Downers Grove, IL--A new, patented self-aligning roller bearing offers significantly increased load capacity in specialized oscillatory applications. Conversely, it can also permit a smaller bearing to be specified for a given load rating.
Targeted towards aircraft primary flight controls--actuator rod-ends and control-surface hinges--the bearing has been selected by the Lockheed/Boeing team for the rudder of the F-22 Raptor. "These applications have a very demanding life-loading spectrum," says Bob Warrick, a retired engineer at Rexnord Corporation, Rex Bearing Division (Downers Grove, IL) and co-inventor of the bearing. "They want the bearings to last the life of the airplane, about 50,000 flight hours."
Under the command of computers, flight controls might make hundreds or thousands of small back-and-forth motions per hour, some as small at half a degree. Mark Grunze, a current Rex Bearing engineer and Warrick's partner in the design, notes that requirements for a recent Airbus program contained a load-motion spectrum with about 10 billion small cycles.
The secret to the new bearing's ability to handle such demands lies in a unique center guide ring situated between the two rows of rollers. Shallow machined pockets cradle the inboard end of each roller. The pockets are canted at a 3-4 degree angle from perpendicular, and it is this slight tilt that provides the bearing with one of its novel properties--precession.
Due to the canted pockets in the guide ring, the bearing turns just a bit more freely in one direction than in the other. This causes the guide ring and rollers to slowly precess in one direction around the bearing--sort of five steps forward and four steps back. "You get about one percent friction in one rotation direction and about one-and-a-half percent in the other," says Warrick. The result is that every 3,000-5,000 cycles (depending on a combination of load and rotation angle) the roller set makes a complete revolution.
Precession not only helps spread wear among all the rollers--an obvious benefit--but also distributes lubrication around the bearing. "Fretting is a typical failure mode for these kinds of bearings," says Grunze, "and fretting is a lubricant starvation failure."
Result: the new bearing offers 20-25% increase in static load capacity for the same size bearing, which works out to more than double the load rating dynamically. For applications with lots of small motions, bearing life typically increases from three to five times. Grunze says he's even seen examples of 100-to-1 life increases.
This isn't Rexnord's first foray into a precessing bearing. During the development of Northrop's B-2 bomber, a change in the load-motion spectrum requirement in one area caused the need for a larger bearing--the typical solution to such a change. But physical limitations prevented changing the bearing size. Rexnord engineers had to find a way to increase the life of the bearing already selected.
Their solution was to cant the long fingers of the retainer found in all such roller bearings. This provided the precession function and improved lubrication and bearing life. But it did not greatly increase the load rating.
Later, engineers tinkered with removing the retainer completely. With the retainer gone, more rollers could be fitted to a given size bearing, thus immediately improving the static load rating. Though a seemingly a simple solution, the retainerless design proved patentable as well. "We've been making bearings for this application for fifty years," says Grunze, "and here we come in and patent a bearing with no retainer at all, which apparently nobody had thought of before."
Natural play in the rollers gave the retainerless some precession capability as well. But the precession was unpredictable; sometimes the bearing races would just change direction and begin precessing the opposite way.
What they needed was a design that had both a full compliment of rollers and controlled precession. Grunze and Warrick's pocketed center guide ring proved to be the answer. Its shallow, canted recesses provide precession, and the stubby fingers are short and small enough to allow more rollers to be fitted than in a bearing with a retainer.
The design offers manufacturing advantages as well. The long-fingered retainer as used on the B-2 had to be machined instead of cast because the canted prongs would have prevented the dies from coming apart. The short prongs of the new design make casting possible, thus reducing cost.
In retrospect, the center guide ring concept might seem obvious. But actually its design took years to prove out, and the subtle details of the ring's configuration were a three-dimensional nightmare. "The shape of the retainer and the interaction with the rollers is extremely complicated," says Grunze.
But the result has been worth it. "It has allowed us to get rid of fretting as a mode of failure," he says.
Additional details...Contact Mark Grunze, Rex Bearing Division, Caller #1482, 2400 Curtiss Street, Downers Grove, IL 60515-0722, FAX: 630-969-8827.
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