Failure Analysis of Lip Seals

Lip seals are products used for sealing in low-pressure medium working conditions. The typical structure of a lip seal is shown below. Leakage is the main form of seal failure, but the causes of leakage are not singular; rather, they are often the result of multiple factors acting together. To understand the reasons for seal failure, it is necessary to obtain at least the shaft or the seal. The best scenario is to observe the failure phenomenon and characteristics of the seal on site and witness the process of removing the seal from the shaft on site.

Based on the data from Freudenberg Sealing Technologie, through the failure analysis of the sealing rings with working hours less than 100 hours or usage performance less than 10,000 km, it is believed that typical failures include:

– 30%: There are problems in the design and manufacturing process of the shaft.

– 30%: Incorrect assembly of the shaft and the sealing ring;

– 10%: Using the wrong sealing ring;

– 15%: Other reasons, such as incompatible lubricating oil, excessive temperature rise, vibration, and excessive dirt such as dust.

– 15%: Obvious leakage in the early stage.

The following are the main forms of failure and the main causes.

1. The sealing ring shows no significant damage and has no fragments.

■ Insufficient interference fit, or a spring needs to be used;

■ High-viscosity lubricating oil leads to excessive oil film.

■ The mechanical wire on the shaft causes oil leakage.

■ Excessive use of oil and grease leads to leakage.

2. Mechanical damage to the sealing lip

■ The shaft or installation groove scratches the sealing lip.

■ Scratches occur during installation due to slots and gears.

■ Insufficient assembly tools;

■ Insufficient protective measures led to the damage of the shaft.

■ Plastic sealing rings, such as PTFE, are more prone to scratches. Inadequate protective measures can lead to a significant increase in the probability of damage.

3. Carbon deposits on the sealing lip and its vicinity

Carbon deposits are mainly caused by the high temperature rise of the oil film in the contact area. Slight carbon deposits often exist in the lip area close to the air side. When the temperature rise is too high and the carbon deposits are severe, they often interfere with the sealing lip, causing leakage. Improvement measures: Improve the heat transfer performance of the system, reduce friction, and replace high-temperature resistant lubricating oil.

4. Axial crack of the sealing lip

Axial cracks can be either fine or large. Small cracks usually require careful inspection to be detected. Cracks in the sealing lip are generally considered to be caused by long-term operation of the sealing lip in a high-temperature environment. High temperature refers to the temperature in the contact area of the sealing lip during operation. High temperature accelerates the aging of the sealing lip. In addition, the aging will be more severe under the combined effect of oil and high temperature.

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5. Compatibility of Sealing Rings with Fluids

The sealing lip may expand due to the effect of the sealing fluid or lubricating oil, resulting in significant deformation of the sealing lip. When the sealing medium undergoes a chemical reaction with the rubber material, a foaming phenomenon occurs.

6. When there is no significant temperature rise, the sealing lip is worn and there are fragments in the sealing area.

■ The fragments in the sealed area indicate that the function of the wiper lip in the product design does not meet the requirements.

■ Metal fragments: It might be due to problems in the cleaning and assembly processes; or it could be because of eccentric assembly.

■ Dust and impurities: This might be due to improper storage.

7. Excessive wear and formation of grooves

■ Excessively wide wear tracks and moderate grooves: This may be due to local dry friction operation or operation under high pressure conditions.

■ Severe wear of the sealing lip and the shaft surface: It may be due to fine abrasive particles on the rotating track.

■ Severely worn and seriously deformed sealing lips: This is often due to excessive fluid pressure, which leads to severe wear.

8. Circumferential damage of the sealing ring

The circumferential fracture of the rubber of the sealing lip is mainly due to the effect of alternating stress during operation.

In conclusion, through the above analysis, it is found that the main failure mode of the sealing ring is leakage, usually manifested as wear. The main cause of failure is the temperature rise caused by the increase in rotational torque. On the one hand, the increase in temperature will accelerate the aging of rubber materials; on the other hand, it will also cause carbonization of the lubricating oil.

Source: Simulation and Innovation Workshop Source: Simulation and Innovation Workshop