Kaydon case studies

The “VariLeg” exoskeleton—thin section bearings from Kaydon enable compact, lightweight construction

Development by Swiss university ETH Zurich offers help for paraplegics

Sophisticated devices known as exoskeletons are the subject of intensive scientific research all over the world. One of the more sophisticated is the VariLeg (VARiable Impedance Leg), which aims to enable paraplegics to walk upright and unaided. VariLeg differs from others in that it features mechanically adjustable stiffness in the knee, minimizing the difficulty of managing slopes and steps. Powerful motors enable the user to stand up and sit down. In short, the device is able to simulate the natural function of a knee joint very closely.

The device is currently in development at ETH Zurich, a university for science and technology in Switzerland, under the auspices of the VariLeg project.

An exoskeleton intended to help paraplegics is currently in development
under the auspices of the VariLeg project at the ETH Zurich
(Photo courtesy of Rodriguez GmbH)

VariLeg implements the concept of variable stiffness in the form of the MACCEPA (mechanically adjustable compliance and controllable equilibrium position actuator) system, integrated in the upper leg. A plate connected to the lower leg via a motor allows rotation and rotates freely on the upper leg. The upper motor serves to bias a spring which is permanently attached to the plate. When the lower motor rotates, the angular position of the plate relative to the lower leg changes, and the lower leg moves. At the same time, the spring connected to the plate absorbs force until an equilibrium of moments is reached. Thin section bearings from Kaydon, an SKF Group brand, help to make this 'scaffold' as compact and lightweight as possible.

Space-saving bearing for the gearing unit

"Compared with the first version of the VariLeg prototype, we wanted to reduce the width of the system in the upper leg area," explains Jannick Oberbeck, project manager for VariLeg. "Therefore, at the very start of the project we began looking for the smallest possible bearing for the gearing unit." The team ultimately decided on Reali-Slim® metric thin section bearings manufactured by Kaydon. The low weight of the bearings and the large inside diameter with minimal space requirement made them the perfect choice for this application.

The VariLeg (VARiable Impedance LEG) exoskeleton differs from existing exoskeletons mainly in that it features mechanically adjustable stiffness in the knee
(Photo courtesy of Rodriguez GmbH)

Two thin section bearings are installed in the VariLeg exoskeleton, one in each knee joint. A gearing unit with motor is mounted in such a way as to be freely movable in each knee. This unit is exposed to relatively large loads, which are absorbed by bearings at both ends. On one side, the gearing is connected to a shaft, which means that a bearing system based on a conventional ball bearing can be used. "In order to achieve the most compact, most lightweight construction possible on the other side, a thin section bearing with a large inside diameter was essential," says Oberbeck. "This made it possible to create a bearing system for the gearing unit that did not entail complicated, space-hogging auxiliary structures."

A total of two thin section bearings from Kaydon
are installed in the VariLeg exoskeleton—one in each knee joint
(Photo courtesy of Rodriguez GmbH)

A gearing unit with motor must be mounted in the exoskeleton knees
so that it can move with complete freedom—the space-saving thin section bearings
thus render complicated, space-consuming auxiliary constructions unnecessary
(Photo courtesy of Rodriguez GmbH)

Thin section bearings in a wide range of configurations

Thin section bearings are notable for their small cross-section and very large bore diameter. They can be used to create compact, lightweight bearings which also leave space for cables or shafts to pass through the bore. Industries including robotics, semiconductor, and medical engineering find thin section bearings ideal for a number of applications.

Kaydon's product line comprises 250 different thin section bearings within the Reali-Slim® series, in both imperial and metric dimensions. Within the series, three different basic types are available. Type C single-row radial ball bearings absorb radial loads with limited capacity to absorb axial loads. Type A angular contact ball bearings feature defined pressure angle and a larger ball set to absorb axial loads in one direction and provide greater bearing capacity and stiffness. They can be used in pairs to absorb forces and moments in two directions.

Four-point bearings designed for various load types

The VariLeg application uses type X four-point bearings. In many applications, one type X bearing can replace two conventional bearings, making them the bearing of choice when it's not possible to install a pair of type A bearings. "Four-point bearings are designed for an enormous variety of different loads," explains Ulrich Schroth, value-added products business unit manager with Rodriguez, Kaydon's German distribution partner. "Their unique internal geometry enables them to absorb radial loads, axial loads in both directions and torque loads, singly or in any combination." After consulting with Uiker Wälzlager AG—Rodriguez distribution partner in Switzerland—the VariLeg development team chose a Kaydon Reali-Slim® thin section MM bearing with metric dimensions. Kaydon's metric line is a big help for designers who no longer have to convert to metric dimensions, thereby reducing the the work associated with making bearing calculations.

The new VariLeg prototype is presently in the trials phase. A paraplegic ex-pilot is training with the exoskeleton in pursuit of the ambitious goal of competing in the Cybathlon in Zürich, an event in which participants with physical disabilities compete in six extremely demanding disciplines.