What Factors May Cause A Leaking Oil Seal

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Automobiles rely on rotating parts to transfer engine momentum to the wheels via the transmission. These components are made of materials such as hardened steel, cast iron, aluminium or other such alloys.

However, oil seals are necessary so that engine oil and transmission oil remain in foreign bodies outside the transmission system. Oil seals perform sealing applications in gearboxes and prevent leakage of non-pressure fluids and grease. Oil seals are usually low cost, but if they are to be replaced they can be costly in terms of labour. Here we look at what are some of the common causes of oil seal leaks.

Driveline problems

If the driveshaft or drive axle is damaged, the transmission seals will start to leak. When a driveshaft or drive axle is damaged, bent or loose, the shaft will experience excessive vibration and uneven shaft wear. Defective U-joints or rear wheel drive vehicles can cause this to happen.

Seals become dry

Oil seals are usually made of natural or synthetic rubber compounds. If they are left unused for a long time, they can dry out. As a result, the seal shaft interface may be lost, exposing the metal surface to oxidation. A rusty shaft can break the seal and therefore in this case the seal will be damaged when driving the car. Similarly, unused seals may develop dry spots due to weathering and cracking.

Aging seals

As a seal ages, it tends to move away from the surface they tend to mate with. This effect can lead to excessive space between the seal and the shaft. It provides space for oil to escape.

Harsh driving conditions

Adverse weather conditions such as severe cold weather can cause seals to break and lead to seal failure — similarly, vehicles on the road can encounter adverse conditions such as rocks, thick dust and temperature changes.

As a result, dirt can build up in the gap between the shaft and the seal. This can be seen usually on construction machinary oil seals. When this happens, the oil starts to leak.

Improper maintenance

Running the engine continuously for a long period of time can make the seal too hot or possibly dry it out, which can cause it to wind up and start leaking. Again, driving with an overheated engine may produce the same result. In addition, adding too much fluid can cause seals to ‘burst’, especially in gearboxes.

Seals are fragile and can easily be damaged by a clumsy mechanic. Therefore, when carrying out any maintenance activities, make sure that the seals are free from dust or burrs.

KDIK OIL SEAL provides a wide range of sealing solutions to meet the various needs of automotive manufacturing. Our seals can be installed in systems of commercial cars, buses, trucks, and motorcycles. Please feel free to contact us.

In Part 2, we explained how to select the right oil seal.

Oil Seals (Part 2): How to select the right oil seal

In order for the sealing property of the oil seal you selected to really shine, attention needs to be paid to how it is handled. In the event of seal failure, it is necessary to take effective countermeasures beforehand. Therefore, this column will explain the handling of seals, seal failure causes, and their countermeasures.

1. Oil seal storage, handling, and mounting

Carelessness in seal handling may cause seal failure. For this reason, it is necessary to take care to perform appropriate storage, handling, and mounting.

1) Storage

Table 1 shows the main points of seal storage.

Table 1: The main points of seal storage

Figure 1: Storage method and conditions

2) Handling

Table 2 shows the precautions for handling oil seals.

Table 2: Precautions for handling oil seals

Do not leave seals out on a table (to prevent the adhesion of contaminants).

3) Mounting

Table 3 shows the precautions for mounting oil seals.

Table 3: Precautions for mounting

Figure 2: Pre-lubrication for seals with minor lip

Figure 3: Recommended shaft profile and machine construction to avoid damaging shaft surface

Figure 4: Recommended seal press-fitting jig

Figure 5: Seal protecting jig for spline, keyway, holes on shaft

Figure 6: Guide jig for inserting of long shaft into seal bore

Figure 7: Avoid old seal lip track

For a more detailed discussion of seal handling, please see the following:
Oil seal storage, handling, and mounting

2. Seal failure causes and countermeasures

Leakage from the seal can be broadly divided into "leakage from lip" and "leakage from seal fitting area."
Figures 4 and 5 show the types of failure and the appearances of "leakage from lip" and "leakage from seal fitting area" respectively.

Table 4: Failure (leakage from lip)

Reduction of tightening interference due to rubber hardening

Table 5: Failure (leakage from seal fitting area)

Tables 6 and 7 show the major failure types, their causes, and countermeasures against them.

Table 6: Seal failure causes and countermeasures (leakage from lip)

Damage on sealing edge

1) Sharp edge or burrs on shaft chamfer
2) Shaft spline or keyway
3) Entry of foreign matter
4) Poor handling

• Remove burrs and polish
• Use shaft-protecting jig (see Figure 5)
• Clean surrounding components
• Improve manner of handling

Lip hardened

1) Temperature exceeded seal service temperature range
2) Poor lubrication
3) Excessive inside pressure happened

• Change rubber material to high-temperature-proof rubber
• Improve lubricating method and lubricant supply volume
• Apply high-pressure-proof seal or breather (vent)

Heavy wear on shaft

1) Entry of foreign matter
2) Chemical wear due to high temperature or excessive-pressure additives
3) Poor lubrication
4) Stick slip

• Attach prevention device for entry of foreign matter
• Take countermeasure to prevent high temperature and change lubricants
• Improve lubrication on lip including pre-lubricating (improve quantity of lubricant or lubricating method)

Heavy wear on lip

Excess heat generation due to
1) Poor lubrication
2) Running under conditions beyond specifications

a) Excess peripheral speed
b) Excessive inside pressure

• Improve lubrication
(change machine structure)
• Examine cause of heat source
• Change rubber to heat-proof rubber
• Apply high-pressure-proof seal or breather (vent)

Blister on lip

Expansion of agglomeration of high-temperature oil entering into the sliding surface

a) Deterioration of lubrication
b) Mirror finish on shaft surface
c) Higher peripheral speed
d) Higher lip radial load

• Improve lip lubrication
• Correct shaft surface finish
• Reduce lip radial load of oil seal

Table 7: Seal failure causes and countermeasures (leakage from seal fitting area)

1) Burrs on housing bore
2) Damage or blowholes on housing bore

• Remove burrs and chips
• Repair housing bore to eliminate damage and blowholes

1) Larger housing bore
2) Smaller oil seal O.D.
3) Improper oil seal press-fit position
4) Deformation of housing

• Use appropriate housing bore diameter
• Correct the oil seal press-fit position
• Improve housing rigidity

For details of failure causes and countermeasures, please see the following:

Seal failure causes and countermeasures

To identify the causes of seal failure and take proper measures, it is critical to observe the seal lip closely and evaluate the failure in all respects, such as shaft surface roughness, contaminants, and lubrication.

3. Conclusion

In this month's column, "Handling of seals and seal failure causes and countermeasures," we conveyed the following points:

1) Carelessness in the storage, handling, and mounting of oil seals may cause seal failure. For this reason, you must take sufficient care in storage, handling, and mounting. A jig should also be used in mounting oil seals.

2) Leakage from the seal can be broadly divided into "leakage from lip" and "leakage from seal fitting area," and various types of failure are categorized according to their appearance.

To identify the causes of seal failure and take proper countermeasures, it is critical to observe the seal lip closely and evaluate the failure in all respects, such as shaft surface roughness, contaminants, and lubrication.