Limit Rods
Limit rods are mechanical safety components installed in untied metal expansion joints to provide controlled movement during normal operation and serve as a backup restraint in the event of system failure.
Key Functions and Benefits
- Movement Restriction: During normal operation, limit rods restrict the range of axial and lateral movement, preventing the bellows from exceeding its design limits and experiencing fatigue or distortion.
- Safety Backup: In the event of a main anchor failure, limit rods are specifically designed to restrain the full pressure thrust load, thereby protecting the bellows from over-extension or over-compression that could lead to rupture or collapse.
- Configuration: Typically installed in pairs or sets (depending on pipe diameter and pressure), limit rods are spaced symmetrically around the joint. Adjustable nuts and washers help define and fine-tune the allowed travel distance.
- Material Construction: Manufactured using high-strength steels or corrosion-resistant alloys, limit rods are engineered to endure both mechanical load and environmental exposure.
- Application: Particularly critical in systems where anchoring is difficult
or where thermal expansion could result in unpredictable pipe movement.
| 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 |