Stern tubes

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The shaft is withdrawn for examination every 3 years.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

1. 1. Fresh water in gravity tank to emulsify and coagulate it, oil pumped around system to seal and lubricated.
   2. Recharge with high viscosity oil
   3. disconnect oil supply line and reconnect to 45 gallon drum which is supported by block and tackle in order to give a variable head. By raising and lowering the drum the oil pressure in the system can be made to match the water pressure from outside (taking into account the difference in gravities.

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 seperator 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

• • 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