hi guys and gals, for years now my team has been using bearing (besides the chassis) to safely and efficiently rotate a driving axle. But we’ve been using less than awesome mount them (screws on 2 sides screwed in to keep it in place, a shaft collar on each side on the bearing, and even j.b.weld and tape!..
we’ve seen it done right but i have to ask…
How do you keep your bearing in? (for example, the Andymark KOP chassis uses precision cut holes that the bearing press fit into, great idea but DIFFICULT to make a hole that precise for newer teams)
Ideally most bearings on an FRC robot should be a light press fit: they won’t come out on their own and can still be installed by hand. To get that though you need to be able to hold a tolerance of a few thousands of an inch, which is very hard to do if you aren’t using precision machinery (mill, router, laser, etc).
If that’s not an option (and even if it is) you can constrain the bearing using the assembly geometry. Sandwich the bearings between two plates with the flanges facing inside. Use retaining rings, bolt and washers, and/or shaft collars on the shaft to make sure nothing can slide freely. Think about the 6 degrees of freedom of every component on the shaft (translation in 3 directions and rotation in 3 directions) and make sure the only one not fully constrained is the rotation with the shaft. 99% of the time you can use this to keep the bearing from escaping.
In the 1% of the time where geometric constraints aren’t a valid option, there’s nothing wrong with using the heads of small screws to capture the bearing outer race. It just adds a bit more complexity and a few more possible failure points.
We often use the flange and clever placement of components (maybe rollers, maybe shaft collars, maybe tapping the shaft end for a screw and washer) for retention. Which has room for improvement, but it’s enough for our needs.
We also use a lot of gussets to give our bearings better fits than we could make ourselves. You can get them from AndyMark, REV, or VEX.
+1 to this, and it’s common practice among some teams (I first saw this on a 148 assembly) to use small rivets instead of screws to hold the flanges down. As a whole, my team generally tries to position the bearings at the absolute ends of the shaft so that we can tap the shaft and use screws & washers to lock the shaft and bearings in place.
Personally, I’ve had good luck with 1-1/8" spade bits (not the ones with the spiral pilots). Not a press fit that can’t possibly fall out, but as good as the KoP chassis, where you do well by having a couple of shaft collars or e-clips to hold things in place. I usually drill from one side until the spade portion is engaged, flip, and drill from the other side. Not only does this reduce the amount of metal turned to shavings, it leaves a bit of flashing which provides a bit of aid in assembly, though it wouldn’t be enough to hold things together in operation. I recall reading somewhere on CD a while back about someone machining the spade bits down a few thousandths to get a press fit.
Added: I think it was in this topic, which has lots of good info on the subject:
We used it for press for this year and found it a little bit tighter than the andymark kit chassis does them. And then because we’re cheap and the pilot hole is the right size for 10-32 we then file the edge of the cut-our section and use that as a washer.
We use a similar one, made by EZARC and available through Amazon. We’ve always found it a good light press-fit for the standard flanged bearings. As long as you have a drill press (even a table top model) you can produce excellent bearing mount holes.
Other than that, we most use the components to make sure they stay in place, like a well-placed shaft collar or putting some other part directly up against the bearing (with maybe a plastic washer to ensure that the only contact is with the inner bearing race.) When this isn’t practical we have been known to JB Weld the bearing into place, but be sure you’ll never need to get it out if you do.
In my experience, the tips of these need ground down before they’ll cut the dimension you want. But once that’s done, yes, they cut quite nicely. Precision instruments, yes, but usually precision clearance, not interference.
Back to main topic.
Another neat tool to put in your toolbox is Loctite retaining compound (green). There are many variants of this (for ‘oily’ environments, for slip fits, for press fits, etc.)- make sure you get the right one, and clean the surfaces you’re bonding before installation (either with primer, or some good solvent like acetone/isopropyl).
With that in mind, it’s rated for about 2000 psi in shear (RTFM to be sure). So, for a 1 1/8" bearing hole in 1/8" aluminum that’s (1.125PI0.125 in^2) * (2000 psi) = 880 pounds of axial retention force.
Geometric constraint (“the bearing literally cannot fall out because things are in the way”) with loctite to keep the bearing from wiggling and fretting is a great solution for mechanisms where low backlash is needed but you can’t make super-accurate parts.
Honestly what you’re doing (except the JB Weld and tape…) are both valid / fine methods for bearing retention. Screws to retain flanged bearings are an easy and effective way to hold them in. This is how many COTS products (like VersaFrame gussets) do this.
Press fits are nice but you don’t want to make the press fit too heavy or you’ll hurt the performance of the bearing, or just seize it up entirely.
Green loctite (gap filling) helps too, particularly with slightly oversized holes.
Relying on a press fit with 1/8 thick and smaller frame matl. is problematic. If you use Loctite, it will be some work to take it back out. Grooves for circlips are huge stress risers. Do not place them on the part of the shaft with torque loads. We try to place bearings where the geometry of the parts retain the bearings.
We have good luck with a plug drill (expensive) and step drills. You can grind the cutting edge of the step drill to get a slight press fit.
We didn’t need to grind down the tips and were immediately getting tight bearing holes, you just can’t go all the way through with it as the OD of the plate above the cutter is larger than the size advertised on it.
First of all, you NEED to be using a drillpress for such tasks if you are not already doing so. It will be extremely difficult to hold a hand drill precisely enough to get the results you are wanting.
Make sure that the table of the drillpress is set as high as practical. Having to extend the quill more than necessary will allow it to wobble around more.
Make sure the table of the drillpress is perpendicular to the axis of the cutting tool. There are many YouTube videos showing how to do this.
Make sure the rpm of the cutting tool is not too high i.e. large diameter = slow rpm. When the linear speed of the cutting tool is too fast, the tool chatters and vibrates, causing the axis of rotation of the cutting tool to wander around. You may have to pause and change the belts from one groove to another but the results are worth it.
To get any sort of precision and repeatability, you MUST clamp the workpiece onto the table of your drillpress using at least 2 clamps, one on either side of the cutting tool. Allowing the workpiece to move relative to the center of the cutting tool will lead to imprecise and inconsistent results. Clamping the workpiece is also a safety measure, especially when using large diameter tools. When a larger diameter tool catches on the workpiece, it WILL rip it out of your hands and spin it around like a lawnmower blade.
It may be a good idea for your team to look for a mentor to teach the students and mentors how to use the shop equipment safely and get the most out of them. Old machinists from the pre-CNC days would be a good choice.
It may also be worthwhile to check the runout of the chuck/quill of your drillpress. There are many videos on YouTube showing how to do this.
… I actually beg to differ. On clamped work I can usually obtain higher positional tolerances by hand. Most drill presses (read: the caliber most people have access to) will not add more stiffness than a hand drill and so you’re not going to be improving diametral tolerance… A mill is a different story …
This also requires that you know what you’re doing and can steer the drill bit effectively
I was going to say… I used to work in aerospace. A GOOD aerospace tech (read as: experienced) can do at least as well as a drill press with a hand drill. A really good (many years of experience) might be able to give a mill a run for its money (if the mill is operated by a high school student).
The key is to set the work up carefully and correctly. Measure once… check. Measure twice… check. Decide that this setup isn’t working, reset, measure again. Check again. Double-check the measurement and check that again. Line up the drill, check one more time, and start drilling.
Vex (and others) provides many gussets that have 1.125 diameter holes for the standard bearings. Picture of standard procedure is below that shows 5/32 rivets securing bearings.
Bench top drill presses can be purchased for as low as $86 (Wen is $86 on Amazon) which means they are within budgetary reach of nearly every FRC team. These make it much easier to drill 1.125 holes safely and accurately when a gusset won’t suffice.
Team 931 has used step bits from harbor freight (use some higher quality ones if your budget allows) and step holes up to the 1 1/8 inch step. That gives a nice press fit. The step bits are also less wobbly than other options. Also we tend to use flange bearings and lock collars for the majority of our designs which alleviates much of the need for an axial constraint (ie. a set screw to hold the bearing in the hole or welding the bearing in to place).
