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Planning and selection of valve seat leakage detection scheme
Measuring only the roundness, concentricity, and runout error of a valve seat may not be sufficient to accurately determine if it will leak. Therefore, it is essential to assess the sealing performance of the valve seat. This test is a functional evaluation that checks whether the valve seat leaks under a specific pressure within a given time frame, ensuring the leakage remains within acceptable limits. For passenger car engines, the typical leakage threshold is around 105 Pa, with maximum allowable leakage not exceeding 400 ml/min.
As shown in Figure 6, the valve seat leakage gauge is a specialized pneumatic plug gauge equipped with a ring nozzle. The gauge’s seal probe is positioned close to the cylinder head valve seat to create a tight seal. An ideal sealing surface, approximately 0.1 mm wide, is machined at both the top and bottom of the gas ring. This surface is then fitted against the valve seat. The gas ring is designed to match the diameter of the valve seat gauge, simulating the actual assembly condition of the valve and seat as closely as possible. If the valve seat and gas ring are perfectly sealed at the mating surfaces, compressed air cannot escape from the gas ring. However, if there are manufacturing defects on the valve seat, some air will leak through the sealing area, allowing for the measurement of the leakage rate. By comparing this value with pre-established standard limits, the leakage of the tested valve seat can be determined, offering a clear visual indication of the quality of the machining process.
The principle behind the valve seat leakage test is straightforward, relying on the sealing and fitting mechanism. The acceptance criteria are also relatively simple. However, the inspection system requires verification of repeatability and reproducibility. Calibrating the maximum and minimum leakage standards can be challenging, as it requires a dedicated calibration device and a standard flow head. These tools can be costly, so they must be factored into the planning of any valve seat leakage detection system.
Additionally, the inspection device demands a high level of cleanliness on both the tool’s fitting surfaces and the valve seat during use. Even small amounts of dust or oil can cause the measured leakage to exceed tolerances, leading to incorrect assessments. Therefore, maintaining strict cleanliness standards is crucial during testing.
Practical production experience has shown that if the valve seat’s roundness is kept within the product tolerance range, the likelihood of leakage exceeding limits is extremely low—essentially a rare occurrence. As such, the key to reliable testing lies in selecting a more accurate roundness measuring tool. If a roundness gauge can accurately measure the valve seat’s roundness, the need for a separate valve seat leakage gauge may be eliminated.