Let me just jump right into this with the key question:
Why do “wheelchair” bearings from durable medical equipment (DME) suppliers cost six to ten times as much as seemingly-identical “generic” bearings sold on Amazon?
I don’t think it’s because they’re better bearings. In fact, I’d bet they are literally the same made-in-China small bearings as you can find on Amazon.
To get a definitive answer to that question, you’d need to know a lot about (e.g.) metallurgy. Not to mention consumer psychology.
Practically speaking, it’s enough to answer a much simpler question:
Are generic made-in-China bearings from Amazon good enough for wheelchair use?
My answer is yes.
In particular, Amazon-purchased generic Chinese steel bearings lasted 21 months in use as caster-wheel bearings on this TiLite chair. That doesn’t seem too bad to me, given that a caster-wheel bearing is open to the elements, located inches off the ground, and so exposed to water, grit, salt, fibers, and who knows what else.
A little post-mortem of those worn-out Amazon bearings taught me some lessons, summarized below:
Source: Amazon.com
Pictured above is an R6-2RS sealed bearing. That stands for inch units size 6 (R6), with two rubber grease seals (2RS). It’s advertised as an “alloy steel” or “chrome steel” bearing, and it’s more than strong and durable enough for use in wheelchairs. These are available on (e.g.) Amazon.com, in a handful of sizes, for less than $1 each. This one is the correct size to fit the caster wheels on this TiLite Aero wheelchair. (All other bearings on this chair are size R8.) YMMV. If your chair uses metric-sized bearings, the size will typically be indicated by a four-digit number instead of the Rx system used by inch-units bearings.
If you change your bearings annually, that’s all you need to know. Use sealed steel bearings of the correct size, from whatever reasonable source. If you can’t find the size of the bearings written down somewhere, you can pull a bearing out of a wheelchair and measure it with calipers. Or, as above, the size may be literally written on the bearing.
Source: Amazon.com.
But, in the gritty, dirty, and sometimes wet environment faced by caster-wheel bearings, you might be better off using the same sealed bearing, but with steel grease seals instead of rubber. That’s pictured above, and in this case goes by the name of R6-ZZ, where ZZ indicates two steel grease seals. These are available for about $1.50 on Amazon. The reason for the upgrade is that the dirt and grit eventually destroy the rim of the rubber grease seal, which then leads to the bearing failing due to a combination of dirt and rust on the inner ball bearings and races. Bearings with steel seals should last longer in that harsh environment.
If you want to kick it up another notch, you can opt for stainless-steel sealed bearings. These are about $8 each on Amazon, but can be found more cheaply elsewhere. The advantage of stainless is that it doesn’t rust. I think that if I lived where the sidewalks and parking lots were routinely salted all winter long, I’d go for stainless.
Near as I can tell, once you know the size of bearing you need, it’s hard to go wrong. In theory, you need “deep groove” bearings, but every bearing I’ve seen for sale, in this size, has been deep groove”. You definitely need “sealed bearings”, because open bearings (without the grease seals on the sides) are slightly thinner and won’t fit correctly. (You’d be hard-pressed to find open bearings in this size anyway.)
Considerable detail follows.
Do wheelchair bearings violate the Law of One Price?
What got me interested in this topic is the big difference in price, for seemingly identical bearings offered for sale on Amazon, or by DME suppliers. (N.B., I’m an economist, and while I am not a wheelchair user, I do routine repairs for a friend who is.)
Below are the real-world prices for identically-sized bearings that I faced the last time I replaced the caster bearings on this TiLite wheelchair. The typical Amazon price is about 80 cents each, the cheapest DME-supplier price I found was $4.50 each.
At first glance, they seem to be a gross violation of the Law of One Price. (For a given product, offered in a given market, competition should drive all transactions to occur near a single price.) How can identical products sell on-line at such vastly different prices?
(To be clear, I don’t mean that a DME supplier had higher prices. I mean that all DME suppliers had higher prices. What you see above is the lowest price I found from any DME supplier. On average, the DME price for these bearings was easily 10x the Amazon price.)
The bearings in this chair appear to be generic, commodity bearings.
Source: Base image from https://uk.rs-online.com/web/p/ball-bearings/2600461
They are “sealed” bearings, meaning that the sides of the bearing are enclosed. In theory, the seals keep dirt and moisture out, and keep the factory-packed grease in.
But, once you cut through all the crap, “sealed bearings” means that they are expected to fail (or be routinely replaced) before the factory-packed grease fails. They are not designed to be serviced in any way, not even re-greased. When they wear out, you replace them.
In effect, these bearings are disposables. They are little disposable self-contained steel bearing units (races, balls, cage, seals, grease) in a package of a precisely-known size and standardized construction.
They are commodity items. They are not unique to wheelchairs. They are tightly-standardized items used in a wide range of applications. You’re supposed to be able to interchange them freely. For a given size, unless you have some exceptional use in mind, more-or-less any sealed bearing offered at that exact size will will probably serve your purpose, to some degree.
In a nutshell, there’s nothing uniquely “wheelchair” about wheelchair bearings. They are just commodity bearings that got used in a wheelchair.
In addition, buying them from Amazon gives you a bit of quality assurance, in that any tendency toward rapid failure of the product will show up as complaints in the user product ratings.
I strongly suspect that these little steel bearing units are all more-or-less the same. Unless you go hunting for something truly exotic. Plus or minus the variations noted in the introduction, in terms of the material used for the grease seals, or the use of expensive stainless steel in place of more common (and rustable) “alloy” or “chrome” steel.
In support of this statement, I note that:
1: My alternative to cheap Chinese-made bearings on Amazon is to buy expensive Chinese-made bearings from a DME supplier. Look carefully at the bearings offered by DME suppliers, and you’ll see a discreet little “PRC” (People’s Republic of China) printed on the bearing race, or a blurry set of Chinese characters stamped into the plastic seals.
Source: “TiLite wheelchair bearings” from wheelchairbearings.com
In fact, every bearing that has come out of the chair I’ve been working on was a made-in-China bearing. It’s not clear to me, at this point, that you can buy anything but made-in-China bearings in the sizes routinely used in wheelchairs.
2: To get a bearing like this to function at all, you have to make it pretty well. The bearing races, balls, and (typically) cages are all solid steel, regardless of whom you buy the bearing from. The seals on most of bearings I’ve pulled out of this wheelchair are rubber, but you can get them with steel seals.
3: Bearings of this type are so widely used in industrial settings that there are agreed-upon standards for size, construction, precision of dimensions, and such. The result of 2: and 3: is that every bearing in this general category of small steel utility bearings ought to have a pretty robust baseline capability.
4: There’s no such thing as “a wheelchair bearing” or “a fork bearing”. There are only standard bearings that wheelchair manufacturers have chosen to use in those locations. Near as I can tell, there is no unique “wheelchair-ness” to the bearing.
For example, what I refer to as “the fork bearing” is just a standard bearing, that was placed in a wheelchair fork. On this TiLite chair, the spoked wheels and the forks literally take the same replacement bearing.
5: Sizes are standardized, and there aren’t many of them if you work in inches.
Source: HCH bearing.com
In particular, inch bearings are named RX, where X is the inner diameter in 16ths of an inch. Thus, an R6 bearing — correct for the caster-wheel bearings on this chair, is (6/16 =) 3/8″, and an R8 bearing — correct for the forks in this chair — is (8/16 =) 1/2″.
The full dimensions of sealed R6 and R8 bearings are below. (Unsealed bearings are thinner from front to back and won’t work in a wheelchair.)
Any R6 sealed steel bearing will fit in the caster wheels of this chair.
Any R8 sealed steel bearing will fit in the forks and the hubs of the spoked wheels of this chair.
Interestingly, it was a little hard to figure that out on-line. Seems like DME suppliers don’t routinely list the dimensions of the bearing. They just tell you which make and model of chair the bearing will fit. It’s almost as if they didn’t want wheelchair users to know the generic size categories for these bearings.
When in doubt, measure the existing bearings with calipers. That’s what I did, and that’s how I could be absolutely sure that what was in the chair had the dimensions of standard R6 and R8 bearings. And not something exotic, e.g.., metric. They were exactly the sizes listed above, to within the resolution of my cheap electronic calipers.
Source: Amazon.
It’s a big help, though, to know that the bearings were “inch” bearings, that is, measured in inches. There appear to be a lot more bearings available in metric sizes than in inch sizes.
The full specification of the bearings I used in the caster wheels is R6-2RS. That is, inch bearing (R), standard size 6 (inner diameter 6/16″), with two rubber seals. If the bearing had metal seals instead of rubber, it would be specified as R6-ZZ.
6: Minimal information on the type of steel used. Virtually all the R6 bearings on Amazon are listed as either “alloy steel” or “chrome steel”. Sometimes both at once. At face value, “alloy steel” just means there some other material in the steel beyond iron and carbon, and “chrome steel” means that one of the metals mixed into the alloy is chrome. So, chrome steel is, technically, a type of alloy steel. In theory, bearings advertised as “chrome steel” should be more rust-resistant than those advertised as merely “alloy steel”. But both types of steel will rust. In practice, I don’t think you can tell much about the quality of the steel from reading the listings on Amazon. Nor do DME suppliers say anything useful about the quality of the steel used in their bearings.
7: “Deep groove” bearings and axial loads. In theory, bearings of this type come in deep groove and shallow groove variants. That refers to how deeply the ball bearings sit within the bearing race. The “deep groove” bearings can withstand higher axial loads. (That is, loads that try to push the center race out of the bearing — loads aligned along the axis of spin of the bearing.) So, in theory, you’d have to make sure you purchase deep groove bearings for use in a wheelchair. But, in practice, everything I’ve run across on Amazon, in steel bearings sized to fit a wheelchair, has been deep-groove.
Sure they fit, but are they strong enough?
Short answer, yes. Typical commodity steel bearings are way more than strong enough for this use.
The ability of these small steel bearings to carry weight is measured in terms of their listed radial and axial loads. Radial loads try to push the inner bearing race into the outer bearing race. Axial loads try to push the inner bearing race out of the plane of the bearing.
If you exceed the bearing’s load rating, the bearing will wear prematurely, may have increased friction, might overheat if used at high speed, and so on.
I have yet to see a DME-sold or Amazon-sold bearing of this type that actually lists the allowable axial and radial loads. As a result, I have to eyeball a few sources to see if I can come up with average or typical values.
For R8 bearings — the size used as a fork bearing and spoked wheels in this TiLite chair — where I can see a load listed, the dynamic radial loads on commodity bearings in this size are almost all around 750 pounds.
This means that the four bearings in the spoked wheels — where most of the weight of the user falls — have a safe working load of around (4 x 750 =) 3000 pounds. That, if they are run-of-the-mill commodity steel R8 bearings.
But, as discussed in the just-prior post, the fork bearings on this chair take an axial load. The weight of the user tries to push the center of the bearing out-of-plane.
The best guidance I’ve seen is that bearings of this type can typically take axial loads that are 10% to 30% of their rated radial loads (Source: SMB bearings.) SKF Bearings pegs that at 25% (reference) for small bearings.
That suggests that these commodity steel R8-sized bearings can be used safely with routine axial loads around (750/4 =~) 185 pounds each. Thus the two front forks can safely be used with a routine load (falling on those forks) of about (2 x 185 =) 370 pounds.
Let me emphasize that’s not their breaking strength. That’s a load at which these bearings will fail to meet some standard of longevity.
Bottom line, near as I can tell, run-of-the-mill steel R8 bearings are far more than strong enough for use in wheelchairs. The weakest weak point is the axial loading of the fork bearings. And there’s no way the routine load on those bearings, in that use, exceeds their combined rating of around 380 pounds.
Possibly you can manage to pick up inferior steel bearings on Amazon. But my guess is, if you just pick steel bearings at random, you’ll get ones that’ll be far more than adequate for this use.
Lessons from a post-mortem on Amazon bearings used as caster-wheel bearings.
All of the above convinces me that all these little disposable generic steel bearing units are interchangeable. They’re all made-in-China, for sure. All I’ve seen so far feel weighty and sturdy, holding them in your hand. Sizing is standardized, and there just a few inch sizes that would plausibly fit in a wheelchair. (But there appear to be more metric sizes.) Most importantly, as far as I can tell, all these generic steel made-in-China R6 and R8 sealed bearings are plenty strong enough to be used in wheelchairs.
I’m still baffled why the bearings sold by DME suppliers cost ten times as much as the equivalent bearings sold on Amazon.
In any case, going on two years ago, I replaced the caster-wheel bearings of this chair with generic steel bearings from Amazon. That worked out well enough that I just bought the same bearings, in a different size, for use in the forks and spoked wheels.
At the end of 2022, I replaced the caster-wheel bearings for this chair with bearings bought from Amazon, as shown above. Those Amazon-sourced generic Chinese bearings worked with no issues as wheelchair caster wheel bearings for a year and a half, but were showing a lot of rolling resistance and starting to lock up, so required replacement after 21 months of use.
Here are those used bearings, after being pulled out of the caster wheels.
You can see a space around the seals — the little black rings. Those seals both hold the grease in, and keep water and dirt out. Note that the pictures of new bearings don’t have that gap.
You can also see some surface rust on the outer races of those bearings. It should come as no surprise, then, that rust killed these bearings. Rust, aided and abetted by grit and grime, no doubt.
That said, I’m pretty sure the full story is that the bearing seals failed. Likely the outer edge of the rubber bearing seals were ground down by accumulated road grit. Then the internal parts (the ball bearings and races) rusted and got packed with crud and otherwise failed to roll smoothly.
I have no way to compare that to the performance of gen-u-ine DME-supplier bearings. No control group. But my guess is that all of these alloy-steel (sometimes chrome-steel) bearings with rubber seals would likely fail at about the same rate, used as caster-wheel bearings.
In any case, I’m willing to say, what I bought on Amazon was good enough for a year of caster wheel bearing use. But not two. They were pretty well locked up from rust and grit before their two-year anniversary. But I doubt any plain-steel rubber-sealed bearing would expected to last two years in that environment.
Rust never sleeps? Next time, I’m buying a better bearing design.
Given how these bearings eventually failed in this use, I have some ideas on making a better choice of bearing for the next replacement.
First, as I see it, these steel bearings failed NOT because of the steel, but because of their rubber seals. The edges of the seals were probably ground away by accumulated grit.
One obvious option, then, is to buy bearings with a more durable seal. Turns out, for around $1.50 a bearing, I can get R6-ZZ bearings (with steel seals), instead of the R6-2RS bearings (with rubber seals) that I used. As shown in the intro.
Better yet, for (typically) $6 to $8 a bearing, I can get R6 bearings made from stainless steel. The point being that stainless does not rust under normal circumstances. Amazon has a few listings for those. (And I see an occasional DME supplier offering stainless bearings.)
So, with 20-20 hindsight, for caster-wheel bearings, living in a harsh, unprotected environment, I should absolutely trade up to steel-seal bearings (R6-ZZ instead of R6-2RS), for no material increase in cost. Or I should pay perhaps $8 per bearing, and install stainless.
The only serious question I have is whether stainless bearings this size can be driven into and out of their housings as “easily” as alloy-steel bearings. If they can, I see no downside to moving to stainless at the next replacement of any caster-wheel bearing.
Separately, stainless bearings will be slightly less strong than equivalently-sized chrome-steel bearings. The metal wear characteristics may not be quite as good. But I don’t see anything to suggest that stainless bearings would be too weak to work in a wheelchair.
Footnote: Going sub-Amazon for stainless bearings
While Amazon offers cheap bearings, there are cheaper sources yet.
Which is why I finally decided to mainline some Chinese goods. I bought a bag of 10 stainless steel bearings, sized to fit the caster wheel, for about $18.
From, God help me, Ali Express.
I hear it’s a slippery slope.
Next thing you know I’ll be tradin’ in my Chevy for a Geely.
Conclusion
There’s nothing unique about the bearings used in wheelchairs.
Near as I can tell, all steel sealed bearings sold for this type of use are far more than adequately strong enough for use as “wheelchair bearings”.
Those bearings are standard commodity items. If you have inch-units bearings, you only have a few standard sizes to choose from. If you don’t know the size of the bearings used in a given chair, it’s not hard to pull one and measure it with calipers. Or see if the bearing size is stamped on the bearing. If it’s an inch-units bearing, the dimensions will match one of the few listings in the bearing size chart above.
But, the most-commonly used steel sealed bearings have rubber seals. This is a bad option for use in caster wheels, owing to the high levels of abrasive grit those bearings are subjected to.
For caster bearings, on the next round of replacement, I’m going to try stainless steel bearings. But I’d bet that $1.50-a-pop R6-ZZ (plain steel bearings with seals made of steel, not rubber) would also hold up well.
I still don’t know how DME suppliers can successfully charge the prices they do, for these bearings. But I don’t feel pressured to pay those prices for genuine “wheelchair bearings”.
The next post outlines the process of pulling and replacing the fork bearings on this TiLite Aero chair.
