Maintenance Guide · Seal & Lubrication
Swing Gearbox Oil Leak: Diagnosis, Seal Replacement, and When the Leak Means You Need a New Gearbox
A swing gearbox oil leak is the most common excavator swing system service issue. In most cases it is a seal replacement job — straightforward, inexpensive, and preventable. In some cases it is the first symptom of a failing gearbox that needs replacement. This guide tells you the difference, and how to fix the seal problem correctly when that is all that is required.
Leak Location Identification
Identifying the Leak Location Before Disassembly
Before opening any part of the swing gearbox or motor assembly, clean the entire swing drive housing assembly with a pressure washer or degreaser and let the machine dry completely. Then run the machine for 15 minutes of normal swing operation and observe where the fresh oil appears first. The appearance location is not always the leak source — oil can travel along surfaces before dripping — but it narrows the investigation significantly.
The output shaft (bottom) seal is the most common swing gearbox leaking oil fix location, accounting for approximately 60–70% of all swing drive oil leak repairs. It is also the seal most directly contaminated by the environment — the output pinion shaft points downward into the slewing ring area, which is constantly exposed to water, mud, and abrasive debris from excavation work. A leak at this position always indicates seal replacement is required; unlike the motor-to-gearbox top seal (which may indicate a motor internal pressure issue), the output shaft seal failure is always a mechanical seal wear or damage issue.
Excavator Swing Gearbox Seal Replacement — Step-by-Step
Position the drain plug over a catch pan and drain fully. Inspect the drained oil — note the colour (black = heavily oxidised), texture (milky = water contamination), and the drain plug magnet (metallic chips indicate internal wear beyond seal replacement). If there are visible metallic chips in the oil, the seal replacement alone will not solve the root problem — internal wear has already occurred and a full gearbox assessment is required.
Before the unit is removed, confirm how much oil was in the gearbox — a swing gearbox oil level check that reveals the gearbox was significantly underfilled (less than 50% of spec volume) means the seal has been leaking for a substantial period. In this case, check the slew ring bearing grease and bearing teeth for oil contamination — gear oil from a leaking output shaft seal contaminates slew ring grease and accelerates ring bearing wear. If the slew ring grease is oil-contaminated, regrease after the seal replacement.
The swing gearbox oil seal type must match the shaft diameter exactly — even a 0.5 mm undersize inner diameter creates a seal that does not seat correctly on the shaft and leaks from first operation. Most excavator swing gearbox output shaft seals are metric TC (double lip) seals. The size is stamped on the seal face — a 50×72×10 designation means 50 mm bore, 72 mm OD, 10 mm width. Always match all three dimensions. If the original seal is not available for measurement, look up the machine service manual seal specification table for the exact shaft position.
Drive the seal into the housing bore using a flat disc driver slightly smaller than the housing bore diameter. Drive squarely — a cocked seal (not perpendicular to the shaft axis) leaks from day one regardless of seal quality. Drive to a depth flush with or 0.5 mm below the housing bore face — not deeper. Applying grease to the seal lip (not silicone — grease only) before installation reduces the risk of dry-running seal failure in the first minutes before oil circulation reaches the seal face.
When the Oil Leak Means You Need a New Gearbox
Seal replacement solves the leak in most cases. But in four specific situations, a new seal will not last — because the leak is caused by a condition that the seal cannot compensate for:
- Scored shaft surface — if the shaft surface under the old seal has a visible groove worn by the seal lip, the new seal will leak at the groove within 200–400 hours. The seal lip cannot maintain contact across a groove. Repair: fit an oversize OD seal with a narrower bore running on an unworn area of the shaft, or replace the gearbox.
- Metallic chips in the oil — indicates active internal gear or bearing wear. Replacing the seal on a gearbox with active internal wear is a temporary fix; the gearbox will fail within 500–1,000 hours and the repair labour will need to be repeated. Assess the gearbox internally before deciding on seal repair vs. replacement.
- Housing bore damage — if the housing bore has corrosion, cracking, or wear that prevents the seal from seating concentrically, no seal of the correct dimension will form an adequate seal. The housing must be repaired or the gearbox replaced.
- Blocked case drain on the swing motor — if the swing motor case drain port is blocked or restricted, the motor case overpressure forces oil out through the motor-to-gearbox interface seal. The fix is clearing the case drain, not replacing the gearbox seal. A new seal under the same motor overpressure will fail within the same timeframe as the previous seal.
For the complete swing gearbox troubleshooting procedure including noise, vibration, and performance diagnosis, see our swing gearbox diagnosis guide. If the oil leak has been accompanied by any change in swing performance — slower swing speed, hesitation, or unusual noise — there is a high probability that internal wear is present and the gearbox requires replacement rather than seal repair.
Our excavator swing gearbox range covers all brands and machine classes from compact 5-tonne excavators to 40-tonne large machines — all with load-tested certificates and confirmed dimensional match within 4 hours of enquiry. MOQ 1 unit.
Swing Gearbox Leaking? We’ll Confirm If It Needs Seal or Full Replacement — in 4 Hours
Send photos of the leak location, the oil condition from the drain plug, and the machine model. We advise seal repair vs. replacement, and if replacement is needed, we confirm the correct unit and quote within 4 hours.
📧 [email protected] · Canada Planetary Gear Drive Co., Ltd · ISO 9001:2015
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Swing Motor Leaking Oil — How to Tell If It’s the Motor or the Gearbox
The swing motor and swing gearbox are bolted together as an assembly, and a swing motor leaking oil situation is frequently misidentified as a gearbox seal problem. The two fluid systems have different oil types — hydraulic oil (ISO VG 46 or VG 68) in the motor, gear oil (SAE 80W-90 GL-5) in the gearbox — and the leak location determines which system is leaking. However, contamination between systems can occur if the motor case drain seal fails, allowing hydraulic oil to enter the gearbox through the input shaft seal, or gear oil to migrate into the motor case drain port.
The diagnostic test: drain and refill the gearbox with a correctly measured volume of gear oil of a distinctive colour (dyed gear oil is available from specialist suppliers, or use a lighter-coloured oil if the gearbox currently contains dark-coloured oil). Run the machine for 4 hours. If the new oil appears at the external leak location, the leak is from the gearbox. If only hydraulic oil (typically lighter in colour and lower viscosity) appears at the leak location, the leak is from the motor case drain system or the motor-to-gearbox interface O-ring. If both appear mixed, the motor case drain seal has failed and both systems are contaminating each other — both systems require service.
In all cases where oil analysis of the gearbox drain shows hydraulic oil contamination — typically identified by lower viscosity and different colour compared to pure gear oil — the gearbox gear oil must be completely replaced before continued operation. Hydraulic oil is not a gear lubricant; it does not have the EP (Extreme Pressure) additive package required for gear surface protection, and a gearbox running on hydraulic oil contaminated gear oil will show accelerated gear tooth wear within 500 hours. See our complete excavator swing gearbox range for all replacement models with full technical support.