Clearance on cutless bearing

[kalanka]

I have pulled the 1 inch propeller shaft on my boat (Sadler 34) prior to fitting a new cutless bearing and bought a 1" x 1.5" x 4" Vetus replacement bearing in a bronze sleeve.

When I hand fit this to the shaft there is a certain amount of play in the fit - I estimate 0.5 to 1mm at the bearing with light hand pressure.

I expected a rather tight fit that would free up in use. When I measure the shaft with a good caliper it measures 25.31mm - which is less than the nominal 25.40mm but not enough, I think, to explain the play encountered. The shaft is 30 years old.

Should I be concerned?

 

[Norman_E]

Cutless bearings have always seemed a bit of a slack fit to me even when new. The last new one I fitted to the 30mm shaft on my boat had definite play, so its probably not a problem. The last time the boat was out of the water was two years ago, and I showed the play which was by then about 1mm to the surveyor who looks after my boat and he said it did not need changing. I will look again when I lift the boat this coming May, and change it if it has got worse, but the boat was motoring with no noticeable vibration or other problems last season.

 

[cmedsailor]

Cutless bearings have always seemed a bit of a slack fit to me even when new. .

Same experience and no vibrations. Little play from brand new. As long the shaft is perfectly aligned and the bearing doesn't wear out unevenly I think a little play is not a problem (speaking from experience at least).

 

[GrahamM376]

Same experience and no vibrations. Little play from brand new. As long the shaft is perfectly aligned and the bearing doesn't wear out unevenly I think a little play is not a problem (speaking from experience at least).

Cutless bearing became an almost an annual event and it turned out that the Vetus ones from ASAP were slightly oversize I.D. There were various threads about this last year and IIRC several people found Countrose bearings a better fit.

 

[contessaman]

Hmm.. I recently replaced mine with one from ASAP and it was quite a tight fit on my original 30mm shaft which has a little wear on it. No play at all and in fact I was worried it was a bit stiff but fortunately when I wet it with a hosepipe then I could turn the propeller a lot more easily. Don't know whether tight or a little play is correct but I guest with use they'll all end up the same?

 

[GrahamM376]

Hmm.. I recently replaced mine with one from ASAP and it was quite a tight fit on my original 30mm shaft which has a little wear on it. No play at all and in fact I was worried it was a bit stiff but fortunately when I wet it with a hosepipe then I could turn the propeller a lot more easily. Don't know whether tight or a little play is correct but I guest with use they'll all end up the same?

Maybe the fit depends on the particular size as the manufacturing moulds for some may have better or worse size tolerances . Mine is imperial 1 1/4". ASAP can also supply other makes than Vetus.

 

[contessaman]

Maybe the fit depends on the particular size as the manufacturing moulds for some may have better or worse size tolerances . Mine is imperial 1 1/4". ASAP can also supply other makes than Vetus.

Yeah mine was some other brand, not vetus. 30mm shaft 45mm naval brass case. I had to get the bronze shaft fitting machined out slightly as the Volvo Penta fitting was 44mm case diameter. I'm sure as long as there's not excessive play there's nothing for kalanka to worry about. after all, if you think how bad the play can be towards the end of a cutlass bearings' life and it still works fine.

 

[Steve_N]

I've had about 0.5mm (certainly less than 1mm) of play in my cutless on a 1.5" shaft for about 10 years now.
I was advised that the amount of play wasn't significant and it didn't need changing but being something of a perfectionist decided that I would change it anyway; I dug a hole big enough to drop the rudder into and set about working out how to do it - and ordered a Vetus cutless from ASAP.
Upon testing the new Vetus cutless on an unworn section of the shaft it seemed to have more play than my 'worn' (30 year old) bearing, so I did nothing.

Then a while back I needed to add something to an order to get free postage so I ordered a GRP-shelled cutless just to be different - again I think it was from ASAP.
Same result - obvious play on my shaft, which measures a perfect 1.5" with a micrometer.

So now I have 2 new cutless bearings neither of which seem to offer an improvement on my now 36 year old one.

 

[Norman_E]

Yeah mine was some other brand, not vetus. 30mm shaft 45mm naval brass case. I had to get the bronze shaft fitting machined out slightly as the Volvo Penta fitting was 44mm case diameter. I'm sure as long as there's not excessive play there's nothing for kalanka to worry about. after all, if you think how bad the play can be towards the end of a cutlass bearings' life and it still works fine.

Are you sure that you bought the correct bearing? For 30mm shafts there are two different sizes. My boat takes a 30mm (Metric) bearing with a 1.75 inch (Imperial) outer case, which is 44.45mm. A lot of French built boats like my Jeanneau take the metric/imperial hybrid size bearing.

 

[contessaman]

Are you sure that you bought the correct bearing? For 30mm shafts there are two different sizes. My boat takes a 30mm (Metric) bearing with a 1.75 inch (Imperial) outer case, which is 44.45mm. A lot of French built boats like my Jeanneau take the metric/imperial hybrid size bearing.

Yes I am. Its a Volvo shaft and Volvo make a special 44mm outer shell GRP bearing that you have to buy from... Wait for it... Volvo at a much elevated price. Standard 30mm x45mm x 120mm naval brass shelled one was a fraction of the price so I got the shaft fitting machined out for that. Now future replacement will be easy and I've bought another for future replacement and put in the spares cupboard onboard.

 

[Neil_Y]

We supply bearings mainly for pumps but they perform the same roles when used in marine shafts for water lubricated bearings and we consider the same details when deciding on clearances for boats. A new 1" bearing as below should have a new clearance min of 0.136 and it starts to wear faster at around 0.63mm or that's when you think about changing it.

First point is they are water lubricated so they are designed for the shaft to run on a film of water, generated by hydrodynamic lift or when the shaft turns fast enough the water will support the forces involved to prevent the shaft touching the bearing. This means they need clearance to have a water film so they should never be tight even when new.

Brings me on to the second point that they will always wear unevenly as most (all in a good system) wear happens during run up and run down before the shaft is turning fast enough to get the water film to support it. This means the wear will be mostly in the bottom of the bearing if the shaft is sitting under its own weight.

Sizes
Bearings can come in two sizes. One for a press or interference fit (most marine shaft bearings are supplied to be pressed in) and one for a clearance fit bedded on a compound. Press means the bearing OD is bigger than the carrier ID. When pushed in the bearing ID will get smaller. The tricky thing is that press can be an almost easy press or a nearly impossible press unless you measure it. Clearance means the ID of the bearing as supplied doesn't change when fitted as you simply slide the bearing in by hand using a bedding such as epoxy to support it. The good thing is you know if it's a clearance as you can test it, slide in by hand and turn it.

There will also be bore closure during use which has to be allowed for. You need a basic min running clearance but to this you add the possible change in dimensions from a change in temperature, in the example below 0.075mm for thermal change. Absolute min from new to allow for moisture, thermal and running is 0.136mm Min radial displacement.

For actual figures here are the sizes we would supply for a typical 1" x 1 1/2"
Shaft max OD 25.40
Carrier Min ID 38.10

Clearance fit bearing would be supplied
(25.54 ~ 25.64) x (38.05 ~ 38.00)

Interference fit bearing would be supplied here you see the ID is squeezed down so the end result ID is the same as above.
(25.56 ~ 25.66) x (38.13 ~ 38.18)

Hope that's helpful, we put a lot more thought into fitting than you think. The figures above are for a 316SS shaft, a bronze carrier and a composite bearing. The figures can alter if the materials are different and we would calculate clearances based on the materials involved.

 

[GrahamM376]

Brings me on to the second point that they will always wear unevenly as most (all in a good system) wear happens during run up and run down before the shaft is turning fast enough to get the water film to support it. This means the wear will be mostly in the bottom of the bearing if the shaft is sitting under its own weight.

Interesting post thanks. As a matter of interest, what would be your thoughts if the wear was even right along the length of the bearing but at 9 o'clock rather than 6 ? Vetus bearings only lasted around 200-300 engine hours.

For info - this is with a two bearing setup, aft one in P bracket, the second (old one never replaced) being in the stern tube. Shaft will slide cleanly in from the stern through both bearings and rotate freely with no tight spots, alignment at the couplings is <0.004" and the assembled shaft , coupling and feathering prop checked on a lathe and shaft checked again with dial gauge in situ. Shaft has an anode clamped on.

 

[alahol2]

Interesting post thanks. As a matter of interest, what would be your thoughts if the wear was even right along the length of the bearing but at 9 o'clock rather than 6 ? Vetus bearings only lasted around 200-300 engine hours.

My cutlass bearings always end up with more free play athwartships (3 and 9 o'clock) rather than up and down. I've always thought it was probably because of side thrust when the engine is running.

 

[Neil_Y]

I guess it is possible with smaller shafts and big props, as you have in yachts, that the act of using lots of power astern as you do when berthing would induce the sideways paddle wheel effect and load up the shaft enough for the water film to collapse? I know on an old Rival 38 the sideways force was significant! and that may result in the 3 and 9 o'clock wear. On power boats where the shaft is long and heavy it''s pretty much always 6 o'clock.

Another aspect is number of waterways, you need some for cooling but too many and the water film collapses earlier.

I have seen P brackets out of alignment, with everything else looking good, again bigger boats. But you can see that in a bigger bearing by looking at the clearance around the front and rear of the bearing and shaft, or test the set up with the shaft chocked and sliding a bearing into the carrier.

We treat it as a bit more of a science as we are often looking at improving pump effiiciency by points of a percent by running minimum clearances.

 

[alahol2]

I guess it is possible with smaller shafts and big props, as you have in yachts, that the act of using lots of power astern as you do when berthing would induce the sideways paddle wheel effect and load up the shaft enough for the water film to collapse? I know on an old Rival 38 the sideways force was significant! and that may result in the 3 and 9 o'clock wear. On power boats where the shaft is long and heavy it''s pretty much always 6 o'clock.

I know paddlewheel effect is usually mentioned in connection with power astern but presumably the same effect is always present when going ahead also. Certainly my turning circle to starboard is smaller than to port, presumably a paddlewheel effect.

 

[chrishelen]

I had a cutless bearing fitted on my inch shaft by the yard fitters about two years ago. The old one had about 1mm play, so I replaced it as my boat does lots of hours. The new bearing had the same play. The fitter said the bearing would swell over time and take up the slack. Last year on survey there was negligible play. So guess he was right.