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From: (no name) (no email)
Date: Thu Nov 03 2005 - 09:58:27 EST
Larry Wrote
Since I posted my original message several years ago, I have had an
opportunity to speak at length to engineers at both Globe and PYI. Both sources agreed
that the problem of using a single element as a mechanical fuse, a thrust and
vibration absorber, and a connection to the prop require compromises that may
make it unsuitable for some boats.
Globe only makes one model which can be fitted to the BW 4-71 transmission.
It must be rigid enough to handle the full torque of engines normally coupled
to that transmission yet weak enough to shear when that torque is greatly
exceeded. I suspect that the shear point for the Drivesaver is reached when the
maximum rated torque is doubled, although I could not get a definitive answer on
that point. For lower powered engines the Drivesaver shear point may be four
or more times the maximum torque. Remember that this is for the Model 504
Drivesaver normally fitted to BW 71 and BW 72 transmissions. Other model
Drivesavers. for other transmissions, are rated from 225 to 6500 ft. lbs. of torque.
PYI makes a similar product but offers several torque ratings for the BW 71
transmission. This is accomplished by adjusting the hardness of the
polyurethane compound. The PYI product also incorporates a mechanical link which will
enable you to limp home after the coupling shears, assuming that your shaft is
not twisted into a pretzel.
If you use a elastomeric coupling element as a mechanical fuse bear in mind
that stronger is not better. The unit should shear at about twice the maximum
torque that your engine and transmission can deliver - else why bother?
In my considered opinion, the Globe 504 Drivesaver, as used on BW 71 and &2
transmissions, is simply too strong for trawler engines of less than 100 hp.
They are marginal in the 100 to 150 hp range. They may break if the prop hits an
unyeilding object, but then again, they may not. If your engine is more
powerful, they will reliably work as expected.
Globe could easily make Drivesavers of lower torque rating suitable for the
BW 71 transmission but they have chosen not to do so. The small trawler market
is to small, they say.
Larry Z
I have been watching this thread go back and forth and was going to stay out
of it but I can no longer control myself. First lets talk about the Globe
Drivesaver and reducing vibration If your considering one of these units I would
suggest that before you install it you check your shaft run out with a dial
indicator. After the unit is installed I guarantee you will find that you have
substantially increased the run out and that will probably create vibration
instead of reducing it. The reason this happens is because the soft urethane disc
distorts as you tighten it even if you pay attention to the torque and
tightening sequence it still happens. The idea of putting a soft disc in-between two
precision couplings in my opinion is flawed from the get go.
As a sheer pin or safety valve for the driveline I don't think much of that
idea either. If you look at the disc and visualize what would have to happen
for it to separate on overload you can see that its not going to be pretty or a
clean break. It will probably throw the shaft so far out of line on separation
it's liable to cause it to bend and or break up the shaft log. If you really
want to add a safety valve to your driveline (and I question the wisdom of
that) then you would probably be much better off to get all the relevant data rpm
torque etc. and get an engineer to calculate a soft metal key and or bolt or
pin in you shaft coupling. That would be a clean smooth break leaving the
shaft in line and causing minimal damage and engineered correctly will have a
pretty exact point of separation or release. Now having the shaft spin within the
coupling will cause some damage but upon impact you will probably immediately
get into neutral so that should be minimal.
The PYI R&D coupling is much better as far as creating shaft run out is
concerned the design is different and in my opinion much better but it will still
create some run out for the same reasons as the globe. It provides no value as
a safety valve or shear pin the get home feature that Larry referred to keeps
it from doing that.
In my opinion if your considering one of these units to reduce vibration you
would probably be much better off getting your drive line in order and
correcting what ever is causing the vibration in the first place. If your thinking
about a safety valve I'd think twice about the need for it but if you really
have to have it find a better way to accomplish it. I've seen a lot of these
thing in use and feel they create more problems than they solve. Well, I feel
better now that I got that out good luck to all.
Brian Palmetto FL
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