NCCER Millwright Practice Exam

A positive-contact seal used on cylinder pistons is referred to as a cup seal. This type of seal is designed to maintain a secure fit against the piston, thereby effectively preventing fluid leakage. The shape of the cup seal, resembling a cup, allows it to flex under pressure and create a tight seal against the cylinder wall when the piston moves. This is crucial in hydraulic or pneumatic applications where maintaining pressure and preventing fluid escape is essential for efficiency and performance. In contrast, O-ring seals, while also commonly used in various applications, are typically circular and rely on compression to seal, making them less effective as positive-contact seals in some scenarios. Flat seals generally provide surface-to-surface contact and are not designed specifically for the dynamic movement involved in piston applications. Rivet seals are not standard terminology in sealing applications and do not apply to the context of cylinder pistons. Thus, the cup seal stands out as the correct answer for its specific function and design in this particular use case.