flexible shaft coupler

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dwkoski
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flexible shaft coupler

Post by dwkoski »

I am currently fitting the propshaft and thought I would use a flexible coupler to compensate for any slight misalignment. I've seen several boats with donut couplers and that appears to be a good solution with good tolerance of misalignment and high torque ratings but they seem extremely overpriced at McMasters Carr and I was wondering if anyone had other suggestions.
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Re: flexible shaft coupler

Post by DetroiTug »

The straight two-piece rigid coupling is very easy to set up and what most all other inboard boats use. Just make up some long tapered wedges of hardwood and put one under each corner of the engine, get the coupler aligned and put the engine mounting bolt right through the wedge. Yep, those flexible couplers are expensive and are really not necessary.

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Re: flexible shaft coupler

Post by Lopez Mike »

I agree that rigid couplers are trouble free. I do find that it is a bit fiddly to get things lined up. I have one in my 36 foot sailboat and if anything shifts then I get vibration from shaft whip.

I have had very good luck with roller chain couplers. They are cheap, can handle any amount of torque and are a snap to make work. Also easy to disconnect if you want to run your engine on the trailer and are concerned about either the cutlass bearing drying out or the dog getting tangled with the prop. Just two sprockets face to face with a length of double row chain wrapped around them to couple them to each other.

I have put 600 h.p at 7500 r.p.m. through a surprisingly small unit and it lasted several seasons of hydroplane racing.

McMaster Carr 6407K42 is a good example. A decent industrial supplier with have some in stock to hold in your hand.

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Re: flexible shaft coupler

Post by Bob Cleek »

Image

http://www.defender.com/product.jsp?pat ... &id=590663

There are "drivesavers" on the market which are probably a good investment. They provide a break in the metal-to-metal contact in the shaft line, which should limit electrolysis problems (which basically don't exist if your boat lives on a trailer.) More importantly, they will "break away" if your prop tangles with a piece of line, a sunken log, or the like. Given the torque that steam engines produce, I'd expect that would be a good thing. A sudden stop of the wheel at power could end up ripping a steam engine right off its mounts (or take the bottom of the boat out!), right? Infernal combustion engines simply stall out when this happens.
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Re: flexible shaft coupler

Post by fredrosse »

There is a lengthy discussion about this topic already on this forum, look up "Torque Calculations and Application to Marine Steam Plants" The tire type couplings are very expensive, the double roller chain couplings cost $29.00 from "Surplus Center". Other options and considerations are in the above mentioned discussion.

The paddle wheel drive shafts on my sidewheeler have "drivesavers" so that if the paddlewheels hit a log they will slip and protect the engine from damage. They transmit full rated torque without any slippage. But these are only costing about $10 each, not several hundred. They are suitable for steam launch engines at several hundred RPM, and are manually settable for breakaway torque.

The picture shows one of these, just to the right of the universal joint. The stub shaft (attached to the universal joint) slides into a tubular sleeve, the sleeve has two longitudinal hacksaw cuts. A two piece shaft collar fits over the shaft sleeve, and compresses the sleeve onto the shaft stub. Tightening the shaft collar keeps the shaft stub and sleeve from slipping, but the tightness is adjusted so that it will slip if too much torque is applied. It is a simple matter to tighten the collar set bolts enough that no slippage occurs at full engine torque, but it will still slip if something strikes the propeller or paddlewheel. Has worked well for three years now.
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Lopez Mike
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Re: flexible shaft coupler

Post by Lopez Mike »

I have never seen a slip drive like yours. I've seen some quick and dirty connections of that sort but they weren't intended to work in any organized way. They don't take care of any misalignment though.

I was shocked at how much those units from Defender cost. Ouch! I think I'll be staying with the chain couplers. I didn't know about the Surplus Center ones. Now my evening is shot. I'll be looking all through their stuff.

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Re: flexible shaft coupler

Post by Oilking »

Big word of caution: A resiliant coupling WILL NOT correct for a bent shaft or out of square coupling.

Allow me to relate an event:
A twin screw Grand Banks Trawler yacht was heading down river when one or the shaft couplings failed allowing the still spinning shaft and coupling to slide back where it tore the stuffing box out of the dead wood! This happened very near the marina and a sinking was avoided. The owner explained that his mechanic had just realigned both engines and was unable to get this one "close enough" :roll: so he had to install a resiliant coupling, like the Deffender, "to correct " the problem.

If it won't align things are bent. Resiliant couplings will make up for minor missalignment of true running parts, but should not be used as a correction for thing that are bent.

I know I'm preaching to the choir, but I had to tell the story.


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Re: flexible shaft coupler

Post by Lopez Mike »

I agree that those couplings that consist of a disk of plastic or rubber with studs are NOT intended to deal with significant misalignment (or much thrust or tension). That said, chain couplers and Lovejoy couplers can deal with quite a bit of misalignment. By quite a bit, I mean a few degrees. If you need more than that, it's U-joint time.

I use a Lovejoy coupler to drive my Hypro feed water pump. No problems. As a prop shaft coupler it would have a problem in that it isn't intended to take any axial thrust. You would need a pillow block or some such bearing to take the prop thrust.

My prop shaft has a pillow block and the engine main bearings are designed to take any amount of thrust. Four sealed ball bearings. Five counting the pillow block. Total overkill. The pillow block is coming out this Winter. Over determining the location of a shaft isn't that good of an idea in a boat hull that is free to twist and bend.

I also have a U-joint at the output of the engine. It will be eliminated when I change over to a chain coupler. My engine to shaft alignment is quite good. I have no idea why the builder did that. Or any number of other weird ass things! Try a shut off valve between the boiler and the safety! And plastic handled water valves all over the place. Need any? I have a large box of box store plumbing fittings that I've removed.

Too make the point that roller chain couplers are robust, the hydroplane application not only required that the coupler take the torque of the engine, it took the entire thrust. This thrust and torque weren't steady by any stretch of the imagination. The prop was in and out of the water moment by moment. They are a remarkably simple and tough little unit.

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Re: flexible shaft coupler

Post by Mike Rometer »

The problem with Hooke's type universal joints (as shown above) is that they do not give true constant rotation, unless in an absolute straight line, in which case there is very little need for one. Ideally they need to be used in 'aligned pairs' to cancel out the increases and decreases of rotational speeds that occur twice every turn, and even that is somewhat of a compromise. Used singly they must be aligned very carefully so that the centre-lines of both shafts cross exactly in the centre of the joint (assuming some shaft out-of-line), but the changes of speed, each rotation, will be putting loads on both shafts, that may show up as a vibration at certain speed ranges. Even in pairs, the out-of-line should really be equal on each joint for the rotation to be equalised, so they are at best a compromise. Good enough for motor vehicles with cushioned drives and tyres, but,

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Re: flexible shaft coupler

Post by fredrosse »

“I have never seen a slip drive like yours. I've seen some quick and dirty connections of that sort but they weren't intended to work in any organized way. They don't take care of any misalignment though.”

Yes, they are simple and easy to make, but they are not intended to accept any misalignment, just there to limit maximum torque.

“The problem with Hooke's type universal joints (as shown above) is that they do not give true constant rotation, unless in an absolute straight line, in which case there is very little need for one. Ideally they need to be used in 'aligned pairs' to cancel out the increases and decreases of rotational speeds that occur twice every turn, and even that is somewhat of a compromise.”

That is correct, and for small angular misalignment the variation in rotational velocity is however very minor. With a steamboat prop turning a few hundred RPM, up to a few degrees would not matter. The linking of two universals in “aligned pairs” giving constant angular velocity is called a “Cardan Joint”, many are used on automobiles and trucks. These units will reliably give constant angular velocity, and would be entirely satisfactory for a boat configuration with significant angularity (up to say 30 degrees) between crankshaft and propeller shaft.

For the sidewheeler I chose single Hooke’s universals on each output shaft just in case there was some flexing of the hull, and they are in good axial alignment within the boat, so no rotational variation there. The other end of the drive shafts are mounted in spherical bearings. I probably don’t need them, but wanted a conservative design.
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