Efficiency gains in the automotive industry are not limited to engine designs. Transmissions are also undergoing significant changes to increase fuel mileage. To reduce friction and cope with more gears (or even an infinite numbers of gears with a Continuously Variable Transmission (CVT), new Ultra Low Viscosity (ULV) transmission fluids and CVT fluids have been developed. Fortunately, Parker has already performed a significant amount of testing in these new fluids.
Automotive transmission seal material usage varies widely by manufacturer and, in some cases, by seal type within a transmission. To keep the length of this blog to a readable size, I will focus on materials most commonly used in O-ring applications within automatic transmissions. Ethylene acrylic elastomer (AEM) is the most common elastomer in North America, but various fluorocarbon (FKM) elastomers are also used.
Each of these material types has its pros and cons. AEM is the most cost-effective, but this material degrades more quickly in transmission fluids. Type 1 FKM is a cost effective FKM and has good fluid resistance, but its low temperature flexibility is relatively poor. Type 3 FKMs offer the best low temperature performance and fluid resistance, but they come at a price.
In terms of compatibility, ULV fluid interactions with elastomers are similar to those of Dexron VI and Ford F-type fluids. AEM tends to swell more than the different grades of FKM. There is almost no swell of Type 1 FKM in the different fluids, but it does experience moderate-to-significant losses in tensile properties over time at 150°C. Type 3 FKM does not suffer this degradation.
|Material Type||AEM||Type 1 FKM||Type 3 FKM|
|ULV Fluid, 1000 hrs. @150°C|
|Hardness Change, pts||-2||+2||-1|
|Tensile Strength Change, %||+10||-45||+1|
|Elongation Change, %||-39||-45||-9|
|Volume Change, %||+17||+1||+2|
*Results for V1475 listed in the table above are based on 1008 hrs. @150°C.
Parker’s Type 3 FKM compound VG286-80 has also been tested in a number of advanced CVT fluids, all with outstanding results.
|Type 3 FKM||Pentosin FFL-2||Idemitsu IB-22||Tutela CVT-PPT|
|VG286-80||500 hrs. @150°C||500 hrs. @150°C||500 hrs. @150°C|
|Hardness Change, pts||+5||+5||+2|
|Tensile Change, %||-21||-20||-12|
|Elongation Change, %||-30||-25||-22|
|Modulus Change, %||+13||+6||+4|
|Volume Change, %||+2||+2||+2|
Low temperature performance
O-rings can typically seal to about 8°C below their TR-10 value in static applications and down to their TR-10 value in dynamic applications. The goal in the automotive industry is to seal at -40°C without leakage. For the AEM and -30°C Type 3 FKM GLT-type compounds, that’s realistic for static applications, but if there is significant movement in the joint, some leakage may occur. Short of redesigning the application, the only solution is to use a seal material with better low temperature performance.
Based on this, ACM and Type 1 FKM O-rings should leak at -40°C. However, the fluid itself helps prevent low temperature leakage. Transmission fluids increase in viscosity at low temperatures, and at -40°C, they exhibit very little natural flow. This increase in viscosity prevents the fluid from seeping past the seals when the seals and fluid are at -40°C. In addition, transmission fluids heat up very quickly upon vehicle startup, and the fluid in turns warms the seals. In practice, these two factors are sufficient to prevent leakage.
However, this phenomenon may not occur with the newer fluids. ULV and CVT fluids tend to have much lower viscosity; this means they don’t thicken as quickly at low temperature and they don’t generate heat as quickly upon vehicle start up. For these fluids, use of improved low temperature seal materials is strongly recommended. This is where Parker compounds VG286-80 and V1289-75 come into play. These Type 3 FKMs beat all of the incumbent materials in low temperature performance by a significant margin.
|AEM||Type 1 FKM||Type 3 FKM||Type 3 FKM||Type 3 FKM|
AEM materials will likely continue to be the low cost solution for sealing transmission fluid. However, low temperature (Type 3) fluorocarbon materials such as VG286-80 offer significant improvements in low temperature and fluid compatibility. While they are more expensive, they offer a near-bulletproof sealing solution for containing transmission fluids.
This article contributed by Dan Ewing, senior chemical engineer, Parker Hannifin O-Ring Division.