Why are all prop shafts water lubricated?

OT: Life on the Water

Well, the Chinese restaurants here are REALLY good, and a nice meal for two with beer is about $10 US.
Nice website! Wish we knew about you the last 3 years as we lived on the Mediterranean in Carthage. I'm still kicking myself for not getting a boat on the water on the Med!. But I'm determined to get some boat somehow here in China..

At some point I want to repower a boat with a DIY surface drive or build a cat/tunnel design with two mid-size auto engines, probably diesel, maybe gas. So please keep telling us what you learn in building yours!!

 

If the bearing is well lubricated, I understand that the bush bearings should work just fine.

 

Automotive shaft seals

One further problem when using standard automotive shaft seals in marine applications is the tension springs in the lips of the seals are made of ordinary spring steel windings, they rust out remarkable quick, and then of course the seal lips have no tension and that horrible salty wet stuff gets into the beautiful polished ball races and destroys them overnight.

If you insist on still doing this, you can remove the springs, undo the join, chop off a tiny bit from the non tapered end and screw them back together. This will increase the face tension on the seal lips (and then cause wear on the shaft more).

Seals in ships are oil lubricated with open ended systems often seen, but they are very expensive, way above the costs of PPI or whatever takes your fancy. The carbon ring seals work very well, they really are not that expensive, and if installed, (as I have done on cruising boats) by keeping the normal packing gland as well, they are 100% reliable (or at least redundable).

 

CDK, a freind is building a twin screw workboat & is fitting grease filled stern tubes, they run a stuffing box inboard set "soft" & have a simple baffle arrangment attached to the shaft at the aft bearing to help keep sand & grit out in shallow water opperation, all thats required is a couple of grease gun pumps every 4 hours to keep fresh grease in & water out, also a particular grease mobil special222 is specified I presume for ease of turning as the wrong heavy type might case issue in cold into gear opperation. Regards from Jeff

 

I never said I'd use automotive parts.
There are many seals specially made for outboards and sterndrives, with a stainless spring.
I intend to use a double lipped seal from SKF that is fully encapsulated in neoprene, so there are no corrosion problems. They also make it with quite exotic polymers for agressive environments where neoprene cannot survive. But I think the neoprene one will be suitable for seawater.

 

Many large ships used oil based tubes. They are now using a biodegradable oil substitute. Remember that in many parts of world a oil slick from your vessel will generate many fines or worse.

I am thinking of modifying my tubes to a pressurize water stern tube system . In other words - I would be taking water from engine cooling and blowing it out tubes. Why, because I going be in very shallow muddy water and dont want sand getting in my bearings.

 

Many Ideas come together....

OK, this is starting to make sense to me...

* Don't intentionally mix Oil and Water.

WhatIf:

- The propeller end of the tube/strut has a water-lubricated bearing of the type successfully used today: Rubber, Plastic composition, Metal alloy etc.

- The engine end has a combination of seal(s) and bearing(s) that are replaceable from the inboard end.

It would be nice to be able to:

- Have a close-clearance point (collar etc.) near the inboard end so that the rate of water coming in when the seal was removed, even with 1 meter of head, was not too high. Much like many packing type glands. Or, how about an inflatable tubular seal that did not normally contact, used only for seal/bearing maintenance when stopped.

- Pressurize the aft-bearing-to-seal tube section with exiting cooling water, to flush the aft bearing of sand particles etc.
- Pressurize (low head) the engine end bearings between two seals, to guarantee that sea water won't enter the bearings, and give warning of a beginning-to-leak seal.

What do you guys think??

PS: Configuration idea: WhatIf the engine end seal-bearing assembly is flange-mounted to the tube and quickly replaceable as a unit? Or at least easy to get at to replace seals and/or bearings. A good locking thrust ring is needed.

 

What do I think...KISS.....there are many things on a boat that we can make better, but playing around for the sake of playing around is only complicating a simple thing.

 

(from waikikin) "all thats required is a couple of grease gun pumps every 4 hours".
I assume this relates to operating hours only, or will he really get out of bed to squeeze the grease pump?
As you mentioned a stuffing box, the stern tubes are conventional and the grease feature is an addition to keep abrasive particles out. That'll surely work, but after a while there will be grease turds floating in your friend's working area....

(from mydauphin) "Many large ships used oil based tubes".
Before I started this topic I really thought I was alone with my weird concept, but this helps putting me at ease. Large vessels have tremendously large prop shafts for which the outer seals are made in segments, so they not as tight as a simmer ring or O-ring. And the amount of oil involved is of another magnitude. Do they also use ball or roller bearings?

(from TerryKing) "how about an inflatable tubular seal that did not normally contact, used only for seal/bearing maintenance when stopped".
A really superb idea. If it doesn't already exist, someone will steal it, have it patented and get rich. A rubber tube with different wall thickness for the inner and outer circumference or a reinforced outer rim will do the trick. That way it'll stay in place when it is not pressurized. I think you just invented the "marine anus"!
Combining the water filled tube with an oil filled inner section seems quite complicated to me. The seal that separates water from oil is located deep in the tube, followed by the thrust bearings and the oil-to-air seal, very hard to reach if something goes wrong.
Also, the water lubricated outer bearing doesn't control the shaft movement as tightly as a ball bearing can, so the oil filled section has to cope with rather large radial forces.
I think it must be either flexible and water filled, or stiff and oil filled.

There is one practical problem with the construction I have in mind. Although slightly off-topic I will mention it here.
Between the prop shaft and the gearbox comes an H-joint, the shortest possible U-joint assembly. It fits to the gearbox with a 4-hole flange, the other side has a sleeve for a 10 spline shaft with 1 3/8" diameter and was made for Berkeley jets and similar products. I will have to find a splined stub that can serve as an adapter for the 1" prop shafts.
Is this 10 spline 1 3/8" shaft a common construction that is used for cars, trucks or other machinery in the U.S.?

 

A drawing??

Maybe the attached quick drawing will be clearer.. I totally agree that the bearings and seals should be able to be accessed and replaced in the field. I would put the thrust point at the engine end of the tube. I would arrange the top section to be removeable so that seals and bearings could be easily removed/replaced without 'pulling' from inside a deep recess.

One issue (often seen in complex assemblies like automatic transmissions) is handling end forces while still managing the seals and bearing preloads and assembly/disassembly. There needs to be some method of producing 'end-clearance' for disassembly, either with an engine coupling that can be disassembled (Probably easier with a CV joint type coupling), or a removeable section of the overall shafting. Note: I can remove the driveshaft in my VeeDrive boat with no additional end clearance, by disassembling a 'U' joint.

OK, about the end-forces: A classic approach is to machine a shoulder on a shaft, but I'd LIKE to be able to have a prop shaft without 'special' machining. There are good 'locking collar' designs, but where should they be positioned? And how is the water-oil seal positioned so that the forward-thrust shoulder or collar is 'inside' or 'through' it?? "Outer" forces are transmitted to the tube, right? One permanent shoulder in the aft of the removeable section would have the oil-water seal bottom on it from the rear, and the thrust bearing race(s) against it's forward edge. A threaded (or otherwise retained) shoulder would come in from the forward end to hold the forward end of the bearing race(s).

I would want to be able to replace the thrust bearings and seals in some sequence like this:

1. Disassemble the engine coupling CV joint, remove center section. (Gets end clearance and bare propshaft end.)

1A. Pressurize the cool inflatable Maintenance Seal (AKA Marine Anus!) to stop incoming sea water.

2. Remove forward locking collar/sleeve (probably includes thrust bearing preload adjust / shims etc).

3. Unbolt flange section of tube, and pull assembly with seals and thrust bearing off the shaft end, possibly pushing prop shaft aft to get more clearance.

4. On "The Bench" inspect seals and bearings, remove/replace as necessary.

...reassemble in reverse sequence.

**OK** I see I need to make a better drawing showing how this goes together and where the forces are transmitted.. !!

I would hope that we can come up with something that can be made by any small machine shop with a lathe that can do typical propshaft tapers, threads and keyways, plus off-the shelf parts. Maybe..

All comments and suggestions welcomed. Heck, even Gotcha's !

**See following post for Detail Drawing (That might actually work...)**

 

Splines

CDK, all I know is that SOME of the agricultural and construction equipment shaft/splines have SOME standardization. None of the automotive stuff I played with was interchangable between manufacturers.. When you wanted to put a Chrysler engine in a Ford, you made a driveshaft with the Chrysler U joint and spline end on one end and the Ford end on the other...

Anyone know about this?? You probably need to ask on the Hot Rod groups somewhere...

 

The attachment shows approx. what I have in mind. It is an early version: I decided to use tapered roller bearings at the engine end, instead of angled ball bearings and also increased the size to make sure the thrust can be handled.
The prop end has two identical seals. The outer one, mounted with grease, may develop a leak by the absence of lubrication, but will still serve to keep dirt outside. Should the inner seal leak, oil can reach the outer seal. If both fail simultaneously, I will anchor the boat in deep water and forget it.

 

OK CDK one of the problems I have in my work (materials handling) is that people do not like change. Especially people that do production work, it is not in their nature.

So, I am not against change, but not change just for the sake of it.

You appear to be recognising those who are with you but not addressing those who question your design.

I have seen bearings collapse in a gearbox that is oil filled. The pressure in the gearbox oil is greater than the outside pressure, there is no poosibility of dirt entering through the seal frrom the outside because when the seal wears it only lets oil out.

But, somehow a speck of grit has got in and all metal to metal parts wear, so a microscopic piece has worn off the bearing and got into the race and the problem starts. When the bearing collapses the shaft thrashes around and usually does some other damage.

Looking at your drawing, the prop shaft is held in place by bearings, but may I suggest that instead of the rubber seals you replace them with a stuffing box. Not only are they adjustable to take up for seal wear, that your rubber seals can't, but they will also hold your shaft in place when the bearing collapses.

Also the tube that holds the top and bottom bearings, could you run a small compressor off the engine and pressurise it with oil lubricated air, which will both, lubricate the bearings and seals and the pressure would keep water out.

Poida

 

Bearings, Bearings...

(edited to add drawing, info):

OK, below is a drawing of the kind of configuration I've been thinking about.

The bearing/seal assembly is flange-mounted to the end of the propeller tube and can be removed with the boat in the water, by removing the engine coupling and then the forward locking sleeve. The bearings and seals can be replaced by unscrewing the end retainer and pushing them out with an appropriate tube, rather than 'pulling' from inside a recess with the shaft still in place.

One reason I like the idea of the propeller-end bearing being a 'conventional' water-lubricated type is that they are very robust and usually have no catastrophic failure modes.

I'd prefer to have ALL the other bearings and seals at the top end, and replaceable in the field.

This leaves the question of the 'best way' to have the two types of bearings at either end, and exactly what (Water? Air? Oil?) is in the tube in between...

Let's keep thinking!

 

I am sorry, but I still do not understand what is your goal.

What is the difference between the last setup drawn, and a combinate use of
http://www.pythondrive.com/800/talen/eng/dload/PD Leaflet 2007.pdf
and
http://www.pyiinc.com/images/pdf/pss/PYI_PSS_Instructions.pdf

The combinate use of off the shelf existing products includes the soft coupling to the engine and separates the seal friction surface from the shaft.
There is no need to machine the shaft to ball bearings or seal tolerance.