Common Faults and Troubleshooting Methods of Skeleton Oil Seals

Skeleton oil seals belong to dynamic seals. The presence of a "critical oil film" is a necessary condition for the sealing of skeleton oil seals. A completely leak-free seal is not permissible nor possible because the presence of the lubricating oil film is essential to ensure the lubrication and friction at the edge of the skeleton oil seal, which inevitably results in some leakage. For rotary skeleton oil seals, if minor leakage occurs within the first 50-100 hours of operation, it is permissible. As the operation time increases, the leakage will gradually stop, and such skeleton oil seals often have a longer lifespan.

Within the effective service life, minor leakage is allowed; otherwise, it must be handled according to the common fault causes and remedies for skeleton oil seals described below.

Poor lip of the skeleton oil seal

Cause Analysis

Poor manufacturing quality, burrs or defects at the edge of the skeleton oil seal

Remedy

Remove burrs or replace the skeleton oil seal.

Poor quality or failure of the spring in the skeleton oil seal

Cause Analysis

Poor manufacturing quality of the spring in the skeleton oil seal

Remedy

Replace the spring in the skeleton oil seal.

Insufficient radial pressure

Cause Analysis

The spring is too loose, resulting in insufficient clamping force.

Remedy

Adjust the spring in the skeleton oil seal.

Noticeable scars on the lip of the skeleton oil seal

Cause Analysis

The lip is scratched by keyways or threads during assembly.

Remedy

Replace the skeleton oil seal; use a protective cover during re-installation to protect the lip of the skeleton oil seal.

Butterfly deformation of the skeleton oil seal

Cause Analysis

Improper installation tools for the skeleton oil seal

Remedy

Re-design and manufacture the installation tools for the skeleton oil seal.

Reversed assembly direction or detached spring at the lip of the skeleton oil seal

Cause Analysis

Improper installation tools for the skeleton oil seal

Remedy 

Re-design and manufacture the installation tools for the skeleton oil seal.

Excessive greasing on the lip and the shaft surface

Cause Analysis

Too much grease applied on the lip and the shaft surface during assembly

Remedy

The grease will decrease and return to normal after the shaft operates for a while.

Poor lubrication causing serious wear on the working surface of the lip, width over one-third, appearing dull

Cause Analysis

Poor lubrication causing dry friction at the lip

Remedy

Ensure proper lubrication for the skeleton oil seal.

Shaft surface roughness greater than 0.8

Cause Analysis

High shaft surface roughness causing serious wear on the lip

Remedy

Reduce shaft surface roughness to below 0.8.

Lubricating oil contains dust, impurities, or lacks a dust cover causing dust and foreign matter intrusion

Cause Analysis

• Dirty oil, the hydraulic pipeline system is too dirty;

  
  

• Dust intrusion into the lip causing abnormal wear;

  
  

• Adhesion of powdery hard particles on the shaft;

  
  

• Metal shavings entering the lip during assembly;

  
  

• Sand from castings entering the lip;

  
  

• Improperly applied paint on the shaft or the lip of the skeleton oil seal;

  
  

Remedy

• Ensure clean lubrication and strengthen the cleaning of the pipeline system;

  
  

• Add dust-proof devices to prevent dust intrusion into the lip;

  
  

• Maintain cleanliness during assembly and remove improperly applied paint.

  
  

Excessive radial pressure at the lip of the skeleton oil seal causing oil film interruption and dry friction

Cause Analysis

The clamp spring of the skeleton oil seal is too tight.

Remedy

Adjust the clamp spring of the skeleton oil seal.

Eccentric installation causing abnormal wear on the lip of the skeleton oil seal; max and min wear widths symmetrically distributed; main and auxiliary lip sliding surface wear pattern symmetrically but oppositely distributed

Cause Analysis

The housing, end cap, and shaft are not concentric, causing eccentric operation of the skeleton oil seal; inappropriate press-in of the skeleton oil seal into a small seat bore hole causing tilt.

Remedy

Ensure concentricity of the housing, end cap, and shaft; ensure proper seat bore dimensions for the skeleton oil seal.

Poor compatibility between the skeleton oil seal and the working medium causing lip softening, swelling, hardening, or cracking

Cause Analysis

Inappropriate matching between the skeleton oil seal and the working medium

Remedy

Select a suitable working medium based on the material of the skeleton oil seal or select a suitable skeleton oil seal material based on the working medium.

Overheating, hardening, or cracking of the lip of the skeleton oil seal

Cause Analysis

The working medium temperature is higher than the design value, exceeding the rubber's durability limit.

Remedy

Reduce the temperature of the working medium or use a heat-resistant rubber skeleton oil seal.

Poor lubrication causing hardening or cracking of the lip of the skeleton oil seal

Cause Analysis

Poor lubrication causing dry friction at the lip of the skeleton oil seal

Remedy

Ensure proper lubrication for the skeleton oil seal

Swelling and softening of the lip of the skeleton oil seal

Cause Analysis

Poor compatibility between the rubber and the working medium; long-term immersion in cleaning oil or gasoline causing swelling of the lip.

Remedy

Select rubber materials compatible with the working medium or choose a working medium suitable for the rubber material; do not clean the skeleton oil seal with cleaning oil or gasoline.

Surface roughness greater than 0.8 or less than 0.2

Cause Analysis

Rough surface causing serious wear; too smooth surface making it difficult to form and maintain the lubricating oil film, resulting in dry friction.

Remedy

Reduce surface roughness to below 0.8; increase surface roughness to above 0.2.

Improper surface hardness, greater than HRC40

Cause Analysis

Tests have shown that when the shaft surface hardness exceeds HRC40, it accelerates the wear of the shaft (except for chrome plating).

Remedy

Maintain surface hardness between HRC30~40, chrome plating the surface is best

Lubricating oil containing impurities causing severe surface wear

Cause Analysis

Unclean lubricating oil

Remedy

Ensure clean lubricating oil

Excessive eccentricity causing radial oscillation noise during shaft movement

Cause Analysis

Eccentric bearings, eccentric shaft

Remedy

Replace the bearings, use eccentric-resistant skeleton oil seals

Dust at the lip causing severe surface wear

Cause Analysis

• Unclean shaft surface with adhesive dust particles intruding into the lip, causing wear to the shaft surface;

  
  

• Intrusion of casting sand causing wear to the shaft surface;

  
  

• External dust intrusion causing wear to the shaft surface;

  
  

• Deteriorated lubricating oil producing oxides intruding into the lip, causing wear to the shaft surface;

  
  

Remedy

• Ensure cleanliness of the shaft surface and the skeleton oil seal;

  
  

• To prevent external dust intrusion, install dust protection devices;

  
  

• Use high-quality lubricating oil.

  
  

Scratches or sand holes on the sliding surface of the shaft

Cause Analysis

• Process cracks or corrosion points on the shaft surface accelerating wear and causing leakage;

  
  

• Scratches, sand holes, etc. on the shaft surface forming gaps with the lip of the skeleton oil seal causing leakage;

  
  

• Scratches or dings on the shaft surface;

  
  

Remedy

Ensure the quality of the shaft surface, avoid damages.

Directional processing marks on the sliding part of the shaft surface

Cause Analysis

Fine thread spiral grooves or other machining marks on the shaft surface, forming a pumping effect causing leakag.

Remedy

Ensure fine processing techniques on the shaft surface. Experiments show that using 0.05mm diameter small glass beads for shot peening treatment is best.

In conclusion, the presence of a "critical oil film" at the edge of the skeleton oil seal is very beneficial for reducing edge wear and extending the life of the skeleton oil seal. This is especially important when using several skeleton oil seals in parallel, as adequate supply of lubricating oil (grease) is crucial to prevent dry friction and burning. Lithium-based grease is generally the best choice.

To ensure the cleanliness of the grease and timely removal of accumulated impurities or other harmful substances in the grease, regular grease replacement is crucial. Therefore, the structural design must include oil injection and drainage holes. To prevent temperature rise in the sealing chamber of the skeleton oil seal, vent holes are sometimes also necessary.