I would just put in a very accurately machined spool piece or steel
disk with proper pilots. As I understand it, having just had the
opportunity to sit at the feet of some masters and listen but not
being an expert myself, the really critical alignment issue is that
the shaft be exactly centered. Steel will do this better than
plastic. Bolts will not maintain it reliably. You need those
machined pilots.
I know that the drivetrains I have used so successfully must develop
some angular misalignment as the engine moves around on the mounts but
the shaft accommodates it without a problem. The single stern bearing
and flexible stuffing box (or shaft seal) on these boats is actually
almost exactly the same setup as on many sailboats.
If you are relying on something between the flanges to accommodate
misalignment, it has to be very, very, soft. Otherwise, there will
still be enough force transmitted to create vibration. Just think how
hard it is to dent or deform that drivesaver disk even a small amount
and then imagine that force on your system a few hundred times a
minute. If the coupling is soft enough to accommodate misalignment,
then you will need a thrust bearing and all sorts of other
complications. I've designed and built successful shaft lines for
auxiliary equipment with big rubber tire couplings, CV joints and
thrust bearings, etc. None ran a smoothly as these high power
research vessels do.
Say, I'm awake too early, how about a sea story?
I worked at Woods Hole Oceanographic Institution converting the ship
that Bob Ballard later used for the first mapping of the Titanic site
and the Alvin dives to the ship. We were taking out the steam engines
and installing a diesel power plant. We took two of the propulsion
motors out of the old diesel electric salvage tug "Chain" the was in
the process of being scrapped and turned them into DC generators to be
driven by big PTO's on top of the new reduction gears. One of my jobs
was to design the shaft system which had to penetrate the main
watertight bulkhead just aft of the reduction gears.
My boss wanted to build two tall pedestals with pillow blocks to
support the shafts. It would have been really tight and complicated
and alignment very critical with the bearings so close to each other.
I suggested that we just let the ends of the shaft hang on it's own
bearings by modifying a big rubber tire flex coupling at the generator
end with a pilot bearing inside. My boss explained that there were
buoys down south of Cape Horn that the ship had to retrieve before the
batteries in the acoustical locator beacons and anchor releases ran
down. If my idea didn't work, years of research would be down the
tubes and several Ph.D. candidates might have to spend and extra year
or two in school to get their doctorates. Was I sure? I was pretty
young but I gulped and said, "Yes".
"What did you say?", he asked, "I couldn't hear you."
"YES"
So, I designed the rig. Time and people were short so I ended up
assembling it myself. The engines for the ship were two surplus
railroad locomotive engines that came from a Navy warehouse still
wrapped up from 1942. Finally, the day came to fire things up and
test it.
We clutched in the port generator. It ran beautifully. Then we
started up the identical starboard engine and clutched in the PTO.
Instantly, the six foot diameter generator and twelve foot long engine
started violently lunging back and forth so hard that you could almost
see the structure of the ship deform. We stopped everything
instantly. Took it all apart, checked the alignments, put it back
together. Same result. Everyone was spending a lot of time staring
at me.
We checked and rechecked. We tried to reproduce the problem on the
other side. It was a total mystery. Finally, my boss said that there
was only one guy who could figure it out and flew someone up from
Texas. The Texan arrived and we fired it up for him so he could watch
the whole thing thrash around. This is tons of machinery solidly
bolted to the heavy engine bed of a ship shaking it so you feel like
you are standing in a moving train. Very scary.
He watched for two seconds and yelled, "Shut it off!". Then he
jumped up on the engine and opened up the cover of the engine
governor. "Give me a drill.", he asked. He drilled a hole, did
something else, put the cover on, and told us to start it again. It
ran smooth as silk and both sides continued to for years until the
ship was sold.
Turns out that the governor of the starboard engine had a device in it
that would give the engine a little boost of power if it suddenly
slowed down. This is so that it wouldn't stall when the locomotive
took all the slack out of the couplers when starting up the train.
When we clutched up the PTO, the inertia of starting the big generator
would wind up the rubber tire coupling and slow the engine. The
governor would give it a goose winding up the coupling some more. The
coupling would then unwind, unloading the engine. The timing was such
that these impulses would travel back and forth through the system in
such a way as to amplify each other with each cycle and build up to
enormous force in about two seconds. All the guy did was drill a hole
and drop a pin into the boost mechanism of the governor. He was on
the ship for ten minutes and then in his car headed back to the
airport.
The next day, we tied pulling power from the generator. The other one
had worked fine so this was pretty casual. When the switch was
thrown, a ring of fire, like a circular lightning bolt appeared around
the commutator of the big, open frame, motor cum generator. I'd
always wondered whether those little round escape hatches in ships
were big enough but, six guys can go up a ladder and through one in
less than three seconds when sufficiently motivated.
Turns out that the guy who overhauled the motor put the brushes in
backwards. He came down and turned them around. No harm done and the
whole system worked perfectly thereafter.
--
Roger Long
"Skip Gundlach" skipgundlach at gmail dotcom wrote in message
...
Hi, Roger, and List/Group,
This really has more to do with a spacer than drive saver, though I
like the flexibility of it in case the alignment isn't perfect.
Those of you who have been around a couple of years know how we
jacked the tranny out of the mounting plate (now replaced, no harm
done other than inconvenience in making up a keeper to stay moving
with the engine, and the packing gland needing redoing cuz of the
weight of the tranny on the shaft when it came out, also redone).
Recent readers also know of my search for line cutters, which would
have prevented that (which shouldn't have happened, but stupid
sailor tricks happen all the time, anyway, so a mooring line
overboard isn't guaranteed against for all time on our boat, as much
as we'd like to think so, and, of course the occasional lobster/crab
pot warp I don't see in the middle of the night) - which will
require a longer shaft, or...
... a spacer - so why not a flexible one?
And, I was looking for input from those who'd had a use for one
preferably where the thing sheared and left the 'limp home' mode in
place.
I agree about proper alignment - and that will happen before we
insert a spacer or drive saver...
L8R
Skip, back to the boat for two weeks in a few days
--
Morgan 461 #2
SV Flying Pig KI4MPC
http://tinyurl.com/384p2 The vessel as Tehamana, as we bought her
"Twenty years from now you will be more disappointed by the things
you
didn't do than by the ones you did do. So throw off the bowlines.
Sail
away from the safe harbor. Catch the trade winds in your sails.
Explore.
Dream. Discover." - Mark Twain
"Roger Long" wrote in message
...
For what it's worth, I deal often with one of the foremost
drivetrain and propulsion specialists in the country in connection
with designing boats like this:
http://home.maine.rr.com/rlma/WHOIrv.htm
The shipyard that builds these boat specializes in fast,
sophisticated, high powered craft. Both told me that they hate
"Drivesavers". The boat in the picture has two 710 horsepower
engines with the engines flex mounted and the shafts connected with
solid couplings. There is just one bearing, in the shaft strut.
They run smooth as silk.
I do have a drivesaver disk in my boat but they cut the shaft short
when they installed it so I have to buy a whole new shaft to take
it out. It's not causing any problems that I can see though other
than making it a pain to repack the stuffing box.
Skip the drive saver. Line up your shaft carefully calculating the
overhanging weight of the shaft and using a scale to hold the end
up. Make sure the flanges are true and the pilot concentric. Then
hard mount it. The metal parts will then be more precision than a
plastic disk can ever be and will stay that way. It will run fine.
There's a whole range of prop strike forces where the drive saver
could break but the solid coupling would still leave you able to
limp off a lee shore with the engine vibrating and shaking. I like
that scenario better. I've heard of a lot more broken shafts and
totally trashed props than gear boxes that failed due to prop
strike. The gears are a lot more rugged than you would think.
Remember that there is a friction clutch in the system that will
give some under an extreme shock load.
--
Roger Long
"Skip Gundlach" skipgundlach at gmail dotcom wrote in message
...
Hi, Lists,
Thanks for all the responses to my line cutter question. To put
one on will require some more space, and, in general, I've come to
think that a break-away (with built-in safeguards to allow
continued use until replacement) spacer is also a good move.
Who here has had one, and, best, has had to use it to save their
gear?
Thanks again.
L8R
Skip
Morgan 461 #2
SV Flying Pig KI4MPC
http://tinyurl.com/384p2 The vessel as Tehamana, as we bought her
"Twenty years from now you will be more disappointed by the things
you
didn't do than by the ones you did do. So throw off the bowlines.
Sail
away from the safe harbor. Catch the trade winds in your sails.
Explore.
Dream. Discover." - Mark Twain