By Tom Miller, Bearings Unit Manager, North America, igus Inc., East Providence, RI
An innovative wheel and bearing design helps a team of young inventors design a promising new wheelchair.
“Dare Mighty Things” is not just a catchy slogan for a team of students from Upper Darby High School in Pennsylvania, but rather a phrase to live by. The team, which also boasts a legacy of award wins at the FIRST (For the Inspiration and Recognition of Science and Technology) Robotics Competition, developed an omni-directional wheelchair to aid people suffering from paraplegia. The brainchild came to fruition after the team won a $10,000 grant sponsored by the Lemelson-MIT InvenTeams Program, a group that inspires creative thinking and innovation among high-school students by providing the resources for a real-world invention experience.
The feature that makes Upper Darby’s wheelchair design unique is its Mecanum wheel system. The variation of the Mecanum wheel designed by Airtrax requires four independently powered and controlled wheels (two left-hand and two right-hand) to provide a complete drive system. Because the rollers are set orthogonal to the wheel hub and because opposing corners of the wheelchair have rollers set in the opposite orthogonal orientation, the wheelchair wheels must be driven independently to move in any direction.
This makes the drive system capable of full holonomic movement. Holonomicity refers to the relationship between the controllable and total degrees of freedom of the wheelchair. If the controllable degrees of freedom are equal to the total degrees of freedom then the wheelchair is said to be holonomic. If the controllable degrees of freedom are less than the total degrees of freedom it is non-holonomic. A wheelchair is considered to be redundant if it has more controllable degrees of freedom than total degrees of freedom.
A custom circuit board provides closed loop control, taking direction and magnitude input from the three-axis joystick and distributing power to each wheel based on the input received. At the same time it monitors the output shaft speed of each motor and makes corrections for any variation.
The dual-layered, omni-directional wheels use iglide Z plastic bearings and DryLin S aluminum shafting supplied by igus. A total of 54 iglide Z bearings and multiple pieces of the DryLin S shafting were used inside each of the four wheels. Because of the unique wheels, the user is able to drive the chair in any direction, including forward, backward and side-to-side.
iglide Z, like all iglide bearings, can withstand high loads and are lubrication-free, maintenance-free and corrosion-resistant, meaning that those caring for wheelchair-bound patients would not have to concern themselves with lubricating the bearings. The iglide bearings can withstand radial pressures between 7,250 and 14,500 psi. The plain bearings are resistant to the edge loads resulting from high surface pressures and are capable of rotational, oscillating, and linear movements.
iglide bearings are meant to be oversized before being pressfit. After proper installation into a recommended housing bore, the inner diameter adjusts to meet specified igus’ tolerances.
Wheel and Bearing Design
Thorough research of the original Mecanum wheel system led to the modified Mecanum wheel designed by Airtrax. There are two distinct variations of this wheel type. The full-width roller version consists of a central hub with rollers placed at a 45-degree angle to the hub and spanning the full width of the wheel. The rollers are supported at each end by an extension of the hub. The second type, which is used for the wheelchair, is the split-roller design, consisting of a central hub with rollers placed at a 45-degree angle to the hub and spanning half the width of the wheel. Rollers are attached to each side of a spoke or fin that protrudes from the center of the hub at a 45-degree angle.
In order to meet the team’s requirements for a drive system capable of maneuvering around obstacles, the split roller wheel design was necessary. This design performs better on uneven surfaces since each side of the roller pair is able to spin independently of the other. It also requires less power. Other considerations included the fact that the machining process is more complex when fabricating from a single piece of aluminum and also that it would have required twice the number of rollers as the single-roller design. The wheels also weigh less and provide added versatility.
The team created a central hub and split roller design. It developed a 10-in. wheel with nine spokes that provided the narrowest possible profile and increased the roller count to 72. The team sought to improve the stability and transition from one roller to another. To accomplish this, it was necessary to use a much harder durometer roller; however, to ensure adequate traction, a softer durometer would be needed to provide the necessary surface grip. The answer was to use both. A method was developed to produce a dual-layer roller. The inner layer or core of the roller would be made using a Shore A 90 durometer urethane while the outer layer or tread would be comprised of a 3/16-in. layer of Shore A 20 durometer urethane rubber. The result was a wheel with drastically improved driving characteristics and better than expected traction on all surfaces.
The team began by developing an initial design of the dual layer rollers. The rollers would have two layers to ensure that they maintained their shape, which is essential to the smooth movement of the wheelchair. The team created Autodesk Inventor drawings of both the inner and outer layers. The drawings were then sent to a company where stereolithography was used to print ABS Plastic models of the inner and outer roller layers. A silicone mold of each layer was created inside an aluminum casing. Urethane rubber was mixed by the students and poured inside the inner core mold first. After the roller had cured for a short time but not fully, it was transferred to the outer tread layer mold. The softer urethane mixture was poured into the mold around the still soft core to ensure that both layers bonded together. Once removed from the molds, the rollers were heat cured at 150° F for eight hours to cure completely. Once six rollers were created, two larger molds were made, so the team could produce more rollers in a shorter amount of time. The team continued this process until they had made seventy-two rollers.