Wheelchairs for off-road use

While standard wheelchairs are an essential tool for many people with disabilities, they are still extremely limited in terms of the terrain that they can handle. This is why Tim Morgan has invented the Mountain Trike, a wheelchair designed specifically for off-road and difficult conditions. Operating in such harsh conditions can put a strain on components and, at times, sourcing parts that could withstand this tough environment proved a challenge. Fortunately, Stieber Clutch was able to offer a solution which has so far proven reliable.

Many wheelchair users want to go out and enjoy the countryside, but activities as simple as family trips in the park or ambling along country paths can be difficult or even impossible in a regular wheelchair due to the uneven ground, steep terrain, and mud. Some traditional wheelchair designs are available with chunky, off-road tires, but they still require the user to push the wheels to move the chair forward, which results in muddy, slippery hands and poor transmission of drive.

The Mountain Trike uses a unique lever drive system that allows the user to power, steer, and brake the Trike by using two levers positioned in front of them. The levers are connected to the wheels using technology similar to that found on a mountain bike and power each wheel independently. A third wheel at the back stabilizes the chair and can be steered using a joystick mechanism attached to one of the levers.
 

Design specifications

Rather than use a standard-bike freewheel, Morgan, the Mountain Trike’s designer, chose to specify an overrunning clutch to transmit the kinetic energy from the levers to the wheels.

“When the wheels are spinning freely on a mountain bike, without drive from the pedals, the freewheel clicks during the rotation,” Morgan explains of his reasoning behind the specification. “The Mountain Trike is intended for use during family walks and other lifestyle activities, so I wanted to make sure it would run silently. I also found that bike freewheels have a small amount of free travel before it engages. Because the levers are quite long, this translated to several inches of free travel with each push, which was quite jarring for the user and resulted in less efficiency overall. Using an overrunning clutch eliminates both of these issues.”

While the initial overrunning clutches on early test models performed well in the short term, it became apparent that over long-term use their durability was not up the standard required for the Trike. Poor seals allowed mud and contaminants to creep inside, causing seizing. Poorly machined internal components led to slipping and, eventually, total failure due to the regular shock loading caused by disengaging and engaging the drive train with every push.
 

Changing suppliers

Morgan needed to source an alternative component that could survive in the most extreme terrain, so he contacted Stieber Clutch.

“I was already aware that Stieber Clutch is well known for its quality and reliability in tough industrial applications, so when we began to have problems with the original bought-in components, I approached the company to see if Stieber was able to offer a solution,” Morgan notes. “I was supplied with the CSK PP overrunning clutch for testing, and the superiority of the component was immediately apparent. The seals were clearly far better protected and the movement was much smoother.”

Stieber Formsprag overrunning clutches offer a number of design features, ensuring that they are longer lasting and more reliable than competing components on the market. The sprags are manufactured using the company’s patented Formchrome process, which diffuses chromium carbide into the working surface of the sprag, increasing its surface hardness to deliver excellent wear protection. They are also precision-machined to ensure that each sprag is nearly identical to the next to prevent uneven rolling movement.

The sprag retainer assembly also features a free-action design that permits each sprag to have free and independent action. This independence allows each sprag to adapt to variations in annular space (eccentricities) so that when the clutch is engaged, the load is proportionally shared among all sprags, eliminating the possibility of clutch damage resulting from the entire load being absorbed by just a few sprags. In operation, springs energize the sprags into position for instantaneous engagement with no backlash.

“During the testing process it was clear that the Stieber clutch offered far more in terms of reliable performance, and they are now specified as standard on all Mountain Trikes,” Morgan says. “Since they have been installed, we have had no reported failures, even from some of our more adventurous customers.”

Stieber Clutch
www.stieber.de
www.formsprag.com

Mountain Trike
www.mountaintrike.com