magnets is likely superfluous.

As for “rifle bearings”, the term is strange enough that I’ve ordered a couple of fans with them to see what they are; but one (marketed as an “air rifle bearing”) was just an ordinary sleeve bearing fan, and the other just a magnetically-counterbalanced bearing. The name suggests that either the shaft or its bearing would be rifled, but I don’t see what the point of doing either would be; it could pump all the oil out one end of the bearing, but that doesn’t seem sensible.

That pretty much exhausts Newegg’s list of names, although a couple of oddballs are left. Of course, as I hope was apparent, this article is not intended to be authoritative or up to date; that would be actual work and would cost actual money. It is just the result of having occasionally ripped apart a fan or two, over the years.


Against state pension funds

For all the talk, these days, of the problems that state pension funds are getting into, I haven’t seen anyone argue against their existence. But the case against them is simple and strong.

To define what is being argued against: state pension funds pay the pensions of retired employees of the state government. Without pension funds, states would be paying these pensions directly out of tax revenues. With pension funds, the government plays the markets, investing tax revenues in stocks, bonds, and such, and then later selling them and using the proceeds to pay pensions to retirees.

If you were to ask anyone of pretty much any ideological stripe whether it’d be a good idea for the government to play the market in the service of any other obligation, he’d likely ask whether you were crazy. The idea that, for instance, maintaining roads should be done by investing money in the stock market, then using the dividends to do the actual road maintenance, would be laughed at — and not just by small-government advocates who doubted the government’s ability to choose winners in the stock market; socialists, from their point of view, might question why you were giving money to the capitalists on Wall Street in the first place, and whether you really could have any hope of getting it back from those lying pigs. But somehow for pensions the political situation in the US is the opposite: at the state and local level (but at least mostly, not at the federal level), pension funds are taken for granted; there is much controversy about some of their details, but generally all parties accept that they should exist. Yet the situation that everyone would laugh at and the situation that is generally accepted are really one and the same: when state money is sent to Wall Street, the official reasons why it is sent make little difference; all that really matters is the amount and the timing. Whether the name on the account be “pensions” or “roads”, the funds used for investing come out of the same pot of money and the proceeds go into the same pot.

Plenty of private companies have pension funds; so it’s easy to think states should, too, especially in this era of much talk about how government should try to imitate the private sector. But for private companies, there is a potent rationale for pension funds: companies often fail; a pension fund is a way to promise that pensions will be safe even if the company ceases to exist. States don’t cease to exist, except via war or troubles that verge on war; and when a state disappears via such events, its pension funds are extremely unlikely to survive the tumult.

The biggest attraction of state pension funds has no doubt been the extravagant promises they make, as to returns. I’ve seen in several sources (Michael Lewis’s recent article on California’s financial troubles being one) that state pension funds generally expect returns of about 8% per year. To illustrate the impact of this, suppose that any given piece of money spends about twenty years in the pension fund. That is the length of a short government career, and also a common length of time spent in retirement, and thus is a reasonable figure for the average interval of time between when a pension obligation is incurred by employing someone, and when that obligation finally comes due and the money is withdrawn from the fund to cover it. Twenty years’ compound interest, at 8%, multiplies the initial amount of money by a factor of 4.6; or if we figure that the 8% is just in nominal dollars, and subtract 2% to adjust for inflation, the multiplying factor is 3.2. So by assuming that 8% yield, they can justify much larger pensions than could be justified if pensions were to be paid directly out of tax revenues: in particular, the pensions can be around three times larger. A modest pension of $20,000 a year can turn into $60,000.

When the market fails to deliver that 8% increase, the result is what many states have now: an “underfunded” pension plan, where even when an 8% return is assumed for the future, the fund won’t be able to meet its obligations. The conventional way of regarding this is to be horrified at it, as a harbinger of state bankruptcy. But if one regards state pensions as things that should just be paid out of tax revenues, without any resorting to Wall Street to amplify money, then the pension fund is a nice big fat asset, and the only thing its “underfunding” is a harbinger of, is a switch to a system of accounting where future pension obligations are not counted as present-day liabilities. There would be nothing dishonest about such a switch; other future obligations, such as schools and roads that will predictably need repair, are not counted as present-day liabilities. As for the promises made, both as regards returns the pension fund would make, and as regards the size of the eventual pensions that would be paid to retired state employees, those were always just fantasies that could never be delivered for long. (For how fantastic some of those pensions have gotten, see this article, as well as Michael Lewis’s above-linked article.)

If an attempt were made to reduce pensions, lawsuits would no doubt be filed; promised pensions have a certain legal standing, as contractual obligations. But it’s not enough of a standing to give them absolute priority over the basic rule of elected government that no legislature can bind its successors. To force a state government to pay pensions that bankrupted the state would be an especially bad violation of that rule. Of course there is never any guarantee that judges will see it that way, especially if the bankruptcy is several years in the future. Still, any judge who tried to enforce payment of every dollar promised would, sooner or later, run into all the usual difficulties of getting blood from a stone. Would he force taxes to be raised? Which taxes? Force cuts in other spending? Which spending? Legislatures don’t have an easy time deciding such things; and judges would find it even harder, especially with the public screaming at them for usurping the legislature’s proper role.

Indeed, to some extent, my whole argument here is merely a justification for what inevitably will be done anyway, barring economic miracles. There is little political will for levying the huge tax increases that would be necessary to restore pension funds to being fully funded, and no short-term downside to leaving them underfunded; simple neglect and inertia would leave them underfunded until they ran out completely, at which point the only things to be done would be to fire the staff administering their investments, and adjust the size of pensions to whatever could be borne out of tax revenue. But to accept that this was actually the goal, rather than just drifting along in that direction, would open up other possibilities. For one thing, the assets in the pension fund could be sold to wipe out other debts of state government, so that the government was no longer, in effect, borrowing money and using it to play the market with. For another, the pension fund administrators could stop trying for unrealistically high returns (something which David Goldman has blamed for their recent losses in mortgage-based investments). Also, the sizes of pensions paid out could be adjusted before the final crunch actually hit; the transition could be a smooth one, rather than an abrupt emergency measure.

Thus far, I’ve focused on the effects of pension funds on government finances; but that’s not all, and likely not even the most important part. When pension funds buy corporate stocks, they get an ownership interest in those companies. They can vote in corporate elections; and they control such large blocks of stock that their votes carry serious weight. Even if they were to abstain from voting, their large purchases have big effects on companies’ stock prices, and thus on how easily those companies can raise more capital. Bond purchases, too, affect what companies do: in many cases, if bonds can’t be floated for a proposed venture, it won’t be done. So for the government to own large quantities of stocks and bonds is a big step towards Marx’s dream of the “workers” (via the government) owning “the means of production”. Not that a Marxist conspiracy to take over the economy is even vaguely possible: today’s Marxists are not intelligent enough to put together a decent conspiracy. Petty corruption is more of a danger, as are politicized investments. But although pension fund scandals and politicization of investments have often made the news, in the grand scheme of things they are minor and occasional problems; the big problem is the everyday mediocrity of the oversight that government pension funds apply to their investments. I have made no particular study of the quality of that oversight; but unless state governments miraculously do it much better than they do everything else, state pension funds must be a large contributor to what might be called the Dilbert-ification of corporate America, in which companies are taken over by people who chase after management fads, while the people who can actually do useful work struggle with silly orders from above, trying to construe them into something sensible. The cartoon of course exaggerates; but the phenomena it mocks are quite common, and a tremendous problem.

Most of what has been said above applies not only to state pension funds but also to those of local governments. The exception is that local governments sometimes do cease to exist: there are plenty of ghost mining towns out West, whose population evaporated when the mine closed. In such a case, just as the mining company may want to promise pensions which will survive the closure of the mine, so may the town government want to promise pensions which will survive the abandoment of the town. But for that, explicit measures would be needed to put the pension fund in some hands that would administer it honestly after the town was defunct as a political entity — a difficult enough proposition that giving control to the payees themselves, via 401(k) plans or the like, is likely better than establishing any sort of collective pension fund. (Not that corporate pension funds are immune from getting hijacked as the company fails; far from it. But politics has a nastiness all of its own.)

There may even be a few cases like this at the state level, where it might be forseen that, due to some economic factor, the population and tax base will diminish drastically. The oil boom in North Dakota might be one such factor: at some point that oil will be exhausted, and people will leave. In such rare exceptions, state pension funds might be justified. Such a justification would, of course, involve a very different attitude from the sort of giddy optimism that assumes that an 8% return will always be available. Also, for the justification to work, the decline would have to be local rather than general; in a general decline, good investments are no more common elsewhere than they are locally — so instead of trying to pick global winners in the market (and distorting it in the process), the government can take the easier and more certain approach of just letting the local winners emerge, and taxing them. In a decline that was national but not worldwide, investments in a foreign country which still had a growing economy might seem attractive — but the catch is that that country might decide, with the newfound power that economic growth brings, that it didn’t care to pay back the money.

In any case, even considering pension funds as an evil, they’re one we’re stuck with for a while, since arguments like this never prevail quickly. Even when everyone with good sense agrees immediately, that still leaves the majority unconvinced. Even if by some miracle this argument did prevail quickly, selling off pension funds’ investments would best be done slowly, so as not to unduly depress the markets and make the sale yield less than it should. And that scenario isn’t so different from what is happening today, since when a pension fund is “underfunded”, it uses up its capital at an increasing rate. Even as regards the effects of pension funds’ oversight of corporate America, that has been a slow process, and can’t be reversed quickly. Good oversight doesn’t magically appear when lousy oversight is destroyed, but rather takes time to build. For the moment, the hope and the threat of it will have to do.

Update: Alexander Volokh, a law professor, has written a nice overview of the legal rules surrounding pension funds. It falls short of considering what might happen when things really get bad, but that’s sort of inherent in legal analyses: they cover precedents (court rulings), not situations that are unprecedented.


Entropy is not chaos

Mediocre physics teachers who are trying to explain the concept of entropy often say that entropy is a sort of measure of chaos, with increases in entropy meaning increased chaos. I found that claim confusing from the first time I heard it; once I got a grip on the concept of entropy, I realized that it’s simply false: entropy has little to do with chaos. Consider, for instance, a bucket into which different-color paints have been slopped, forming a chaotic mess of colors. That mess has less entropy than it will after you mix it to a orderly uniform color, which is the opposite of the way the entropy-means-chaos idea would have it. Likewise, a room filled with a chaotic mixture of air at different temperatures has less entropy than it will after the temperatures all equilibrate to the same value. Or take a situation in which you have two cylinders, one filled with air and the other evacuated, and connected by a pipe with a valve. Once you open the valve, half the air will rush from the full cylinder to the empty; this will increase the entropy. But which situation is more chaotic than the other? Relative to the everyday meaning of chaos, it’d be hard to say.

As for what entropy is, if it’s not chaos — well, as with other things in physics, a definition could be given simply enough, but wouldn’t mean much to anyone who didn’t already know how to put it in context. (“The logarithm of what?”) The concept takes a lot of understanding; I didn’t really get a grip on it until I spent a lot of quality time with Enrico Fermi’s book Thermodynamics. That book explains it probably as simply as it can be explained, but it’s still not easy.

It’s a worthwhile concept, though. One can get the impression from casual physics talk that entropy is only good for making gloomy statements about the heat death of the universe, and how everything is doomed to run down and deteriorate. (Or in the above case, how it’s easier to mix paints than to unmix them.) There is that aspect of it, but entropy is also a practical tool. Using it it one can, for instance, derive the Clausius-Clapeyron equation, which relates the vapor pressure of a liquid to its heat of vaporization. Or one can use it to calculate the exhaust velocity of a rocket engine, under the assumption of shifting equilibrium.

While on the subject of chaos, it’s also worth mentioning that the “chaos” defined in the branch of mathematics known as “chaos theory” also isn’t chaos in the usual sense of the English language. In chaos theory, water dripping from a faucet is a “chaotic process”. That’s because the exact size of each drip and the exact interval between drips is hard to predict, even though to the eye it looks like a steady drip, drip, drip, and though the average person would say you were nuts to call it chaotic. This has rendered scientific papers a bit more difficult to read, since it can be hard to tell whether “chaotic” is meant in the ordinary sense or in the chaos-theory sense. Unlike in the case of entropy, I have difficulty labeling this technical concept of “chaotic” worthwhile, since I’ve never encountered anyone making any practical use of it, and since I don’t know why labeling something “chaotic” would help with anything: you couldn’t predict it precisely before, and you still can’t predict it precisely.


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