Many operators get worried when their excavator slewing bearing leaks grease, fearing that the machine might be faulty. For example, someone reported that after 700 working hours and just three grease injections, oil started leaking.

Actually, don’t panic — this is not a major problem. What you should do is recall whether the machine had ever entered deep water (above the top of the tracks) or worked in muddy water. If neither happened, you can continue using it with confidence. If you are still concerned, you can open the grease-injection flange on the slewing bearing (usually a square flange plate fixed with four bolts) and check inside for degraded grease or mud. This is the last-resort method — the “secret technique.”

Think about it: for any grease-injection point on machines like excavators or loaders (excluding automobile wheel hubs, which require disassembly to replace grease), when you inject fresh grease, where does the old grease go? For example, the grease you inject daily between the bucket arm and the bucket — where does it go? It wears away! If it didn’t wear away, you wouldn’t need to inject grease, right?

So, if the grease in the slewing bearing area doesn’t leak out, that would actually be a bigger problem. It would mean the used grease — containing worn material or even metal shavings — is staying inside the machine, entering the slewing reduction gear! If you check the parts manual carefully, you’ll understand this point.

Some operators ask: why didn’t this grease leakage happen before? The answer lies in design philosophy. It’s simply that the path of old grease discharge has been changed. The seal ring cannot fully block grease leakage.

In some high-end loader brands, pin joints are not lubricated with grease but with thin oil (engine oil). These are checked less frequently (according to the manual) and replaced periodically. That’s because those joints use high-precision mechanical seals instead of simple, inexpensive seal rings.

Structurally, for excavators under 30 tons, the slewing bearing is a relatively inexpensive, simple ball-type slewing bearing. So-called high-end brands may have higher precision bearings, greater assembly preload, or even some interference fit. Only larger machines use double-row or triple-row cylindrical slewing bearings (just like large wind turbines!). The sealing of these slewing bearings is always rubber rings, since their rotation speed is low — at most 11–12 rpm, and that’s under extreme operating conditions. The larger the machine, the slower it rotates. (This principle was already applied during WWII, when the T-34 tank exploited its fast turret traverse to counter the more precise but slower-turning Tiger II.) For reference, wind turbines rotate only about 5 rpm.

Therefore, grease leakage does not mean the slewing bearing is damaged — the situation must be analyzed according to operating conditions.

If you truly encounter slewing bearing seal failure or detachment, don’t worry — it’s a minor issue. Simply prepare the seal strip (ordered by length), measure the required length, cut the ends into an angled joint, align with the installation direction (don’t reverse it), apply adhesive to the joint, and bond it together. That’s all.

Points operators should keep in mind:

• An excavator is not a submarine. It must not operate in water higher than the track plates. If it does, once it leaves the deep-water area, you must immediately drain any mud or water, then inject new grease until the old grease is completely flushed out.

• If you check carefully, you’ll find that many brands of slewing bearing mounting surfaces are not fully sealed. So, if the machine enters deep water, water will definitely seep into the slewing bearing. CAT excavators have fully sealed slewing bearing mounting surfaces, but even that cannot completely prevent deep mud and water from entering. Moreover, small excavators have three drainage holes in their mounting surfaces, which can also allow water in. Operators should be aware of this!