A

Rubber covered O.D., metal insert, sealing lip with garter spring.

AS

Rubber covered O.D., metal insert, sealing lip with garter spring and additional dust lip.

B

Outer metal case, sealing lip with garter spring.

BS

Outer metal case, sealing lip with garter spring and additional dust lip.

C

Outer metal case with reinforcing metal inner ring, sealing lip with garter spring.

CS

Outer metal case with reinforcing metal inner ring, sealing lip with garter spring and additional dust lip.

Working Principle

The area between the sealing edge and the shaft is the most important. The sealing effect is achieved by preloading the sealing lip, making its internal diameter slightly smaller than the shaft diameter. The garter spring ensures constant mechanical pressure and maintains the radial force to the shaft, flattening the sealing edge to a defined width. Sealing is provided by the surface tension of the hydrodynamic oil film between the seal flattened area and the shaft.
Oil thickness must be between 1 and 3 µm to avoid leakage. The meniscus acts as an interface between the outside air and the fluid. Any break in the meniscus will result in leakage. This can occur if the shaft contains scratches along the seal path.

Metal case

The metal insert or case is used to give strength and rigidity to the seal. Normally it is made of cold rolled steel in accordance with DIN 1624.

Garter spring

The garter spring maintains the radial force exerted by the sealing lip around the shaft surface. Normally produced in garmonic spring steel wire SAE 1074 (DIN 17223) or stainless steel wire Chrome Nickel AISI 302 (DIN 1.4300).

Nitrile rubber NBR
This elastomer is a copolymer of butadiene and acrylonitrile and is used for the majorty of conventional fluid sealing applications.

Physical data
– Working temperature range: -30°C
– +120°C in oil (at the sealing lip)
– Tensile strength: up to 20 MPa
– Standard colour: black

Advantages
– Good resistance to mineral oil and grease
– Good resistance to water and radiator fluid
– High tear strength

Limitations
– Poor resistance to high-alloyed hypoid oil
– Poor resistance to ozone, weathering and sunlight
– Not resistant to automotive brake fluid (glycol based)
– Poor resistance to polar fluids (ketones, ethers, esters)
– Poor resistance to chlorinated hydrocarbons (carbon tetrachloride, trichloroethylene)
– Poor resistance to aromatic solvents

Fluorinated rubber FPM (VITON from DuPont, FLUOREL from 3M

Physical data
– Working temperature range -20°C – +200°C (at the sealing lip)
– Tensile strength: up to 15 MPa
– Standard colour: brown

Advantages
– Excellent resistance to mineral oil and above all high-alloyed hypoid oils
– Excellent acid resistance
– Good resistance to aromatic and chlorinated hydrocarbons
– Excellent resistance to ageing, ozone and weathering

Limitations
– Limited cold flexibility 
– Poor resistance to polar fluids (ketones, ethers, esters)

Rotating speed

This table below gives an indication of the rotary or linear velocity of the shaft in relation to various elastomers which are permissible under normal conditions of use.

Chemical resistance

Shaft

The shaft hardness and surface finish are of primary importance for efficient sealing and for achieving a useful life. Basically the hardness should increase with increasing peripheral speed. According to DIN 3760 minimum hardness required is 45 HRC. At a peripheral speed of 4 m/s the hardness should be 55 HRC and at 10 m/s 60 HRC. Recommended minimum hardness depth. 0,3 mm if shafts are not fully hardened.

Surface finish as specified by DIN 3760 must be Ra 0.2 to 0.8 µm, but we recommend 0.2 to 0.5. Rougher surfaces generate higher friction, hence higher temperatures. Machining defects and scratches on the shaft must be avoided.

Even very small defects could be sufficient to increase the film thickness, eventually rupturing the meniscus and causing leakage. It is also important to avoid spiral grinding or marks, because they can cause a pumping effect and leakage.

Finishing operations employing forward feed processes should only be used in exceptional cases. The use of a grinding wheel with grit size “60” to “100” is recommended. Dressing of the grinding wheel should be done with a cluster diamond at a dressing speed of c. 0,25 m/min.

– Individual advances of the grinding wheel: 0,03 – 0,04 mm
– Grinding wheel revolutions: c. 1500 rpm
– Shaft revolutions (counter-rotating) 80 – 100 rpm

Recommended machining tolerance is ISO h11 according to DIN 3760.

The mounting end or the shaft should have a chamfer inclined by 15°-30° with rounded and polished edge.

The mounting end or the shaft should have a chamfer inclined by 15°-30° with rounded and polished edge. Therefore the sealing lip must compensate for it. The higher the rotation speed is, the smaller can be the permissible shaft run out which can be compensated by the sealing lip, because the inertia of the sealing lip prevents it from following the shaft movements. It is therefore advisable to install the seal immediately adjacent to the bearing and minimize bearing play.

Eccentricity between shaft and housing bore centers must be avoided as much as possible so as to reduce unilateral load (wear) of the sealing lip.

A good press fit of the shaft seal onto the housing bore is vital. The result is stable installation. Machining tolerances of the housing bore diameter for rotary shaft seals are H8 according to ISO standard UNI-6388-68.

 The recommend ate roughness of the housing:

– covered seal: Ra =1.6 to 4 µm. R = 4 to 12.5 µm
– external outer ring: Ra =1.2 to 8 µm. R = 3 to 8 µm

To ease installation the entrance of the groove should have a chamfer inclined by 10° – 20° and a depth according to the thickness.

Note: If the housing is made of a material with a high coefficient of expansion, must this be taken into consideration when defining the interference (tightness) with the seal.

Axial positioning and alignment

1. The seal is mounted against a stop on the rear side. This presents no particular problem, provided that pressure is applied at “A” to insert it and not at “B”.

2 . Here there is no axial stop. The mounting tool positions the seal both axially and perpendicularly.

3. The seal is mounted against a stop on the front side. This should be avoided as the elastomer at “B” could be compressed and the seal will tend to move out of position.

4. The housing has a shoulder as in 3, but the seal is positioned by the mounting tool. This case is preferable to case.

Recommendations for the assembly tool

Time should be allowed during assembly in order to allow the elastomer time to settle. The seal must be held in position for a few seconds once mounted, to avoid too large a return movement.

It is very important that the seal not remain diagonally in the house. The angle tolerance may not exceed the values in the table below. Bigger angle lead to a pumping effect.

Pressure

The effective pressure to which a seal is submitted is the difference between the pressures of the fluids on each of its two sides (one of which is often the atmosphere). It is clear that the sealing lip should be found on the side which has the higher pressure. In theory, the lip seal for rotary shafts is not a pressure seal.

However, most seals will resist pressures of the order of 0.5 bars without special precautions, if the velocities do not exceed 3m/s. At higher pressures, there is a risk that the lip may be turned back on itself or pressed onto the shaft with a force which gives rise to an unacceptable tightness and frictional torque. At low velocities most seals will bear pressures of up to 3 or 4 bars with the addition of a supporting bush.

Lubrication

House

While the first means of avoiding damage to the outside of the seal is to pay attention to the housing characteristics, the second means, which is jus as important, is lubrication:

– be it of the housing,
– or the outside of the seals,
– or both at the same time.

This not only avoids damage to the seal, but also ensures a better axial positioning.
A seal whose outside diameter is not lubricated will certainly be damaged on the outside when it is mounted in a dry housing (elastomer cover cut or ripped, sealing lacquer removed).

Shaft
Lubrication is very important for a good function and life of the seal. The sealing lip does not actually run on the shaft directly, but on an oil film, called meniscus. The thickness of the meniscus is usually between 1-3 µm, but is affected by many factors such as oil viscosity, shaft surface finish and seal radial load.

Also when the unit is started up, the oil will always take some time before it reaches the lip of the seal (from a few seconds to a few tenths of seconds depending to the application). If it is the first start, and if the lip has not been lubricated at assembly, it will function “dry” dynamically, which will lead to great wear and the risk of total deterioration.

The first few hours of operation is called the “bedding-in” time. This is necessary not only for the meniscus to form, but also for the sealing edge to flatten. 
During this time limited leakage is possible.

Adequate lubrication strongly reduces friction between sealing lip and shaft and also acts as a coolant to the generated heat. The lower the temperature can be kept, the longer will be the life expectancy of the seal. Should the fluid have poor lubricating capability (water and aqueous solutions), dust lip-type (AS, BS or CS) rotary lip seals must be used. In such a case make sure to fill the space between the two lips with grease. The friction heat also depends on the peripheral speed of the shaft.

WAO and WBO are specially designed rotary shaft seals to be used for needle bearing applications. The seals are reinforced with steel insert and have a single lip without spring.