Expansion Joints Explained

An expansion joint or movement joint is an assembly designed to safely absorb the temperature-induced expansion and contraction of construction materials, to absorb vibration, to hold parts together, or to allow movement due to ground settlement or earthquakes. They are commonly found between sections of buildings, railway tracks, piping systems, ships, and other structures. To ensure durability in these environments, many engineers specify flexible rubber bellow joints to handle repetitive cycling and mechanical stress.

Building faces, concrete slabs, and pipelines expand and contract due to warming and cooling from seasonal variation, or due to other heat sources. Before expansion joint gaps were built into these structures, they would crack under the stress induced.

Ducted air systems

Expansion joints are required in large ducted air systems to allow fixed pieces of piping to be largely free of stress as thermal expansion occurs. Bends in elbows also can accommodate this. Flexible rubber bellow joints also isolate pieces of equipment such as fans from the rigid ductwork, thereby reducing vibration to the ductwork as well as allowing the fan to “grow” as it comes up to the operating air system temperature without putting stress on the fan or the fixed portions of ductwork.

An expansion joint is designed to allow deflection in the axial (compressive), lateral (shear), or angular (bending) deflections. Expansion joints can be non-metallic or metallic (often called bellows type). A high-performance rubber expansion joint for exhaust is a common non-metallic solution, often consisting of a single ply of rubberized material or a composite made of multiple layers of heat and erosion-resistant flexible material. Typical layers include an outer cover to act as a gas seal, a corrosion-resistant material such as Teflon, a layer of fiberglass to act as an insulator, and several layers of internal insulation.

A bellows is made up of a series of one or more convolutions of metal to allow the axial, lateral, or angular deflection.

Pipe expansion joints

Pressure balanced expansion joints

Expansion joints are often included in industrial piping systems to accommodate movement due to thermal and mechanical changes. When the process requires large changes in temperature, metal components change size. Expansion joints with metal bellows or flexible rubber bellow joints are designed to accommodate certain movements while minimizing the transfer of forces to sensitive components in the system.

The unique concept of pressure balanced expansion joints is they are designed to maintain a constant volume by having balancing bellows compensate for volume changes in the line bellows. An early name for these devices was “pressure-volumetric compensator.”

Manufacturing of rubber expansion joints

Wrapping fabric reinforced rubber sheets

Rubber expansion joints are mainly manufactured by manual wrapping of rubber sheets and fabric reinforced rubber sheets around a bellows-shaped product mandrel. Besides rubber and fabric, reinforced rubber and/or steel wires or metal rings are added for additional reinforcement. This manual method is frequently used to produce a specialized rubber expansion joint for exhaust where custom dimensions are required. After the entire product is built up on the mandrel, it is covered with a winding of nylon peel ply to pressurize all layers together.

Molded rubber expansion joints

Some types of rubber expansion joints are made with a molding process. These molded rubber products are typically medium-sized expansion joints with bead rings, which are produced in large quantities. These are manufactured on a cylindrical mandrel wrapped with bias-cut fabric ply. At the end, the bead rings are positioned and the end sections are folded inwards over the bead rings. This part is finally placed in a mold and vulcanized.

Automated winding of rubber expansion joints

New technology has been developed to wind rubber and reinforcement layers on the mandrel automatically using industrial robots. This is fast and accurate and provides repeatable high quality. This precision technology is also found in the production of high-grade extruded rubber products used across various sectors. With individual reinforcement, it is possible to add more or less fiber material at different sections of the product by changing the fiber angles over the length of the joint.