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Old 26-09-2021, 18:51   #1
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Convert from solid drive shaft to u-joint

Has anyone converted from a solid stainless drive shaft to a drive shaft with u-joints. How many u-joints,and pillow blocks. I have a solid drive shaft stainless around 1 1/8 inch and it is over 10 foot long. I was thinking of shortening the solid shaft. My packing gland hose is over 5 feet long. So it can get to somewhere serviceable from cabin. I can build a locker to the bilge under the cockpit and put the packing gland there and was thinking of shortening the solid shaft for a swivel shaft with u-joints. One that I can just uncouple like a tractor and no more tedious lining up. Has anyone else tried this, is it doable.
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Old 26-09-2021, 22:58   #2
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Re: Convert from solid drive shaft to u-joint

IMHO, it is certainly doable. You will need a double thrust bearing on the shortened end of your solid prop shaft and that thrust bearing will need to be held in a housing that is firmly attached to the hull. That will keep the shaft centred in the stern tube and will take the thrust of the prop in both forward and reverse. These kind of thrust bearings are available from engineering supply companies and are not rare items. They have an eccentric collar that locks the bearing onto the shaft. You can buy the housings from the same company.

The propshaft will go through the thrust bearing, protrude maybe 100mm, and have a flange on its end (similar to the flange on the output of your gearbox). Your first "U" joint will have a mating flange and will bolt onto that. This "U" joint will be fitted to the secondary shaft. On the other end of the secondary shaft will be another flange. The second "U" joint fitted to the shaft will have a flange that will bolt to the transmission flange.
You could use CV (constant velocity joints instead of "U" joints).
I would get a good engineer to specify all the relevant part sizes...
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Old 26-09-2021, 23:01   #3
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Re: Convert from solid drive shaft to u-joint

Sorry, I forgot to add that the secondary shaft must have some sort of sliding spline assembly to allow for slight movement of the engine forward and aft on its mounts.
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Old 27-09-2021, 05:05   #4
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Re: Convert from solid drive shaft to u-joint

Wow! nuku34 really laid it out for you. Pay particular attention to the issue of thrust. At present the prop shaft slides slightly forward and pushes against the tranny, which transfers the thrust to the engine mounts. The engine mounts push your boat. You'll have to design a thrust bearing mount attached to the hull and robust enough to take that thrust, including in reverse. Here are basic thrust bearings as a starting point to your thinking. And yes, this is a design job for a mechanical engineer.

https://www.mcmaster.com/thrust-bearings/
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Old 27-09-2021, 05:08   #5
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Re: Convert from solid drive shaft to u-joint

Why ???
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Old 27-09-2021, 05:13   #6
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Re: Convert from solid drive shaft to u-joint

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Originally Posted by goboatingnow View Post
Why ???

Quite.

Something like this perhaps:


https://shop.tnorrismarine.co.uk/pages/aquadrive-3

or

Pythondrive The ideal flexible and vibration-free drive unit
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Old 27-09-2021, 09:13   #7
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Re: Convert from solid drive shaft to u-joint

SIMPLE: DO NOT DO THIS - NEVER!

A well balanced shaft is in one piece
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Old 27-09-2021, 09:37   #8
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Re: Convert from solid drive shaft to u-joint

Quote: "SIMPLE: DO NOT DO THIS - NEVER"

Whyever not??

Hundred of thousands if not million of front wheel drive cars have used CV joints beginning with Hardy Spicer joints on such conveyances as Austin "Mini"s more than half a century ago, and subsequently adapted by the likes of Chrysler Corporation and still in use on such as Dodge Caravans to this very day.

Obviously the number of rotations required of a propeller shaft in a boats number of hours of use in its lifetime is infinitesimally small compared with that required by the shafts in garden variety "daily driver" motor cars. Thus the durability test has long since been surmounted by CV joints.

The only issue in a boat is the arrangement of the thrust bearing, and given the rather small thrust forces required to drive your garden variety cruising sailboat, that "problem" hardly qualifies as a problem at all!

In fact a competent CVJ/Thrust bearing installation will totally eliminate vibration originating from the shafting, but not, of course, from the diesel itself or from the propeller.

TP
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Old 27-09-2021, 09:43   #9
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Re: Convert from solid drive shaft to u-joint

We changed the unbalanced mass of our splitted shaft to a single shaft and the result was PHANTASTIC!

And our engine is only making 1800 RPM!!!
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Old 27-09-2021, 09:47   #10
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Re: Convert from solid drive shaft to u-joint

It is certainly possible, and details have been provided and if engineered correctly will work well.

Is there a problem you are trying to solve?
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Old 27-09-2021, 11:31   #11
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Re: Convert from solid drive shaft to u-joint

The marine CV's listed by Pete7 look like a viable option with the addition

of the load bearing hull attachments specified.


BUT truly, I fail to see a problem listed in your original post.

Seems like adding a lot of complication to a very simple system.


If you're asking, "Could I simply cut the shaft and add a U-joint?"

I think you'll find the answer to be no across the board. Not without

additional structure.
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Old 27-09-2021, 11:47   #12
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Re: Convert from solid drive shaft to u-joint

Quote:
Originally Posted by toerail View Post
Has anyone converted from a solid stainless drive shaft to a drive shaft with u-joints. How many u-joints,and pillow blocks. I have a solid drive shaft stainless around 1 1/8 inch and it is over 10 foot long. I was thinking of shortening the solid shaft. My packing gland hose is over 5 feet long. So it can get to somewhere serviceable from cabin. I can build a locker to the bilge under the cockpit and put the packing gland there and was thinking of shortening the solid shaft for a swivel shaft with u-joints. One that I can just uncouple like a tractor and no more tedious lining up. Has anyone else tried this, is it doable.
Some form of U joint is not uncommon. When I worked at a metal boat maker we built small boats for the armed forces with a double U joint system. I cant remember all the details but one was that you actually WANT an angle at the Ujoint rather than a straight line system. IIRC that angle was 3.5 degrees minimum.
That sounds a really hokey system you have and improving it would be good. a 10 ft long x 1.12 diameter shaft sounds like a wobble ready to happen.

I would draw up your system and talk to a drive shaft shop. They know all about this. In the above mentioned situation, it was the drive shaft supplier who designed the system.

When you have a prop driven system SOMETHING needs to transfer the thrust to the boat. That can be via the engine, or some form of thrust bearing support mounted to the hull between the prop and the engine.
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Old 27-09-2021, 12:39   #13
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Re: Convert from solid drive shaft to u-joint

my Irwin has the system you describe. I like it, it uses a drive shaft from a 3/4 chevy truck. with a slide yoke in it. I have a big thrust bearing on a bulkhead that supports and takes the thrust of the shaft. it works well. when I want to get down into my 5 ft deep bilge i can just unbolt the shaft and take it out of the way. no load on the engine mounts and no alignment worries. More complicated but works well.
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Old 27-09-2021, 19:21   #14
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Re: Convert from solid drive shaft to u-joint

I have same set up as you propose on my boat - it works well
You can also incorporate a flexible coupling into the design - and there are a few options but they will help reduce any noise, vibration and even damage in the event of a prop foul or hitting something

Good luck
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Old 27-09-2021, 20:11   #15
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Re: Convert from solid drive shaft to u-joint

The reason for the angular offset in a Spicer type joint is to keep the needle bearings circulating. You could use R&D style couplings (sort of an improved rag coupling). There used to be another type -can't remember the name- that used radial bolts with rubber bushings to take slight mis-alignment and vibration.
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