The Tie rods need to be checked for proper tension at intervals laid down in the maintenance schedule, and also after any scavenge fire. This is done hydraulically for modern engines.
Large tie bolts are tightened with a hydraulic jack which loads the tie-bolt in tension. The tie bolt nut is usually drilled to take a toggle bar or, slotted to take a hook spanner, and when the correct pull is on the tie bolt, the tie nut is pulled up hand tight; the pressure in the jack is then released leaving the tie bolt tight. The load placed on the tie bolt by hydraulic jack is controlled by the hand pump pressure, which is indicated on the pump pressure gauge.
Tie rod tensioning is done by the following procedure:
- Connect two pre-tensioning jacks to two adjacent tie bolts.
- Obtain the required hydraulic pressure and maintain this.
- Check the clearance between the nut and the intermediate ring and adjust if required.
- Release the pressure and remove jacks.
By tensioning the Tie rods, a pre-determined tensile load is induced in the Tie rod, and a pre-determined compressive load is induced in the Entablature, ‘A’ Frame and Bedplate. During the firing stroke, the tensile load on the Tie rod increases.
The compressive stresses on the entablature, ‘A’ frame and bedplate are reduced, thereby keeping the components at very low fatigue levels. In other words, if the Tie rods were not pre-tensioned properly, the fluctuation of the stresses in the components between the firing and non firing periods could be higher, leading to fatigue failure of components.
To minimize bending, Tie rods are placed as close to the crankshaft axis as possible. This reduces the bending stress on girders and prevents unbalanced loads from being transmitted to the welds.
Reference and Images
“Marine Engineering Practice” by Vikram Gokhale and N. Nanda