Designing for thrust needle roller bearings? - The Hobby-Machinist
Designing for thrust needle roller bearings? - The Hobby-Machinist
Hello!
I have a question about designing for thrust needle roller bearings.
My question relates to the commonly available and inexpensive thrust needle roller bearings like the type you find on Amazon or eBay which consists of a cage holding rollers and two washers:
My question is:
How do the OD and ID of the washers and cage not rub up against, or rattle around between the housing and shaft that is retaining them?
For context, I have also used ball bearing type thrust washers, where the two washers have a groove in them:
In that case, the OD and ID of the cage holding the balls is undersized and won’t touch either the retaining housing or shaft of the part, but instead “float” riding in the grooves of the washers. One of the washers has a larger OD and ID, so it will be retained by the ID of the retaining housing, and one of the washers has a smaller OD and ID so it will be retained by the shaft of the part. The only contact between the retaining shaft and housing is via the balls and there will be no other rubbing or contact.
In the case of the various thrust needle roller bearings I ordered off of Amazon and eBay, the needle cage and washers all have the exact same OD and ID, so wouldn’t that mean that those parts will rub against the shaft and housing or rattle around if there is clearance?
I could imagine machining some precise steps into the retaining shaft and housing of the parts, but I feel like I haven’t seen that in the thrust needle roller bearings applications in the wild. 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.
I have a question about designing for thrust needle roller bearings.
My question relates to the commonly available and inexpensive thrust needle roller bearings like the type you find on Amazon or eBay which consists of a cage holding rollers and two washers:
My question is:
How do the OD and ID of the washers and cage not rub up against, or rattle around between the housing and shaft that is retaining them?
For context, I have also used ball bearing type thrust washers, where the two washers have a groove in them:
In that case, the OD and ID of the cage holding the balls is undersized and won’t touch either the retaining housing or shaft of the part, but instead “float” riding in the grooves of the washers. One of the washers has a larger OD and ID, so it will be retained by the ID of the retaining housing, and one of the washers has a smaller OD and ID so it will be retained by the shaft of the part. The only contact between the retaining shaft and housing is via the balls and there will be no other rubbing or contact.
In the case of the various thrust needle roller bearings I ordered off of Amazon and eBay, the needle cage and washers all have the exact same OD and ID, so wouldn’t that mean that those parts will rub against the shaft and housing or rattle around if there is clearance?
I could imagine machining some precise steps into the retaining shaft and housing of the parts, but I feel like I haven’t seen that in the thrust needle roller bearings applications in the wild. 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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