Elastomers In Automotive Shaft Seals

Common elastomer  or rubber mainly used , Seals are divided into categories according to the following parts:

Engine Part                 Past used : VMQ               Current : FKM    Future  : HNBR or FKM
Transmission Part       Past used : VMQ or P.A.   Current : EAM    Future  : FKM
Pinion Part                  Past used : NBR or P.A.    Current : FKM    Future  : FKM
Wheel Part                  Past used : NBR                Current : EAM    Future  : EAM
Steering Part               Past used : NBR                Current : FKM    Future  : HNBR
Bearing Part                Past used : NBR                Current : NBR    Future  : HNBR or NBR

Seal must withstand effect of oxidized oils and oil additive , surface hardening of fluoroelastomers. For oil seal ; Seal functional mode is under static and dynamic load .

Properties of seal that must be consider

• Dimensional stability
• Modulus
• Permanent Set resistance
• Abrasion resistance , dry and/or lubricated
• Resilience
• Tear Strength
• Tensile and Elongation properties
• Excellent oil resistance
• Good fuel resistance

Life Prediction of O-Rings

Life prediction of O-Ring can use compression stress relaxation test because most polymeric part are intended to be in service for several year. O-Ring experience changes in properties under heat , high temperature, mechanical stress and fluids. Service life prediction of  an O-Ring is of practical and engineering interest to predict the behaviour of the component over the service life cycle. Main factor for O-Ring degradation can shown in picture 1

The two important properties of O-Ring are compression stress relaxation and resilience. Resilience is a measure of the ability of the seal to retract to its original shape after a constraint has been removed. The force decay of elastomer component under constant compressive strain is known as compression stress relaxation. The test measures the sealing force exerted by seal or O-Ring under compression between two plates.

Test condition for compression stress relaxation. 

• Accelerated aging : Accelerated Oven Aging at 100 , 125 and 150 degree celsius. 
• Sample were aged inside jigs with 25% compression.
• Failure Criteria : Compression stress relaxation 40%

Compression stress relaxation test data is used to Service Life prediction by Arrhenius equation and WLF equation. 

What is the standard test for oil seal.

In designing oil seals what is important is the type of rubber used as material oil seals must be oil resistance properties well under working condition. Important information for user-defined testing properties of materials used to make oil seals are as follows.
1. Type of oil such as lubricant , hydrolic oil.
2. Temperatue of oil at working condition
3. Pressure of oil at working condition
4. Shaft Speed (In the case of the rotating shaft)

The standard test used to test the resistance of the oil is ASTM D471 Standard Test Method of Rubber Property-Effect of Liquids. This test method only those failures caused to determine the likehood that an oil might cause premature sealing system failures in field use.

Measure
1. volume swell (% Volume Change)
2. excessive rubber hadening
3. elongation loss
4. Stress Relaxation

Testing of the material properties of the oil seals of NOK Oil seal material found that Viton® or Fluorocarbon (FKM) is the rubber has heat resistance. It also has excellent resistance to oil and chemical. It is the best rubber for oil seals due to its well-balanced charecteristics. However, this material is not resistant types of brake fluid (DOT 3 , DOT5 glycol base) refrigerants (R32 and R134a)  Anti freeze Solution ans water stream.