Machine Design

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.

GGB’s MB DX bearings consist of a sliding porous bronze sinter coated with acetal copolymer and supported by a steel substrate. A pattern of indentations keeps lubricants in contact with the moving surfaces.

Hocoma’s orthosis supplies precise joint movement to people learning how to walk after spinal-cord injuries. MB DX bearings hold the tight tolerances required.

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