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Old 24-02-2010, 05:33   #1
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What Is the Best Material for Cutlass Bearings ?

Asking around the boat yards today was suppose to clear up some questions about the cutlass bearing, as well as get quotes on the slipway hire.

The opinions were divided evenly as to the best material for cutlass bearings. Nitrile and compressed fibre. Ummmh, so which one to use ? I am inclined to get the Nitrile bearings as the original bearings in the vessel were nitrile and they were only nominally worn after 4 years in the boat. The fibre ones we put in at the last slipping did not last very long at all. nitrile are more expensive but slipping is much more expensive !!!
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Old 25-02-2010, 17:23   #2
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Have been looking at the
Johnson Duramax Cutless bearings.

Are they as good as they claim ?
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Old 25-02-2010, 20:17   #3
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Quote:
Originally Posted by ribbony View Post
Have been looking at the
Johnson Duramax Cutless bearings.

Are they as good as they claim ?
They are one of the most well know manufactures around. And what ever material they use is likely the best you can get at a decent price.

Two notes; if the material is too soft it will wear out too fast. If it's too hard it will wear out the shaft costing even more in the long run.
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Old 25-02-2010, 20:35   #4
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Wear out the shaft, OUCH ! That is incentive enough to strike the correct balance.

Thanks
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Old 12-05-2010, 07:23   #5
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Not quite what we see in practice, this is born out by research and practical trials on commercial vessels.

Here is an intersting piece of research which agress with our findings.
http://www.wartsila.com/Wartsila/glo...ls-article.pdf

Firstly lets clarify that a water lubricated bearing when running does not touch the shaft so the only opportunity from wear is at start up and run down. Solid hard bearings have solid lubricants and can support the shaft with low friction when dry or during this period.

Soft materials such as nitrile rubber will allow sand and grit particles to enter the water film area which supports the shaft, if they find their way in from a water groove the gap gets smaller, at some point certain sizes of sand or grit will not be able to pass between shaft and bearing. With a soft material the particles can imbed in the bearing surface creating an abrasive surface.

Hard materials faced with teh same grit or sand particles will see the grit/sand in the water channel. as it tries to enter the boundary between shaft and bearing at the narrower areas if it won't fit it will bounce along the edge until ejected it can not force its way in without moving the shaft, the water pressure of the supporting film will not allow this.
If the sand/grit particles can enter the boundary area it will be small enough to pass through, it may at very wosrt tear a piece of the bearing material out whilst scoring the shaft slightly but it won't stay there to continue to wear the shaft.

Currently the best material for water lubricated shaft bearings is a phenolic composite with surface chemistry which resists marine growth (another large factor in wear rates) advanced surface polymer that helps prevent marine growth on the shaft and integral dry lubricants. Aquarius is the most advanced of these new materials but there are a wide variety to choose from. Many are made from different polymers and have widely differing characteristics here are some Orkot, Railko, Thordon. If you search for composite marine bearing you'll find most of them.
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Old 12-05-2010, 08:46   #6
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I have plain bronze/rubber tubes and they have been good for like 10 years. Maybe your problem is not relaed to the material but there is some uneven load / pressure on the bearing? Do they wear evenly? (they can be turned ...)

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Old 12-05-2010, 15:46   #7
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The wartsila info is an interesting read. Though we do not use the vessel in what we would consider abrasive waters the marine growth issue is very real where we moor the vessel. I am sure that a lot of barnacle bits must contribute to wear over time.

Installed now we have composite and nitrile tubes and the vibrations we had before have disappeared, time will tell as to service life.
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Old 12-05-2010, 16:23   #8
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The marine growth to destroy the bearings is beyond doubt. It is not the barnacles - they are too big to fit in. But there are all sorts of minute calcium covered creatures that like to live in there if the shaft remains without movement for any extended time. Been there seen that.

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Old 20-05-2010, 08:04   #9
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We have found the Duramax Bearing to have been built to sustain at least .003 clearance around the shaft.
I prefer to have a better material and a glove fit around the shaft.
Rit
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Old 20-05-2010, 16:18   #10
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There certainly is a small amount of play in the bearing, hopefully it does not increase rapidly.
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Old 20-05-2010, 16:30   #11
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what sort of composite bearing did you install?
thanks,
rit
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Old 20-05-2010, 16:49   #12
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Duramax !
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Old 20-05-2010, 16:50   #13
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MMMMMMMMmmmmmmmmmmm, they are the very company I'm having problems with.

Thanks, appreciate the feedback.

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Old 20-05-2010, 17:03   #14
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We got less than 2 years out of our previous bearings (unbranded compressed fibre bearing). The duramax does run much smoother, but we do not want to have to slip the vessel again just to fix bearings as that is too expensive an exercise. The druamax were relatively cheap when bought from the US compared to similar bearings available here.
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