Simplex shaft seal A very cmmn arrangement fr il lubricated stern tube bearings. A simplex seal arrangement is fitted t bth inner and uter ends. The replaceable chrme liner prevents damage t the prp shaft which wuld be expensive t repair. Nt shw is a rpe guard blt t the hull which prevents material frm being 'wund' int the gap and damaging the seal. Rpe cutters may be fitted with a fixed blade attached t the hull and a mving blade t the prpeller. Oil pressure is fed t the area between the tw ppsite facing seals. This pressure is gverned by the draught f the vessel and is ften supplied via tanks situated at set heights. This pressure balances the sea water pressure n the seal and prevents sea water ingress; by pening the crrect tank the pressure exerted by the il is insufficient t cause il t leakage ut. Stern tube seals with il lubricatin have tended t use rubber rings increasingly. Fluric rubber (Vitn) with additives has been shwn t be mre effective than nitrile butadiene rubber fr seal rings
Fitting Shaft seals in service. It is pssible t replace lip seals withut remval f the tail shaft by vulcanising split seals n the shaft. The ld seal is remved and the shaft and husing carefully cleaned A precut seal is assembled int the vulcanising machine The vulcanising machine is then set up ff the shaft and the psitin f the seal checked.
The vulcanising agent is mixed and applied t the seal ends. The vulcanising machine is then fitted t the shaft and cnnected t an electrical supply. A heater within the machine heats the seal t a predetermined temperature fr a set time determined by ambient temperature, material type etc.
Split type stern tube (Rss-turnbull) Main advantage f this system is that tail end shaft, stern tube bearing and tapped blts can be inspected withut dry dcking. System allws stern tube t be drawn int the vessel fr inspectin The bttm half bearing is supprted n chcks which in turn rest n tw fre and aft machined surfaces within stern tube bss; these chcks gvern the height f shafting. A detachable arch is attached t the lwer bearing and carries the utbard il seal, the face f which cmes int cntact with a seal seat which is fastened t and rtates with tail shaft flange.
The tp half f the bearing mdule makes a seal n the face f the arch and a seal alng the hrizntal jint n the bearing. The bearing is held in place vertically by 4 x 50 tnne pilgrim type jacks, these jacks als hld the tw half bearings tgether. Lateral psitining is by 4 x 30tnne pilgrim type jacks, tw each side. A running track is arranged abve the bearing fr easy remval f tp half. A rlled race skid is prvided s that the bttm half can be transprted. Remval-The hydr mechanical seal is actuated making a seal n the frd face f the prpeller and lcked mechanically in psitin. The space is then drained f water. Tp half f bearing can then be remved by taking ut the tp vertical jacks and using the lifting jack t allw the tp half t be brught inbard n the running track. These jacks are nw fitted under the lwer half bearing t raise bearing and shaft sufficient t allw the chcks t be remved. The jacks are then lwered until the prpeller rest n the prpeller rest built int the stern frame. Further lwering allws the bearing t mve away frm the shaft until bearing is resting n rller skids. The lwer half bearing cmplete with il seal can then be remved int the vessel fr examinatin. Reversing the prcedure enables the bearing t be replaced Odd facts-anti vibratin lcking gear fitted t jack nuts. As with a CPP it is usual t fit a flange munted prpeller eliminating taper and keyway with their assciated prblems. The tap blts securing prpeller t tail shaft flange can be remved ne at a time, crack detected and returned t their wrking psitin. Stresses in tail shafts Due t the cnsiderable weight f the prpeller, the tail shaft is subject t a bending stress. There is hwever ther stresses which are likely t be encuntered. There is a trsinal stress due t the prpeller resistance and the engine turning mment, and a cmpressive stress due t the prp thrust. All these stresses cupled with the fact that the shaft may be in cntact with highly crrsive sea water makes the likelihd f crrsin attack highly prbable. Examining a tail shaft and stern tube Befre the peridic inspectin the bearing wear dwn shuld be measured. After shaft remved given thrugh examinatin. On water lubricated shafts the integrity f the fit f the brnze liner shuld be checked by tapping with a hammer alng its length listening fr hllw nise indicating a separatin. Measure wear f shaft. Examine key way fr cracks especially the nut thread area. Replace rubber rings