Oneof the world's largest vibratory pile drivers, the Octa-Kong, will be used toerect the Hong Kong-Zhuhai-Macau Bridge in China. The machine, being built byAmerican Piledriving Equipment Inc., uses 14 gearboxes to regulate the timingon 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 theshaft keyways aligned properly in orientation to the spiral bevel gear teeth,"says N.K. Chinnusamy, president of ExcelGear Inc. "The difficulty involved here was staggering."

Chinnusamy notes that special fixturing had tobe built to facilitate the orientation of the keyways in the shaft and holes inthe spiral bevel gears. But the very first test runouts resulted in keywayalignments within 0.0005 inch of parallel.

In only 10 weeks, Excel Gear fullymanufactured, tested, assembled and shipped the gear boxes. Using a "design formanufacturability" approach, the company manufactured the spiral bevel gearsand machined the weldments and forgings, along with engineering special Teflonseals. It also created special stainless-steel breather caps and holes toaccommodate the pressure build-ups and air dissipation requirements of thesystem. The drive shafts and bevel gears were then heat-treated, assembled, andfinally tested in the company's quality inspection lab. Each component washand-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 syntheticoil and protected with special Teflon bearing seals, are used in pairs at eachof eight stations to provide the sequential vibration to the massive assemblythat holds the piling above the insertion tube. As the piling vibrates, itliterally shakes its way into position.

Designed for Travel and Operation

Since the system will betransported on a barge, if the pile driver system has any problems with thegearboxes when it arrives, the entire system will not work. To withstand verysevere vibratory conditions, the unit construction needed to be rigid. All ofthe screws were wired, so nothing could come loose, and a "shrink fit" was usedto guarantee the integrity of the assembly.

"When we shrink fit a bevel gear to a shaft, there is a concern wemight lose the precision timing required for the application. So we had to makea special fixture for it to achieve the manufacturability goals of building thegearboxes right the first time," says Chinnusamy. "The challenge was to buildthe units in eight to 10 weeks, and also make it work in the application."

The timing of the gearboxes was the number onecritical 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 thegears.

"We didn't have time to make a prototype andthen 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 thecustomer's specification and subsequent units were all close to each other."

Eight structures in the pile driver machineare configured in an oval shape to drive the piles with the drive shaft on eachof the two gear boxes used to connect a set of two. An oval-shaped raceway andthe larger structures are placed between the gear boxes with the long shaftconnecting the structures. Using mechanical timing as a critical part of thedesign, the gearboxes were timed so well that American Piledriving Equipmentcould use a standard coupling.

To get the proper backlash and contact patternin the system, the spiral bevels needed to move in and out very precisely. Ifone pattern on the bevels was too far in or too far out by only five thousandthsof an inch, the contact may not have been good enough and could createvibrations or break the teeth in the mechanism. Plus, all of the gearboxes hadto be inspected to be sure the gearbox parameters were within tolerances.

Bevel Gear Considerations

Anotherimportant part of the design was making surethe keyways were mounted in parallel witheach other. The reason for this is that the bevel gear and mounting shaft bothhave holes in them, and the bevel gear is mounted on top. The keyway in the shaftalso has mounting holes, and the shaft is mounted to the bevel gear withmatching holes, as well.

"Aligning the keyways and holes in the shaftto the spiral bevel teeth is very difficult," says Chinnusamy. "The problem isthat spiral bevel teeth are curved and the holes in all 30 of them need to beexactly the same. When we assembled the gearboxes onsite, we had to be surethey were in the same orientation. We made a special fixture, so before thegear went onto the shaft, it was all oriented properly."

For the assembly process, you only have 10 to 15 seconds. If youdon't do it quickly,the heat transfer immediately swells the part. Everything has to be timedprecisely. Another difficulty, because it was also shrink fitted, is the needto heat the part and hold it when it's hot.

Setting the proper backlash and good contactpattern of the bevel gears is vital for quiet operation. Precise axial play ofthe Timken bearings used also proved to be very important to control bearingtemperature at high speed, and the special fixtures helped measure the bearingoffset precisely.