What maintains contact between the two surfaces of a mechanical seal?

The correct answer, spring pressure, is fundamental to the operation of mechanical seals. In mechanical seals, spring pressure ensures that the sealing surfaces remain in close contact with each other, which is essential for preventing leaks of the fluid being contained. The spring provides a constant force that pushes the sealing faces together, compensating for any wear or thermal expansion that might occur during operation. This is crucial because, without adequate pressure, even the smallest gap between the sealing surfaces could lead to fluid leakage.

Friction, while a factor in the operation of mechanical seals, does not directly maintain the contact between the sealing surfaces but rather occurs as a result of that contact. Temperature can affect seal performance but is not the mechanism that maintains the intimate contact between the surfaces. Gravity may play a role in certain applications but does not actively maintain contact in the way that spring pressure does.