Seal selection eliminates powertrain noise

Features - Sealing

When trapped air created rough shifting in a multi-speed automatic transmission, the manufacturer turned to SFC Koenig to improve performance.


As 8-, 9-, and 10-speed automatic transmissions become more common, system complexity and sensitivity increases. With additional speeds comes the requirement for additional clutch actuation ports to be added to the transmission shaft, resulting in less space between ports and thinner port walls. While delivering increased fuel economy and better performance, these modern transmission systems have much less operational tolerance.

When a major automotive manufacturer noted noise and rough shifting in their new, multi-speed automatic transmission systems, engineers began a system-wide analysis. They eventually focused on dead space within the cross-drilled channels that connected the clutch actuation ports. These channels allowed for even distribution of transmission fluid, but created approximately 6" of space between an actuation port and the transmission shaft seal. The dead space became an area where air would collect, which was determined to be a major source of the performance issues.

The manufacturer brought the project to SFC Koenig for evaluation. SFC Koenig engineers reviewed the cross-drilled channel design and noted that the manufacturer continued to use the same cup plug seals used in their older, less advanced transmissions. The cup plugs, which sealed holes at the surface of the transmission shaft, created the dead space that allowed air pockets to form. While these air pockets were not problematic in the operation of their older designs, the new transmission systems were much more sensitive.

“While often selected for their low price, cup plugs present a number of challenges, especially in precision and deep bore applications,” says Nate Moore, product manager at SFC Koenig. “We knew that sealing at the surface was not a good solution, and cup plugs couldn’t be installed deep within a drilled shaft, deep in the transmission. Ensuring proper installation of a cup plug in that type of location would be extremely difficult. In this design there would be no way to visually inspect the seal integrity or to access an improperly installed seal for a repair. A failed seal would result in complete transmission failure.”

Eliminating cup plugs

Cup plug installation offers little tolerance for variation. The installation system must provide accurate alignment, apply even and controlled pressure, and deliver a precise stroke to create a secure seal. The aspect ratio of cup plugs (wide in diameter but short), make alignment more difficult. Additionally, pressure points created by installation tools make the cup plugs susceptible to deformation and can lead to a reduction in seal effectiveness. There is also an inherent risk of contamination from sealant, which most cup plugs require to maintain seal integrity.

Other common sealing solutions, such as threaded pipe plugs, were also not viable options. A threaded plug would require the port to be tapped to the cross passage, introducing additional machining processes, increasing costs, production effort, and the risk of contamination from metal debris. Such a solution would ultimately require another form of thread locker to secure the plug in place.

SFC Koenig engineers suggested a solution that could be reliably positioned deep within the bore. A push-type Koenig Expander MB 850 Series plug would be installed at the edge of the clutch actuation port, eliminating the space that allowed air pockets to form.

Koenig Expander plugs feature a serrated sleeve and ball. During installation, the ball is forced into the sleeve, causing the plug to expand into the base material. This creates a permanent, metal-to-metal seal that does not require sealant, and can be installed in the existing gun-drilled port without additional machining.

“Our Koenig Expander plugs were the type of engineered sealing solution a high-performance application like this requires,” Moore says. “We have installed more than 3 billion components with field failure rates less than 1 part per million. After testing and evaluation, our customer was also able to realize the value in our one-piece, engineered expander seal.”

Seal installation

SFC Koenig worked directly with the manufacturer, providing the sealing technology and developing a solution to integrate their products into the manufacturer’s production lines. While high-speed installation tools were accurate, there was concern for variation caused by positioning tolerances in the automated environment. SFC Koenig engineers developed an additional positioning method to ensure optimal placement.

During installation, a temporary, removable pin was inserted into the actuation port. The Koenig Expander was inserted into the deep gun-drilled channel, pushed until it contacted the pin, and expanded. After the seal was installed, the pin was removed, ensuring the plug was not inserted too deep in the channel, which could block part of the actuation port. The process provided maximum contact between the plug sleeve and the port walls, ensuring the most direct fluid path and blocked access to the dead space behind the expander.

With the Koenig Expander plugs in place, problems associated with air pockets were eliminated. Testing of the new transmission was successful and the SFC Koenig solution was used in large-scale production of the transmission systems.

Given the Koenig Expander’s performance and reliability, engineers have begun to work with SFC Koenig to evaluate other opportunities for expander seal technology, such as hydraulic manifolds, pumps, cylinder heads, and engine blocks.

SFC Koenig