Common causes and troubleshooting methods for skeleton oil seals
Oil seals are dynamic sealing elements. The existence of a "critical oil film" is a necessary and sufficient condition for a seal to be effective; a leak-free seal is neither allowed nor possible. This is because the presence of a lubricating oil film is essential for ensuring lubrication and friction locking of the seal's cutting edges, and this film inevitably leads to a certain amount of leakage. For rotary oil seals, minor leakage during the initial 50-100 hours of operation is permissible. As operating time increases, the leakage will gradually cease, often resulting in a longer lifespan for such seals. Within the effective service life, minor leakage is acceptable; otherwise, the common causes and troubleshooting methods for oil seal failures must be followed.
1. Poor quality of the skeleton oil seal causes early leakage.
| No. | Fault | Cause Analysis | Troubleshooting |
|---|---|---|---|
| 1 | Poor lip condition of the oil seal | Poor manufacturing quality, with burrs or defects on the cutting edge of the oil seal | Remove burrs or replace the oil seal |
| 2 | Poor quality or failure of the oil seal spring | Poor manufacturing quality of the oil seal spring | Replace the oil seal spring |
| 3 | Insufficient radial pressure | The spring is too loose, resulting in insufficient clamping force | Adjust the oil seal spring |
2. Poor assembly of the skeleton oil seal components led to leakage.
| No. | Fault | Cause Analysis | Troubleshooting |
|---|---|---|---|
| 1 | Obvious scratches on the lip of the oil seal | During assembly, the oil seal lip is scratched by passing through keyways or threads | Replace the oil seal; use a protective sleeve during reinstallation to protect the lip |
| 2 | Oil seal deformed into a butterfly shape | Improper installation tools for the oil seal | Redesign and manufacture proper installation tools for the oil seal |
| 3 | Lip of the oil seal flipped in the wrong direction, or spring detached | Improper shaft chamfer, high surface roughness, or excessive force during assembly, causing the lip to flip or the spring to come off | Polish the shaft chamfer with fine sandpaper, apply grease, and install the oil seal carefully |
| 4 | Excessive grease applied between the oil seal lip and the shaft surface | Too much grease applied between the oil seal lip and the shaft surface during assembly | After the shaft runs for a period of time, the excess grease will reduce and return to normal |
3. Lip wear
| No. | Fault | Cause Analysis | Troubleshooting |
|---|---|---|---|
| 1 | Poor lubrication; severe wear on the working surface of the oil seal lip, with wear width exceeding 1/3 and appearing dull | Poor lubrication causing dry friction on the oil seal lip | Ensure proper lubrication of the oil seal |
| 2 | Shaft surface roughness higher than 0.8 μm | Excessively low shaft surface roughness leading to severe wear of the oil seal lip | Reduce the shaft surface roughness to below 0.8 μm |
| 3 | Lubricating oil contains dust/impurities, or lack of dust cover allows ingress of foreign matter | Contaminated oil, dirty hydraulic system; dust ingress causing abnormal wear; foreign particles (metal chips, casting sand) scratching the lip; paint mistakenly applied to the lip or shaft | Ensure clean lubrication, clean the hydraulic system; install a dust cover to prevent contamination; clean the lip/shaft during assembly and remove any mistakenly applied paint |
| 4 | Excessive radial pressure on the oil seal lip, breaking the oil film and causing dry friction | The oil seal spring is too tight | Adjust the oil seal spring |
| 5 | Eccentric installation causing abnormal wear of the oil seal lip (symmetric wear distribution); wear marks on the primary and secondary lips are symmetric but opposite in position | Misalignment of the housing, end cover and shaft causing eccentric rotation; undersized seal bore leading to tilted installation | Ensure concentricity of the housing, end cover and shaft; ensure the correct bore size for the oil seal |
4. Poor compatibility between the oil seal and the working medium, resulting in softening, swelling, hardening, or cracking of the lip.
| No. | Fault | Cause Analysis | Troubleshooting |
|---|---|---|---|
| 1 | Poor compatibility between the oil seal and the working medium, causing the seal lip to soften/swell, or harden/crack | The oil seal material is incompatible with the working medium | Select a suitable working medium based on the oil seal material, or choose an appropriate oil seal material based on the working medium |
5. Rubber aging
| No. | Fault | Cause Analysis | Troubleshooting |
|---|---|---|---|
| 1 | Lip of the oil seal hardens or cracks due to overheating | The working medium temperature exceeds the design value, surpassing the rubber material's temperature resistance limit | Lower the working medium temperature, or replace the oil seal with one made of heat-resistant rubber |
| 2 | Lip of the oil seal hardens or cracks due to poor lubrication | Poor lubrication of the oil seal causes dry friction | Ensure adequate lubrication of the oil seal |
| 3 | Lip of the oil seal swells and softens | Poor compatibility of the rubber with the working medium; prolonged immersion in cleaning solvents or gasoline causes the lip to swell | Select rubber materials compatible with the working medium, or choose a working medium suitable for the rubber; do not clean the oil seal with solvents or gasoline |
6. Reasons related to the shaft
| No. | Fault | Cause Analysis | Troubleshooting |
|---|---|---|---|
| 1 | Surface roughness is higher than 0.8 or lower than 0.2 | Rough surface leads to severe wear; overly smooth surface makes it difficult to form and maintain the lubricating oil film, resulting in dry wear | Reduce the surface roughness to below 0.8; increase the surface roughness to above 0.2 |
| 2 | Improper surface hardness (higher than HRC40) | Tests show that when the shaft surface hardness exceeds HRC40 (except for chromium-plated surfaces), shaft wear is accelerated | Maintain the surface hardness at HRC30~40; chromium-plated surface is preferred |
| 3 | Lubricating oil contains impurities, causing severe surface wear | The lubricating oil is contaminated | Ensure the lubricating oil is clean |
| 4 | Excessive eccentricity, with noise when the shaft radially oscillates | Eccentric bearing; eccentric shaft itself | Replace the bearing; use an oil seal designed for misalignment tolerance |
| 5 | Dust at the lip causes severe shaft surface wear | Unclean shaft surface with dust particles entering the seal lip and abrading the shaft; foundry sand ingress; external dust contamination; degraded lubricating oil forming oxides that abrade the shaft surface | Keep the shaft surface and oil seal clean; install a dust-proof device to prevent external dust ingress; use high-quality lubricating oil |
| 6 | Scratches or sand pits on the sliding surface of the shaft | Process cracks or corrosion points on the shaft surface aggravate wear and cause leakage; gaps formed between shaft scratches/pits and the seal lip lead to leakage; shaft surface scratches or impact damage | Ensure the quality of the shaft surface and avoid collisions |
| 7 | Directional machining marks on the sliding part of the shaft | Fine thread whorls or turning/grinding marks on the shaft surface create a pumping effect, leading to leakage | Ensure precision machining of the shaft surface. Experience shows that shot peening with 0.05mm diameter glass beads is optimal |
The presence of a "critical oil film" at the oil seal cutting edge is highly beneficial for reducing wear and extending the oil seal's lifespan. This is especially true when several oil seals are used in parallel; proper lubrication (grease) supply is crucial. Otherwise, insufficient lubrication will lead to dry friction, causing the oil seal to burn out. Lithium-based grease is generally the best choice. To ensure the cleanliness of the grease, it is essential to promptly remove accumulated impurities or other harmful contaminants and to replace the grease regularly. Therefore, the structural design must include oil filling and drain holes. Sometimes, a vent hole is also necessary to prevent the temperature of the oil seal's sealing cavity from rising.