O-ring seals are widely used, mainly in static seals and dynamic seals. Static seals include box cover seals, bearing outer ring seals, etc., and dynamic seals include piston seals(commonly used glyd ring with great advantage), rod seals, rotary seals, etc. Generally, static seals have high reliability and fewer failures, while dynamic seals have relatively worse working conditions and are more prone to failure. Here are the main failure phenomena, causes and measures of O-rings:
1. Cutting during installation:
If cutting occurs during installation, as shown in the figure below, the compression amount at the cutting position is reduced or broken, and the regular operation of the O-ring cannot be guaranteed.
Causes: ① Damage during installation through threads or sharp corners; ② Insufficient assembly chamfers; ③ Twisting or extrusion during installation; ④ Improper lubrication of the O-ring; ⑤ Improper O-ring size or O-ring groove size (such as excessive compression or insufficient deformation space for the O-ring groove, etc.)
Measures: ① Cover the thread or edge chamfer during installation; ② Design 15-20° assembly chamfers; ③ Correct installation: O-rings are installed with oil or smoothed after installation; ④ Lubricate the O-rings before installation (such as oil); ⑤ Use O-rings of appropriate size and reasonable O-ring groove design;
2. Expansion:
Usually due to the incompatibility between the O-ring material and the lubricant, the O-ring absorbs excessive lubricant, which causes the physical properties of the material to decrease and the interference to increase after the O-ring expands, resulting in abnormal extrusion, as shown in the following figure.
Cause: O-ring material is incompatible with lubricating oil;
Measures: Select O-ring material compatible with lubricating oil;
3. Compression deformation :
refers to the plastic deformation of the O-ring after being squeezed for some time, as shown in the figure below, and it cannot be restored to its original state.
Cause: ① Use materials with poor resistance to permanent deformation; ② Improper interference design leads to excessive extrusion; ③ Too high temperature and material ageing; ④ O-ring expands due to incompatibility of lubricating oil;
Measures: ① Use materials with good resistance to permanent deformation; ② Design reasonable interference; ③ Reduce system temperature; ④ Select O-ring materials compatible with lubricating oil;
4. Wear:
This usually occurs in dynamic sealing. Wear will scratch the sealing surface, causing the O-ring compression to decrease or break, as shown in the figure below, and the sealing effect cannot be achieved.
Causes: ① The roughness of the moving surface in contact with the O-ring is poor; ② The O-ring is improperly lubricated: ③ Lubricant contamination (dust/metal powder, etc.); ④ The running speed is too high;
Measures: ① The roughness of the moving surface meets 0.8~1.6Ra; ② Improve lubrication (soak in oil or apply grease before installation, and consider relubrication of grease for long life); ③ Do a good job of structural sealing and use more wear-resistant sealing materials; ④ Choose better materials or optimize lubrication or reduce the running speed;
5. Explosive decompression:
Gas may be trapped in the O-ring material in high-pressure application scenarios. When the system pressure decreases, the gas trapped in the O-ring expands and causes the surface of the O-ring to crack or bubble, as shown in the figure below;
Cause: The system pressure decreases rapidly;
Measures: Increase the pressure release time and use materials with improved gas permeability;
6. Distortion:
Usually found in long-stroke motion scenes, when the seal stops sliding along the sealing surface and starts rolling, the O-ring is found to be twisted after disassembly and inspection after use, and deep spiral cuts appear on the surface, as shown in the figure below, resulting in seal failure or O-ring breakage;
Cause: ① Unreasonable gap design; ② Insufficient seal lubrication; ③ Poor surface roughness (axial section); ④ Too slow stroke speed; ⑤ Distortion of O-ring installation; ⑥ Too soft O-ring material;
Measures: ① Reasonably design the gap in the compression direction and deformation direction; ② Improve O-ring lubrication; ③ Dynamic sealing surface roughness requirements 0.8-1.6Ra; ④ Use rectangular cross-section sealing ring; ⑤ Correct installation: O-ring oil installation or smoothing after installation; ⑥ Use harder O-ring material;
7. Extrusion:
In high-pressure applications (here, high pressure refers to the large pressure difference between the inside and outside of the cavity), the O-ring may be pressed into the mating surface of the cavity, causing the low-pressure side of the O-ring to show “chewing” or “biting” appearance damage, as shown in the figure below. In some extreme cases, more than 50% of the O-rings may be squeezed out from the low-pressure side;
Causes: ① The gap between the cavity mating surface is too large; ② The pressure is too high; ③ The O-ring material is too soft; ④ The O-ring material is incompatible with the lubricant; ⑤ Improper design or processing of the gland, sharp corners of the O-ring groove; ⑥ The cross-section of the O-ring is incorrect;
Measures: ① Reduce the gap between the cavity mating surface; ② Use an O-ring retaining ring (as shown below); ③ Use a more complex or more compatible sealing material; ④ Choose an O-ring material that is compatible with lubricants; ⑤ Edge design and processing chamfers; ⑥ Choose a reasonable O-ring size and a reasonable O-ring groove size design;