The working principle of the sealing element: The essence of a sealing element is "using elastic deformation to fill the gap and using contact stress to hold the medium in place". Regardless of their appearance, all sealing elements have the same core principle - creating a barrier between the two mating surfaces that prevents the medium from passing through. 

The fundamental physical principle at the bottom level: All three conditions must be met simultaneously. 

For a seal to remain leak-proof, it must simultaneously meet three conditions: 

First, the gap must be filled. 

When any two metal parts are fitted together, they may appear flat from the surface, but microscopically, they are all full of peaks and valleys. When the two surfaces are pressed together, the peaks of one meet the peaks of the other, and the valleys are all empty. This is the leakage channel. The sealing parts are soft (such as rubber, silicone, polytetrafluoroethylene and other elastomers), and they are compressed or squeezed during installation, causing elastic deformation. They flow into every microscopic valley and fill up all the empty spaces. 

Second, the stress on the contact surface must be greater than the pressure of the medium. 

Just filling it up is not enough. The medium itself has pressure and will squeeze into the gaps. The sealing element must generate sufficient contact stress (compressive force) on the mating surface. This force must be greater than the force the medium tries to push it away. To give an analogy: If you use your finger to block the opening of a water pipe and you press it tighter, the water will be less likely to spray out. The sealing element is that "finger", and it is always pressing. 

Thirdly, the sealing material itself must not be damaged by the medium. 

If the medium is a strong acid, the rubber will be corroded and dissolved; if the temperature exceeds 300 degrees, ordinary rubber will carbonize; and if it is high-pressure hydrogen gas, the molecules are extremely small and ordinary materials cannot withstand it. Therefore, the material of the sealing component must be able to withstand the chemical erosion, temperature and pressure of the medium. 

All three conditions must be met simultaneously for sealing to occur. If any one of them is not satisfied, leakage will occur. 

Core mechanism: Elastic restoring force 

This is the fundamental reason why the sealing parts can work for a long time. 

The sealing elements are usually made of elastic materials (such as rubber, silicone, fluororubber, polyurethane, etc.). When installed, they are compressed and deformed, but they have a tendency to "return to their original shape" - this is the elastic restoring force. This force keeps the sealing element always in close contact with the mating surface, regardless of how the parts vibrate, how the temperature changes, or how the pressure fluctuates, it will automatically "stick on". 

Metal gaskets won't do. Metal gaskets seal by the preload force generated when tightened by bolts. Once the bolts loosen or the temperature changes causing thermal expansion and contraction, the preload force decreases, and the gasket becomes loose and leaks. However, elastic sealing components have their own restoring force and do not rely on external clamping, so their lifespan is much longer than that of metal gaskets. 

Advanced principle: Self-tightening effect (tighter under pressure) 

This is the most ingenious aspect of many seals - the higher the pressure of the medium, the tighter the seal becomes. 

Instead of relying on elastic deformation, it uses "curved channels" to dissipate pressure. Several grooves are made on the shaft, and several raised platforms are added on the cover. For gases or liquids to leak out, they must follow an S-shaped path. With each turn of the shaft, a portion of pressure is lost. By the time it reaches the outlet, the pressure is already close to zero. Steam turbines and compressors often use this method. The advantage is that there is no friction, no wear, and an extremely long service life. 

Packing seal - relies on multiple compressions. 

Around the pump shaft, apply a layer of soft packing material (such as graphite, Teflon, or carbon fiber) in a circular pattern. Use the gland bolts to press the packing tightly onto the shaft. If the medium tries to leak out, it will be blocked by each layer of packing. Eventually, there will be almost no leakage. The structure is simple and can withstand high temperatures, but there will be a small amount of leakage. Regular adjustments of the gland are required.