Metering Rates of a Valve-Stem Seal

Valve-Stem Seals are designed to meter oil at precise metering rates. Currently Corcos has design standards for a variety of seal grades (or metering rate ranges). These ranges are divided down into two categories, single lip seal designs and double lip designs. It is important to note that the single lip design should be considered the standard design and that the double lip design is an enhancement of the standard design to extend the function of the seal in more demanding operating conditions (higher pressures and/or larger valve guide ID to OD concentricity). As such, the functional range of the two different lip designs overlap, as they are not intended to function in the same environment. The double lip seal design is more robust, which allows it to adapt to a larger variety of operating conditions. In normal operating environments (generally operate in vacuum with low dimensional variation) the single lip design will provide excellent performance over the entire seal life. The use of lower friction, reduced wear materials is strongly recommended for longer seal life when using lower metering rate grade seals due to more concentrated pressure distributions on the seal lip. The following is a table of the seal grades with target metering rate and expected ranges for each design.

The standard lip designs that we have are based on experience and testing derived from application of valve stem sealing theory. Valve stem sealing theory led to the development of the following formula for estimation of oil flow for a valve stem seal:

From a design standpoint, lip angles, lip radius, and lip offset from spring pocket are the geometrical factors, that combined with radial lip load define oil-metering rate. These are the geometrical factors that define the pressure distribution (pressure gradients) of the seal lip to the valve stem. Additionally, lip flexibility and material heat resistance impact metering rate due to the effects of lip geometry changes during operation from stem oscillation and heat. Finally, the viscosity of the oil also determines the actual metering rate of the seal in the engine. As viscosity increases the film thickness on the valve stem also increases, thus allowing more oil to flow past the seal lip on a given stroke.

The double lip design seal lip is also derived by valve stem sealing theory with adjustments made to lip dimensions to allow for geometry changes due to the contact of the “second” lip with the valve stem. Actual dimensional standards for each grade and lip type have been defined through testing and experience.

The operating environment within the engine is what ultimately determines the actual metering rate of a seal in a specific application. Due to the variation from one engine type to the next it often becomes necessary to adjust the standard seal designs to meet the actual needs of a particular customer. This is done by applying valve stem sealing theory, past experience with similar valve train/engine types or by testing to determine the lowest possible leak rate grade that still gives acceptable performance in scuffing, wear, noise, and yet be assembly friendly.
 

Integrated Spring Seat Design (VA09)

The integrated spring seat design valve stem seal was developed as a way to reduce the number of parts required for the valve train assembly. The VA09 style seal utilizes a metal flange that fits between the valve spring and the cylinder head. This flange protects the cylinder head from wear from the valve spring as it cycles. This is especially important on cylinder heads made from aluminum. In addition to protecting the cylinder head, there is usually a spring pocket built into the metal case of the seal to help align the valve spring. In applications that do not utilize a VA09 style valve stem seal (VA03), it becomes necessary to install a separate spring seat to the cylinder head assembly. This increases the number of parts required for cylinder head assembly and adds to the assembly complexity.

Since the VA09 style valve stem seal must withstand the cycling of the valve spring for the life of the engine, the seal metal case must be hardened for durability. This adds additional process steps to the manufacture of the seal, and thus increases cost to produce. Overall, the decision to use a VA09 verses VA03 style seal is generally driven by customer application and the assembly/manufacturing methods used by the customer. Due to the fact that the change from VA09 to VA03 style seals does not impact the seal lip design, there is no change in seal function between the different types of seal.

A special version of the integrated seal is the addition of a sealing bid below the washer. This is necessary when due to the kind of production process, a leakage remains between the valve guide and the head.
 

Test Benches

Valve Stem Seals development is based on extensive testing. Mathematical tools are extremely useful during the development stage but test benches are needed to confirm the leak rate and the seal behaviour over time. We can currently test the seal with two kind of test bench.