TWL: Re: Shaft Alignment
John,
The basic procedure is to loosen the coupling where the shaft connects to
the transmission, and then measure the clearances between the coupling flange
and the transmission flange. I use a set of feeler gauges for this job.
Measure top, bottom, and both sides. You want the clearances to be equal all
the way around. A target for alignment is to get the differences to be less
than .001 inch per inch of coupling diameter.
The corrective action depends on what you find when you do the measurement.
Simplest correction will be if the side measurements are equal, and the top
and bottom measurements differ a bit. If the top clearance is tighter than
the bottom clearance, then the front of the engine has to come down a bit.
The math is just a ratio thing, involving the size of the coupling and the
distance between you engine mounts. If you have a 4 inch coupling and 36
inches between mounts, then your ratio is 4:36 or 1:9. Then if the top
clearance is .010 tighter than the bottom clearance, the front of the engine
wants to come down 9x.010 or .090 inch. My mounts are 16 threads per inch, so
this would be about 1 1/2 turns on the nut.
Hope this helps.
Al Schober
Bristol 38 'Duet'
All this info about alignment and measurement is great, but how do you
account for the "dip" at the end of the shaft from where it enters the
boat/engine room through the 4-5 foot run to the transmission once you've
removed the bolts holding it to the transmission?
I would assume that even on a fairly substantially sized shaft there is
some downward moment due to gravity that has to be factored in to a correct
alignment. A mechanic who has aligned a couple of engines at our marina
said he sortof plays with the end of the shaft, moving it up and then down,
until it seems to be straight. Is that as good as it gets?
At 08:12 AM 1/13/2004, Al Schober wrote:
John,
The basic procedure is to loosen the coupling where the shaft connects to
the transmission, and then measure the clearances between the coupling flange
and the transmission flange.
<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
Bob McLeran and Judy Young Manatee Cove Marina
M/V Sanderling Patrick Air Force Base
Hampton 35 Trawler Melbourne, Florida
No! Tolerance with feeler gauges should be .002 inches. Many installations
with that much shaft exposed use a bearing to support the shaft and prevent
sag. Are the bearings called "pillow blocks?"
Ron Rogers
----- Original Message -----
From: "Bob McLeran" rmcleran@ix.netcom.com
| All this info about alignment and measurement is great, but how do you
| account for the "dip" at the end of the shaft from where it enters the
| boat/engine room through the 4-5 foot run to the transmission once you've
| removed the bolts holding it to the transmission?
The shaft will "dip" a bit at the coupler end from its own weight plus that
of the coupler. To compensate, multiply the length of shaft times the
weight per foot, divide by two and add the coupler. This is the approximate
force weighing on the shaft end. Devise a weight equal to that, tie a line
to the weight, run the line through a pulley clamped overhead and tie to the
end of the shaft at the coupler and let the weight bring the shaft end up a
bit. This should approximate where the shaft should be at rest with no
weight to make it dip and will hold it there for alignment.
Procedurally, I always make any significant side to side adjustments to
bring the engine into lateral alignment with the shaft coupler first because
this requires more than just adjusting the leveling nuts on the motor mounts
and can involve a bit of muscle to move the engine around to get the
couplers aligned laterally with equal gaps on each side, then any major
vertical adjustments, then the fine tuning of mating the couplers. Keep in
mind that most engine mounts only give leveling adjustment, so any lateral
adjustment requires twisting the engine in relation to its mounts.
If an engine has been quite a bit out of alignment and has no soft coupler,
I highly recommend having the coupler faced to the shaft by a good machine
shop to make sure you have a good coupler face to align the engine to. This
coupler facing to the shaft can be measured in the boat with the right
measuring tools.
In the end, a well aligned engine will mate vertically and laterally with
the shaft coupler and the gap between the two coupler halves at each of the
four quadrants will be within zero" to .003" of each other, the closer to
zero, the better.
Shaft alignment should be checked a week or two after launching from any
haul-out, to give the boat a chance to settle into shape from the haul-out.
Alignment should also be checked once a year, preferably at the beginning of
the season. This can prevent minor problems from developing into major
problems.
Good luck,
Bob Lowe
All this info about alignment and measurement is great, but how do you
account for the "dip" at the end of the shaft from where it enters the
boat/engine room through the 4-5 foot run to the transmission once you've
removed the bolts holding it to the transmission?
I would assume that even on a fairly substantially sized shaft there is
some downward moment due to gravity that has to be factored in to a correct
alignment. A mechanic who has aligned a couple of engines at our marina
said he sortof plays with the end of the shaft, moving it up and then down,
until it seems to be straight. Is that as good as it gets?<<<<