Shaft-Stern Tube Interface

Not convinced pd, I and I don't think many people do it, install the oil seals on my trailer wheels back to front. I am more concerned with water coming in rather than grease going out.

It also enables me to flush the bearings with grease to push any water out.

Further to this the head of pressure from oil is less than the same head of water, so the extra head needs to be calculated together with water thrust from waves and reversing.

The biggest problem with bearings under water is sea water getting in.

 

Oil bath shaft logs , with positive head pressure, are very reliable. You dont see them on small craft because of cost...they are not cheap....and physical size... they are challenging to install, set up for the builder. In 40 years of sailing with oil bath shaft tubes I have never had a problem.

A popular stern tube supplier.

http://www.rubberdesign.nl/products_services.aspx?id=48&subnav=7

 

The system is in use since 2007 and has not shown any signs of water intrusion or oil escaping. Only during reversing does the water pressure rise and could water enter if there was any room for it.
But there isn't. When I installed the tubes the boat was on the trailer and I added oil nearly every day because small variations in air pressure and temperature caused air to escape until there was none left, a process that went on for approx. 2 weeks.
Water trying to enter meets 2 seals, the chamber between them filled with lithium grease, silicone oil and probably a small amount of engine oil that entered through a tiny groove in the outer ring of the ball bearing. the bearing itself also has seals on both sides.

I am completely satisfied with the current construction, but should I ever make something like it again, I would probably use needle bearings so the tube could have a somewhat smaller diameter.

 

Thanks CDK, it is just what I thought would be necessary from my pumping experience in preventing the pumped medium from entering a pump.

However it appears I am not alone as two searches on "Stern Tubes" have come up with a configeration as I suggested would be desirable. One not too clear but the other a defined drawing of a stern tube with one seal facing in and two facing out to prevent the ingress of water.

I don't know how to link other web sites except to write the full address.

Here goes: http://www.kemel.com/product/pdf/SNAMEAirSealFinal07_30_30.pdf

http://www.brighthub.com/engineering/marina/articles/62728.asp#imgn_2

I have never seen needle bearings used in heavy load transmission, as you said smaller diameter but that also equals faster revs and normally the needles are not kept apart by cages meaning they revolve very close to each other. Because of this they only need a small amount of dirt grit etc to stuff them up.

I have no idea what's on the back of my boat, I'm too scared to look. The flange at the gearbox end of the shaft has been welded on (a previous owner) so heaven knows what's down the other end.

I spend more time working on it than using it as it is, without looking for trouble.

I'll keep your drawing incase I need to work on it.

regards
Poida

 

Its possible there are two lip seals, there are in the small oil filled systems we've supplied as one seal resists oil pressure coming out and the other is to stop water coming in (on the wet end) Internal end they have single lip seal.

The tube will have a bearing set up which could be white metal roller(unlikely) or possibly composite. If you have white metal a direct and easier replacement and it is likely to last longer is Maritex.

Lip seals need a good bearing system to keep the shaft central so the lip seals are not pushed around by the shaft.

I'm curious to see the system Michael has? sounds like a Seatorque type of set up?

 

Simplest thing in the world to make, Take a slug of bronze and cut ot a hole bigger than the shaft then cut for seal front and back.

Mount with a flexible rubber coupling, drill a 1/4 hole and cut BSP fit a nipple and a motorcycle brake resoiviour add Auto gear oil .

I have these on my surface drives , made them 7 years ago no maitainence since then , boat in water all the time,--not a single drip,--ever.

 

Pretty much what I'm doing except on Mike Johns' recommendation I'm using a chunk of Vesconite instead of the bronze and seals only on the outer end, tube filled with oil as per CDK design. All the bits are made, should be fitting them pretty soon.

PDW