Expansion joints are designed to provide stress relief in piping systems that are loaded by thermal movements and mechanical vibration.
To deal with the various forces on the joint they require fiber reinforcement, which guarantees both flexibility and strength. Rubber expansion joints combine a rubber matrix and a reinforcing material, so high strength to weight ratios can be achieved. The reinforcing material, usually a kind of fiber, provides the strength and stiffness. The rubber matrix, with low strength and stiffness, provides air-fluid tightness and supports the reinforcing materials to maintain their relative positions. These important positions influence the resulting mechanical properties.
All rubber expansion joints currently available in the market are reinforced using prefabricated cord fabric, like nylon tire cord fabric. The reinforcement cords in these fabrics are woven in a predetermined pattern, mostly 90-deg/0-deg. During a calendering process, the rubber compound is pressed on and into cords. The result is reinforced rubber sheets, which are wrapped around a green form or mandrel to build the expansion joint.
The use of these fabric plies makes it impossible to control the orientation of the fibers on complex shapes such as the bellow of an expansion joint. As any other unrestrained expansion joint, these types of rubber bellows will extend under pressure. The reason is that the internal pressure exerts forces on the reinforcement. The reinforcement fabric consists of individual cords that can only withstand tensile forces (similar to a rope being pulled). If these cords follow their optimal paths, they can use 100 percent of their strength. If not, they will try to relocate to their optimal path, so that they can take up as much force as possible. This relocation of the cords results in a shape deformation of the bellow, which causes the extension of the conventional expansion joint under pressure.
The pneumatic muscle uses this concept as a working principle. Each fiber in this pneumatic muscle is deliberately placed under a non-optimal angle. By inflating the muscle, the fibers are forced toward their optimal paths, causing either a pulling or pushing force. The theory behind optimal fiber paths for cylindrical shapes, such as this pneumatic muscle, is well known in the industry today. However, applying the same principles for the reinforcement of complex shapes such as a bellow is more difficult and not as well known. Even if the principles were known, the current production process with cord fabric does not allow for controlling the exact orientation of the individual fibers.
Optimal Fiber Reinforcement
With technology invented and patented by Delft University of Technology and further developed by a research company on optimal fiber reinforcement, it is possible to make optimal fiber reinforcement designs for complex shaped products such as air springs or expansion joints. Special software has been developed to calculate the optimal geometry of the expansion joint in combination with the optimal path of each individual fiber of the reinforcement. A mathematical model of the design allows the manufacturer to predict the behavior of the product using special simulation and FEA programs.
To control the exact positioning of the fibers in the final product, they are placed using an automated CNC production process. The fact that the position of each individual fiber in the product can be controlled means that the behavior of the joint can be predicted and controlled.
The performance of the fibers can be maximized throughout the whole structure. In this way, the product can be made lighter or sustain higher forces. Secondly, it can be designed in such a way that the rubber matrix material does not carry load. This leads to zero shear stress in the rubber, improving the durability of the product. It also means that reinforcement structure is in equilibrium, so the product will not expand, shorten or lengthen under pressure.
In general, the advantages for the rubber expansion joint are:
- Control and prediction over expansion behavior (No extension under pressure)
- More flexibility and durability
- Lower material costs
- Automated design and production process
- Repeatability with constant quality
These aspects were first proven with the development of a five bellow rubber joint. The joint is 14-in in diameter has a total length of 4-ft. The joint has a wall thickness of 6-mm and is flexible. It was burst tested at 92-bar (approximately 1,330-psi). During the entire burst test, the joint did not extend and remained its exact designed shape.