A chipper's disc or drum spins on a main bearing. When that bearing fails, the machine stops earning until somebody fixes it, and if it seizes hard enough it can take the shaft or the housing with it.
But the bearing is not what totals chippers. The bearing is cheap. What totals chippers is how the manufacturer sells the parts around it, and what the machine is worth on the day the quote lands. On a $6,900 machine, a $700 assembly plus shop labor is not a repair decision. It is a conversation about whether you still own a chipper.
The good news: this is the one failure you can screen for by hand, in about five minutes, before any money moves.
Heat and dirt, not hours
Bearings die from heat and contamination. The hour meter has very little to do with it.
Run one short of grease and friction builds heat until the races work loose. Get dirt, water, or metal shards from some other failed component into the grease and the bearing turns into its own grinding compound: races against rolling elements against debris.
How much does maintenance actually matter here? One arborist's original chipper bearing lasted 22 years before heat and contamination finally worked the races loose. The same class of part can fail in a small fraction of that on a machine run with an open access door. Two decades or two seasons, same part. The bearing is not deciding that. The seals and the operator are.
The clearest documented case: on a Morbark M12R, both lower feed roller bearings failed together, and the mechanic pulled the housing apart to find it "jam-packed full of Filth because that's what kills these bearings." He tied it directly to running the machine with the bottom access door down. That is not a design defect. That is a door somebody left open.
What late-stage failure looks like
On a Vermeer BC-XL machine, a bearing replacement job found the drum and feed-roller bearings "dry, seized up" and needing full replacement, with the protective cover already broken and cracked before the repair started. They went in as a pair alongside a new belt.
Read that sequence in order, because it is the whole story of how these machines die. The cover broke. Grease escaped and dirt got in. The bearings dried out and seized. That machine was throwing warnings the entire way down, and somebody kept feeding it until it stopped.
The parts-policy trap
This is the mechanism that actually totals machines, and it has real numbers on it.
On a Vermeer feed pump repair, the drive shaft had snapped. The owner called the dealer. The shaft by itself, OEM and in stock, was quoted at about $120. Reasonable. But the dealer would not sell the internal bearing separately. The only way to buy that bearing across the parts counter was to buy the full pump and shaft assembly, at $700.
The owner bought the $120 shaft and sourced and pressed in a bearing himself. So the same repair costs $120 or $700 depending on one thing: whether you can do press work, or know somebody who will. That $580 spread has nothing to do with the machine's condition.
For a main disc bearing bought as a part, the figure that circulates is "a couple hundred dollars." That comes from a cross-brand disc-chipper repair rather than a specific manufacturer's parts counter, so treat it as an order of magnitude and not a quote. The point survives either way: the bearing is cheap, the assembly is not, and nobody publishes the labor. Get a written quote before you decide a machine is worth buying.
The free-spin test
The single best pre-purchase check on this system costs nothing and takes a minute.
With the machine off, turn the feed wheel by hand. A feed wheel that spins unusually freely is the tell-tale symptom of a broken drive shaft. The field confirmation, per the mechanic who documented it: pull the feed pump off the side and check the shaft, because "if half the shaft comes out with it, you probably have a broken shaft."
One minute of your time catches the costliest documented feed-system failure on this page, before you have committed a dollar. Do it on every machine you look at.
Inspecting the main bearing
Four things, in this order.
Spin it by hand, engine off. You are feeling for anything other than smooth rolling. Roughness, grinding, a stick-slip catch, or resistance at one point in the rotation all say the bearing is damaged. This is the check that ends conversations.
Listen with it running empty. Engine on, nothing feeding. A healthy bearing is close to silent. Grinding or rumbling means the races are already damaged and what you are hearing is metal on metal.
Feel the housing after a run. Warm is normal. Too hot to keep your hand on for more than a few seconds means heat is getting away from it. If you can get the seller to run material through for ten minutes, check it again under load. A housing that climbs into cannot-hold-it territory under load is a bearing in distress.
Look at the seals. Cracked, deformed, or flattened seals let contamination in. Bone-dry and cracked usually means the bearing has been running dry. Dark grease weeping out means the seal is already compromised. Visible rust on races or rolling elements means water got in and the life is already spent.
Reading the listing
Low hours plus a hot bearing housing after a short run means the meter is telling you nothing useful. Get a repair quote in writing and subtract it, or walk.
"Bearing just replaced" or "freshly repacked" is genuinely good news, provided you can verify the date and see the invoice. A recent bearing job means somebody found a problem and fixed it properly, and that is worth paying for. Unverified, it is a sentence in an ad.
And a seller who cannot tell you whether the access doors and seals have been intact has just told you what killed the last set of bearings.
Prevention, if you already own one
Keep the access doors closed and the seals intact. That is the entire prevention program for the most-documented bearing failure across these machines, and it costs nothing. The M12R case above traces to exactly that and nothing else.
Follow the greasing interval in your operator's manual rather than a number somebody quoted you at a job site, and know that over-greasing is its own failure path. Push too much grease through and you drive it past the seal, which opens the bearing to the same dirt you were trying to keep out.
After wet work, check the seals. Water in the grease is a bearing on a shortened clock.
Parts availability decides your downtime
Before you buy, find out who stocks bearings for that machine. On a common platform the parts move through normal dealer channels. On a model with a thinner resale market, fewer shops carry anything for it, and you are either waiting on an order or driving the machine to another town for the press work.
That is real money on a machine that only earns when it runs. Put it in the repair budget before you sign, not after.
Pull the records for the machine you are considering before the inspection: the Vermeer BC1000XL for the documented dry-and-seized bearing case and the feed-system architecture, the Bandit 250XP for the feed-roller-bearing weak point, and the Vermeer BC700XL if you are shopping the small end, where a $700 assembly quote eats a much larger share of the machine. Then do the free-spin test and the hand-spin on the disc before anybody says a price. Five minutes, no tools, and it decides whether you bought a bearing you budgeted for or the one that ends the machine.