The top half of the bearing module makes a seal on the face of the arch and a seal along the horizontal joint on the bearing. The bearing is held in place vertically by 4 x 50 tonne pilgrim type jacks, these jacks also hold the two half bearings together. Lateral positioning is by 4 x 30tonne pilgrim type jacks, two each side.
A running track is arranged above the bearing for easy removal of top half . A rolled race skid is provided so that the bottom half can be transported.
Removal-The hydro mechanical seal is actuated making a seal on the ford face of the propeller and locked mechanically in position. The space is then drained of water.
Top half of bearing can then be removed by taking out the top vertical jacks and using the lifting jack to allow the top half to be brought inboard on the running track. These jacks are now fitted under the lower half bearing to raise bearing and shaft sufficient to allow the chocks to be removed.The jacks are then lowered until the propeller rest on the propeller rest built into the stern frame. Further lowering allows the bearing to move away from the shaft until bearing is resting on roller skids. The lower half bearing complete with oil seal can then be removed into the vessel for examination.
Reversing the procedure enables the bearing to be replaced
Odd facts-Anti vibration locking gear fitted to jack nuts. As with a CPP it is usual to fit a flange mounted propeller eliminating taper and keyway with there associated problems. The tap bolts securing propeller to tail shaft flange can be removed one at a time, crack detected and returned to their working position.
Stresses in tail shafts
Due to the considerable weight of the propeller, the tail shaft is subject to a bending stress. There are however other stresses which are likely to be encountered. There is a torsional stress due to the propeller resistance and the engine turning moment, and a compressive stress due to the prop thrust. All these stresses coupled with the fact that the shaft may be in contact with highly corrosive sea water makes the likelihood of corrosion attack highly probable.- Examining a tail shaft and stern tube
- Before the periodic inspection the bearing weardown should be measured.
- After shaft removed given thorough examination.
- On water lubricated shafts the integrity of the fit of the bronze liner should be checked by tapping with a hammer along its length listening for hollow noise indicating a seperation.
- Measure wear of shaft.
- Examine key way for cracks especially the nut thread area.
- replace rubber rings