1. A catalogue as an aid to defining and recognizing the causes of leakage in radial shaft seals

1.1 Introduction

This survey is to help define and recognize the cause of leakage in radial shaft seals.  The possible causes of leakage are listed and clarified by means of pictures in the Appendix.  If the damage can be seen it should be possible to define the relevant cause of leakage with the help of the pictorial examples.

The pictures of damage relate to the following areas:

A: Sealing edge area 

B: Return pumping feature

C: Dust lip area

D: Area of the outer covering

E: Area of the spring retaining lip

F: Spring area

G: Area of the sealing lip/metal insert interface

H: Area of the back covering

K: Radius

2. Leakage definition

2.1. According to DIN 3761

Using prescribed operating conditions, the lack of tightness is determined by the amount of the sealed fluid - over and above any moistness which may occur - which gets past the sealing edge and can be collected and measured, when the seal is run for a definite time on a test-rig.  The quantity is defined as the leakage.

2.2. Leakage terms

Four leakage terms are distinguished for Simmeringâ  radial shaft seals:

Tip:

Results from this area of testing have shown that radial shaft seals which are moist or wet may stop leaking at their own accord during the course of further testing.  It is therefore recommended that radial shaft seals in these states should not be replaced.  The reasons for the temporary moisture could be for example the running-in behavior of the radial shaft seal or a small particle under the sealing edge.

Should the radial shaft seal show a definite leak (for example 3 ml) then this will increase with increasing running time in 80% of cases.

However 20% of radial shaft seals which are leaky exhibit only short term leakage.  Here the seal contact has been disrupted for a short period (for example, dirt under the sealing lip).  After wiping off the leakage, it is possible to see whether the radial shaft seal is functioning correctly.

2.3. Classification of the leakage occurring in series monitoring and release of the components in accordance with DIN 3761

For monitoring production parts with a 240 h running time, the following leakage classes are to be used:

Table 1 - Leakage classes

The various test conditions are to be agreed upon.

In addition, the so-called zero-leakage with test-rig tests of 240 h with 12 specimens can be arranged for release for construction, or for critical places of installation with special safety requirements.  These zero-leakages can be sub-divided according to the following criteria:

1. Film of moisture at the sealing edge only.
2. Film of moisture over the sealing edge area but not passing beyond the back abutment face; no formation of droplets.
3. Film of moisture passing beyond the back abutment face and/or formation of droplets, but no dripping occurs.

In tests on assemblies and vehicles, the zero-leakage defines that state of the radial shaft seal during static and dynamic conditions where the sealed medium does not leak beyond the radial shaft seal.

3. Recognizing the causes of leakage

Two kinds of leakage are distinguished with radial shaft seals.  Static leakage which is possible at the press-fit and at the sealing lip, and dynamic leakage which is possible at the sealing lip.  These leaks can have many causes which arise not only from the radial shaft seal but also from unsuitable operating conditions.

Radial shaft seal manufacturing faults which are not permissible are described in detail in DIN 3761 - Part 4 and are supported by pictorial material.

The following list of the causes of seal leakage refers to fitting faults and to unsuitable operating conditions.

3.1. Static leakage

3.1.1.

At the press-fit

3.1.1.1.

The housing bore is too rough which is especially critical in B1-radial shaft seals

Nominal:

R_max  £ 16 m m  for B1-radial shaft seals

R_max  £ 25 m m  for BA-radial shaft seals

3.1.1.2.

Sharp-edged chamfer region at the housing bore

B1-radial shaft seals get longitudinal furrows

BA-radial shaft seals are damaged (Figure 1)

3.1.1.3.

Chamfer angle which is too steep at the housing bore

Rubber can be sheared off BA-radial shaft seals

(recommended chamfer angle 15° to 25°) Figure 1

3.1.1.4.

Blow holes in the cast housings

B1-radial shaft seals have too little sealing gap length

BA-radial shaft seals are damaged (Figure 1)

3.1.2.

At the sealing lip

3.1.2.1.

Shaft is too rough, possibly with longitudinal furrows caused by the pressing-on of a bearing.

3.1.2.2.

Damage to the sealing lip caused by sharp-edged chamfer or keyway on the shaft in the area of the sealing lip (Figure 2 to 7)

3.1.2.3.

The sealing edge lifts up due to pressure at the sealing lip by return pumping action.  The sealing lip rests only on the helix  (for example, with non-ventilated units).

3.1.2.4.

Shaft diameter is too small and/or the shaft offset is too great.

3.1.2.5.

Chamfer at the shaft is too small or too steep (the sealing lip can tip over or turn under and the spring can come off).

Comment:

During dynamic operation of the radial shaft seal, these causes of leakage can result in the leakage increasing or decreasing.

3.2. Dynamic leakage at the sealing lip

3.2.1.

Manufacturing faults on the shaft

3.2.1.1.

Helix or scratches on the shaft

3.2.1.2.

The shaft is too smooth (insufficient lubrication of the radial shaft seal) - see Figure 8.  The shaft is too rough (severe seal contact wear)

Recommended roughness:

R_a  = 0.2 ... 0.6  m m,   R_z  = 1 ... 4  m m

R_max  £ 6.3 m m,  Figure 9

3.2.1.3.

Blow holes in the running track area (damage to the sealing lip).

Pores  £  0.05 mm in the running track area are permissible.

3.2.2.

Leakage due to the lubricant

3.2.2.1.

Chemical incompatibility between two lubricants (for example, synthetic oil at the sealing lip and mineral grease between the dust lip and the sealing lip - the mineral grease can cause removal of additives form the synthetic oil.  The elastomer can then be attacked by the concentration of additives in the grease and by the lack of antioxidants in the oil).  Figures 10 to 13

3.2.2.2.

Chemical incompatibility between the lubricant and the elastomer (etching phenomena and blisters in the sealing lip area).  Figures 10 to 13

3.2.2.3.

Additives from the oil are deposited on the shaft in the sealing lip area (severe wear at the sealing lip).  Figure 14

3.2.2.4.

Thermal over-stressing of the lubricant (due to lack of lubrication and/or incorrect choice of lubricant).  Carbonized oil gets into the runout at the helices of the radial shaft seal and becomes deposited in the areas of the sealing lip and the running track thereby damaging the sealing lip (formation of galleys, longitudinal furrows, rough running track, thermal cracking, discoloration and hardening). Figures 15 to 23

3.2.2.5.

Amount of lubricant is insufficient (rough running track, severe wear; often occurs in the initial operation of the assembly - possible remedy is coating the radial shaft seal with wax).  Figures 24 to 26

3.2.3.

Leakage due to ambient effects

3.2.3.1.

Dirt in the oil or entering from the air side (wear at the sealing lip, or the lifting up of the sealing lip).  Figures 27 to 33

3.2.3.2.

Pressure on the side of the medium raises the specific contact pressure of the sealing lip thereby increasing the wear and the thermal stress on the oil or the lubricant respectively.  Figures 34 and 35

3.2.4.

Leakage due to thermal effects

3.2.4.1.

Oil temperature is too high for the chosen elastomer, which hardens the elastomer, causing axial cracks or blisters.

3.2.4.2.

Poor heat transfer (hollow shaft, or elastomer coated wear sleeve) and high frictional output due to rough sealing lip profile leads to a high temperature at the sealing edge of the radial shaft seal.

Assessment of the photos is based on existing individual cases.  An assessment obtained from similar photos could turn out quite differently.