pic: Team 125 sideplate close-up

[Daniel Brim]

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[Woody1458]

Hey Iw as just wondering because you guys have obviously done it well, how do you mount bearings? Our team is having a lot of problems with them falling out, we tried hammering them in to barely too small holes but that would only last about a day, and epoxy cant take the stress.

[s_forbes]

Looks good! Can't wait to see it up close in AZ. How much does the whole thing weigh? (and what kind of wheels are those?)

[Daniel Brim]

Originally Posted by Woody1458

Hey Iw as just wondering because you guys have obviously done it well, how do you mount bearings? Our team is having a lot of problems with them falling out, we tried hammering them in to barely too small holes but that would only last about a day, and epoxy cant take the stress.

Woody, we press them in there. Generally, with aluminum, if you cut a hole .003 undersize a bearing should stay in. Mind you, in a situation like what is pictured, a flanged bearing is a better application. It was a lip that you can press against one side, so it prevents it from being pushed out in one direction. You should not need to epoxy bearing.

Originally Posted by s_forbes

Looks good! Can't wait to see it up close in AZ. How much does the whole thing weigh? (and what kind of wheels are those?)

We don't have an accurate scale in our lab, so we can't give an exact answer to it, but my estimate is about 50 pounds. The wheels are custom made. They are 4" delrin with aluminum hubs and urethane treads.

[henryBsick]

Originally Posted by Woody1458

Hey Iw as just wondering because you guys have obviously done it well, how do you mount bearings? Our team is having a lot of problems with them falling out, we tried hammering them in to barely too small holes but that would only last about a day, and epoxy cant take the stress.

Press fit tolerances and flanges, all built into the original design.
For press fitting, Dan (above) has the correct dimension. Use an Arbor press to press (finesse) in the bearing, not a hammer which could lead to eccentricity and bearing damage.

You can capture and un-flanged bearing with two small bolts on either side of the bearing hole. If the bolts are close enough the the hole the heads of the bolts will extend over that area of the bearing hole limiting the travel in that direction. If you need more wall thickness for the new bolt holes than the proximity a normal bolt head would provide, drill the holes (to be tapped) farther away from the bearing hole and extend their "covering reach" with washers. If space allows this can be done on both sides of a bearing (if necessary), with 4 total holes and 4 total bolts, all ideally at 90 degrees from each other.

Lets say our drive train bearing pictured was not flanged. The only direction it would be able to move in our configuration would be out (away from the wheel) because of the full shaft on the other side. We could drill 2 #6-32 tap holes 180 degrees from each other around the bearing hole and tap to thread in the bolts. In this hypothetical scenario the bearing is thicker than the side plate so it leaves a lip that is not flush with the outside, so we can't tighten down to flush. Some of the things that can be done from here:
1.Locktight the bolts at given depth and voila.
2.Have the holes placed further away, toss a washer onto each bolt and thread a nut on them to keep the washers fixed on each bolt, then tighten down as far as you can (providing the the thickness of the nut is the ideal offset from inner washer face to outer side plate face. I would still suggest locktight in the later scenario for lack of a locking nut on the other side of the bolt.

That was a book on a simple explanation, a good break from thermodynamics though

//We molded the urethane onto the delrin ourselves. Brandon can describe the process and details as that was one of his (many) project leads for the students.

[sanddrag]

Originally Posted by Daniel Brim

enerally, with aluminum, if you cut a hole .003 undersize a bearing should stay in.

With a hole .003 undersize, it'll do more than stay just in, it'll seize. That is far too heavy of a press fit. This will damage the bearing and decrease efficiency. A good medium press fit will have a ~.0004 interference. This will be sufficient to retain the bearing in most scenarios and will not excessively load it. A really tight press fit on for example, an R8 bearing into aluminum should have no more than about .0008 interference. These numbers come from my personal experience. Machinery's Handbook probably has some more standardized guidelines for designing bearing press fits.

As stated earlier, flanged shielded bearings are a good item to have around and use on FRC robots. The flange makes it so you don't need a counterbore or screws to retain it. Flanged shielded bearing part numbers follow the format FRXZZ. F for flanged, R for radial, the X corresponds to the size in 16ths of an inch, and the ZZ signifies double shielded. They are available for about $4 ea from Small Parts Inc (company specific part numbers brf-06 and brf-08) and from other sources such as VXB, Motion Industries, eBay.

Be careful when selecting flanged bearings. There are many unground non-rated flanged bearings out there. These can handle only a small fraction of the load and speed that ABEC-1 rated precision bearings can.

[henryBsick]

Originally Posted by sanddrag

With a hole .003 undersize, it'll do more than stay just in, it'll seize. That is far too heavy of a press fit. This will damage the bearing and decrease efficiency. A good medium press fit will have a ~.0004 interference. This will be sufficient to retain the bearing in most scenarios and will not excessively load it. A really tight press fit on for example, an R8 bearing into aluminum should have no more than about .0008 interference. These numbers come from my personal experience. Machinery's Handbook probably has some more standardized guidelines for designing bearing press fits.

As stated earlier, flanged shielded bearings are a good item to have around and use on FRC robots. The flange makes it so you don't need a counterbore or screws to retain it. Flanged shielded bearing part numbers follow the format FRXZZ. F for flanged, R for radial, the X corresponds to the size in 16ths of an inch, and the ZZ signifies double shielded. They are available for about $4 ea from Small Parts Inc (company specific part numbers brf-06 and brf-08) and from other sources such as VXB, Motion Industries, eBay.

Be careful when selecting flanged bearings. There are many unground non-rated flanged bearings out there. These can handle only a small fraction of the load and speed that ABEC-1 rated precision bearings can.

Pretty amazing that our bearings our still turning then I guess .
At Engineer's Edge tolerances max are 2.3 thou on a 1.125 inch hole (R8 bearing O.D). Toss in some slop from the machine/jig and bam. From 7 ten thousandths over the maximum suggested tolerance to a perfect press fit in no time flat. Even without machine slop, straight measurement from one hole to its insert's O.D. we have done 3 thou. Funny thing is on a 1.125" hole that "good medium press fit" of ~.0004" is under the minimum interference listed.

I can't speak for any super high end bearings though, we stick at abec-1 (if that) I assume that doing 3 thou under for an abec-12 would be a big problem... but the tolerance on that bearing itself is an outlandish 50 millionths


Side note:
I think that if I ever asked a student or fellow mentor to machine a hole with a 1.1246" diameter hole, they would probably throw the print back in my face.

//edit
Here is what I wanted

[Brandon Holley]

Originally Posted by s_forbes

Looks good! Can't wait to see it up close in AZ. How much does the whole thing weigh? (and what kind of wheels are those?)

We started with a ~4 inch diameter piece of delrin. Brought it on the lathe and faced, put a 1/8" deep 3/4" wide (overall width is 1") groove into and counterbored a place for the hub to sit.

Then we made aluminum hubs that slide into the center of the wheel, and sit flush with the face because of the counterbore. Put a bolt hole pattern in there and hex broached the hubs.

Then we made a mold out of a white plastic, i believe UHMW. We made teh ID of the mold to be 4.25" (so enough for 1/8" of tread over the edge). Using a 2 part urethane as well as a very accurate scale we whipped up batches of wheel tread, and SLOWLY and CAREFULLY poured the urethane into the mold with the wheel in there already. After letting it sit for an hour or 2 you popped a wheel out and called it a day.

[ducttapedude]

Originally Posted by Woody1458

Hey Iw as just wondering because you guys have obviously done it well, how do you mount bearings? Our team is having a lot of problems with them falling out, we tried hammering them in to barely too small holes but that would only last about a day, and epoxy cant take the stress.

As others have said, flanges are definately the way to go, every bearing on our robot has a flange on it, it really makes things great to assemble and to dissasemble, and of course, for functionality.

With flanged bearings, pressfitting is almost always not necessary.

[MrForbes]

Originally Posted by Henry_222

Side note:
I think that if I ever asked a student or fellow mentor to machine a hole with a 1.1246" diameter hole, they would probably throw the print back in my face.

That's funny!

The only thing on our robot that approaches being a precise press fit, is some Igus plastic bushings that sit in holes in relatively thin metal, which we punched on a Rotex punch.

Ahhh...the joy of limited machining capabilities!

[Cory]

Originally Posted by Henry_222

Side note:
I think that if I ever asked a student or fellow mentor to machine a hole with a 1.1246" diameter hole, they would probably throw the print back in my face.

It's not THAT hard to hold tenths, if you're using a CNC

[sanddrag]

Originally Posted by Henry_222

Pretty amazing that our bearings our still turning then I guess .
At Engineer's Edge tolerances max are 2.3 thou on a 1.125 inch hole (R8 bearing O.D). Toss in some slop from the machine/jig and bam. From 7 ten thousandths over the maximum suggested tolerance to a perfect press fit in no time flat. Even without machine slop, straight measurement from one hole to its insert's O.D. we have done 3 thou. Funny thing is on a 1.125" hole that "good medium press fit" of ~.0004" is under the minimum interference listed.

The above link is for press fitting solid hubs onto solid shafts, not fragile bearings into bores. Press fitting a bearing into delrin, 3 thou will work because the delrin has a lot of "give." In aluminum, I simply wouldn't do it. Even if it doesn't totally seize your bearing, you'll be putting unnecessary load on it taking away from its load capacity and potentially leading to accelerated wear.

[henryBsick]

Originally Posted by Cory

It's not THAT hard to hold tenths, if you're using a CNC

true story, but things happen
it would be nice if we had a legit cnc too.... prototrack gets the job done though