March 23, 1998 Design News
From the regional editors
Precessing bearing increases life and
Patented design takes full
compliment of rollers, continuously distributes lubrication in oscillatory applications
by Mark A. Gottschalk, Western Technical
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."
For oscillatory applications
with small rotation angles, bearings with
center guide ring offer dramatically improved
life compared to standard self-aligning roller
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.
Patented center guide ring
has slightly canted pockets (3-4 degrees) that
cause retainer and rollers to slowly precess
around the bearing as it oscillates back and
forth, distributing lubrication, eliminating
fretting, and increasing load capacity.
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
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
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,"
Additional details?Contact Mark Grunze,
Rex Bearing Division, Caller #1482, 2400 Curtiss Street,
Downers Grove, IL 60515-0722, FAX: 630-969-8827.