At the for'd end of the engine room a light box emitting light through a pin hole is fixed from the design height of the crankshaft
Using the sighting gear in stern frame boss with solid piece fitted. The stern frame boss is marked off for boring . The solid piece is then exchanged for a sighting piece.
A second sighting gear with sighting piece is fitted to the bore hole in the aft peak bulkhead. This is adjusted until the light source can be seen through the boss and aft peak bulkhead sighting pieces. The sighting piece is replaced by the fixed piece and the bulkhead may be machined. The stern tube is scribed out and the p.c.d. of the bolts which will support the stern tube flange marked off. A similar procedure us repeated for other bulkheads. When boring out is completed the stern tube is hauled into position, wood packing being fitted under the flange before bolting up at the aft peak bulkhead, the external stern tube nut is screwed up hard making a rigid connection at the after end. The tail end shaft is now fitted into the stern tube, the flange of the tail end shaft is now the standard by which the remaining line shafting will be aligned.
The trailing block (or towing block), of fitted, sometimes an ordinary plummer block is fitted (bearing material all round) is mow fitted around tail end shaft using feelers and wedging, chocked and bolted sown. The bearing acts also as an auxiliary thrust with a large clearance so that there is no possibility of it taking over from the main thrust under normal conditions other wise the towing block would shear.
This takes the form of a split brass ring fitted to the for'd end of the towing block which allows the tail end shaft to be disconnected from the intermediate shaft and hence rotate freely whilst the ship is under extended towing . The after face of the connecting flange then rides against this brass ring.
Coupling relationship method
The rest of the intermediate shafting is dropped into position on lower half bearings and using tail end flange as a standard they are lined up . This is done by using feelers between the faces of adjoining flanges, wedging the lower half bearings until faces are parallel with a 1/10mm gap between.
A parallel block is used around the periphery of the flanges. The intermediate bearings are chocked up by cast iron chocks about 50mm thick and bolted down. Couplings are continually rechecked. The thrust block is now aligned coupling to coupling and secured.
Main engine alignment
Bedplate and crankshaft now landed on hardwood blocks in approximately the position, slightly lower than true. It is now raised and jacked into position by lining the mating couplings on thrust and crankshaft. Cast iron chock thickness now measured, a small allowance being made to allow for individual fitting after machining. As each chock fitted its corresponding stud bolt is screwed through to engine seating and secured top to bottom. Checks made to ensure that shaft alignment is maintained, interference fit coupling bolts fitted and nuts screwed up.
It should be understood that the lining up of the shaft will only be true for one set of conditions such as on the building stocks or floating in a light condition. During service with variable loading some hogging and sagging takes place but there is sufficient flexibility in the shaft system to take care of this variation. Any bearing which runs chronically hot is almost certainly due to bad initial alignment.
Optical sight line method
This method uses a micro alignment telescope which generates a sight line between an illuminated reflective target at one end of the shafting and the telescope mounted at the other end. The sight line is generated at a uniform height above the shaft vertically above the centreline of the shaft. A movable scale or target is employed on the intermediate shaft at bearing support points to measure distance from shaft to the sight line. The reflective target and movable scale consists of a magnetic 'v'-block fitted with transverse inclinometer and vertical stand with micrometer and scale.
Similar to optical arrangement except that a laser housing generates a collimated red laser beam above the shaft which is detected by a centring detector at the other end of the shafting. A moving scale detector is used at intermediate bearing position.
Taut wire method (Pilgrims wire)
Consists of steel wire anchored above shaft at one end of system and led over a pulley with suspended weight at the other end.
The height of the pulley and fixed anchorage are adjusted so that they are the same distance above the shaft and are positioned vertically over the shaft centre line. A microstaff is employed to measure the differences in height at bearing support points between shafts and wire, an allowance being made for wire sag.
A master inclinometer is employed to monitor ships movement during the aligning process.
Bearing load method
Top cover off and horizontal alignment checked by measuring the side clearances of the shaft within the bearings. By using the system shown the shaft is carefully jacked up and a graph plotted. Initially a curve will be plotted as the ships structure stress relieves itself from the weight of the shafting, shaft still sitting on bearing material.
When curve assumes a straight line shaft has left bearing and in order to avoid damaging the shaft only sufficient plots will be taken to establish the slope of the straight line. The slope of the line for each bearing is put into a computer program which establishes the shaft system characteristic.
Elongation of a bar produces a related reduction in cross sectional area.
A bar with the same elastic properties in all directions will have a constant relationship between axial strain and lateral strain. This is termed the Poissons Ratio and given by the symbol n.
A bolt when tightened similarly causes a loss in area and diameter. In a clearance hole this is not a problem. With a fitted bolt however, the positive contact or 'fit' between the accurately machined bolt and reamed hole would be affected.
Shaft coupling bolts are tightened to force the faces of the flange together so the friction between the faces will provide some proportion of the drive. However, fitted bolt shanks are also designed to take a proportion of the drive. A clearance bolt could provide the first requirement but not the second. A fitted bolt when tightened and subject to reduction in cross section would also fail on the second count and probably be damaged by fretting. A tapered bolt may be used instead of a conventional coupling bolt to obtain a good fit and required tightening.
Taper fit bolt
Parallel shank fitted bolts
have Interference fit in holes so that in the event of loss of frictional grip between flanges then each bolt will take on equal share of the shear stress due to torque transmission.
Parallel bolts become slack after one or two refits. Therefore taper shank bolts have been used. An alternative is the sleeved coupling bolts.
The fit of the bolt is achieved by the tensioning of the taper shank bolt. Should wear occur in the sleeve then this can be renewed, reusing the rest of the assembly.
Hydraulically fitted bolts.
The pilgrim or margrip hydraulic bolt uses the principal embodied by poissons ratio to provide a calculated and definite fitting force between bolt and hole.
Center load rod fitted into hollow coupling bolt and hydraulic head fitted. High pressure oil pumped into head pushing down, seal, piston and rod .This action stretches the bolt ( within its elastic limit ) and reduces its diameter sufficiently for a sliding fit into the hole. Fluid pressure is released allowing bolt to expand and tightly grip within the hole with a radial grip of about 2.36 Kg/mm2 . Simultaneously longitudinal contraction of the bolt having already fitted the nut hand tight, exerts considerable compressive force which is about 2 1/2 x greater than that which can be achieved by normal torque tightening.
Hydraulic head and loading rod now removed and a protective cap and seal screwed back on
Reduction in fitting and dismantling time, bolts can be used repeatedly, no replacements required , known loads applied
Tailshaft keys and keyways
KEYS AND KEYWAYS There are approximately 100 reported cases per year of partial or total tail shaft failure and 200 reported cases of lost props. Causes of this are quoted as inadequate force fit between prop and tailshaft causing loss of peripheral grip which allows prop to move and make contact with key. This causes excessive dynamic load to fall on key and shaft adjacent to keyway. This causes incipient cracks (small and superficial ) which usually begin at high stress concentration areas i.e. around the leading edge of the keyway
These fatigue failures may be corrosion N.B. Temperature variations in sea water can alter the force fits
Keys and Keyways
Abrupt changes of shape of section cause stress concentrations to build up due to interruption of the stress flow lines.
This build up in stress causes cracks to develop and supports crack propagation. With this in mind it can be seen that shapes or sections which may be subject to great stresses; should be well rounded or gradually tapered off to give smooth stress flow.
Round end keys used and the keyway in prop boss and cone of the tailshaft are to be provided with a smooth fillet at bottom of keyways, fillet radius at least 0.0125 of shaft diameter at top of cone. Sharp edges at top of keyway to be removed. Two screw pins secure key in keyway and the for'd pin should be at least 1/3 of key length from for'd end. Pin holes should have a depth not exceeding pin diameter. Hole edges bevelled.
Water lubricated bearings
The stern tube is normally constructed of cast iron slightly larger at the forward end to ease removal. The forward end is flanged and bolted to a doubler plate stiffened aft peak bulkhead. The forward end is supplied with a stuffing box and gland, the after end with a bearing comprising lignum vitae or similar, the wood is dove tailed into a brass bush, the wood is machined and cut on end grain. Can be lined with Lignum Vitae , rubber composition (cutlass rubber) or an approved plastic material (Certain plastics possess good bearing properties being inert and very tolerant of slow speed boundary lubrication conditions. Cresylic resin bonded asbestos such as Railco WA80H give good results in condition of heavy water contamination in the lubricating oil of almost 100%)
For water lubricated bearing not less than 4 x the diameter of the steel shaft. If the bearing is over 380mm diameter forced water lubrication must be used, a circulating pump or other source with a water flow indicator.
The shaft is withdrawn for examination every 3 years.
Modern Water tolerant oil lubricated stern Tube
With the increase in size of VLCC's shipping companies required a stern tube bearing capable of operating with high degrees of water comtamination. The alloys in white metal tend to oxidise and the clearance is removed leading to seizure. In addition as shaft revs reduced in search of improved propeller efficiency the hydrodynamic forces available become limitedfor oil film generation.
For this reason Railco WA80H bearings where developed.These contained a phenolic resin impregnated asbestos yarn. The next generation contained non-asbestos material. This material tended to be tainted due a series of overheating problems. (later found due to the combination of stiff high power transfer shafts and flexible hull design).
The modern material is called SternSafe and comprises an inner bearing surface with an overwound outerlayer. This has greater tolerance to overheating and reduced swell in the event of water contamination. The latter allows for reduced running clearance and thereby greater control of the shaft position reducing oil loss, seal damage and water ingress.
A wear gauge is incorporated into the bearing as our temperature sensors.
Oil lubricated bearings
Unlike for the water lubricated stern tube a shaft liner is unnecessary. Generally a small one is fitted in way of the aft seal bolted on to the propeller boss. In this way it excludes sea water contact with the main shaft and provides an easily replaceable rubbing surface for the seal. Lined with white metal are to have a bearing length so as not to exceed a bearing pressure from the weight of the shaft and propeller of 5 kg/cm2. The limitations of a bearing are the load it can withstand without metal cracking or squeezing out and the temperature it can withstand without melting. Length of bearing not less than 2 Н D in any case. Cast iron and bronze bearings must have a bearing length not less than 4D. Lubrication system must be capable of maintaining oil tightness despite varying temperature. Gravity tanks fitted with low level alarms, Usual for aft peak to be filled with water to provide cooling low suction valve to be fitted to be locked shut.
Wear down for the white metal should not exceed 2mm to avoid hammering out and the period for inspection is 6 years. A highly resilient reinforced plastic may be used in place of the white metal. It is claimed to have greater load carrying capacity, high resistance to fatigue and shock loading, with good lubrication properties. Ceramic liners can also be used
This system depends upon Hydrostatic lubrication stern tube oil charge remaining in stern tube until pressure test is carried put to ensure that oil supply line is not blocked. This is done by manipulation of valves at header tank and operation of pump which slightly over pressurises stern tube. Oil returning to tank indicating clear oil lines. Top half of white metal bearing is usually machined to give a left hand and right hand helix, this gives a small pumping pressure forward to aft to provide lubrication and to assist in maintaining oil tightness of the oil seals.
If outboard seal leaks, the following steps are to be taken
When large propellers are fitted the heavy overhanging weight greatly increases the load at the after end of the stern tube breaking down the hydrostatic lubrication causing metal to metal contact and seizure towards the aft end of bearing. To obviate this it is usual to angle the shaft downwards for about 8mm over 100m length thus attempting to ensure than the weight of the bearing is taken on the full length of the bearing. It is good practice to leave the oil tank open to the stern tube when in port with machinery stopped, this prevents sea water leaking into the system. However, water has been known to contaminate lubricating oil systems causing rusting of tail shaft particularly when shaft is stopped for periods long enough for water to settle in bottom of bearing. Fit only water separator I,e, a coalescer or cyclonic or osmosis system.
In ships with large changes in draught it is usual to fit two gravity tanks. The upper tank is used when fully loaded or there is water leaking in.
Water based oil replacements
Available are water based sterntube lubricants having the advantages of oil but with a more eco-friendly face. These lubricants must have an adequate viscosity, resistance to sea water contamination as well as biodegradability.
They typically have a water content greater than 90% and are highly soluble. Friction is reduced in comparison to equivalent mineral oil/white metal bearing.
Other benefits include increased heat transfer rates and better protection against galvanic corrosion of dissimilar metals found in the shaft/prop arrangement.The fluid has no measurable flash point.
Simplex shaft seal
A very common arrangement for oil lubricated stern tube bearings. A simplex seal arrangement is fitted to both inner and outer ends.
The replaceable chrome liner prevents damage to the prop shaft which would be expensive to repair.
Not show is a rope guard bolt to the hull which prevents material from being 'wound' into the gap and damaging the seal. Rope cutters may be fitted with a fixed blade attached to the hull and a moving blade to the propeller.
Oil pressure is fed to the area between the two opposite facing seals. This pressure is governed by the draught of the vessel and is often supplied via tanks situated at set heights. This pressure balances the sea water pressure on the seal and prevents sea water ingress, by opening the correct tank the pressure exerted by the oil is insufficient to cause oil to leakage out.
Stern tube seals with oil lubrication have tended to use rubber rings increasingly. Fluoric rubber (Viton) with additives has been shown to be more effective than nitrile butadiene rubber for seal rings
Fitting Shaft seals in service.
It is possible to replace lip seals without removal of the tailshaft by vulcanising split seals on the shaft.
The old seal is removed and the shaft and housing carefully cleaned
A pre cut seal is assembled into the vulcanising machine
The vulcanising machine is then set up off the shaft and the position of the seal checked.
The vulcanising agent is mixed and applied to the seal ends
The vulcanising machine is then fitted to the shaft and connected to an electrical supply. A heater within the machine heats the seal to a predetermined temperature for a set time determined by ambient temperature, material type etc
Split type stern tube (Ross-turnbull)
Main advantage of this system is that tail end shaft, stern tube bearing and tapped bolts can be inspected without dry docking. System allows stern tube to be drawn into the vessel for inspection
The bottom half bearing is supported on chocks which in turn rest on two ford and aft machined surfaces within stern tube boss, these chocks govern the height of shafting. A detachable arch is attached to the lower bearing and carries the outboard oil seal, the face of which comes into contact with a seal seat which is fastened to and rotates with tail shaft flange.
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
The intermediate shafting if supported in plain or tilting pad bearings, has an after most bearing which is lined top and bottom. Roller bearings are installed in many vessels.
Plain and tilting pad bearings
The shaft supported in a plain journal bearing, will as it rotates, carry oil to its underside and develop a film of pressure. The pressure build up is related to speed of rotation. Thus oil delivered as the shaft turns at normal speed, will separate shaft and bearing, so preventing metal to metal contact. Pressure generated in the oil film, is effective over about one third of the bearing area because of oil loss at the bearing ends and peripherally. Load is supported and transmitted to the journal, by the area where the film is generated. The remaining two thirds area does not carry load
Replacement of the ineffective side portions of the journal by pads capable of carrying load will considerably increase its capacity. Tilting pads based on those developed by Mitchell for thrust blocks are used for the purpose. Each pad tilts as oil is delivered to it so that a wedge or oil is formed. The three pressure wedges give a larger total support area than that obtained with a plain bearing. The tilt of the pads automatically adjusts to suit load, speed and oil viscosity. The wedge of oil gives a greater separation between shaft and bearing than does the oil film in a plain journal. The enhanced load capacity of a tilting pad design permits the use of shorter length or less bearings
Any bearing instability, regardless of its nature is called 'oil whip'.
Bearing instability falls into two types
The most effective bearing to prevent oil whip and dampen shaft vibration is the tilting and multiple shoe bearing.
Oil film operates at a lower temperature than a comparable full sleeved bearing.
Tilting pad bearings are in common use on steam turbines, high speed reduction gears, centrifugal compressors and line shafting.
Split Shaft bearings
Inner Ring Rotating inner ring is in two halves with a scarf or diagonal joint so that the tendency of the joint to open due to the weight of the shaft is reduced when the joint is at bottom centre. The scarf also allows more progressive transition of the roller over the joint reducing noise and vibration
Cage And rollers The cage and rollers are in a matched pair with a diagonal split
Pedestal Cap Clamps the splt cartridge. The joint is spherical allowing upto 2 1/2' of swivel without effect.
Split cartrdige Is located by dowels and holds the outer ring in position. radial socket screw attach the outer ring securely to the split cartridge
Clamping ring This rotating ring secures the split inner ring assembly and is joined by socket head screws
Clearance exists between the inner ring and thesplit cartridge. This allows movement of the shaft between thrust pads during ahead and astern movements. This also allows for thermal expansion of the shaft.
Plane white metal bearings offer a relatively high frictional resistance to rolling
but are cheap and not subject to brinelling or corrosion.
Roller bearings are expensive but offer little resistance to rolling. However, they are susceptible to brinelling when stationary.
The above design removes the major disadvantage of assembling the bearing onto the shaft which would normally require shaft removal.
SKF (Muff) coupling
Outside dia's at end of outer muff measured before fitting
After fitted, dia's should be approx. 0.5mm greater Restraining devices must be fitted to prevent the muffs separating too quickly
Emergency astern arrangement
This is fitted to ships, the purpose of which is to prevent the shaft from slipping out of the stern tube if the muff coupling should fail. Its purpose is not to transmit torque.