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Wear in stern tube bearing

Posted: Thu Dec 01, 2016 8:13 am
by popeye62
Good day all

I'm looking for a ball park figure for wear in a single (aft) oil lubricated stern tube bearing and its steel backing.

The shaft is 470 mm diameter. Material unknown
The bearing is 4 mm thick. Material is PbSb10Sn6
The steel backing is thick. Material is EN-GJL-250
Say the propeller and the T/S are 10 t each so call it 200kN in total for the Force (forget buoyancy and dynamic propeller forces)
Shaft revs are 120 rpm
and finally...the stern tube is full of seawater!

What is the rate of wear through the bearing and what is the rate of wear through the steel?

Simply: could the wear down be 14 mm (yes, 14 mm! 4mm of bearing and 10 mm of the bearing backing) after 10 days? There were no high temperature alarms.
A maths solution or somebody's experience will be fine. The vessel is old enough to negate any initial alignment issues

Re: Wear in stern tube bearing

Posted: Thu Dec 01, 2016 11:06 am
by Merlyn
Sounds to me you have a serious problem here possibly requiring letting go the coupling bolts to check shaft alignment to begin with.
We had a similar problem many years ago when a ship ran aground on rocks and we had a photo for years on the wall at work of a rock up through the engine room plating.
Dry dock repairs completed and back in the water one shafts sterntube bearings rapidly wore out ( like machining it out almost ) kicking up a din and vibration all round.
Letting go the coupling indicated a massive drop in the alignment.
The reason?
Engine bed out of alignment with original settings with ships bottom and mountings owing to grounding at speed.
Yours sounds horrific, to have that clearance and through the bearing into the steel backing must be a problem of I would have thought a rather obvious nature?
Engine alignement perhaps via fractures/ welds let go possibly in mounting plates?

Re: Wear in stern tube bearing

Posted: Thu Dec 01, 2016 4:10 pm
by D Winsor
I don't think there is any way to calculate the rate of wear without knowing the coefficient of friction between the babbit bearing and the cast sleeve on the shaft when immersed in water. I would venture to guess that the babbit bearing failed very quickly, possibly within hours of the tube being flooded with water due to the shear loss of the lubricating oil. From then on the shaft sleeve was riding directly on the base of the sterntube again with little or no lubrication. If it isn't already known I would investigate what caused the water to displace the oil in the sterntube.
It also doesn't surprise me that you did not get high temperature alarms considering any heat generated would have been dissipated with the water.
I also experienced an issue on a ship where the staves in a Lignum Vitae sterntube wore beyond the maximum limit, as a result the shaft began to roll up the side of the tube as it turned and would fall down into the bottom of the tube. Later when the ship went into drydock, the shaft was pulled and it was found that the hammering damaged the stave guides at the bottom of the tube and the bronze shaft sleeve was knocked a 1/4" out of round.

Re: Wear in stern tube bearing

Posted: Fri Dec 02, 2016 2:21 am
by Merlyn
The million dollar question remains,
Is the shaft damaged?
Deep shit.

Re: Wear in stern tube bearing

Posted: Fri Dec 02, 2016 7:05 am
by Big Pete
Yes, I would be very worried about the Tail shaft, straightness, surface hardening all sorts of things could have happened to it, you don't mention how much wear there was on the shaft?.

Million dollar question is did the stern tube fail because of the excessive wear/play between the shaft and bearing or did the seal fail first, flooding the stern tube with Sea water and cause the wear?

Was the "wear" caused mechanically or was electrolytic action involved?? Was the shaft properly Earthed/Grounded??

Normally there is sufficient head of oil in the stern tube header tank to exceed the head of Sea Water, so that any leakage is oil outwards rather than water inwards past the stern seal. Therefore the first sign of trouble should have been a low oil level alarm from the astern tube header tank, why didn't that go off?? Had someone closed the oil valve from he header tank in dry dock and failed to open it before flooding the dock?

Some ships have 2 header tanks one for deep draught (loaded) and one for shallow draft (Ballast) if this was the case, was the right tank in operation?

If the draught had been increased from the original design, the stern tube header should have been raised in proportion, in order to maintain the designed positive oil pressure at the seal, for every meter increase in draft the header tank would have to be raised by the density of sea water / the density of the stern tube oil. ( say 1.025/0.8 or 1.28 meters.)

It will be interesting to hear more.


Re: Wear in stern tube bearing

Posted: Fri Dec 02, 2016 8:20 am
by popeye62
Thanks Merlyn, the misalignment is a symptom not a cause. The original calculation and the real world measurements were all within IACS rules for bearing reaction and misalignment tolerance. There is no sign of bottom damage.
Thanks D, a mathematical solution does have many variables that is why I was hoping for some personal experience and rule of thumb for wear.
Thanks Big Pete, the t/s is most likely condemned but there is a spare on site and is being fitted to the propeller as I write. The bearing failed because the emulsified oil was unable to support the load due to the failure of the seals - no doubt at all. Nice point about electrolytic corrosion. The earthing gear is totally u/s and the ICCP was turned off, probably wisely. The shaft journal is showing pitting but I believe this issue is running in parallel to the ingress of water. There is no way I can prove that the bearing was spark eroded as no records have been kept of the shaft potential or the ICCP output. Yes you are right about the relative heads of seawater and LO header tank but dynamic forces of propeller and weather will pass water through the final seal (No 3 of a B&V 'multisafe') into the stern tube. No, not quite, the first alarm is the aft SEAL tank high level. As seal 1 and 2 fail water flows to the aft seal tank which is below the ballast water line. The only option is to close the valves and vents to the tank to prevent a massive amount of bilge water. This is not a problem so long as you do something about it at the next port and monitor the water in the lubricating oil. The vessel was fully laden and the upper tank was in service, as it should be. There were no changes to the design draft.
Mineral oil was in the stern tube not an EAL e.g. Hydrox

Re: Wear in stern tube bearing

Posted: Sat Dec 03, 2016 4:51 am
by D Winsor
I'm a little baffled as to how water entering the sterntube through a failed outer seal would come out of the header tank for the inner shaft seal with no noted loss of oil from the upper header tank. It is also my understanding that the static oil pressure from the header tank should always exceed the maximum outside water pressure when the shaft is stopped or subjected to thrust from the prop when going astern.

Is there any kind of cooler and possibly an oil circulating pump fitted to the external oil supply for inner seal and or sterntube or is the sterntube kept cool by submersing it in water carried in the Aft Peak? I've seen on the inner seal oil system and in some cases the oil system of the sterntube some sort of a water cooling system with oil circulated through cooler by either the pump or the turning of the shaft to help keep the inner seal and or the sterntube cool. In the specific installation I'm referring to, the cooler looked like a small sump box under the shaft with a coil in it and one would not recognize it as a cooler except for the, then blanked, water lines running in and out of it.

If there is a leaking cooler located at possibly the lowest point in the system, and if the water pressure supplied to the cooler is close to the static head pressure of the oil system and or the oil circulating pump was able to pump the collected water into the system. It is possible that water leaking from the cooler could enter and displace the oil in the sterntube. This could also explain why water is coming out of the header tank for the inner seal and overflowing into the bilge and possibly explain why there didn't seem to be any loss of oil from the upper header tank.

It is also possible that water could enter the stentube from the aft peak tank when the ship is in the ballast condition if there is a crack in one of the oil supply or vent lines to the sterntube. Over time enough water could collect and settle out in the bottom of the sterntube and migrate back to the aft bearing displacing the oil around the bearing

If water were to enter the sterntube through either of these scenarios it is possible the bearing failed before the aft seals and the aft seal damage was the result of the bearing failure. The damage would most likely also have happened while the ship was in ballast when the aft bearing would be at it's lowest point due to the vessel's trim

Re: Wear in stern tube bearing

Posted: Sat Dec 03, 2016 8:24 pm
by Revolver
To my understanding the way Popeye describes the seals, and putting water into the seal Tk, there are:
water seal/water seal/stern seal tk\oil seal.
So if two outer water seals fail then water will ingress to the seal water tank.
If the 3rd seal, the after most oil seal, fails then head pressure from the 2 header tanks (whichever is in operation - high tank for loaded) should keep water out.

According to Reed's ;) hah:
Weardown for the white metal should not normally exceed 2mm to avoid hammering out and the period between inspections is about 6 years.

There should be an oil cooler and a circ pump for the system, eh?
...what kind of pressure does the pump produce, Popeye?

Re: Wear in stern tube bearing

Posted: Sun Dec 04, 2016 1:32 am
by popeye62
Thanks D and Revolver
There are no pumps or coolers (apart from the forward seal 'circulatory' tank which cools the oil by convection). The only pump is a transfer pump for topping up the header tank. The cooling comes from a cooling tank which surrounds the stern tube and is permanently filled with FW. This tank is separate from the AP tank. All the pipework runs through the stern tube not the AP
There is no doubt that seawater has contaminated the stern tube LO. How the seawater has overcome the head of the upper header tank is not relevant. The question is; could the shaft have dropped this much in ten days? The emulsion has reduced the load carrying capacity of the oil film, boundary lubrication has replaced hydrodynamic and metal-to-metal contact has followed, accelerating wear.
The last wear downs show reasonable amounts of wear and an acceptable clearance in the bearing but are four years old (Revolver, the vessel should be inspected at the intermediate survey which is every 2 to 3 years and occurs between the class renewal surveys. If the survey for the first intermediate is done 'in water' or the wear downs were not taken then the minimum period for inspection is five years which is still too long for me as we engineer's love to spot trends in data)
Agreed that an excessive bearing clearance will hammer out the white metal and possibly bend the shaft, the drop in the shaft will also exceed the misalignment tolerance of 0.3 mrads. It will also have the effect of increasing the vertical positive offset of the intermediate bearing, increasing the reaction beyond that stated in the calculation which has a further knock on effect at the after most main engine bearings by unloading them.
The question remains that; is there enough lubricity in the ductile cast iron bearing backing, lubricated and cooled by seawater to some extent to cause a wear down of +10 mm (plus the 4 mm of bearing) in ten days or has this occurred over a much longer period bearing in mind that the condition of the LO has deteriorated over two years with little maintenance other than occasional draining of water and topping up with oil?

Re: Wear in stern tube bearing

Posted: Sun Dec 04, 2016 2:18 am
by Merlyn
For the shaft to have 10 mm ( which I take to be 5 mm a side ) and the associated support bearing to have 4 mm ( 2 mm a side ) from a shaft only 470 mm OD planed off in a timescale of only ten days seems to me to be akin of no less than a machining operation being carried out by a lathe tool.
Even over a two year period it seems extraordinary to me.
As regards to running clearances surely the sufficient to ensure a 360 degree support of the shaft ought to suffice?
The more the shaft is supported by the lubricant the less wear all round.
Where does the ten day timescale enter into it all?

Re: Wear in stern tube bearing

Posted: Sun Dec 04, 2016 3:03 am
by popeye62
Thanks Merlyn
The wear is all in the bottom of the bearing as the mass of the tailshaft and propeller is approx. 20 tons and is always acting downwards (negating transverse propeller thrust from hard turns or the 'hand' of the propeller). There is no wear in the upper portion so saying 'per side' is not strictly true as the bearing/backing is no longer concentric. Apart from heavy scoring on the shaft journal there does not appear to be any 'wear' (by that I mean a reduction of the overall diameter of the journal) in the shaft. The timescales come from a difference of opinion. Ten days was the last safe port.

Re: Wear in stern tube bearing

Posted: Sun Dec 04, 2016 5:02 am
by Merlyn
So it would appear that except for the barbed wire effect around the tail shaft all of the 14 mm of excessive wear is in the bearing and subsequently into the bearing housing assembly?
As per most worn shafting etc owing to the weight of the shafting most of the wear is present in the lower housing thereby eating into the lower seal lip and indeed creating a gap at the top of the seal causing a prompt lube exit.
Plus with a prop tailshaft the helical turning effect would possibly produce a lifting moment causing " chatter " in the shaft/bearing areas to accelerate the wear process.
An irreversible process of rapid destruction of the clearances between shafting diameter and support bearings/seals now commences.
But rates of wear?
Coefficients of friction big time here.
Ten days versus two years?
Rates of wear V time?
Only the man with the sandals on can answer that one methinks.
If you are really stuck I still have some Belzona from the sixties ( only one owner -me )

Re: Wear in stern tube bearing

Posted: Sun Dec 04, 2016 6:00 am
by D Winsor
I've read your response and the responses of others but it is still baffling me as to why there seemed to be no indication of a loss of oil from the system, considering the primary purpose of the shaft seal are to keep the oil in not the water out and that the outer water seals are there to protect the inner oil seal. Over my career I've seen numerous outer seals quite literally destroyed by ice, fishing line and other such debris. In just about every case the first indication of a problem was the loss of oil from the system indicated by a loss of oil from the header tank or a visible oil sheen around the stern of the vessel.

I don't mean to be condescending when I say this but I assume that the upper header tank was inspected including the oil level indicators (Gauge Glass or Float Gauge) and the Low Level alarm to verify that they were functioning properly and not giving a false reading. IF so and everything was correct this leads me to believe that there may be some issue with the oil supply line from or the vent line back to the upper header tank, which is somehow restricting or blocking the flow of oil or preventing the sterntube from being vented effectively. If collected sediment in the header tank had carried over into the piping system or an uncontrolled growth of bacteria in the oil (an increasingly more common problem)could potentially block any horizontal or elbow section of pipe supplying oil to or block the vent from the sterntube or there was a malfunctioning or damaged valve in system, would reduce the effective static head oil pressure in the tube and water could enter the tube from the outside.

As it is believed that the cooling tank surrounding the tube may actually serve a dual purpose, that is to provide besides a means of cooling the sterntube, to also act as means of preventing contamination of the after peak and prevent the accidental discharge of oil in the event of an oil line failure through vibration or corrosion. It is also possible that this tank could have been designated as a cofferdam in the original design of the vessel. It is also assumed that the level of the fresh water cooling tank surrounding the tube is checked on a regular basis and is maintained at 100% capacity at all times. If so the possibility of water entering the tube from this tank through either a crack in tube itself or tube supply or vent line passing through the tank cannot be ignored. If the tank is pressed to overflowing on a regular basis or there were to be a crack in deck between the fresh water tank and the after peak, the static head pressure of the water in the tank, under certain conditions, could exceed and overcome the static head pressure of the oil.

Re: Wear in stern tube bearing

Posted: Sun Dec 04, 2016 6:39 am
by popeye62
Thanks D, I understand your line of questioning but unfortunately I was not on-board the vessel at the time, plus, the information I am being given is the information that the owner wants me to have. My own investigation is limited and therefore there are certain assumptions that I have to make which must be plausible. I am between two vested interests and there is a lot at stake.
Having said that, of course the level in the LO header must have been falling due to an egress of oil and draining of water followed by regular topping up. The most pertinent question is time; how long it would take for the shaft to grind its way through the bottom of the bearing if there was nothing in the stern tube other than seawater?

Re: Wear in stern tube bearing

Posted: Sun Dec 04, 2016 7:48 am
by D Winsor
Thanks for the response, if I were a betting man I venture to guess that the bearing failed quite quickly (within a day or so) most likely after it was observed that an excessive amount of oil was being lost from the system and the oil supply valve to tube was either throttled in a little too much while running or shut off completely in port and not reopened before departure. Then as I mentioned and others have suggested that once the bearing failed the seal would have been destroyed almost immediately as pieces of the failed bearing went through it, allowing water to flood the tube. removing much of the effective lubrication for the cast iron backing shell for the babbit bearing.
If you can obtain in order to help your investigation any kind of oil consumption records for the sterntube, I'd say up to 6 months to a year prior to the failure or any kind of recorded increase in engine load, fuel consumption or loss of propeller pitch or if any reports of the shaft bouncing while turning were made around the time of and just after the incident was believed to have occurred. As I mentioned earlier with respect to my experience with an over worn Lignum Vitae sterntube, as wear increases the shaft will grab and roll up the side of the tube as it turns hammering in this case the cast iron bushing along with an marked increase in fuel consumption, a marked loss of pitch and a corresponding loss of vessel speed. In my case the loss of vessel speed was observed by the charterer and I had to try and explain it as diplomatically as possible as so not upset the operations manager as he tried to extend the vessel drydocking a year past the recommended 5 year cycle.
If it turns out to be a case of human error that lead to this failure Good luck in finding someone to own up to it.