Machined Parts: No Clearance

[David Winer]

I’m making a cart for my canoe club. It will haul boats back and forth from the canoe shed to the river. Mostly it’s of wood, but the ¾ inch axle and ¾ inch bearings are of steel. Apparently the two are made so precisely that there is no clearance for the axle to slide through the bearings.

I have tried smoothing and lubricating both articles but to no avail—the mating is still too tight to allow slipping the axle through to the desired position in the bearing. I’ve include photos to illustrate that a ¾ inch bit won’t go through the ¾ inch bearing hole.

Suggestions from Creekers should help. Many of you are skilled in metal-crafting as well as woodworking.

[Gerry Grzadzinski]

Sand the axle down with some 400-600 grit sandpaper. It shouldn't take much at all to get it to go in.

[David Eisenhauer]

Or use Clover compound and/or emery cloth.

[Charles Taylor]

Heat the bearing (and the casting into which it fits) directly atop a 100 watt incandescent bulb for a few minutes. Also, if it will fit, put the axle in the freezer for a while beforehand. Don't forget gloves to handle the bearing.

[David Winer]

Sand the axle down with some 400-600 grit sandpaper. It shouldn't take much at all to get it to go in.

As I said, I have tried this (using 600 grit wetordry paper). I'm uncomfortable with smoothing the axle because it would need to be to kept round over its whole length.

I'm hoping for a solution based on someone's experience rather than "have you tried?" For example someone might say to "warm the bearings and cool the axle, or take the bearings to a machine shop to enlarge the holes--that's the way I do it."

[David Winer]

Heat the bearing (and the casting into which it fits) directly atop a 100 watt incandescent bulb for a few minutes. Also, if it will fit, put the axle in the freezer for a while beforehand. Don't forget gloves to handle the bearing.

Thanks, Charles. Have you done this before? In this case I would want to try it first, then take the bearings off for re-assembly later in their final position on the axle. I'd hate to have the parts seized together so I can't remove them for later assembly.

[Lee Schierer]

Thanks, Charles. Have you done this before? In this case I would want to try it first, then take the bearings off for re-assembly later in their final position on the axle. I'd hate to have the parts seized together so I can't remove them for later assembly.

If you heat and cool the parts to get them together, they won't come apart willingly without a good press.

[Russ Ellis]

I would not heat the bearings without knowing the rated operating temperature.

Dry ice to shrink the diameter of the shaft is an option. Just don't drop it.

A good machine shop can assemble these with a hydraulic press.

Bearings are typically intended to have an interference fit with the shaft. This makes sure the bearing race is spinning and not the shaft spinning inside the bearing race. The interference fit would be in the range of +0 / -0.002 inches.

[Gerry Grzadzinski]

As I said, I have tried this (using 600 grit wetordry paper). I'm uncomfortable with smoothing the axle because it would need to be to kept round over its whole length.

Do you need to slide the entire axle through it, or just the ends?
Do you have a caliper or micrometer that you can measure the axle with? I would expect that the bearing is pretty close to .750", but the shaft is probably a bit oversize. Even .001-.002" oversize and it won't go in, but should be easily sanded enough to fit. And yes, I've done this before.
For what you're using it for, it shouldn't matter how precise your sanding job is, as long as the axle slides in.

[Charles Taylor]

Thanks, Charles. Have you done this before? In this case I would want to try it first, then take the bearings off for re-assembly later in their final position on the axle. I'd hate to have the parts seized together so I can't remove them for later assembly.

I have, when reassembling a drill press. Probably will be doing it again soon with my table saw, which I have taken apart to replace the old bearings. I didn't realize you were planning a "trial run", and I agree with Lee's warning that to get them apart again, you will likely need a press or puller.

[Matt Day]

I've used the freezer/oven method and had success. I think i did so when I replaced the my TS arbor bearings.

Regarding the statement about keeping the shaft round, I don't think you're taking off nearly enough material for that to be a concern.

[David Kumm]

Heating bearings is a common way to mount them. Get a heat stick and mark the inner and heat until the mark liquifies. Once on you won't likely get it apart without wrecking the bearing but they don't look like expensive ones. If heat or cool doesn't do it, you could take the shaft to a machinist. Shouldn't cost much to take a .001 off the shaft. Dave

[Keith Weber]

A good machine shop can assemble these with a hydraulic press.

Bearings are typically intended to have an interference fit with the shaft. This makes sure the bearing race is spinning and not the shaft spinning inside the bearing race. The interference fit would be in the range of +0 / -0.002 inches.

Russ beat me to the punch. Bearings usually are press fit on to the shaft. Find someone with a hydraulic press, or hefty arbor press. You'll need to know where you want to stop the bearings when being pressed onto the shaft, so you can assemble everything later. Another option might be to source a bearing with a slightly larger inside diameter. When I say slightly larger, I mean a few thousandths, not the next fractional size up. This is a canoe cart and not a planer head, so a press fit bearing is probably overkill anyway.

[ian maybury]

Hi guys. Pardon a different view. Stock pillow blocks and flanged bearing units like David's are normally sized so that standard bright or drawn shafting is a close sliding/clearance fit in the bore of the bearing. The bearing inner race is locked on to the shaft so it doesn't turn/spin using the provided set screws. It's basically intended as a cheap and cheerful (= no machining, sealed, self aligning, easily assembled) means of mounting a not too heavily stressed shaft in a bearing. They can over time with a bit of corrosion become locked on and need cleaning and/or a pullers to remove. (it won't anyway survive very long if dunked in water)

The table near the top of the first page of this chart lists typical shaft and bearing inner race bore tolerances in inches: http://www.emersonindustrial.com/en-...Form_9579E.pdf There's a similar table in metric units on page 13 in this FAG bearing write up: http://www.schaeffler.com/remotemedi..._219_de_en.pdf

Drawn/bright shafting is typically produced a hair under nominal size for this reason (perhaps 1/2 thou at this size), and bearing inner races a shade over. They have long since been made to a stock tolerance class determined by standards - so fit problems are unusual. At 3/4in dia the total clearance is likely to be something less than 1 1/2 thou - which should mean the shaft slides freely in the race, but doesn't rattle about.

All it takes it a small ding or a bit of rust on the shaft to mess up the fit, or maybe a burr on the inside of the bearing race. It'll jam tight on a few specks in the wrong place. Maybe as the guys say a touch with emery cloth or whatever would sort it out? Go easy on the bearing getting the shaft out or back in - support the inner race using a square ended tube/sleeve something. Unsupported hammering will destroy the ball race.

If cleaning up doesn't sort it and the shaft is truly oversize for the bearing, then something is likely wrong and it's a case of changing for replacement parts. If you have a micrometer you could measure the shaft and decide whether the problem is the shaft or the bearing...

[Ellery Becnel]

Ian is spot on for this application. Not all bearings are made to have an interference fit with the shaft. The issue is most likely with the shaft. The only way of knowing is to measure both.

Ellery Becnel