Geared Hub Technologies

Description of the Problem
There are more than 1.4 million wheelchair users, and about 75% of them (about 1.05 million) use manual wheelchairs.  The population of individuals using wheelchairs is estimated to be growing at a rate of 10% per year into the year 2002. Manual wheelchair users generally have lower-extremity weakness, paralysis, or amputation, making walking unsafe or difficult at best.  They may include individuals with spinal cord injuries, hemiplegia and other types of paralysis, multiple sclerosis, cerebral palsy, spina bifida, arthritis, and lower-limb amputations.

A user’s independence, access to environments, health, and safety are all affected by their ability to propel the wheelchair.  There has been little change in the basic design of manual wheelchairs over the past decade.  The user typically propels, steers, and brakes the motion of the wheelchair by pushing against or grasping the pushrim.  The user must apply large forces to accomplish these actions.  The force directed inward toward the wheel hub does not contribute to propulsion or braking but is necessary in order to generate friction between the user’s hands and wheelchair pushrims.

Injury is common among manual wheelchair users. Shoulder-related injuries have been shown to be present in up to 51% of manual wheelchair users.  In addition, the prevalence of elbow, wrist, and hand pain has been reported to be 16%, 13%, and 11%, respectively. The incidence of carpal tunnel syndrome and rotator cuff tendonitis, for example, is greater than 50% for people who regularly use manual wheelchairs as compared to 3% for the general population.

Manual wheelchair users generally brake or slow their wheelchairs by producing a frictional force, opposing wheel motion between their hands and the pushrims.  Both propelling and braking the wheelchair can produce blisters and calluses on the user’s hands.

Environmental factors can increase the force required to propel a manual wheelchair. These factors include soft rolling surfaces (e.g., sand, gravel, dirt, mud, thick carpeting); rough, broken, or uneven surfaces (e.g., potholes, sidewalk cracks); inclined surfaces (e.g., hillsides, curbs, ramps), and weather-affected surfaces such as snow and ice.

Environmental and human factors can reduce a user’s ability to produce and apply the forces necessary to propel and brake a manual wheelchair.  Rain, sleet, snow, and perspiration can reduce the friction between the user’s hand and the pushrim.  Cold can reduce the user’s ability to grasp the pushrim.  Fatigue reduces the user’s ability to generate the force for propulsion or braking.  Heat can accelerate the onset of perspiration and fatigue.

Propulsion of manual wheelchairs improves a user’s cardiovascular fitness and upper body strength.  As one would expect, a user’s strength and stamina typically diminish over time. Of necessity, these users must transition to a power wheelchair or be pushed about in a manual wheelchair by a caregiver.  Frail elderly and persons in the later stages of progressive disabilities typify such users.  Extending the period during which these user’s can self-propel would provide important health and independence benefits.

Current State of Technology
The forces needed to propel, brake, or steer a manual wheelchair depend on pushrim and wheel diameters, pushrim design, tire and surface properties, surface contour, and propulsion biomechanics, which vary from user to user.  Geared hubs allow a user to select the level of propulsive force appropriate to their physical abilities, personal preferences, and environmental conditions.

The mechanical advantage provided by a geared hub is characterized by its “gear ratio.”  For a standard hub without gearing, there is a 1-to-1 ratio between the rotation of the HANDRIM and the rotation of the wheel.  For a geared hub having a 2-to-1 gear ratio, a complete rotation of the HANDRIM results in a one-half rotation of the wheel.

• A low gear ratio allows a user to propel with decreased force albeit at low speeds.  In general a low gear ratio would be useful when the user is going up inclines or over soft surfaces, is fatigued, or has decreased strength and stamina.
• A high gear ratio requires a user to generate more force in order to propel, steer, or brake but allows the user to propel at higher speeds.  In general, a high gear ratio would be useful for fast travel over hard, level ground or declines or for active or athletic wheelchair users

The mechanism by which the user selects a gear ratio is an important design consideration.  A common mechanism is a system of levers and cables (similar to road bikes).  However, levers and cables tend to get caught on clothing and environmental obstacles, can impede the user’s movements, and are considered, by users, to be unsightly.  Using this method to change gear ratios, the user must interrupt the continuity of propulsion by moving their hand from the pushrim to the gear-selection lever.

Multi-rim geared hubs have been designed whereby each rim drives a different gear ratio.  The user selects a gear ratio by selecting and pushing against a matched set of rims.  Multiple sets of rims must necessarily be offset vertically or horizontally.  Horizontal rim offset increases wheelchair width and decreases the user’s access to confined environments.  Rims that are vertically or horizontally offset are more difficult for the user to reach and change propulsion biomechanics.  A multi-rim approach is not likely to be compatible with continuously variable gearing.

Technology Needs and Barriers
Information gathered from users, manufacturers, clinicians, and other stakeholders has identified characteristics of an ideal geared hub system.  Minimum requirements are identified as items that “must” be addressed to make a design acceptable; other attributes that will benefit the design are identified as items that “should” be addressed.  Specifically, a geared hub system…
 

1. must be part of a drive wheel that replaces a standard drive wheel rather than being a retrofit or add-on to a standard drive wheel.

2. must retain a single, “standard” pushrim as part of its design.  This pushrim should resemble a standard pushrim in both its form and function.

3. should have continuously variable (linear) gearing.  If continuous variable gearing is not practical, then the geared hub system should have (at least) four discrete gear ratios in the  (approximate) range of 1-to-2 through 2-to-1.

   Note: It may be appropriate to have a few geared hub systems whose gear ratios reflect the needs of distinct user populations (e.g., frail elderly versus active, athletic).  Consumer or clinical trials could better define the optimal range of gear ratios and specific gear ratios.

4. must provide “reverse gearing” for propelling backward.

   Note: Group participants suggested that a single reverse gear ratio equal to the lowest forward gear ratio would be optimal.  Consumer or clinical trials could better define the optimal reverse gear ratio.

5. should offer manual selection of gear ratios rather than an “automatic” gear ratio selection—gear ratio automatically changes in response to applied torque.  Gear ratio selection must have the following characteristics:
   • a single user interface—as opposed to separate interfaces for the left and right wheels—to shift both wheel hubs simultaneously.
   • shift smoothly through all (the entire range of) gear ratios.
   • shift smoothly while the chair is in motion—i.e., being propelled.

6. must offer a user interface for gear ratio selection with the following characteristics:
   • minimal user dexterity required to operate
   • minimal user force required to operate
   • simple, intuitive actions required to operate
   • no interruption of propelling required to operate
   • no obtrusive levers, cables, or similar hardware
   • easily mountable in many locations on the wheelchair
   • easily mountable or configured in a range of orientations

   Note: Manual wheelchair users suggest that an interface similar to pressure pads (like big button switches) may be the optimal user interface.  They suggest that the geared hub system should also be able to utilize other user interfaces (pressure pad, lever, etc.) in order to accommodate diverse user abilities.

7. should “feel about the same” as a standard manual wheelchair from the perspective of a person pushing either wheelchair.. must have the following physical characteristics:
   • weigh 7 pounds or less
   • not increase the width of the manual wheelchair
   • not change the look of the wheelchair—i.e., unobtrusive appearance
   • not change any other wheelchair functions—e.g., portability, disassembly,folding

8. should generally be quiet during use.  When the user shifts gears, the geared hub system should provide an auditory “click” (similar to mountain bikes) confirming the gear ratio change.

9. must have a safety mechanism that allows the user to independently revert to a 1-to-1 gear ratio.

10. must not add significantly to the rotational inertia of the drive wheel—i.e., flywheel effect.