of the world's largest vibratory pile drivers, the Octa-Kong, will be used to
erect the Hong Kong-Zhuhai-Macau Bridge in China. The machine, being built by
American Piledriving Equipment Inc., uses 14 gearboxes to regulate the timing
on the vibratory power transmission for the apparatus needed to secure the pilings in the riverbed or sea floor.
"The toughest part of this job was to get the
shaft keyways aligned properly in orientation to the spiral bevel gear teeth,"
says N.K. Chinnusamy, president of Excel
"The difficulty involved here was staggering."
Chinnusamy notes that special fixturing had to
be built to facilitate the orientation of the keyways in the shaft and holes in
the spiral bevel gears. But the very first test runouts resulted in keyway
alignments within 0.0005 inch of parallel.
In only 10 weeks, Excel Gear fully
manufactured, tested, assembled and shipped the gear boxes. Using a "design for
manufacturability" approach, the company manufactured the spiral bevel gears
and machined the weldments and forgings, along with engineering special Teflon
seals. It also created special stainless-steel breather caps and holes to
accommodate the pressure build-ups and air dissipation requirements of the
system. The drive shafts and bevel gears were then heat-treated, assembled, and
finally tested in the company's quality inspection lab. Each component was
hand-washed and inspected for chips, then alcohol-wiped, run with oil,
inspected and re-cleaned. Watch videos of the machine design, as well as an overview of the bridge project.
The 14 gearboxes, filled with Mobil 626 synthetic
oil and protected with special Teflon bearing seals, are used in pairs at each
of eight stations to provide the sequential vibration to the massive assembly
that holds the piling above the insertion tube. As the piling vibrates, it
literally shakes its way into position.
Designed for Travel and Operation
Since the system will be
transported on a barge, if the pile driver system has any problems with the
gearboxes when it arrives, the entire system will not work. To withstand very
severe vibratory conditions, the unit construction needed to be rigid. All of
the screws were wired, so nothing could come loose, and a "shrink fit" was used
to guarantee the integrity of the assembly.
"When we shrink fit a bevel gear to a shaft, there is a concern we
might lose the precision timing required for the application. So we had to make
a special fixture for it to achieve the manufacturability goals of building the
gearboxes right the first time," says Chinnusamy. "The challenge was to build
the units in eight to 10 weeks, and also make it work in the application."
The timing of the gearboxes was the number one
critical issue, especially to achieve the timing specification of 0.005 inch.
This unit is a timing gearbox and is structured to rely on the precision of the
"We didn't have time to make a prototype and
then test it and make adjustments. We also made all 16 units at the same time,"
says Chinnusamy. "The very first unit we inspected was within 10 percent of the
customer's specification and subsequent units were all close to each other."
Eight structures in the pile driver machine
are configured in an oval shape to drive the piles with the drive shaft on each
of the two gear boxes used to connect a set of two. An oval-shaped raceway and
the larger structures are placed between the gear boxes with the long shaft
connecting the structures. Using mechanical timing as a critical part of the
design, the gearboxes were timed so well that American Piledriving Equipment
could use a standard coupling.
To get the proper backlash and contact pattern
in the system, the spiral bevels needed to move in and out very precisely. If
one pattern on the bevels was too far in or too far out by only five thousandths
of an inch, the contact may not have been good enough and could create
vibrations or break the teeth in the mechanism. Plus, all of the gearboxes had
to be inspected to be sure the gearbox parameters were within tolerances.
Bevel Gear Considerations
important part of the design was making sure
the keyways were mounted in parallel with
each other. The reason for this is that the bevel gear and mounting shaft both
have holes in them, and the bevel gear is mounted on top. The keyway in the shaft
also has mounting holes, and the shaft is mounted to the bevel gear with
matching holes, as well.
"Aligning the keyways and holes in the shaft
to the spiral bevel teeth is very difficult," says Chinnusamy. "The problem is
that spiral bevel teeth are curved and the holes in all 30 of them need to be
exactly the same. When we assembled the gearboxes onsite, we had to be sure
they were in the same orientation. We made a special fixture, so before the
gear went onto the shaft, it was all oriented properly."
For the assembly process, you only have 10 to 15 seconds. If you
don't do it quickly,
the heat transfer immediately swells the part. Everything has to be timed
precisely. Another difficulty, because it was also shrink fitted, is the need
to heat the part and hold it when it's hot.
Setting the proper backlash and good contact
pattern of the bevel gears is vital for quiet operation. Precise axial play of
the Timken bearings used also proved to be very important to control bearing
temperature at high speed, and the special fixtures helped measure the bearing