O-Rings and Seals for Automotive Transmission Fluid

As improvements in the Automotive Industry are constantly being introduced, engines and transmissions are being changed to offer increased fuel economy.

As a result of new Ultra Low Viscosity (ULV) transmission fluids, CVT fluids have been developed for Continuously Variable Transmission (CVT).

The choice of sealing materials for transmissions can vary depending on a range of factors including seal type. One commonly used material for O-rings in automatic transmissions is Ethylene Acrylic Elastomer (AEM), also known as Vamac®. In addition to this material, various Fluorocarbon (FKM /Viton®) elastomers are also used in these applications.

Both these material types have their advantages and disadvantages. For large production quantities, AEM is cost-effective, however, this material degrades more quickly in transmission fluids.

FKM Type A is a cost effective FKM and is generally classed as ‘standard’. It offers good fluid resistance, however, low temperature flexibility is relatively poor.

FKM Type GLT and GFLT offer better low temperature performance and fluid resistance, but they are more expensive.

Fluid compatibility

AEM generally swells more than the different grades of FKM in ULV fluids. There is minimal swell of FKM Type A in the various fluids, however, it experiences a loss of tensile properties after use at 150°C. FKM Types GLT and GFLT are not susceptible to this degradation.

Low temperature performance

O-rings can typically seal approximately 8°C below their TR-10 value in static applications, and down to their TR-10 value in dynamic applications. The aim in the automotive industry is to seal at -40°C without leakage. For AEM and FKM Type A compounds, this is usually acceptable for static applications. However, if there is significant movement in the application, some leakage could occur.

In view of this, it is possible that AEM and Type A FKM O-rings could leak at -40°C. However, the fluids help prevent low temperature leakage. Transmission fluids increase in viscosity at low temperatures, and at -40°C, they exhibit very little natural flow. This prevents the fluid from seeping past the seals when the seals and fluid are at -40°C. Also, transmission fluids heat quickly at vehicle start up. The fluid then warms the seals and this combination helps to prevent leakage.

It is possible that this may not occur with the newer fluids which usually have much lower viscosity. They do not thicken as quickly at low temperature or generate heat as quickly upon vehicle start up. For these fluids, use of improved low temperature seal materials is recommended. This is where our compounds V75 GLT and V75 GFLT are useful. These compounds offer much better performance at low temperatures.

<td-15°C<td-29°C-29°C

AEM
FKM Type A
FKM Type GLT
FKM Type GFLT
TR-10
-23°C-15°C-29°C-29°C

The low temperature fluorocarbon materials offer significant improvements in low temperature and fluid compatibility. Although they are more expensive, they offer a better sealing solution for use with transmission fluids.

For more information on any of the materials mentioned above, please visit our AEM materials page and our Fluorocarbon (Viton / FKM) page. To discuss your application and requirements, please contact our Technical Sales team – call us on +44 (0) 1670 840529 or email sales@easternseals.co.uk.

Download our Automotive Sealing Solutions brochure