Just a hunch since I don&#;t know what exact bearings are in the vise, but by separating the load and having the ball bearing handle the radial load and alignment, and the thrust bearing handle the thrust load, each is likely stronger in its given roll than a single taper roller bearing doing both roles would be.
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It&#;s also possible it&#;s cheaper depending on the exact bearings that were specified. Sometimes you can get bearings for a smoking good deal because they are a super common bearing, so that may be a factor. The common and cheap tapered roller bearings often have a shallow taper angle, so they&#;re mainly for radial loads, and a little thrust load, not a lot of thrust load and a little radial load (as it would see in the vise). That could mean an expensive low volume tapered roller bearing VS 2 inexpensive high volume thrust and ball bearings.
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Again, just speculation on my part based on experience and the shock I&#;ve gotten in the past for the cost of some bearings. I doubt @Fireball_Jason chose the bearing setup on a whim, so there&#;s got to be a reason, maybe even the two I suggested.
At the price point that the amazon/ebay thrust bearings come.... I don't split hairs.
That said, the very nature of a needle thrust bearing means that ZERO force will be translated into making the races (washers) rub on a shaft. The force from gravity is all you're gonna get. For the applications that would warrant these inexpensive bearings (which are a lot of applications, don't get me wrong.... I'm not knocking the bearings at all), a drop of oil or grease on the shaft will probably allow it to ride there without ever bothering. The rollers will follow their circle, and provided that they're preloaded just a whisker, they'll self center straight away. That's how they get away with such loose dimensions. The races (washers) need not be anything but flat. The "roundness" is just for convenience. They could be square, so long as the needles never rolled off of them. Centering them is irrelevant, except as you say, the rubbing.
If you're making a thing that's sensitive to that, you've got to pocket one washer, and set the other as a slip fit on the shaft. If it's sensitive enough that it takes a press fit on the shaft to meet any design criteria, it's probably time to not be looking at the discount bearings. (Or custom fit the part to the actual bearing that you've received). I've broken several at work trying to do that. They're in some PTO units on the fleet at work, a couple of trucks have a helical gear to serve as the driver for the PTO, which generates a thrust on the PTO input shaft. Either the ID tolerance is off from advertised (snap....), or the race (washer) is just flat out too hard, (maybe not annealed at all?), and again, snap.... Buying a bearing from the bearing shop across town (for probably six times as much money, or the OEM for the same money) solved the problem. Yeah, if I had a few chances to iterate the project, I'd bet dollars to donuts that I could get the cheap ones to do the job. But again, the application.... Every minute I'm buggering around trying to save a buck, we've got a truck not making money. And every unsuccessful attempt leaves a truck down somewhere. What's your end goal. On the other hand, the five shelf rotating nut and bolt rack... It's a vertical pipe, every shelf gets a shaft collar like thing, with a spring pin in it, and a thrust bearing sits on top of it. Never needed a bearing replaced, but we have had to take it apart and clean the dust out of the bearings about every five or ten years. Those bearings have an eighth of an inch of flop in 'em. The washers are a little heavier than usual, but it's still the same needle thrust bearing. Just toss 'em together, and they don't care where they land. Kinda opposite ends of the spectrum I guess.
How much is time worth? Just how imperative is it that the fixed race not touch the shaft in any way? Those are questions you've got to answer based on your project.
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