Tie Rods
Tie rods are essential mechanical components used in tied metal expansion joints to absorb pressure thrust generated by internal system pressure. Primary role is to restrain the bellows axially, ensuring the forces resulting from axial thrust are not transmitted to adjacent piping or supports or anchors. Unlike control or limit rods, tie rods are continuously load-bearing and form a fundamental part of the joint’s structural design.
Key Features
- Function: Restrains axial movement and absorbs thrust loads from internal pressure.
- Design: Consists of rigid rods mounted externally with fixed connections at both ends.
- Movement Allowed: Permits lateral and angular movement, depending on the number and orientation of rods.
- Material: Constructed from high-strength steels or alloys suitable for temperature and load conditions.
- Applications: Used where axial movement must be prevented and where the piping system lacks the structural support for thrust anchoring such as in boilers, high-pressure piping, heat exchangers, and pumps.
| Feature | Control Rods | Limit Rods | Tie Rods |
| Primary Function | Limit movement of each bellow in universal expansion joints. | Restrict excessive movement during normal operation and anchor failure. | Absorb pressure thrust and restrain axial movement. |
| Pressure Thrust Load | Not designed to resist pressure thrust. | Designed to resist pressure thrust in case of anchor failure. | Continuously resist pressure thrust as part of normal function. |
| Usage Scenario | Used in universal joints to control individual bellow travel. | Installed in untied expansion joints as a safety feature. | Installed in tied expansion joints to handle axial pressure loads. |
| Movement Allowed | Allows controlled axial, lateral, or angular movement within set limits. | Allows limited axial or lateral movement within safe limits. | Prevents axial movement; lateral and angular movement may be allowed depending on design. |
| Load-Bearing Role | Do not carry pressure thrust even during anchor failure. | Bears load only under fault conditions (e.g., anchor failure). | Bears load during normal operation. |
| Construction | Rods with stops or washers to prevent over-extension/compression. | Rods with nuts and stops to define maximum travel. | Rigid rods with fixed ends for load transfer and restraint. |
| Design Consideration | Prevent uneven movement distribution and over-travel of bellows. | Protect bellows from over-compression or over-extension. | Enable use of expansion joints in systems with limited anchoring capability. |
| Common Applications | Universal metal expansion joints in piping systems with multiple bellows. | Low- to medium-pressure systems with potential for anchor failure | High-pressure piping systems, boilers, heat exchangers, etc |