Tie Rods / Control Units

  • Tie rods / Control Units
  • Design
  • Spherical Washers

Rubber expansion joints cause force on the adjacent sliding or fixed points when under pressure (active bellows cross-section surface area x operating pressure). The force created by this pressure is designated as pressure thrust.

Where the pipe supports are not designed to absorb this force, tie-rods must be incorporated across the joint from flange to flange so that the expansion joint is restrained in axial direction and can move lateral only. It can be eliminated also by using angular expansion joints with hinges and pin, such that the pipe anchors and guides are unburdened accordingly.

Based on the Pressure Equipment Directive PED 2014/68/EU the number and size of tie-rods must be calculated to take the full pressure thrust plus extra safety margins at the required hydraulic pressure test. Most commonly tie-rods are directly integrated into the backing flanges which influences their construction thickness depending from the design pressure and dimensions.

The use of gusset plates placed behind the mating flange is an alternative but introduces pointwise forces into the flange. This technology works for steel pipes but their use is restricted for GRP pipe flanges which could break under these extra unconsidered forces.Tie rod materials can be according to DIN or ASTM standard which defines slightly different tensile and yield strength which is considered in our calculation. In most cases tie-rods and nuts are hot dipped galvanized but can receive PTFE coatings also.

Mating flanges of lateral expansion joints shall be parallel aligned.

Typical Tie Rod Designs

Recommended on most applications to prevent damage due to excessive pipe movements, each rod shall be borne in spherical washers to allow lateral movement secured by double-nuts on the in- and outside to restrict axial expansion and compression.

Moderate axial installation tolerances can be accommodated by adjusting the nuts and washers according to the real installation length of the expansion joint. The spherical washers were specifically designed and machined for the application to compensate large lateral movements and are not comparable with available market standards.

To resist long-term environmental impacts they shall be made of suitable stainless steels especially in sea climate areas. Though our spherical washers are always PTFE-coated frictional forces must be considered in addition to the spring rates.