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lever drive wheelchair addresses all associated disadvantages
Added: (Thu Jul 21 2005)
LEVER DRIVE WHEELCHAIRS.
BRIEF HISTORY.
Since the early 1900’s patents have been regularly filed for designs for this type of wheelchair transmission, yet very few have succeeded commercially.
Our research shows that nearly all the designs have the pivoting drive levers placed either side of the occupant then connected to each wheel independently. The levers being connected to the wheels in traditional ways such as: -
Chains & sprockets.
Belts & pulleys.
Rod linkages.
These methods usually have some form of ratchet mechanism enabling the drive to be performed on the push stroke & re-set on the pull stroke. By driving each wheel independently slight steering can be achieved, if a brake is added to each wheel then far more precise steering results.
LEVER DRIVE WHEELCHAIRS HAVE WELL DOCUMENTED ADVANTAGES.
When sitting upright & pushing the levers forwards, the occupant is able to double the power to the rear wheels, essentially halving the effort required.
The seating position is more comfortable with the backrest acting as a brace to push against & not the abdominal muscles.
The occupant is not at risk of carpal tunnel syndrome or developing calluses.
The occupant’s hands & clothing stay clean.
WITH THESE SIGNIFICANT ADVANTAGES ALREADY WELL KNOWN WHY ARE THEY NOT READILY AVAILABLE?
Unfortunately, traditional lever transmission methods have many disadvantages.
For example: -
1. Handles not synchronised.
Research has revealed that occupants need the handles to be synchronised, this eases operator control & has been found to be more comfortable. Other advantages of synchronised handles are yet more power & helps with straight-line motion.
With the handles traditionally driving each side independently the occupant is constantly correcting the direction of the wheelchair & a smooth straight path is almost impossible or requires a great deal of concentration.
One handed driving is impossible, as the wheelchair will simply travel in a circle.
If the handles are linked together then steering is lost.
Conclusion.
The power from both handles must be harnessed simultaneously before the drive to the wheels, without losing the ability to steer.
Although it may seem unnecessary this system can be adapted to give a driving action whereby:
As one handle pushes forward the other is pulled, this gives a neutral effect on the occupant twisting as one force on the left is counteracted with an equal & opposite force on the right. No turning force is exerted on the wheelchair as both driving forces are harnessed together before driving the wheels. This system may be preferred.
One-handed propulsion is vital when required.
2. No gearing.
Our tests show that even a modest incline significantly increases the amount of power required to push the levers to propel the wheelchair. This leads to occupant fatigue & a decrease in control, resulting in the wheelchair being unable to follow a reasonable straight line. The fixed gear can be set to a low ratio that will help on inclines but will become unreasonably slow under flat conditions. Different levels of occupant fitness, strength & mobility will require different fixed gear ratios therefore a common ratio will only suit a small minority of potential users.
Conclusion.
Several different gears are required to suit the occupant’s circumstances & the conditions.
Better still, a constant effort system that constantly monitors the pressure on the levers & adjusts the ratio to suit the conditions. This system needs to be easily pre-set to suit the occupant’s ability & comfort.
3. Rollback on inclines.
Most systems require the occupant to hold the levers or brake when at rest facing up an incline; there is a force on the levers trying to pull them back towards the occupant. This is also the case when travelling up an incline. When the occupant pulls the levers back to re-set them for another power stroke the wheelchair will probably not have enough momentum to carry on travelling forwards. This results in two actions: either the occupant is simply slowing the reversing or holding the wheelchair at rest (using energy) whilst re-setting for another power stroke.
Conclusion.
The occupant must be able to release the levers at any time when travelling up an incline. This is an essential safety feature & increases operator confidence.
If this system is fitted then ascending an incline will be far easier as the occupant is not holding the wheelchair on the return stroke of the levers. Each stroke will be independent of the last & the occupant can stop & rest at will without applying the parking brake.
With some systems this can be overcome by applying the brakes on the return stroke but this is far from satisfactory.
4. Turning efficiency.
With many of the traditional style wheelchair transmissions various types of turning are possible.
Pushing one lever harder than the other can make slight directional changes.
This results in the occupant deliberately not applying their full force all the time & therefore inefficient.
Pushing one lever only & slightly applying the brake to the opposite side can make a more pronounced change in direction. This is even more inefficient as only half of the available power is transmitted to the wheels.
Pushing one lever & applying the brake fully gives the tightest turning circle. Once again half of the available power is being used.
Most lever transmission systems do not allow the wheelchair to turn on its own axis unless both hand rims are used.
A combination of these actions has to be used all the time in order to travel in the required direction. To be fully efficient both handles need to be used all the time & no loss of power must be taken up by the brakes.
Conclusion.
A system needs to be devised whereby both handles are always used regardless of the direction required. There must be no power transferred to the brakes unless for stopping. When attempting the tightest turns the wheelchair must be more responsive typically turning on its axis without having to use both hand rims.
5. Exposure to dirt.
Most transmission systems have their working parts exposed to the elements grit, mud, water etc.
This can lead to premature failure, high maintenance & a depreciation in performance.
Conclusion.
The transmission system must be fully enclosed to prevent any ingress of water & dirt.
A WHEELCHAIR TRANSMISSION SYSTEM HAS BEEN DESIGNED THAT ADDRESSES & SOLVES ALL THESE ISSUES.
1. IT HAS SYNCHRONISED HANDLES YET CAN STILL TURN WITH NO LOSS OF POWER.
IT CAN BE DRIVEN IN A STRAIGHT LINE WITH ONE HAND.
2. IT HAS FOUR GEARS: - SLOW - MEDIUM – FAST & NEUTRAL.
IT HAS A SYSTEM THAT ALLOWS THE RETURN STROKE TO DELIVER POWER TO THE WHEELS IF REQUIRED. THIS ALSO HAS FOUR GEARS, SAVING UNNECCESSARY UNPOWERED LEVER MOVEMENTS.
3. ONCE THE OCCUPANT HAS FINISHED THE POWER STROKE REGARDLESS OF THE PUSHING OR PULLING GEAR SELECTED, NO ROLLBACK OCCURS & NO RETURN PRESSURE IS EXERTED ON THE LEVERS EVEN WHEN ASCENDING SLOPES.
4. THIS WHEELCHAIR CAN TURN UNDER FULL POWER FROM BOTH HANDLES. BRAKE ONE SIDE; PUSH BOTH HANDLES & ALL THE POWER GOES TO THE UNBRAKED WHEEL.
IT WILL TURN ON ITS OWN AXIS USING ONLY ONE HAND RIM.
5. THE TRANSMISSION SYSTEM IS FULLY ENCLOSED.
This transmission system does not need a parking brake. When in any gear, the transmission automatically prevents both forward & backward movement unless driven by the handles.
Parts of the transmission have been built & tested; these components have performed really well. The first fully functioning prototype will be completed & fitted to a Remploy Roller wheelchair by October 2005.