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Compact electric drives for wheelchairs

Increased friction for a more dynamic response, load capacity and compact design

Alber has significantly increased the torque of its add-on drives for wheelchairs by inserting friction shims that had first been put through the toughest of stress tests.

Fact or fiction...?

... unequivocally fact, say those in the know. It’s a well-known part of company history that Alber exists in its current form today because Ulrich Alber, the founder, for a long time found it a constant source of ire that it took two people to move washing machines up and down stairs. This annoyance turned out to be a source of creative inspiration, as Alber subsequently designed a mechanism that was particularly suitable for negotiating stairways when transporting heavier loads. At some point, these “loads” also started to include humans, namely people with limited or total loss of movement: Alber’s stair-climbing mechanism was registered for patent approval – and a new product was born!

Alber, the company, ...

has long since expanded its portfolio of products. Operating under the umbrella of the Invacare Group since 2004, the company has since become a worldwide market leader with its drives for mobility equipment in the medical technology sector. In addition to stair-climbing equipment, add-on and active drives for wheelchair users have become the top products in their portfolio. These are sold with diversely adapted mounting kits or brackets – which, according to Alber product manager Jens Senner, “are offered for almost all available wheelchair models. They’re generally sold as add-on products through medical supply stores, where they’re also installed for the end customer.”

A problem can often become the inspiration for creating a truly novel solution

Harald Kauffmann

When redesigning the E25 – an electric drive from Alber's e-fix series – the developers were looking for a solution that would ensure optimal transfer of torque for the drive from transmission to wheel hub. The loads caused by the required torque were proving to be too much for the planetary gear to bear in its form at the time, with the screws literally being sheared off when subjected to continuous stress. Finding a solution that generated the desired torque under the given requirements proved to be more difficult than expected. Kauffmann, who was project manager for development of the E25 at the time, recalls: “We basically only had two different parameters available that we could manipulate to improve the torque, namely installation space and friction. Increasing the installation space was not an option for us, because it would have restricted the mobility of the wheelchair user. With a bigger drive on the side, wheelchairs would simply no longer have fitted through standard doors.” Their first attempts to increase the torque by improving the friction initially failed. Then during an Internet search, they stumbled across…

friction shims.

Kauffmann says: “We then created the corresponding drawings with our perforation patterns and ordered the relevant samples from 3M. After the initial trials, we were quite optimistic that it could work. A continuous load test – which we carry out as standard procedure in such cases – was ultimately the deciding factor for determining whether the friction shims could actually be used for our purposes. The stresses to which the components were subjected in this test go far beyond the standard norms for the product and are much more in line with the real stress that occurs during operation of a wheelchair. The use of the shim was tested over a period of a thousand hours under alternating loads. The result was an unmitigated success:

the friction shims had withstood the loads in every phase.

Added to the excellent increase in friction up to transmission of the desired torque, the shims – being only 0.1 mm thick – effortlessly ensured that the installation space for the drive could be kept as small as we had wanted it to be. The minimal space requirement also meant correspondingly little weight, so that the product slogan of the e-fix – “small, light, maneuverable” – was once again underscored, and is a critical requirement of our customers.”

Conclusion:

Once the design was finalised and incorporated in the production process, the e-fix E25 gradually became a firm favorite in Alber’s product portfolio. This is proven as it’s still being built and sold in certain regions to this day – at least thirteen years after it was first designed.

Kauffmann says: “Even in hindsight, I can honestly say that in the shims we had found a truly optimal solution to our problem. In fact, there have never over the entire period been any difficulties related to the high load on the power transmission. The shims keep doing their job, just as they have done from day one.”

Information

The friction shims consist of a thin, flexible metal foil coated on both sides with a nickel matrix, which has a fixed amount of ceramic or diamond particles of a defined size embedded in it. When the disk is mounted between two components, the hard ceramic or diamond particles press into the surfaces of these components as they are pressed together, thereby creating a micro-positive lock. Thanks to these friction shims, greater forces and torques can safely and easily be transferred in a simple and cost-effective manner – without any changes in design.

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