shaft wear - bearing and seal life

[Bosun Higgs]

My set up is a 25mm drive shaft to a Radice folder via a shaft log with a water lubricated bearing and a Volvo seal at the other end. Now the bearing has some slop after just 18 months and before my accident reported elsewhere, the Volvo shaft seal had started to drip after the same length of time. Both these events seem quicker than they should be, so what is wrong?

The shaft itself has wear that you can feel with finger tips, but the most I can detect with a micrometer is 0.08mm or 3 thou. The surface is smooth and shiny.

Any ideas? Any engineers out there who can comment on wear limits? Personally I wonder about the bangs and bumps from the prop.

 

[Neil_Y]

Each bearing manufacturer will give their own wear limits and new running clearances so I can only give accurate figures for Maritex water lubricated shaft bearings.

For a 25.00mm shaft
New clearance ID of bearing will be 25.10 - 25.20
Our advised worn clearance when bearing should be replaced is when clearance approaches 25.625.

Accelerated wear can result from
1.Misaligned carrier/bearing (with one bearing as I understand yours is this can be corrected with normal engine alignment adjustment)
2.Too big a clearance to start with (shaft runs on a smaller area when dry and at extremes can't support a water film)
3.Abrasives in the lubricating water, and type of abrasive.
4.Insufficient lubricating water, which can result in a build up of heat.
5.Growth of abrasive crustacean on the shaft or bearing surface.
6.Excessive shaft movement due to worn or broken engine mounts.
7.Debris on shaft or unblanced shaft/prop causing shaft to exert lateral forces.
8.Not enough bearing support for shaft length/weight and prop combination.

I think that covers all the possibles...As bearings when running should run on a water film wear should only occur at start up and run down, so your wear on a perfect system will be greater the more you start and stop your engine. Many commercial ships we supply have very low wear rates as they don't stop that often.

 

[scottie]

Do you have either a flex coupling or flexible mounts?

 

[vyv_cox]

Adding to Neil Y's list - operation at resonant frequency. Operation of my previous engine and gearbox (Bukh 20) produced no stern gear vibration or noise. I replaced the Bukh with a Yanmar 27 about 10 years ago. Since then there has been a resonant frequency at about 2000 engine rpm which has proved impossible to eradicate. Unfortunately 2000 rpm is almost exactly where we want to be when cruising, so we run slightly above or below this figure, and suffer a bit of extra noise.

I have replaced propeller, shaft and P-bracket, added an Aquadrive, changed the engine mountings to different hardnesses, realigned countless times and checked for even mounting loadings. None of these has eradicated the resonance.

The consequence of the resonance is that the shaft moves excessively, in a vertical mode. The principal consequence is that the cutless bearing wears at top and bottom. I seem to get about two seasons out of one, although since replacing the shaft two years ago it may be that some improvement has been made.

 

[brianhumber]

I have similar set up to you a fairly lengthy 25mm shaft with volvo stern seal and outboard p bracket with cutlass bearing - fairly common set up. When I bought Ronhilda in 1995 I replaced the lightweight folding two blade prop with heavy three blade Gori.

Ronhilda was built in 1987 and I have replaced the Cutlass bearing only 3 times, twice due to pick up of fishing nets and fleece.
I have slight parallel weardown over the length of the cutlass bearing c0.15mm but have not bothered to get a 'special' bore from Countrose to take into account this weardown and current standard cutlass bearing is some 4 years old. Yearly inspection indicates to me this will need replacing in approx 2 years at current 1,000/1,500 miles per year.

I drew the shaft at the same time as last replacing the Cutlass as the Volvo seal was showing signs of dripping after nearly 20 years service. Some wear surface rings on the shaft where the lips pressed onto the shaft but putting the new seal on couple of mm down solved that.

So my exprience suggests you have something amiss as your seals and bearings should last far far longer.

You have to have a film between the bearing /seal lips and shaft or rapid wear will take place.
Things that destroy this film are
Basic misalinement
Big bearing clearances leading to shaft wobbling
Unbalanced prop ( blades damaged etc )
Unbalanced anodes
Critical rev vibrations leading to resonance
Soft engine mounts
Soft flexible coupling

Its problably no comfort to you to know several modern cruise liners have have stern tube bearing failures when water lubrication came back into fashion after a couple of decades of oil filled bearings.

Brian

 

[Neil_Y]

Exactly what I was thinking Brian, Vyv's set up with an Aquadrive should give a smooth low wear unit aft of the Aquadrive. He may still be subject to engine resonance/vibration which could be changed with different mounts (softer or harder to change the resonance characteristics) But the shaft/bearings/prop should be straight and only subject to minimal torque and fairly small thrust.

However...a 25.00mm stainless shaft is quite bendy and if allowed to, will flex and twist.

Bearings longer or harder can reduce this, if you do the maths and look at possible deflection at the P bracket due to gap and/or rubber deflection, this can add up to a few mm mid shaft or at the next bearing. I'm fairly certain that a hard bearing would be an improvement over rubber in these cases, thats what we get told.

Your right about the ships, there have been a few issues with bearings running water lubrication and cooling, engineers have to allow for thermal expansion and moisture absorption in vessels that can run in sea temperatures from -2 to + 32C which presents some problems, whilst trying to keep a shaft from flexing causing noise and vibration. As yet none of the incidents have involved Maritex.

 

[Bosun Higgs]

Thanks for the replies. I'm principley concerned about tolerances that the Volvo shaft seal will tolerate.

The set up is a flexible coupler with the Volvo seal maybe 200mm away. The total shaft length is 1.1m, the diameter 25mm nominal, and instead of a P bracket the shaft pops out of the stern tube in a shaft log and the cutlass bearing is in the end of the stern tube. No more than 50mm of bare shaft and then the Radice folder which is a heavy old lump of brass.

As I said earlier the shaft has worn over 15 years use and the min diameter I can find in the seal area is now just on 24.9 mm with a rippley but not grooved surface. Nice and smooth.

 

[Bilgediver]

Any ideas? Any engineers out there who can comment on wear limits? Personally I wonder about the bangs and bumps from the prop.

Volvo make an issue in their sales manuals for these seals of a maximum unsupported shaft length and NOT suitable for use with shafts using a flexible
coupling.

This suggests to me that the housing might be a stiff structure and therefore it is possible that it is very important to check it is mounted so as not to put any side loading on the shaft. I have looked to see if I can find a recommended clearance on the web and not found anything however feel that the limit should be more than 0.15MM . This is a situation where there might be a higher rate of wear initially while shaft polishing occurs and then providing the seal is correctly greased the ongoing wear rate should diminish.

I shall continue to use my packed seal. Only uses a turn of packing every five years

Incidentally. A new shaft costs many beers . Being a tight welshman I might be tempted to move the seal to a new position on the shaft. Many ways of achieving this depending on how the seal is mounted or the shaft fitted.

 

[NorthUp]

For a 1" dia shaft, Volvo specify (+0.0.-0.1) shaft size,no reference to wear or grooves but I would guess that none are acceptable.
details here-
http://www.volvopenta.com/sitecolle...parts brochures/rubber stuffing box (eng).pdf

The propeller overhanging by twice the shaft diameter might be a concern if the prop is a heavy one. A longer cutless bearing may help here.

(edit- wear limit is presumably in mm..)

 

[Bosun Higgs]

Volvo make an issue in their sales manuals for these seals of a maximum unsupported shaft length and NOT suitable for use with shafts using a flexible
coupling.

Just read twice through the manual that comes with the seal and I can find no reference to flexible couplings one way or the other. Where did you see this? The coupling is the RD type which might take up a few degrees of axial alignment but certainly doesnt allow any lateral movement

 

[Alcyone]

Before Neil mentioned it, I was thinking about abrasives in the water. You sail mainly in the Bristol Channel, BH, IIRC? I think the amount of suspended sand and mud in the water here must be just about as much as it is anywhere.

I've not heard it mentioned that wear is particularly bad in the BC, but it's at least possible, isn't it?

 

[Tranona]

Just read twice through the manual that comes with the seal and I can find no reference to flexible couplings one way or the other. Where did you see this? The coupling is the RD type which might take up a few degrees of axial alignment but certainly doesnt allow any lateral movement

That surprised me - but it does say so on the data sheet in Northups post. However pretty sure all the Bennys and Jennys that use it as OE have flexible couplings. Does not seem logical as the purpose of a flexible coupling is to reduce movement of the shaft relative to the engine.

 

[scottie]

It is normally considered good practice to keep one solid element in the set up of stern bearing coupling and engine mounts to keep the shaft from whipping. A R&D coupling is often used as a solid coupling but all worth checking

 

[Heckler]

That surprised me - but it does say so on the data sheet in Northups post. However pretty sure all the Bennys and Jennys that use it as OE have flexible couplings. Does not seem logical as the purpose of a flexible coupling is to reduce movement of the shaft relative to the engine.

Stew
No flexible couplings on the 351, 381, 36cc Benes and 42 Jene from my first hand knowledge
Stu

 

[nedmin]

The advantage of the Volvo seal is that it has its own cutless type bearing built into the seal running on the shaft ,so the lip seals follow the movement of the shaft. it will therefore stand some movement of the shaft. Also dont forget that your fingers can feel well under a thousands of an inch of wear on the shaft.I run in the Humber and Yorkshire Ouse,probably some of the worst silted rivers in the UK. My last boat ran 10yrs on same seal without leaking,this one 8 yrs. I can feel a slight wear on the shaft but seal doesnt leak.If it does start leaking I shall renew seal and try to move shaft either forward or back to let the lips run on new metal.It may be possible to fit a packer between gear box and coupling and then shorten shaft.Much cheaper than buying a new shaft.