Jay Wright wrote:They do go kaput once in a while but they are usually easy to replace- some have a screw down inside the chuck, some don't. I replaced one with a keyed chuck so I could put some hurt on bits that were still slipping with the chuck as tight as I could get it by hand, and replaced one on a smaller old drill because it just locked up and wouldn't budge. In both cases I replaced the factory plastic with all metal chucks. They're not expensive.
If the chuck does need to be replaced, there are many choices, at various price points. I would recommend the best chuck you can afford/budget if the drill itself is worth keeping for the long haul. Living with chintzy chucks is frustrating. Keyed or keyless, pick your poison.
You'll need to determine how the chuck is mounted to the drill. It is almost certainly a thread mount (screwed to the motor/gearbox output), as Jay mentioned. Heavy duty stuff (but more customarily drill presses) may be taper mounted (a truncated metal cone on the output - or an adapter - fits into a mating conical hole in the chuck). We have a DeWalt mag base drill at work which is set up like this. There are also chucks with integral straight arbors (e.g. 1/2" diameter Weldon shank) but these are very unlikely in consumer grade equipment.
As jay said, if the chuck is threaded, there is, for any modern reversible drill, almost certainly (but not 100%) a left handed hex drive socket head or straight slotted screw which retains the chuck to the threaded output arbor of the motor/gearbox. That is, this will be a righty-loosey screw. Using the left handed thread prevents the chuck from unscrewing from the threaded mount when the drill is run in reverse. The exception would be if someone has already replaced the chuck on a reversible drill, but used an older style chuck with no provision for the lock screw because the chuck was built for a non-reversible drill. A hand impact driver (the kind you give a belt with a hammer - I like to use a shot hammer with these) may be useful when backing out the lock screw with a straight slotted head - just remember to use turn the screw clockwise to loosen it. If the screw has a hex socket (Allen) head, then you can use a ratchet driver or whatever else falls readily to hand to put some mustard on it (box end wrench on the short end of an Allen key, etc.). The hand impact driver may be helpful here, too. Worth a try.
Once the lock screw has been backed out (which may be a chore - don't hesitate to apply some penetrating oil and wait a while), you can unscrew the chuck from the arbor, lefty-loosey. Gripping a hex head screw in the chuck and running the drill in reverse while gripping the screw in a bench vise or with a pipe wrench may work. If the chuck is truly junk, you can just put the pipe wrench on the chuck, but I am always hesitant to resort to destructive means. There's always the "smacking it" routine! Once removed, check the pitch and diameter of the mounting arbor by comparing to screws/bolts of known diameter and thread pitch. 1/4"-20 and 3/8"-16 seem to be pretty common in the US. I'd guess 6 or 10mm coarse pitch for the rest of the world, but don't know, for sure. Test with a known nut, to confirm the conjectured thread. My local hardware store is always accommodating of me dragging in half-dead projects to check and compare hardware (most recently a pole lopper head). I would guess most any big box home improvement center would be similarly disposed, but I have limited experience.
Before remounting, a clean-up of the arbor threads with a wire brush/wheel and a bit of light machine oil would not be remiss. You may want to pick up a fresh left handed lock screw, also (the head may be munged up, or it may be a bit crusty with rust). Left handed screws are findable in most good hardware stores, at least for common sizes and thread pitches. Lock screws are usually fine pitch. For a 3/8"-16 threaded chuck mount, a 1/4"-28 left hand lock screw is customary. In the worst case, you'll need to mail order (internet) one of the left handed screws from the usual suspects, though at least two hardware stores within a short distance of my home stock some left handed screws. In the US, McMaster Carr and MSC (Manhattan Supply Company) are big online retailers of manufacturing supplies. They will sell to anyone, even though they market to industrial customers. They are definitely not the low cost leaders, however. Zoro and Grainger are somehow connected, as best I can tell, but Zoro's online prices is significantly cheaper for identical items. If you live outside the US, there are likely other vendors which cater to industry, and will have quality products and a good selection, even if the price is a bit high.
For general tool replacement parts, I have had a good experience with eReplacementParts.com for items/models which are in their catalog. This even includes off-beat stuff like a Makita 5402 16-5/16" diameter blade circular saw (old enough it doesn't have an electronic brake - "Danger, Will Robinson! Danger!" - die cast housing [no plastic], and a flimsy stamped steel shoe). I buy a lot of stuff which is a bit distressed, because I get deals that way. Buying replacement parts, many of which I have found at eReplacementParts.com, is part and parcel of that. It is worth checking there to see what is available. They'll usually flag out if the items are OEM, or if they are generic commodity replacements.