The Case of the Counterfeit Cages

DN Staff

July 19, 2004

4 Min Read
The Case of the Counterfeit Cages

I often am asked how I got into forensic engineering. One reason is that I don't like an unsolved mystery. To paraphrase the bank robber staring into the business end of Dirty Harry's .44 Magnum and not knowing how many rounds are left, "I got to know!"

As an example, here is one of the first conundrums I came across while working as an engineer for a lift truck manufacturer. This case whetted my appetite for more and more-ultimately leading me down a whole new career path.

In the mid-1980s, the U.S. Navy Atlantic Fleet was in naval exercises off the coast of Sweden. Onboard one of the U.S. aircraft carriers were high-ranking U.S. and Swedish Naval officers, there to conduct and observe exercises in combat readiness, which included loading bombs and rockets onto the racks and launch rails of combat aircraft.

In the ammo holds and on the flight deck, lift trucks were used to carry bombs and rockets while naval personnel carried out armament loading. With precision choreography, these sailors turned, what in lesser hands would have been a cattle stampede, into a ballet of speedy motion and order.

Unfortunately, in front of officers, enlisted men and dignitaries, a lift truck faltered, listed to one side, ground to a halt, and tumbled the unfuzed bomb it was carrying out onto the deck. With quick action the deck crew stopped the rolling bomb before calamity struck.

Before long, reports came into the Atlantic and Pacific fleet headquarters that other lift trucks were experiencing similar failures. The Navy rapidly "sidelined," or quarantined the entire fleet until the cause of the failures could be determined and the situation corrected. The pressure on the manufacturer, my employer, to resolve this problem was understandably heavy and relentless.

As Manager of Product Safety, my mission was to find the cause and fix the problem. Best described as a stand-up rider, end-control, straddle-type, electric-powered fork lift, this type of vehicle is propelled by two linked, steerable motor/wheel units mounted to the underside of the truck frame by a heavy-duty slewing ring or bearing. Navy personnel reported that the drive-unit bearings were breaking apart, causing loss of the complete motor/wheel drive unit and disabling the lift truck. Post haste, the company's military contract administrator and I set off to inspect the broken lift trucks on carriers in for refitting at ports in Florida and Virginia.

I found bearing cages and loose bearing balls rolling around inside the disabled trucks. The failed drive units had segmented plastic cages that appeared to have "squirted" out of the space between the inner and outer bearing races. In contrast, each still-functioning truck had bearings with a one-piece, circular, cast-bronze cage to contain and separate the balls.

With field inspections done and samples of both good and bad bearings to examine, analysis commenced. A review of the specifications and our design group's records showed that engineers had properly applied the correct bearing. Yet, the combination of axial and radial forces was enough to pull the bearing apart!

Differences in the bearing cage design appeared to be a primary clue. My persistent badgering of the drive unit company revealed that although they were still using the same bearing supplier, they had made a change to the inner and outer races and switched to plastic cages. The original bearing had close-fitting races with an elaborate (and I am sure, costly) ball-filling method, but which gave proven strength and mechanical integrity. Our engineers had not been notified of this fact, and no information was provided to indicate that a smaller hole was required in the truck frame to trap the bearing cages and prevent them from squirting out of the bearing.

By the time the mystery was solved, the lift truck division for which I worked had (perhaps not coincidentally) been sold. I was now employed by a trust fund, set up to administer remediation of polluted sites, sell redundant equipment, and resolve various military contracts. I now had to change hats and resolve monetary, as well as technical, issues with the drive unit supplier and the U.S. Navy. The supplier furnished enough original-design bearings to replace all failed or questionable bearings and funded the conversion labor.

Even though the final resolution involved fixing some 300 lift trucks and took about six months to complete, the story had a happy conclusion: The mystery was solved, and though a few heads may have rolled, no more bombs did.

Sign up for the Design News Daily newsletter.

You May Also Like