Fluid coupling to save drive?
08-27-2010, 02:23 PM
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Fluid coupling to save drive?
Hi all,
Has anyone ever tried using a fluid coupling (like a torque convertor, but far simpler) between the motor and outdrive to give the drive an easier time?
The way I'm thinking is that during hard launches, re-entries etc, the fluid coupling would remove the harshness on the drive gears caused by a mechanical connection. It would also have the effect of stopping the prop spinning up too fast if you're slow on the sticks during a jump, whilst keeping the prop speed close to what it should be when off the throttles in mid air, ready for re-entry.
Seems like it could be an easy solution to helping a drive's gears survive to me?
If anyone's tried it, someone in the US must have! If not, I might just give it a go!
Cheers, James
Has anyone ever tried using a fluid coupling (like a torque convertor, but far simpler) between the motor and outdrive to give the drive an easier time?
The way I'm thinking is that during hard launches, re-entries etc, the fluid coupling would remove the harshness on the drive gears caused by a mechanical connection. It would also have the effect of stopping the prop spinning up too fast if you're slow on the sticks during a jump, whilst keeping the prop speed close to what it should be when off the throttles in mid air, ready for re-entry.
Seems like it could be an easy solution to helping a drive's gears survive to me?
If anyone's tried it, someone in the US must have! If not, I might just give it a go!
Cheers, James
08-27-2010, 04:19 PM
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Great Idea, but a few things that come to mind:
There would certainly be some horsepower loss. Any time you introduce a fluid coupling like that, it will rob some power.
If the coupling was strong enogh to transfer enough torque to move a 12,000lb boat 80+ mph, wouldn't it also be strong enough to transmit enough shock to still break drives?
There would certainly be some horsepower loss. Any time you introduce a fluid coupling like that, it will rob some power.
If the coupling was strong enogh to transfer enough torque to move a 12,000lb boat 80+ mph, wouldn't it also be strong enough to transmit enough shock to still break drives?
08-27-2010, 05:22 PM
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Well y'see that's the thing with fluids. It's not like a mechanical joint where the torque is solidly transferred, it's all about momentum in the fluid. The momentum of the fluid created by the paddles in the driving side turns the driven side at a constant speed and torque (albeit with about a 5% loss), but any sudden changes up or down are absorbed by the fluid as they don't have time to change the momentum of the fluid , so it absorbs any sudden changes as though they're not there.
Another advantage is that due to the 'slip' given by the fluid coupling as it builds up fluid momentum and transfers the torque, when launching it would act in the same way as prop slip and allow the engine to build up revs before the prop really starts to bite. Handy with a turbo diesel motor or similar.
Better still would be having the fluid coupling built into the prop hub, so the forces are negated before they even reach the drive!
Here's a funny thing with fluid couplings. Imagine coupling a 500hp BBC and a 200hp 4cyl to the same shaft driving a prop for instance. With mechanical joints they'd always fight each other and you'd get no gain, probably ending with 2 broken motors and 0hp. With fluid couplings on each one, they'd purely help each other out no matter what the differences in torque and RPM. Allowing for the losses, you'd still have at least a useable 650hp. Weird eh!
Think I'm gonna have to try this one!
James
Another advantage is that due to the 'slip' given by the fluid coupling as it builds up fluid momentum and transfers the torque, when launching it would act in the same way as prop slip and allow the engine to build up revs before the prop really starts to bite. Handy with a turbo diesel motor or similar.
Better still would be having the fluid coupling built into the prop hub, so the forces are negated before they even reach the drive!
Here's a funny thing with fluid couplings. Imagine coupling a 500hp BBC and a 200hp 4cyl to the same shaft driving a prop for instance. With mechanical joints they'd always fight each other and you'd get no gain, probably ending with 2 broken motors and 0hp. With fluid couplings on each one, they'd purely help each other out no matter what the differences in torque and RPM. Allowing for the losses, you'd still have at least a useable 650hp. Weird eh!
Think I'm gonna have to try this one!
James
08-27-2010, 05:57 PM
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Fluid couplers (torque converters) are inefficient on the top end because of the slippage. I think all auto converters lock up now. Maybe something like the springs in a clutch disc would work better.
Last edited by thirdchildhood; 08-27-2010 at 06:03 PM.
08-27-2010, 10:37 PM
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I've broken bigger gears in car rear ends than what are in Bravo drives. This was with less than 500hp an built Turbo 400 tranny. They still transfer plenty of power to break gears.
08-28-2010, 12:04 AM
#7
Interesting idea. Here are some thoughts though:
We already run spring or rubber damper plates on the flywheel which is supposed to do this job. The propeller itself sort of works like a torque converter as well. We've gone and put 5 and 6 blade props on drives to limit the slip, and then, now we're going to put a fluid coupler in to make heat in the drive? I say go back to a 3-blade prop instead.
We already run spring or rubber damper plates on the flywheel which is supposed to do this job. The propeller itself sort of works like a torque converter as well. We've gone and put 5 and 6 blade props on drives to limit the slip, and then, now we're going to put a fluid coupler in to make heat in the drive? I say go back to a 3-blade prop instead.
08-28-2010, 01:00 AM
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Thirdchildhood - A clutch wouldn't work due to it needing to be set to an exact pre-load at which it would let go. This would have to be maximum engine torque, which would be hard enough to achieve in itself not knowing each person's spec, but would result in it being totally locked unless you happened to make a re-entry at flat out throttle. Fluid is adaptive, it would have the same give no matter what the rpm or torque level.
Cars run torque convertors, which are far less efficient than fluid couplings due to the presence of a stator to allow them to function the way they do. A torque converter will run at about 80% efficiency best, so needs a 'lock up', whereas a fluid coupling will achieve 95% or better. Not a bad trade off to allow your drive to live happily ever after!
Griff - Granted, yes, but that was in a car, which probably had a torque converter and a large gear reduction in the gearbox. Torque convertors multiply torque at first, resulting in some big figures.
If you put a 1200hp engine in front of a bravo with a fluid coupling I don't doubt it would break it a lot, as the torque of the engine alone would shred it with ease, but looking at the majority of people running 500-700hp, a fluid coupling would probably smooth things out enough for their drive to live.
tcelano - Yep, but go feel how stiff those rubber bushes/cush drive are, then note you can freely turn a fluid drive by hand, two totally different kettles of fish. The rubber bush allows a degree or two of movement at best under extreme circumstances, but the shaft is ultimately mechanically locked to the flywheel. A fluid drive can allow an infinate amount of give and will negate even the slightest of shocks being totally progressive, whilst allowing several revolutions of give until the torque gradually builds back up if required.
As you rightly point out, it's well known that a 3 blade prop is easier on the drive by far, that's why I've always run one.
James
Cars run torque convertors, which are far less efficient than fluid couplings due to the presence of a stator to allow them to function the way they do. A torque converter will run at about 80% efficiency best, so needs a 'lock up', whereas a fluid coupling will achieve 95% or better. Not a bad trade off to allow your drive to live happily ever after!
Griff - Granted, yes, but that was in a car, which probably had a torque converter and a large gear reduction in the gearbox. Torque convertors multiply torque at first, resulting in some big figures.
If you put a 1200hp engine in front of a bravo with a fluid coupling I don't doubt it would break it a lot, as the torque of the engine alone would shred it with ease, but looking at the majority of people running 500-700hp, a fluid coupling would probably smooth things out enough for their drive to live.
tcelano - Yep, but go feel how stiff those rubber bushes/cush drive are, then note you can freely turn a fluid drive by hand, two totally different kettles of fish. The rubber bush allows a degree or two of movement at best under extreme circumstances, but the shaft is ultimately mechanically locked to the flywheel. A fluid drive can allow an infinate amount of give and will negate even the slightest of shocks being totally progressive, whilst allowing several revolutions of give until the torque gradually builds back up if required.
As you rightly point out, it's well known that a 3 blade prop is easier on the drive by far, that's why I've always run one.
James
08-28-2010, 09:36 AM
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James,
I've been wondering the same thing for years. There should be no problems using a fluid coupling between the engine and drive. I currently own a Ferret and a Fox armored cars which use a fluid flywheel. The fox has a Jaguar J60 and weighs about 14000# and I know from other owners that the gearbox will fail before the coupling.
If you go forward with the project post pix and let everybody know what happens, sounds like a budding business! Fluid coupling for farm tractor, $200.00 ---------Hi-Tech polished chromium plated Fluid coupling for performance boat......$1200.00
I've been wondering the same thing for years. There should be no problems using a fluid coupling between the engine and drive. I currently own a Ferret and a Fox armored cars which use a fluid flywheel. The fox has a Jaguar J60 and weighs about 14000# and I know from other owners that the gearbox will fail before the coupling.
If you go forward with the project post pix and let everybody know what happens, sounds like a budding business! Fluid coupling for farm tractor, $200.00 ---------Hi-Tech polished chromium plated Fluid coupling for performance boat......$1200.00
08-28-2010, 10:44 AM
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Torque converters produce lots of heat so you would need a fluid coupler, dry sump, pump and fluid cooler and it still probably would not stop drive breakage. The idea that I was trying to indicate with the clutch disc is that the springs would be fully compressed under power but would release when airborn and provide some cushioning effect on re-entry. There would not be a clutch but rather a spring damper system.
