A large diameter machined billet aluminum
prop shaft is only made possible by a major breakthrough in thin
section ceramic hybrid bearings.
Paul Lamar
...ALSO ... with aluminum shaft and a "press-fit on the rotating member"
I cringe a bit.
I have no experience with press-fits onto aluminum. I can just see the
bearing peeling off a layer of aluminum at assembly time and ruining
one's sizable $investment in the aluminum shaft. You see any problems
in this?
Robin Bergland
Use a slip fit and Lock Tite Bearing Lock.
http://www.loctite.com/
Lock Tite is super glue basically.
I know of at least one Tracy PSRU that had silicone rubber locking
the bearing to the housing. There is also a mechanical axial lock
consisting of a plate and a lot of screws.
The hybrid ceramic thin section bearing has a steel inner and
outer race. As the shaft expands it locks the inner race of the
bearing. If the aluminum housing expands it grows away from the
outer race. In that case a flexible bond namely, red silicone rubber
makes sense. The shear load on the outer race is minimal as that is
what bearings do. Reduce the torque load caused by the bearing.
I know of another Tracy gear box case where a high pressure oil fed
plain bearing failed due to inadequate oil supply after working
for many hours. The builder made the decision about the oil supply.
Not Tracy's recommendation. Finally, under extra severe conditions,
the bearing failed but it did not lock up until after the housing
cooled.
Paul Lamar
Paul, Large diameter bearings are often installed warm to avoid
press fit problems particularly on long shafts. If the bearing is
oil lubricated you would heat it in oil with a candy thermometer to
350 degrees and then slide it on and hold it up against the shoulder
to it locks on the shaft. With a sealed bearing or one that is
grease lubricated I pack them with grease or coat the sealed one
with grease and put them on a steel plate. Then heat the underside
of the plate until the grease begins to melt then slide them on. I
do this on spherical roller, timkin, and ball bearings on
hammermills, chippers and big band saws which have bearing bores
ranging from 3-15/16 to over 6" diameters. I have never dismounted
a large bearing from an aluminum shaft, however a first try would be
to get a puller tightened up on it and pack it in dry ice for a half
hour.
I might also mention that only thin wall bores increase
with temperature. With thick wall sections the OD gets larger and
the ID gets smaller. An extrusion press die at 650 degrees F would
be destroyed trying to remove it from the bolster but at room
temperature it will fall out if it had been coated with milk of
magnesia from the drug store as an anti seize.
George grimes
"I might also mention that only thin wall bores increase
with temperature."
Is that really true? That IS counter intuitive :)
BTW using silicone rubber to bond on the bearing needs a bit
of explanation. If the bore is 4 inches and the width of the bearing one
half inch the bonding area is about 6 square inches. If silicone rubber
shear capability is only 500 psi that is a shear force over 3000 pounds.
Torque fail wise it is 6000 inch pounds or 500 foot pounds. To fail
that... the bearing would need to seize. If epoxy is used than that goes
way up to an incredible 3000 pounds per square inch shear capability.
Fail torque than is 3000 foot pounds. Fail torque is defined as that
torque required to twist the bearing out of the holder or off the shaft
after it has seized. If it seizes. Ball and roller bearings rarely seize.
That is why Lock Tite and silicon rubber work so well.
Silicone rubber has the additional advantage of being flexible to absorb
different expansion rates in aluminum shafts with steel bearings.
To prove this is easy. Bond one square inch test strips of aluminum
and steel and apply a shear load until it fails.
Paul Lamar
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