This was posted in a relative haste, so comment on any corrections/clarifications if necessary. I may edit to add more actual photos later on.
Step 1: Disassemble the Axle Assembly
A future instructable project is coming concerning frame alignment. Please see it first if applicable!
Have at least 3 separate rags for this procedure. Rag 1 is for the raw glop of removal, rag 2 is for finer cleaning after rag 1, and rag 3 is for reassembly.
Have some non-chlorinated brake cleaner aerosol, and a glass jar to catch the overspray for best results. The overspray will still be useful to you in the future.
For front axles as covered here, the situation is essentially the same. Best method is as described below:
If you do not have a cone wrench (a recommended tool), try to determine the side that will give way first by unscrewing the locknuts away from each other. The side that moves relative to the axle is your first bet, then retighten the "stubborn" side to the fork as tight as possible, once this procedure allows. Then loosen and remove the nut from only one side of the front axle from the fork, and then loosen the cone locknut. Sometimes the cone locknut will stubbornly hang onto the cone. loosen it enough to get any form of wrench or pliers onto it to break the two free from each other in as few turns as possible. Since you are repacking the hub, use of penetrating oil such as "PB Blaster" is recommended, even if the parts are not rusted/seized.
Once you have them free from each other, spin them off completely and put them on rag #1. Scour off as much old sludge as possible with this rag from the cone and locknut first, then from the axle and other cone/locknut on the other side. Perfection is not required yet, just get the main glob of goo off.
Step 2: Clean Bearings and Cups
If the bearings are in a cage, and you have additional ball bearings of the same gauge to make it a "loose ball" bearing, discard the cage and clean them all by soaking a portion of the rag in solvent and "shuffling" them in the rag by creating a pocket with your hand and rubbing the bearings into the rag.
By now, you should have a clean-looking set of parts. I hope that you have salvaged an old hub to make this a "loose-ball" bearing kit, but if not, be very sure to use the hem of the rag to clean out the cage. Remove balls from the cage by pressing them inward to push them out. Again, this is not clean yet, and if you plan to re-use the cage, you will have to follow additional steps.
Step 3: Inspect the Bearing Surfaces
DO NOT USE A MAGNET TO HOLD ANY PART OF A BALL-BEARING ASSEMBLY TOGETHER FOR ANY REASON! Doing so will magnetize the parts and actually draw damaging metallic particles into the bearing and cause a premature failure later on in it's life.
Use the same rag to clean the inside of the hub and especially the bearing cups and cones as thoroughly as possible. Now it's time for inspection.
An ideal is that you should see a relatively even wear line on the cones that looks like a polished line that is centered in the curvature of the cone taper. This should be matched with the same impression on the inside of the bearing cup (race) in the hub.
Inspect each ball itself for defects such as pitting or galling. Remember these parts are "case-hardened", meaning they are harder on the very outside than on the inside. Visualize a steel M&M...This; goes for the cone and race as well. Pitting will appear like the metal was "pinholed", and is usually the result of abrasive contamination. Galling looks more like some kind of acid burn, and is a result of contamination and/or lack of lubrication, where the metal has eaten itself. Only extremely minor pitting is something you can get away with on occasion. Galling will result in you never being able to properly adjust bearing preload and will just roughen every other contact surface like a contagious disease; dispose of any part demonstrating a galling effect. If the cup (race) in the hub is galled, junk the entire hub assembly regardless of the apparent condition of the individual parts. The only salvageable part of such a condition is the axle stud itself if anything, if it is not sprung.
Pay attention to the wear line around the circumference. If the line gets wide, you can see the load-bearing area, but if it gets too wide, it shows a bearing well-lubed, but poorly adjusted or not maintained, for too long a period. A wear line should not deviate more than 5% in width from thinnest to thickest. If it does, junk the part as it has worn way too far out of round to be useable. You didn't get to it in time to save it....
I cannot get accurate photos of the difference. so an illustration will have to do. The first is an acceptable wear line, the second is a good reason for recycling and demonstrates a potentially bad bearing group that has soured the entire hub operation. If your bearing cone looks like this, odds are that you are in the market for a new hub. The example is of a cone from opposite the galled bearing on the other side. This is how the effect ruins the other bearing, and why I say to toss the whole assembly when cups go bad. Only in certain special circumstances can a cup be saved from the hub with any hope of positive results.
The illustration provided shows an ideal condition. You can never repack your hub bearings too often.
Step 4: Repack
After assuring that all bearing surfaces are clear and clean at this point, you are ready to rebuild.
Gently squeeze the bearing cage "fingers" down to hold each ball snugly but without binding. The cage is there to keep the balls equidistant for optimal strength, but it should not bind against the balls or any other bearing surfaces. Simply pressing the balls in will likely open the cage enough to allow free movement, but check to be sure.
Now, smother the cage assembly in grease, pushing grease from the inside out well past the point where grease starts to ooze from the outside. Positively drown it in grease, and add a significant bead of grease to the inside of the hub's bearing race, and place the cage assembly in the race, using the grease to hold it in place.
You simply cannot use too much grease for this step, and you should not be conservative on this step. Best is to saturate the bearing with as much grease as you can cram into it. Excess will be cleared later.
Step 5: Reassembly
For those making the conversion and not sure of how many extra balls to add, just add enough to fit about in the wear line. One too many will demonstrate itself as too full to reassemble properly, so in such case, take one out. The bearings should fit in the same space as the caged version did, and one too many will be clearly evident. Use the same number of bearings for both sides, usually only adding 2, maybe 3 more will suffice. 7 balls caged usually equals 9 balls loose on a full-sized axle. Only fill a gap larger than 110% the diameter of a new ball. A little space between loose balls is acceptable.
Again, it is better to put too much grease than not enough at this point. I recommend using "high-temp disc-brake bearing grease" (automotive grade black lithium base) since these are low-speed high-load bearings per typical lube specifications. The thicker the grease, the better at this point, and water repellance is a nominal and desired property, regardless of bearing configuration.
A cone wrench is strongly recommended at this point to ease assembly and preloading.
Step 6: The Art: Proper Preload Adjustment
Loosely assemble the hub assembly and install the "fixed" side, and secure that side's stud nut (lug). With a cone wrench in hand, slowly tighten the cone as you spin the wheel opposite the tightening direction until you feel a slight growl from the bearings through the wrench. The growl says "too tight". Loosen 1/8 turn at a time and moderately seat the locknut against the cone. Remove the wheel from the fork.
Spin the axle by hand, it should spin freely (gooey actually) without any grittiness, yet without an excess of free-play (looseness). You should just barely be able to detect looseness and never feel a "gritty" sensation. Keep in mind that the preload will increase when you secure the axle lugs, so you want a slightly loose setting to counteract this effect. Wipe away any excess grease as well, as some will ooze out past the seals (indicating overfilling, but a little bit over is desired), and the bearing will automatically purge excess grease to the amount it can use as it rotates.
Reinstall the wheel with the "fixed" side tightened down to full torque and the other side free...Spin the wheel very lightly to look for the sign of "stepping", where the wheel seems to lock into a "detent". If you do not see this, tighten the other lug loosely, hold the wheel steady and try to rock it laterally to feel for the slightest sensation of a "clunk". Ideal is that this should be barely perceptible. No perception of this means to loosen the cone by 1/6 turn at a time until you feel it. Do not mistake the flex of the fork for freeplay.
When you are confident you have this set to a barely perceptible sense of free-play, lock the cone locknut down firm, but not tight yet. Torque the remaining lug to proper torque. Spin the wheel barely, again checking for "stepping". Check for freeplay again. if there is none, it *maybe* is too tight still. Loosen "adjustment-side" by 1/4 turn, locknut and all, and retighten all as if it was final assembly. Check again for freeplay. If you can perceive the slightest bit of freeplay, tighten the cone 1/8 turn, lock it back down, and repeat until the clunk is completely gone, but NO tighter. If you have reached this point and the wheel has no lateral freeplay but spins freely, you are just about done.
Time and experience with performing this service will make this easier and quicker each time, until you can do this in your sleep and even skip a few steps and still come out perfect.
Step 7: Troubleshooting
The ideal threshold for tapered-ball bearings such as those for bicycle hubs overall is just a *hint* tighter than the threshold of freeplay to pre-load the bearings for varying stresses so that they wear properly. There are two bicycle axle gauges, and the larger the better. If the rear axle is thicker than the front, your front axle is in danger of breakage due to some moron's idea of lowering manufacturing costs. If they match with a silver color, they are soft steel and are probably already sprung and bending your dropouts further. Investing in "black steel" or Chro-Mo axles will save you troubles from warped/bent axles and are far more resistant to becoming "sprung" than the soft-steel versions on cheaper hubs. I have landed a jump from over 50 feet high without damaging the axle at all. No comment on the frame or the wheel though.....
Misaligned dropouts almost always lead to sprung axles, and in turn bearing failures, so look for a future instructable on frame alignment by me, coming soon. I do not recommend attempting to straighten them on your own because there is a specific method involved to minimize weakening them. A broken dropout while riding can ruin your day in ways you can't imagine, so don't even attempt it without the assistance of a qualified bike-builder. Furthermore, over-correcting when cold-bending metals weakens them twice as much for every bend you make.
Step 8: Final Steps
If all is well, pay attention to the hubs for about 30-50 miles and wipe away any excess grease that may ooze out of the seals to prevent attracting more dirt. Keep your hubs free of grass and hair (somehow it'll find it's way from someone's head to wrapped around your axle, it is still an unexplained phenomena in quantum relativity), because they both eat seals like asphalt eats away flesh at 30+mph.
As a further tip, if you dunk your bike (whether off a jump or discovering uncharted water caves in a city-endorsed pothole), you may need to do this service far more often to preserve parts. Salt and lithium don't always play well together, so you sea-side residents may have to do this sooner than the typical 30,000 mile maximum.
Step 9: You Are Done!
I have ridden a mountain bike at over 70MPH regularly on what would be considered "suicide runs", despite being unnatural for it's intended construction. I am a specialist in attempts for the land-speed record for bicycles (and a practiced attempter), so speak up if you have an issue relevant to this project. I'll be glad to hear your queries.