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Old 21-05-2005, 15:09   #1
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cutlass bearing

I am replacing the cutlass bearing on my sailboat. The shaft is 1.125", I ordered one from West Marine telling them the diameter of the shaft. They indicated that the new cutlass bearing would have an ID of 1.125". However it appears to be to loose, while there is no wobble, it stills slides rather easily. Is there anyone here who can give me some advice as how to size for the bearing? Maybe it is correct, I,m just not sure. Thanks in advance for any help

S/V Tivoli

Mike
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Old 21-05-2005, 21:04   #2
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Yes as long as you have no or very very little movement, the bearing is correct. These bearings are lubricated by the water and should not be to tight.
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Old 22-05-2005, 11:17   #3
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Cutlass bearing

Thanks alot for your feedback, I am attempting to install a new diesel and this is only the first of what I am sure will be many questions.

So again, thanks alot Wheels

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Old 24-05-2005, 03:23   #4
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Press fit ... changes the 'installed' dimentsions

There are three critcial dimensions when installing/replacing a cutless bearing .... or any other 'plain' bearing. Plus, there are always 'tolerance' allowances +/- 0,001 inches that are normal 'manufacturing' variabilities.

1. the shaft size is important to be accurately measured to 0,001 inches (measure three times, etc.)

2. the cutless housing diameter is just as important as the shaft diameter .... maybe more important here's why: The cutless (and all other such plain bearings) are held in place by a 'press fit' or 'intereference fit' - meaning that the OD of the uninstalled bearing is slightly larger (by about 0,001 inches per inch of housing diameter than the OD of the bearing. This will hold the bearing in place by the 'hoop stress' of the bearing forced into the slightly smaller geometry. When you force or press the bearing into the housing the ID of the bearing will 'close-down' to the magnitude of the 'interference' of the housing - the inside of the bearing gets smaller when you push/press it into the bore. Yes, there is sometimes a 'set screw' but this is NOT to hold the bearing in place but to secure the fit and prevent the bearing from coming loose IF the housing for some reason becomes hot (thus changing its ID ... and the bearing will/can easily slide out, etc.). The 'press' fit is what holds the bearing in place, not the set-screw (a back-up device).

3. Normal INSTALLED dimensions should be such that there is approximately 0,002 to 0,003 inches (per inch of shaft diameter) CLEARANCE between the shaft and the bearing surface. This provides a 'hydrodynamic space' or allows a film of water between the two surfaces on which the shaft 'rides'. You dont want the bearing touching the shaft!!!! you want water to be between the bearing and the shaft.

The important message is - you MUST accurately measure the (actual dimensions of the) shaft, and accurately measure the housing into where the bearing is 'pushed' into). Then carefully measure the differences so that there is sufficient clearance.
Why you need to accurately measure the shaft is that even if there is a hydrodynamic film keeping the shaft off the bearing, over the course of running time particulate in the water will abrade/polish the shaft; hence, 'catalogue' dimensions will probably result in replacement of the same 'looseness' that you had before you changed.
I doubt that West Marine has the 'smarts' to know this. There are 'accurate' sources of marine cutless bearings on the web if you do a websearch .... and have and know how to use an 'inside' and 'outside' micrometer set. Its not always the bearing that wears out, sometimes the shafting changes dimension because of abrasive polishing.

How to check a cutless without removing it: put a dial indicator on the shaft near the bearing, wiggle the shaft and if you get upwards of > 0,010" wiggle on an approximate 1" shaft ... then 'begin' to think about replacement. A 'well set' cutless on a well aligned engine will last almost indefinitely (unless the water you are operating in has a LOT of undissolved particulate).
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