Trackball bearing comparison
A comparison of trackball bearings: (cheap) rollers, BTUs and static bearings – by wiwilwi.
Published November 25, 2022
So I designed a trackball add-on for my Skeletyl (posted a while ago; github repo of my project for those interested), and after using it for a while I was rather dissatisfied with the roller bearings, so I decided to try out a few different bearing solutions. I thought I'd share the results for other's to be better able to make a choice for their own project.
So I tried 3 types of bearings. And for TLDR, these are in order of my preference:
- Static ceramic bearings (3mm ZrO2, form Aliexpress)
- Steel BTU's (Veichu VCN310 7.5mm, from Aliexpress)
- Steel roller bearings (MR63-3x6x2.5mm, from a local webshop)
It is worth noting that for the rollers and BTU's, money probably makes a big difference; if you are willing to spend good money, you may arrive at entirely different conclusions. A good quality BTU or roller bearing can be quite expensive.. The rollers, BTU's and ceramic bearings I bought all cost me roughly the same (all cheap), so they are a fair budget comparison, there's no real monetary incentive to pick one over the other.
In order to make good comparisons, it's probably best to first establish the parameters by which I evaluated the performance of each bearing. Basically there is 4:
- Stiction: more accurately described as static friction. This is the initial force required to start moving the ball. When static friction is high, it feels like the ball "sticks" to the bearing when initiating movement, or when moving at low speeds (precision movements), hence the term stiction. Rated 1-5: 1 is minimal stiction, 5 is high stiction.
- Friction: this is the amount of force required to keep a moving ball moving. When friction is low, you can give the ball a spin, and it will keep spinning for a while. When friction is high, it will come to a stop much faster, so it takes (slightly) more force to roll the ball. Rated 1-5: 1 is very low friction, 5 is very high friction.
- Consistency: some people like to talk about smoothness, but I think that is easy to confuse for stiction/friciton as well. Consistency is a more accurate descriptor to me. Basically, does the ball behave consistent and predictable in all rolling directions. Rated 1-5: 1 is very consistent, 5 is very irregular.
- Sound: this wasn't very important to me, but some people will find it relevant so I have included it here as well. Rated 1-5: 1 is very silent, 5 is very loud.
Starting with the roller bearings. I used steel roller bearings (MR63-3x6x2.5mm) from a local webshop, but probably equivalent to what you may find on Amazon or Aliexpress. I used them, because the Bastardkb Charybdis uses the same measurement of bearings, and I based my design off that. I had heard good things about the ploopy rollers, so I thought it would be a good solution. However, I was kind of disappointed with them in the end - my only guess about people's ravings about the ploopy bearings is that either they are much higher quality than what I bought, or most people are less critical than I am (or both); but I was definitely not impressed.
Let's start with the good: Stiction is fairly low, as compared to commercial ruby bearings. It doesn't take much effort to get the ball moving. However, there are certain rolling directions in which that performance isn't consistent.
Friction is a bit more difficult to determine, which is due to the inconsistency, which is where it gets really bad. The rollers roll in only one direction, whereas the ball rolls in all directions. When the ball moves perpendicular to a roller, there is a lot of friction, when it moves parallel, there is very little friction - so the amount of friction varies a LOT depending on which direction you roll. On top of that, the rollers are somewhat gritty and inconsistent by themselves, which I guess is just the poor quality of a cheap bearing. The result is that at all speeds, but particularly so at low speeds, it is annoying to make accurate predictable cursor movements.
- Stiction: 2-4 (Mostly good (2), but with some variation)
- Friction: 2-5 (Great to awful, depending on rolling direction)
- Consistency: 5 (Awful)
- Loudness: 4 (Pretty loud)
BTU's (ball transfer units)
So, after the disappointment of roller bearings, I decided to buy a bunch of cheap BTU's from Aliexpress (Veichu VCN310 7.5mm), as well as some 3mm ceramic bearings. First the BTU's. They are a definite improvement over the rollers, no question, but still leave something to be desired.
Stiction truly is minimal, there is hardly any force required to get the ball moving.
Friction is also minimal, it is very very light to move and keep moving. It doesn't spin as long as you see in some youtube videos of people that bought expensive high quality BTU's, but it is an obvious difference with the rollers or static bearings. However, this is not necessarily a good thing. Because it is so light to move the ball around, it can be difficult to be steady in your movements. It is like writing with a pen: you require a bit of friction of the paper to write properly. Writing steadily with a stylus on a tablet is much harder. This is what the BTU's feel like as well.
Consistency is better than the rollers, but still lacking a bit, which surprised me. They do not have the flaw of the roller bearings that only roll in one direction. However, the BTU's are gritty, particularly at low speeds. This grittiness makes for some unexpected hick-ups that can unpredictably occur in any one of the bearings, which throws off the consistency. So again it can be a bit annoying to make slow precision movements, though it's not nearly as bad as the rollers. And at average and higher speeds the grittiness doesn't really interfere, and it becomes supremely smooth. I suspect though that shelling out for some high quality BTU's may solve any issues here.
- Stiction: 1 (Excellent)
- Friction: 1-2 (Okay-ish, its low friction, but that doesn't necessarily equate to good).
- Consistency: 3 (Okay-ish)
- Loudness: 3 (surprisingly a little less loud than the rollers)
Static ball bearings
The static ball bearings are what we are commonly used to in commercial trackballs, for which ruby bearings are most common, and can be notorious for stiction. Also to clear up some common confusion, these ball bearings do not rotate (hence static), but function as low friction points for the trackball to glide on.
Ceramic bearings (Zirconium oxide) are the most popular type for replacing the ruby ones, in order to reduce stiction (with mixed results), so that is also what I used. I got 3mm ones, which are quite a bit larger than the ones that are in my M575 for example; I used 3mm because it would be easier to fit into a 3D printed holder, without having to worry about tiny tolerances too much.
Ultimately, I found that the static bearings are the most preferable. Stiction is definitely there, but it is acceptably minimal. Definitely not as bad as some ruby bearings I experienced in some kensington trackballs, and even a little better than my M575, which surprises me because it also uses zirconium bearings, and I am using the same ball. Whether that has to do with the bearing diameter, the position of the bearings, or the fact I use my current setup with finger instead of thumb, I have no clue. But the stiction is not problematic.
Friction is clearly higher than the BTU's, but as mentioned before, that is not necessarily a bad thing. It makes it easier to be steady, and there's no reason to spin the ball for long durations in any practical application (I find spinning is probably the most idiotic way some people measure trackball bearing performance).
Consistency however is where the static bearings shine. There is no moving parts, so the only thing that can cause inconsistency is imperfections in the trackball surface itself. This makes the behavior of the ball very predictable - there may be some stiction, but it is always the same in every rolling direction. The friction does not vary depending on direction. And there's no unexpected hick-ups due to grittiness in the internals of the bearings. And that all helps in being precise.
One thing to note with static bearings is probably that the quality of the trackball surface becomes much more relevant in the overall performance as well, which may be of less relevance for the BTU's in particular. So shopping for another ball may be the next thing I'll try.
- Stiction: 3 (acceptable) (would rate the rubies in a kensinton orbit fusion at 5 for comparison).
- Friction: 3 (excellent)
- Consistency: 1 (excellent)
- Loudness: 1 (pretty silent)
So for those wanting to build or mod a trackball on a budget, static ball bearings are probably your best bet (I suppose there's the reason commercial trackballs use them as well). But for those willing to shell out on high quality bearings, you may arrive at different conclusions; but to me it probably wouldn't be worth the dough.