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The Only One In New York

The Third Avenue Bridge in New York is an important traffic link carrying more than 70,000 vehicles each day between Manhattan and the boroughs of Bronx and Queens. It’s the only swing bridge among the fourteen bridges owned by New York City that cross the Harlem River; so it’s the only bridge on the river with a bearing that has to be absolutely dependable in daily operation and have a working life of fifty years or more.

As a first step to ensure reliability and longevity, bridge designers Hardesty and Hanover specified a spherical roller thrust bearing, rather than a spherical plain bearing: the type usually used in swing bridge applications.

nother concern was the demanding seismic loading conditions that made the rolling bearing preferred choice over the plain bearing.

They made their decision after consultation with SKF who invented spherical roller bearings in 1918 and have been at the forefront of their development ever since. Today, all 29,000 tons of the Third Avenue bridge rests and rotates on a single spherical roller thrust bearing that is said to be the largest of its type ever used in a swing bridge application.

The particular solution suggested by SKF had another key benefit; energy and cost savings by virtue of its low friction. This enabled the drive motor to be reduced from an expected 175 Hp to 150 Hp and that also downsized all of the drive train components resulting in ‘significant savings’.

The bearing recommended by SKF specialists was from their SKF Explorer™ range of spherical roller thrust bearings. It has an outside diameter of 1520mm, a width of 372mm and a bore of 900mm. The cage, rings, and rollers are made from SKF Xbite high performance steel, patented by SKF and now used in many of their bearings.

Essentially, SKF Xbite steel is produced by applying a unique and patented heat treatment process to SKF’s standard bainitic steel. Doing this gives a steel with much greater hardness than other steels but with no lessening in toughness. It was once thought to be impossible to increase steel’s hardness without reducing its toughness. SKF Xbite has shown that belief to be mistaken and as a result, bearing performance and endurance have been significantly improved.

As well as improved performance, SKF Xbite gives a bearing longer service life, higher load carrying capacity, greater crack resistance, and wear resistance up to three times that achieved with bainitic steel. It’s an impressive cluster of advantageous properties and it’s what makes SKF Explorer bearings highly suitable for applications with demanding environments and operating conditions - such as the Third Avenue Swing Bridge. Another advantage for this application is that a spherical roller thrust bearing can have a high contact angle without increasing friction. This gives the bearing and the bridge added stability and safety in the event of some unpredictable threatening situation.

Because of the bearing’s exposed position, and its intermittent operation, SKF and Hardesty and Hanover paid special attention to effective lubrication - a vital factor if the bearing was to meet reliability and longevity requirements.

A salad-bowl-shaped housing, roughly 2 metres in diameter supports the outer ring of the bearing and this housing bowl is kept full of oil. This keeps the bearing, its rollers and raceways completely immersed in lubricant. Although this gives plenty of protection from rust and contamination, it’s not enough. When a swing bridge is opening for river traffic, the turning movement is slow. For the bearing this means that its rollers are turning slowly, and that’s not ideal. For ideal lubrication, rollers need to be rotating quickly in order to pick up a film of oil. The rollers will then ‘aquaplane’ on a hydrodynamic oil film and there is no metal-to-metal contact between the rollers and the raceways. Metal-to-metal contact is one cause of high friction and wear, which can lead to deteriorating bearing performance and eventual failure.

SKF’s solution to overcome metal-to-metal contact was to coat the bearing’s rollers with an SKF ultra low-friction ceramic coating called NoWear®. Because the NoWear coating is permanently attached to the roller surface, the rollers and the raceway are always in a low-friction state, no matter how slowly the bridge turns. And there is no metal-to-metal contact.

Unlike other low-friction coatings, such as PTFE, the NoWear coating is hard (1200 HV). Also, it has a high load carrying capacity, which PTFE coatings lack. To prove the value of their NoWear coating SKF put it through extensive testing in a wide variety of demanding applications, such as paper mill rollers. In one application where existing bearings normally failed within one to three months, replacement bearings coated with NoWear were still running three years later.

The coating also makes the rolling surfaces last longer, so a service life of 50 to 100 years is being predicted for the Third Avenue Bridge bearing.

Even so, failure of the Third Avenue Bridge could lead to serious problems. So much so, that it is expected that New York City Department of Transportation will commission SKF Reliability Systems to carry out periodic checks on the bridge bearing in case unexpected circumstances occur that would affect the performance of the bearing.

Assessing the condition of a working bearing and predicting future problems (jf any) is a specialty of SKF Reliability Systems and they have many ways of monitoring bearing performance through vibration analysis. However, one application area where accurate monitoring using computer assisted data collection becomes difficult is in slow moving mechanisms - such as the slow moving sections of a bridge. Specialists at SKF have a name for this tricky monitoring problem. They call it the Ultra Low-Speed Challenge.

To beat the challenge, the vibration analysts resort to methods used long before computers were available. These tried and trusted methods from the past collect vibration information by timing. It’s called Time Domain Analysis and it remains the most effective way of accurately collecting vibration data in ultra-low-speed systems.

When working on a noisy bascule bridge in 2004, SKF analysts used Time Domain Analysis to determine that the noise was not related to bearing problems. They then used other sophisticated methods to obtain a further 93 pieces of data that gave the bridge operator peace of mind and the assurance that there was no immediate threat to bridge operations.

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