Valve stem seals are devices engineered to allow a small amount of oil to pass through their lips to lubricate the valve stem/valve guide interface of a combustion engine.
SKF’s range of valve stem seals
“If too much oil passes through the lip of the seal, emissions are worsened and coke builds up on the valve, potentially causing the engine to lose power or even fail. If too little oil passes through the lip of the seal, the valve does not receive enough lubrication and will scuff, ultimately causing the valve to seize. The ideal metering rate lies between these two extremes,” explains André Weyers, SKF South Africa’s platform development manager.
The primary function of a valve stem seal is to meter oil to the valve guide/stem interface. Although the requirements for each engine differ according to the design parameters and operating conditions, the oil metering rate is generally within the range of 0,1–1,0 mg per valve per hour. These values have been developed through extensive comparative testing on cylinder heads using SKF’s standard test cycle.
Experience has shown that the standard SKF design provides the correct oil metering rate for most applications. This is true for diesel and petrol applications as well as for intake and exhaust valves. There are however, applications where the oil metering rate must be adjusted. This can be tuned to the exact requirements of a specific engine by changing the geometry of the sealing lip.
The major factors influencing oil metering rate are the radius of the sealing lip and the lip angle to the oil side. The radial force of the seal, while being important for assuring lip contact to the valve stem under operating conditions, is not a major factor.
SKF has developed and patented a valve stem seal for high pressure engine applications such as those found in heavy-duty vehicles (trucks and buses) when an exhaust brake is in operation. By withstanding the high pressures in the exhaust and intake ports of the engine, the new seal maintains its primary function as an oil-metering device and thus reduces emissions and enhances engine operation. A further benefit is that because the lip does not open under pressure, the capacity of the oil separation unit in the breather system can be downsized as the potentially high gas volumes which flow through the lip under pressure are eliminated.
The new valve stem seal design allows the primary sealing lip to function independently of the pressure applied, by protecting it with an independent secondary lip. This lip resists high pressures in the ports of the engine without opening.
SKF products provide complete automotive sealing solutions for engine, transmission, driveline, steering and suspension systems. This single source approach is possible through a worldwide network offering one-source accountability.
For more information contact André Weyers, SKF South Africa, +27 (0)11 821 3500, [email protected], www.skf.co.za
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