CONTROL RODS

Control rods are mechanical components used primarily in universal expansion joints, especially those with multiple bellows elements.

Their primary function is to limit the movement of each individual bellow, there by preventing over extension or compression beyond the designed movement capacity.

Key Features

Purpose: Control rods are installed to distribute movement equally across each bellow in a universal expansion joint, avoiding excessive strain on any single element.
Movement Limiting: They restrict axial, lateral, or angular movements to within pre-defined limits, ensuring operational safety and longevity of the joint.
Not a Substitute for Anchors: Control rods are not designed to withstand pressure thrust. In the event of an anchor or main restraint failure, they should not be relied upon to carry the full thrust load generated by internal pressure.
Common Materials: Typically fabricated from high-strength carbon or stainless steels, depending on the operating environment and service condItions.
Installation: Often used in sets of two or more, spaced 90° or 120° a part around the circumference of the joint, and fitted with washers and nuts to control travel distance Read more

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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