VEX pro bearings tolerances

I can’t seem to find any information on the tolerances for the VEX-pro bearings. I want to be able to press fit some bearings into a plate; particularly 217-2733 and 217-2731. Any help would be greatly appreciated.

Generally for a press fit, you would provide the machinist with an example.
When I was machining bearing holes, I made the holes between -0.001 or -0.002" smaller than the bearing OD, and that seemed to work okay.
Be wary of CNCing bearing press fits quickly; we got some oval-shaped holes that needed to be rebored from a sponsor due to improper CNC use.

Basically the only way to be sure that you’re getting the exact fit you want is to test the fit of the bearing as you machine. If you CAD the bearing hole at 1.124" for example, it doesn’t really matter if that’s the dimension the CNC (or machinist) is cutting. Surface finish of the cut or cutter deflection/kerf can make the fit deviate from what you want, even though the dimension cut might be what you specified. It’s a lot more helpful for the machinist to know to get a press fit for a certain bearing than to tell them to hit some arbitrary dimension exactly. If you give us more info on what machine the hole is being cut on, we can give better advice on getting the fit you want.

On the other hand, it’s usually possible to get a press fit on a CNC the first time around if you know the machine, material, and cutter your using fairly well, and do finish passes at the correct speeds/feeds. But again, that’s more about the machinist and machining process than the specified dimension.

In some cases, it may be easier manufacturing or assembly wise to cut the bearing hole as a slip fit and have other things (rivets, screws, stuff on the shaft) retain the bearing. Sometimes, retaining the bearing in other ways just isn’t possible, or doesn’t do what you want, but it’s usually a ton easier to machine a slip fit than a press fit.

Anecdotal evidence usually isn’t very helpful, but in case it’s what you’re looking for, I’ve never measured a R6 or R8 bearing at anything other than the nominal dimension with digital calipers.

For our flanged 1.125 OD and .875 OD bearings, they all measured exactly 1.125 and .875, accurate to the nearest thousandth. If you make your hole exactly the the diameter of the bearing, it is not a loose fit, but the bearing can be pushed in and out by hand. If you’re about .001 under, you need to press the bearing in, and it won’t come out by hand.

What material are you using, and how do you plan to machine it?

.125 6061 and laser or waterjet cutting

Just to further reinforce the points everyone else is making, for our CNC, our tolerances, etc. we found that we needed a 1.127 hole for an easy, if slightly loose, press fit. Your mileage will vary. If you’re using a CNC to make these holes I would suggest making a template of holes in 1/4" plate from 1.124-1.128 and seeing how bearings fit differently in each hole.

One thing to be aware of with laser cut bearing bores. Typically, the laser will leave a small burr on the edge of the hole at the entry/exit point. If left untouched, this burr will prevent the bearing from pressing in, or compress the outer race enough to inhibit the bearing’s motion.

We send our sheet metal sponsor parts with bearing holes drawn as the nominal bearing diameter, and they press just right, maybe just a little loose, when this burr is removed. We always use rivets and washers as bearing retainers for some extra insurance, but in properly designed mechanisms, I think they’d be fine without them.

Ah the fun games of tolerances…

Basically all the bearings you get will come dead nuts, IE 1.125 bearing will be 1.125 +/- .0005 at worse probably better. To do a proper pressfit, (ie you need a arbor press to install/remove the bearing) for the FRC sized bearings generally it’s +.001 to .0015. Press-fits are precision operations.

In my experience laser/waterjet generally at best hold maybe +/- .005 consistently. So trying to achieve a pressfit, your luck may vary. It really depends on operator skill and then fine tuning of dimensions to the machine. Also you have to worry about burrs and tapers, which this type of cutting is infamous for.

Since I don’t have constant access to a waterjet/metal cutting laser I generally CNC my gearbox plates and etc. To do it right you have to do test cut some pieces generally, and test your tool for wear and add some tool compensation. And worry about surface finish, deflection etc. Not that it’s hard, but it takes a bit of practice.

If you are set on laser/waterjetting these, figure out what kind of quality you can get from them. If it is to poor just allow for a slipfit and design in some mechanical retention of the bearings.

Sounds like you’re getting a sheetmetal shop to do parts for you.

Lasers can cut press fits, or at least relatively close slip fits. I know teams like 971 laser their gearbox plates, although I’m not sure their bearings are press fits. Of course, YMMV based on the machine and operator.

Based on my experience, you’re not going to be able to get a press fit off of a waterjet. If I had to do a press fit on a waterjet, it would likely need to be with a multi-axis headed machine, with an operator that knew his speeds and feeds, and after cutting a good number of test parts to determine the jet diameter, kerf, and feed rates for that material with the orifice being used that day. Navid Shafa is sometimes on these forums, and does waterjet R&D, so he might step in here with more info on press fits on waterjets.

Basically, given the machines your parts are being cut on, I’d plan on either mechanically retaining the bearing or reaming/sanding the hole to the right fit. If the parts come back as press fits, awesome, but I’d at least have a contingency plan unless they don’t.

When we waterjet cut press fit bearing bores we would have them cut at the exact or slightly under sized dimensions and then use a flap wheel on a drill to enlarge it until we are happy with the fit.

We’ve been asking for -0.0005 to -0.001 when we remember. You can also hit the material around the hole with a hammer to close it up a bit if you forget…

If it’s thicker plate, you can use a spring punch (the kind for marking holes) on the inside of the hole to turn it into a press fit. That was an interesting trick to learn.

I’ve found that to be inconsistent at giving you a good press that doesn’t become loose later. YMMV.

We didn’t put a lot of load on that bearing (failed prototype) for a long time, so I’ll keep that in mind.
Mainly we did that because I failed at boring the hole correctly and made it ever-so-slightly too big for a press fit. It’s usually better to just get it right the first time.

In light gauge sheet metal for a press fit the Mfg engineer can make the hole a D shaped hole. The engineer adds a small .001 flat in to help keep the part retained. The Design engineer should draw the hole to the size of the bearing.

Annotate the CTF drawing that the hole is a press fit bearing hole.

Better to be under than over in size when it comes off the machine. Always consider heat, vibration, load, bearing length, etc in the design to retain the bearing.