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Old 27-09-2010, 10:19   #1
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Are Flexible Shaft Couplings Worthwhile ?

After my vendor disappeared with my shaft and prop, I'm buying the whole coupler/shaft/dripless/prop setup. The flexibles promise anti-vibration, anti electrolysis, but at some point there are still bolts that must mount from hard flange to hard flange, right? Or am I missing something? Thanks!
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Old 27-09-2010, 10:26   #2
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Go Here...scroll half way down the page.

Installing a 3JH4E *^%#! Yanmar (Rant)

Del is a supper guy..he will walk you through it...just ask
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Old 27-09-2010, 16:04   #3
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One half of the coupling is mounted to one side of the driveline and the other half is mounted to the other side. Inbetween the 2 halfs is a flexible element. It transfers the force from one side to the other and allwows for some missalignment. Even when dialled in properly there is still likely to be .002 inch (2 thou ) misalignment on some plane. Also the element provides a cushion between driving forward and reversing. Plus if the prop and shaft got jammed hopefully the element would disintegrate protecting other parts of the driveline that are costly to repair or replace.
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Old 27-09-2010, 16:22   #4
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Are you talking about Drive Savers? They are fine until they break, leaving you without an engine. In my experience they break too easy. I know that's their job but I think they have them tuned to break too easy.
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Old 27-09-2010, 16:33   #5
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Quote:
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Are you talking about Drive Savers? They are fine until they break, leaving you without an engine. In my experience they break too easy. I know that's their job but I think they have them tuned to break too easy.
David'
Can you give a few examples of the way in which the drivesavers failed. Not dis-believing you, but in 50yrs around boats, including building some 60-80ft workboats, I have never seem one fail.
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Old 27-09-2010, 16:38   #6
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I had them break on my boat twice. I stopped using them because of that. I need a reliable boat. That's the only example I have. I have two 315 Hp Cummins turbo diesels each turning 24 x 24 four bladed props with a 2:1 reduction. I did not wrap something around the prop nor run it aground either.

As far as the installation goes, the nuts were re-torqued after the break in period.

Perhaps DriveSavers are better suited to smaller engines with much less torque? I don't trust them to not blow up on me again.
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Old 27-09-2010, 16:59   #7
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Are Flexible Shaft Couplings Worthwhile ?

Quote:
Originally Posted by Anzo View Post
After my vendor disappeared with my shaft and prop, I'm buying the whole coupler/shaft/dripless/prop setup. The flexibles promise anti-vibration, anti electrolysis, but at some point there are still bolts that must mount from hard flange to hard flange, right? Or am I missing something? Thanks!
A couple of points.
1. As previously posted, each hard flange is bolted onto the flexible element, not right through to the other hard flange.
2. Apart from allowing for minor misalignment and providing for shock absorbing of sudden torque changes, they provide for some flexibility between a soft mounted engine and a hard mounted shaft.
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Old 27-09-2010, 17:14   #8
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I had one of these on my Pathfinder (42 hp) motor/shaft and it did quiet down the vibration a bit and I didn't have any problems with it. PYI Inc. Max-Prop PSS Shaft Seal Seaview Radar Mounts R&D

My new Yanmar it would probably be OK but I was having other problems (shaft whip). So I've put in a thrust bearing and don't need it any more.

As far as breaking, these have two cross bars opposite sides and apposing positions that will, at least, hold it together if it were to fail. On large motors w/large props I would think they wouldn't make any difference, so I would not use them. A 300 hp motor/trany doesn't turn very fast.

BTW if you need one I have one for a yanmar with only 20 hours on it, at a reasonable price.
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Old 27-09-2010, 17:22   #9
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I'm not so sure. A 2600 RPM, 315 horsepower diesel reduced by a 2:1 transmission is still turning the shaft at 1300 RPM. When you reduce the speed by a factor of 2:1, you also double the torque, minus any losses from friction.
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Old 27-09-2010, 17:42   #10
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With my Pathfinder the shaft turned at 1500 rpm at full out speed. I doubt one would run a diesel at full speed for very long, just in short bursts. It's that first powering up that really stresses shafts. A piece of plastic in there is really going to get stressed if one were to go from fwd to rev too quickly.
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Old 27-09-2010, 20:30   #11
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Maybe there is a max HP for the Drivesaver, but mine has been in service for 15 years on my 135hp Perkins and has saved my butt/transmission twice. The Drivesaver after one incident of super over-torque looked like a wave washer with about 1 inch of displacement. But the next morning it was back flat again and has worked fine for the last 10 years although I keep a spare in my spares locker.
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Old 01-12-2010, 16:22   #12
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I would think the manufacturer would proved the horsepower rating for their product. They work really well for damping the engine pulsation at idle.
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Old 01-12-2010, 17:15   #13
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The ultimate is a CV joint with thrust bearing. It is a double universal joint with telescoping shaft inside the rubber bellows, The prop shaft is held rigidly to the thrust bearing that is connected only to the engine bed. This way the prop shaft can only rotate, not shake and move around, which the engine does freely. It mkes our PSS shaft seal work better, and if we snag a crab pot line, it could save the transmission. It also increases the life of the engine feet, as they do not take the forward thrust from the prop shaft at all, and the engine can be TOTALLY out of alignment!

We have over 15 years on this one, and love it. It is pricey... probably over a thousand bucks now... but well worth it if your boat is a keeper.
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Old 02-12-2010, 02:26   #14
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I will second Mark's comments on the CV joint setup. We have had one for 23 years and I would not be without it. Never have to worry about engine alignment , within a few degrees is fine.
When we changed from Volvo to Yanmar, just had a new connecting shaft made up.
Also makes for a very quiet installation.
Regards, Richard.
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Old 02-12-2010, 20:49   #15
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I just installed one. It seems like the best thing for Yanmars and a PSS!


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