Each day, some 60,000 passenger and commercial vehicles cross the Interstate 5 Bridge, which connects Portland, Oregon and Vancouver, Washington. And virtually all of the region’s maritime commerce passes under the lift-span bridge en route to various ports. So when a potential problem surfaced with one of the bridge’s bearing shafts, the need for action was urgent. So, too, was the need for exceptionally tight scheduling – to keep the bridge up and running as much as possible.“We couldn’t afford much downtime on this bridge, which is the busiest in the area,” explains Steve Lovejoy, senior mechanical engineer, Oregon Department of Transportation. “We had to identify the least risky option for correcting the problem, while maintaining the highest quality workmanship and the fastest turnaround time possible.”
   

For engineers at SKF USA, those requirements posed several challenges. First, a replacement bearing that precisely matched the original had to be fabricated on a tight time frame. Next, the bearing had to be mounted to meet extremely tight tolerances. Perhaps most challenging of all, the removal of the old bearing assembly and installation of the new assembly had to be done in a matter of days. Any delays beyond the schedule would cost the prime contractor a penalty of $4,000 per hour. Clearly, there was no room for error here.
Early signs

Built in 1917, the Interstate 5 Bridge is based on a vertical lift design that uses a series of pulleys to lift it, allowing boating traffic through. Over the course of its 80 – plus years, the bridge has undergone several modifications. In 1960 the bridge’s journal bearings, originally supplied by SKF, were replaced with new SKF spherical roller bearings.
In 1980, a transportation department on the East Coast found a crack in a bearing shaft on one of its lift-span bridges – the same type of shaft found on the Interstate 5 Bridge. “That prompted us to take a closer look at the shafts on the Interstate 5 Bridge,” says Lovejoy.
   

Through a combination of ultrasonic and acoustic emissions testing, the Oregon DOT found indications of an impending failure on one of the shafts. It was clear that a replacement had to be made.
Beating the clock

With so many commuters, commercial vehicles, and maritime vessels dependent on the bridge, it was imperative that repairs be made as quickly and efficiently as
possible.
   

“We hired a consultant to advise us on the least risky approach to repairing the bridge, and to estimate how long that approach would take,” Lovejoy says. The consultant’s recommendation: Replace the existing bearing and shaft with a similar design, which he estimated would require closing the bridge for about three weeks.
To assure that downtime would be kept to a minimum, the DOT specified the project’s exact starting time and ending time in its request for a proposal (RFP). General contractor Christie Constructors, which was awarded the contract, would be fined $4,000 for every one-hour delay beyond the deadline. Conversely, the contractor would receive a bonus of $4,000 for every hour the job was completed ahead of schedule. As a subcontractor on the project, SKF was expected to do whatever was required to help the contractor complete the job early.
Recreating the past

According to Gill Detweiler, senior applications engineer, SKF USA, the original bearing installed in 1960 was an SKF roller bearing made of high-grade 52100 steel, with an inside diameter of 420 mm. Exposed to the elements for almost 40 years, the bearing had held up extremely well and showed no signs of excessive wear or impending problems.
   

“The only reason we didn’t re-use the existing SKF bearing was because of time constraints,” Lovejoy says. It would take too long to remove the bearing, transport it to a shop to replace the shaft, then bring it back for installation, he explains. Instead, the DOT hired SKF to create an exact duplicate of the original bearing assembly and to oversee both its mounting and its installation on-site. Inspection of the old bearings after removal revealed that it would be possible for SKF to refinish the rolling surfaces, so that they could be stored for re-use at the next change-out.
   

“SKF’s involvement on-site was key to this project; in fact, it was specified in the contract,” Lovejoy says. “SKF has been doing this kind of work for a long time and their engineers are used to dealing with high-precision parts. Because the bearings are an integral part of this bridge, we felt the SKF engineers had to be intimately involved in the installation.”
Valuable advice

According to the contract, the new bearing assembly had to be installed during a three-week window beginning September 16, 1997. But well before that time, there was much groundwork to do. (See sidebar.) Detweiler says that, along the way, several decisions proved critical to the project’s ultimate success.
   

“We only had one chance to install the new assembly and we were dealing with very narrow tolerances,” he explains. “So we crafted a frame that would simulate the exact specifications of the bridge, to use in the mounting process.” The frame replicated the footprint of the bridge, allowing for accurate testing before installation.
   

The fabrication of the pillow blocks to house the bearing also proved a challenge. The originals were made of cast steel, which requires a long lead time for manufacture and can be costly when fabricated in small volumes. SKF engineers recommended flame cutting as an alternative – allowing for faster turnaround at a lower cost, while providing a higher grade of steel. “The use of flame cutting got us a high-quality product on a very tight timeframe,” Lovejoy says.
   

Detweiler also suggested that the DOT add a bearing cover to better protect the assembly from the elements, and recommended an alternative method for lubricating the bearing to more effectively flush out any water. In both cases, the DOT took Detweiler’s advice.
   

“When we specified that SKF had to be on-site, we were looking for an expert to help guide us,” Lovejoy says. “That’s precisely what we got.”
   

The bearing assembly removal and installation work began at precisely 10:00 a.m. on September 16 and ended just seven days later. Christie Constructors netted a large financial incentive for the early completion. And the Oregon DOT obtained a new bearing assembly which Lovejoy describes as “a product made with greater care and precision than we even specified. We believe the new bearing assembly has an infinite fatigue life. It should last forever.”