Used Excavator Slewing Bearings: The Play Must Not Exceed This Number

March 8th, Qingyuan. A 2016 36-ton excavator, 9,800 working hours, asking price 410,000 yuan.

Condition looked good — clean engine, no seepage on the hydraulic pump, even fresh floor mats in the cab.

I had the operator climb in and swing the upperstructure empty. I crouched by the undercarriage, hand on the slewing bearing, listening.

Halfway through a rotation — a clunk. I already had my answer.

I set up the dial indicator and measured: 3.1 mm.

The seller leaned in and said the gauge was off. I pulled it, re-zeroed it, measured again: 3.0. He switched stories: thermal expansion these days.

I didn't argue. March weather doesn't expand three millimeters of play into a bearing.

How to measure the play

The slewing bearing connects the upper and lower frames, dozens of steel balls running in the raceways. Wear them and there's play between upper and lower.

The method isn't complicated. Set the bucket on the ground to lift the upper slightly, mount the dial indicator on the undercarriage, and put the needle against the bearing's outer race.

Pry the upperstructure; the needle's swing is the play.

Without a dial indicator, feeler gauges barely do — but feelers read static clearance; dynamic play needs the dial.

Three points to watch: the gauge mount must hold firm, pry evenly, measure in four directions, take the maximum.

Some machines show more play on one side — eccentric wear, and those are worse than evenly worn ones.

Eccentric wear usually comes from years of one kind of work — always swinging spoil to the same side on a slope, the bearing loaded on one flank, and in a few years one side of the raceway pits out.

Replacing the bearing on such a machine may not fully fix it; the upper frame's deformation has to be checked too. That's another day of checking.

What counts as over spec

Factory standards: new-machine assembly clearance generally runs 0.1 to 0.3 mm. In service, most models allow up to about 2 mm of wear.

Brands differ slightly — Japanese tighter, domestic looser — but the 2 mm line is industry-wide. Past it, it's time to replace.

My own rule of thumb: on 20-ton-class machines, get wary past 1.5 mm. Above thirty tons, past 2 mm I don't buy.

Minis are looser — a 5-ton machine within 1 mm is normal.

What does 3.1 mm mean? The raceways on that bearing have worn into steps and the balls are jumping in pits. Keep running it: first the swing sticks, then the motor overloads, and worst case the bearing disintegrates and the upperstructure tips.

Tip-over isn't a figure of speech. An upperstructure weighs a dozen-plus tons; coming down is an accident. On a real site, what you pay stops being about money.

What a replacement costs

The seller said a little extra play is nothing — five more years in it.

I asked him: a slewing bearing for a 36-ton machine — 58,000 yuan genuine, 30,000-odd aftermarket, plus 15,000 labor to remove and install — fifty thousand minimum, accidents extra. Are you paying that?

He went quiet.

Swapping a bearing isn't swapping a part. Separate upper from lower, crane work, alignment, clearance adjustment — seven days down.

What's a day of downtime worth on a site? A 36-ton excavator bills around 3,000 yuan a shift — over 20,000 for seven days. Fifty thousand parts, twenty thousand downtime: the buyer swallows that whole ledger.

So with play, what you save at inspection you pay back on site.

The 49,000 yuan I lost

In 2021 I bought a machine at 2.6 mm of play, telling myself it could hold.

Third month with the customer, the bearing started knocking. We covered the full replacement — 49,000 yuan, including rush air freight on parts.

What 49,000 bought: mechanical clearance never grows back; it only grows bigger.

Since then, over-spec play means no purchase. No exceptions.

On that 2021 machine, the seller had sworn on his chest it had just finished break-in. Nine thousand hours — break-in. I believed him; I paid.

So now I don't listen to what sellers say. I listen to what the needle says.

That 3.1 mm machine — I offered 200,000, parts-machine pricing. The seller called me a shark. I didn't buy it.

Half a month later I heard it sold for 210,000. If the buyer knows a bearing swap costs fifty thousand, the math works; if not, he's the next one paying tuition.

Three field tricks when you look at a machine

No dial indicator on hand? There are rough ways.

One: listen on an empty swing. Clunks and clicks are all wrong. A healthy bearing hums evenly.

Two: watch the swing arc. Rotate the upper slowly a full circle and fix your eyes on the bucket-tooth tip; if the path isn't round or it hitches, the raceway is likely damaged.

Three: check the grease. Pump the bearing's grease fitting and look at the old grease — glittering metal powder means walk away.

How do you read metal powder? Smear the pumped grease on white paper and hold it to the sun. Fine glitter is normal wear; flakes and needles are raceway spalling. Learn to tell the two apart.

These three tricks won't call borderline cases, but they screen out eighty percent of the junk. For borderline machines? Bring the dial gauge. Don't cheap out.

Remember these numbers

On a used excavator, hours can be rolled back and paint can be sprayed — only mechanical clearance doesn't lie.

Under twenty tons, 1.5 mm is the line. Over twenty tons, 2 mm is the line.

People ask: if an over-the-line machine is 30% cheaper, can you buy it, fix it, and resell? Yes — but you price the repair in and tell the buyer plainly. Say nothing, and you're running a scam.

An over-the-line machine, however cheap, isn't equipment you take home — it's a landmine with no timer marked on it.