A rubber expansion joint, also known as a flex joint or expansion bellows, is a flexible connector made of natural or synthetic rubber used to absorb vibration, misalignment, and thermal expansion in piping systems. They are commonly used in HVAC, plumbing, and fire protection applications.Rubber expansion joints are an important part of many piping systems as they allow for movement while still maintaining a seal. Without them, pipes would be subject to excessive stress and could eventually break. While they are mostly used in industrial applications, they can also be found in homes. For example, they are often used to connect the water line to the washing machine.