I’d thought a bearing was a bearing, but it looks like i’m sorely mistaken, and I hate to think i’m missing out.
Ball bearings, linear ball bearings, needle roller bearings, thrust bearings, pressure bearings, press bearings, sleave bearings. I think I grasp most of the distinctions, but in terms of FIRST what are the part/application relationships? Specifically, which of these are useful for a press-fit in a gearbox? Which of these make sense to use inside the free-gears of a dog-shifting system? Are some of these not viable with a keyed shaft (needle rollers?). Which can handle our driving loads? Which can handle side loads as per swerve module needs?
most gearboxes will use plain ball bearings, as will a bearing inside a rotating gear in certain shifting mechanisms. I would assume that if you were to construct a swerve drive properly, you wouldn’t have any need for a special bearing. As I’ve never constructed one myself, I can’t give you a definitive answer.
I understand that ball-bearings should work fine for just about anything-- thats what I designed last year’s gearbox with. However I notice that other stripes of bearings, particularly roller pin and sleave bearings, have a much lower OD-bore ratio, which I could see being highly advantageous if you’re pressing them into tiny gears. And for applications like a swerve drive module, I have to think there are compenents designed with linear forces in mind and I don’t see why I wouldn’t want to use them. I could go on forever just using the same components and designs every year, but I’d really like to at least learn about what else it out there.
I would recomend you take a look at Small Parts P/n brf-06 and brf-08 That is what we use for everything. They aren’t in the catalog but they do sell them for about $5 each or 10 for $40. For specs go to www.firstcadlibrary.com
I feel I can give an answer, since I have. You do need “special” bearings for swivel drive. Since the entire weight of the robot is on the drives, and they rotate like casters, you need thrust bearings in there. Or, you can just go for angular contact roller bearings, like bikes. All the bearings in bikes are angular, because all joints take side loads and radial loads. But… finding angulars can sometimes be a pain if you have size limits. MVRT’s swivel had thrust bearings on every module. We didn’t do it too efficiently this year, every module used 2 thrust and 2 radial on a .75" steel shaft. You need two thrust bearings, one to take the up vertical load (weight of robot) and one to take the down load (weight of swivel module when you lift it off the ground). A little excessive, since you don’t really need the second one. And of course, you have to have radial bearings in there, so the shaft stays on the same rotational axis as it should.
There are a few types of bearings, I’ll list off the ones that come quickly:
Radial: these take the rotational forces of a shaft running in them, as well as take the weight that comes at 90 degrees to that shaft axis.
-Linear: these take sliding forces of a shaft sliding through the bearing, as well as the weight that comes at 90 degrees to the shaft axis.
Thrust: these take weight. Most are in the rotational style, they are designed to take the rotation of a shaft, and the weight that comes parallel to the shaft. The shaft needs a flange on it, to ride on the bearing itself.
Angular Contact: these are just like radial bearings, but the contact the balls roll on is at an angle, so they double up as thrust bearings.
Bushings: these are just inserts made of oil impregnated cast bronze. They are self lubricated, and can take lower rpms and pretty good weight loads. Every bushing has its own rating, so they aren’t always usable. They come in linear, radial, and thrust styles.
However, there are MANY styles of the above bearings (except bushings). Roller bearings use rollers instead of balls, angular contacts come in different angles, radial and angular contact bearings come in one, two, or 4 contacts. There are mostly different types of radial bearings. Thin section, miniature, superprecision, heavy load, etc… the list goes on forever.
And if you’re crazy enough, you can make your own bearings. That’s what I’m doing this summer, since I can’t find the right size for cheap ($150 each and I need 12!!!). I’m just using rollers and cutting steel inserts, hardening and grinding them. When they press into the gearbox and the gears go into them, the rollers can’t go anywhere so no flanges or seals needed. Works just as good as ones you can buy.
For all future needs like this, I heartily recommend Engineer’s Edge as a starting point. I will say that I’ve seen some erroneous or misleading details there, but for the most part, it’s very helpful. I also recommend EFunda as a basic reference. And finally, buy Machinery’s Handbook if you can at all afford it. It is so massively useful that I can’t even begin to describe it. I’ll be purchasing my own copy in a few weeks. If you can’t afford a copy, I have one more link that got me through a design project this last week, Machinery’s Handbook
EDIT: I’m somewhat doubting the need for thrust or angular bearings in a FIRST robot with swerve. Especially with a 3/4" shaft. Deep-groove radial bearings should be more than enough, as they can handle some axial load in addition to radial load. a 3/4" bearing is rated at 1700 lbs radially, so it could easily handle 200lbs axially.
Not to let you get confused, flanged bearings may be ball bearings. We use flanged ball bearings in our drive modules. It allows one to contruct a transmission where there is relatively low side loads on the shafts and keep everything in alignment. If you turn the shaft down to the ID of the bearing and leave a shoulder on the shaft, then the bearing will not move in on the shaft. The two side walls of the enclosure then have the bearings pressed into place from the inside. When you assemble the walls the flange on the bearing will prevent the bearing from moving out and the shoulder on the shaft prevents the bearing from moving in. That way all parts remain in alignment and nothing falls apart and no shafts pull out of the bearings.