Today's post is an important post, because it's a post about bearing posts. Thanks for bearing with me through that. From here on out there are going to be less puns and more technical details.
For as important as the bearing post is in yoyo design, there isn't a whole lot of variety when it comes to them. We've reached a point where the guts are generally standardized, with only a few minor variants made for different types of response. That doesn't mean that they aren't rife with challenges and complications. Point of fact: The bearing post is one of the more frustrating parts of the yoyo.
Before we start talking about posts, we need to look at a bearing. The C bearing we use in the majority of yoyos now has an inner diameter (ID) of 6.35mm (0.250"). However nothing is perfect, especially in machining. Because of the variations that occur in production you can not assume every bearing has an exact ID of 6.35mm, this is where ABEC ratings come into the picture. An ABEC rating is a classification by an oversight body that a bearing will meet a specific dimensional tolerance. More clearly, an ABEC 7 bearing has a smaller range of possible sizes than an ABEC 3 bearing.
Unfortunately, ABEC 7 bearings are significantly more precise than is economically possible for machining yoyos and this means that bearing posts have an even wider variation in sizes. OneDrop prides themselves on holding a 0.0002" (0.005mm or 5μm) tolerance on the guts of yoyos produced in their shop. Generally this means that the size a bearing post can be either wider or thinner by that margin. Now you have to add those variations by the possible dimensional variations of the bearings, and you have a compounding variance in your production. What's worse, anodizing typically introduces even more variation into your production. Worse still, the chemical process of anodizing means that the dimensional variation is different with each batch of anodizing, meaning you have to account for an even wider range of possibilities!
How do you solve the problem? Unfortunately the most common solution is to minimize your bad outcomes.
Given the nature of production, you know your bearing posts are going to be either too large, too small, or just right. Since bearings aren't flexible, a bearing post that is too large is a critical failure; you're going to chance deforming the bearing post if you press fit it and introduce vibe, and that's if it will even fit at all. If the bearing post is too small you'll have similar issues with vibe, and at extremes you'll have bearing slippage which will very negatively affect your spin times. When the post is just right, you'll have a slip fit that should reduce vibe and minimize the amount of galling you'll suffer over time.
Because a bearing post that is too large creates a yoyo that is simply unplayable, the safest option is to skew your production range smaller. At this point, it becomes a balancing act of finding the right size that fits your production ranges to minimize the number of critical failures. If you skew too small, you won't have any posts that are too large, but you're increasing the likelihood of posts that are too small and you're solving your problem by creating a new one.
After the machining difficulties, just about everyone that produces eventually comes to abhor anodizing. It's an expensive process that can end up introducing more scraps into your production run than the actual machining. I asked a few anodizers about the variance of their anodizing process and got responses from "There is no noticeable change to diameter" to "Adds around 0.2mm" and there are even variables for the colors that are used. The best mitigation for this is to find an anodizer you trust, and adjust your designs to compensate for the outcomes of their specific process.
What are the outside the box solutions for the difficulties of bearing posts? OneDrop is the one on the ball. The Side Effect system allows a lot more room to breathe with anodizing and machining because of the tapered fit of the Side Effect to the body of the yoyo. I'm working off an assumption here, but I would guess it reduces their production costs as well, as they can machine Side Effect yoyos a little more quickly and easily, and can set up side effects as a large production run that are standardized across all of their throws.
True to form, OneDrop is also working on another solution to bearing post woes that they haven't used in a production run yet. I've seen Shawn mention a brass press fit cover that they would apply after anodizing. Brass has significantly higher resistances to galling than aluminum does, and since the brass wouldn't be anodized it wouldn't be as variable when it came to fit. The issue is the difficulty of machining because of the size of the piece. Shawn has been working on it for awhile, and he's still hopeful they can bring it to market. I hope he does too, because I'm already developing a healthy fear of sending prototypes to some anodizers.
Thanks for making it to the end! I hope you gleaned some new information about bearing posts, and a new found respect for the difficulty of producing a good yoyo. Of course, the bearing post is only one part of what makes a yoyo, but it is also the least variable. Zach Lerner put it very well in his Youtube yoyo design tutorials: An error in the guts is a mistake, and error anywhere else can be a chalked up to a design feature.
Corrections: The diameters in the bearing sketches above are missing about five thousandths of a millimeter on their label. They're also not drawn to scale. Woops!