Plain bearings shake a leg
New metal-polymer plain bearings helped solve a wear problem in precision joints that let patients relearn how to walk.
The joints are part of a framework called an orthosis.
Computer-controlled motors
in the orthosis move the patient’s
hip and knee joints through
walking motions on a treadmill.
Engineers who developed
the orthosis at Hocoma AG in
Switzerland noted that bearings
in the hip and knee joints
were wearing out quickly. The
original metal-polymer bearings
were designed to run dry.
But the lubricating polymer
layer was wearing away, introducing
too much play into the
joints which are perpendicular
to their rotation axis.
Hocoma replaced the original
bearings with plain MB DX
bearings made by GGB, also
in Switzerland. The new bearings
last longer and require less
maintenance, according to Hocoma
engineers.
The MB DX bearings consist
of a steel backing overlaid with
a sliding layer of porous bronze
sinter and acetal copolymer.
The bearings are designed for
minimal lubrication and have
indents on the bearing surface
that hold lubricant. Hocoma
had the bearings’ IDs precision
reamed to match tight tolerance
requirements. The bearings are
rated for static stresses of up to
20 ksi and dynamic stresses of
10 ksi. The manufacturer recommends
a maximum sliding
speed of 8.3 fps. In this application,
the coefficient of friction is
0.06.