SIMPLIFIED STRENGTH TESTING OF MANUAL WHEELCHAIRS IN DEVELOPING COUNTRIES

Dwight O. Johnson, MSME, Consultant to Wheeled Mobility Center San Francisco State University San Francisco, California 94132

Abstract

Many small manufacturers of wheelchairs throughout the world's developing countries do not have the resources and technical background to test their products for safety and durability. This paper suggests some essential tests which they can easily perform. The tests are based on fifteen years of experience with the Whirlwind type wheelchairs (which are manufactured and used in developing countries), and on the ANSI/RESNA Wheelchair Standards.

The paper is intended for use by manufacturers in developing countries who are working to improve wheelchair quality.

Background

There are many small to medium size wheelchair manufacturing shops throughout the developing world. In order to successfully market their chairs and serve their customers these manufacturers must provide chairs that are safe, durable, and good performers, and that are producible at affordable prices. This is an enormous challenge where money and materials are very limited. Although many wheelchair designer-mechanics demonstrate a high degree of skill and innovation in building chairs with available materials and tools, they often lack the background -- either educationally or experientially -- to evaluate their designs for safety and durability.

In recent years, standards for evaluating "Western" wheelchairs have been established. They are the ANSI/RESNA Wheelchair Standards (1,2) (in the U.S.) and ISO Wheelchair Standards (in Europe). These extensive Standards cover many subjects including: A uniform description of the basic seating dimensions of chairs so that consumers and health workers can accurately place orders; ways of determining tipping stability; minimal strength requirements to assure that the chair will not bend or break due to occasional high forces; and standard methods for testing long term durability where the chair is subjected to the equivalent of a lifetime of riding on rough surfaces and being dropped.

There are several facilities in the United States and Europe equipped to perform the tests (3). Ideally, manufacturers in developing countries should submit sample chairs for testing at one of these facilities. However, the cost of transportation and testing is often prohibitive.

Problem Statement

Manufacturers in developing countries need simplified standards for testing their chairs. The testing methods, and minimum specified values, need to be appropriate for their testing capabilities, and their customers' riding environments. They need to include at least the tests essential to preventing catastrophic or serious functional failures.

Tests and Testing Method

Fortunately, the following essential strength tests can be performed by manufacturers using simple equipment and procedures. However, the durability tests, and some of the impact tests, specified in the ANSI/RESNA Standards require complex machines that small manufacturers cannot afford. The issue of durability will be covered later under Results and Discussion.

The tests, testing methods and minimum values have been developed by Ralf Hotchkiss (see Acknowledgements), and others, during the more than fifteen years history of Whirlwind-type wheelchairs. They are based on mechanical analysis, laboratory testing to ANSI Standards, and extensive field testing by Ralf Hotchkiss and other riders. They found that a chair that passes these simple static or impact force tests is likely to pass the ANSI/RESNA tests. More importantly, it will also survive active use in rugged Third World riding conditions.

The diagram below shows the static and impact strength test forces a wheelchair of any design should withstand without breaking or sustaining permanent structural deformation. Tests which have been added to those in the ANSI/RESNA Standards, Part 8, or that have higher test values, are marked with an "*". The rest are similar, but not necessarily identical, to those in the Standards. The related "Clauses" in the Standards are in brackets, [Cx.x]'s. A description of each test follows its minimum test value. All the tests can be performed using a tape measure, spring scale, and a lever-arm made of strong pipe for measuring moments. For impact velocities, the tester can practice propelling the chair at 1.5 meters per second by, say, learning to traverse 12 meters in 8 seconds -- a fast walking speed.

[Diagram of a wheelchair with lettered force vectors is omitted. Available from Author upon request.] TM The lettered arrows on the diagram show the direction and point of application on the test forces. A 91 Kg (200 lbm) active adult rider (either a dummy or willing person with a weight in their lap) is assumed.

To check for deformations, before-and-after test measurements should be made using the tape to measure the distance from:

- Rear wheel-to-rear wheel at the front and rear (toe-in or toe-out).

- Rear wheel-to-rear wheel at the top and bottom (camber).

- Caster axle to a rear-most point on the bottom of the each side frame.

- Foot rest mounting points on the side frames to rearward points on the frames.

- Side-to-side between side frames at the foot rest mounting points, caster pivot bearing barrels, rear of bottom frame, and push handles, and between the front and rear of the seat tubes.

Test Values and Descriptions

A 1,000 N (225 lbf)[C6.3]-- Force of rider's leg pushing, or a "hitchhiker" standing, on Footrest.

B* 1,800 N (400 lbf)[C7.7]-- Impact force on one Footrest or front of chair running into a solid object, or test by hitting a solid curb 1.5 m/sec (4.8 ft/sec) -- a typical rolling speed.

C* 1,800 N (400 lbf)[C7.5]-- Peak force when one Caster Wheel with a soft rubber or pneumatic tire impacts a solid step, or test as for B. This tests both the caster fork and casterwheel.

D* 280 N-M (200 lbf-ft)-- Moment on a Caster Fork due to C.

E* 950 N (200 lbf)[C7.9]-- Force from wheelie off an 18 cm (7 in) curb, chair tipped sideways 10 deg., pneumatic tires on rear wheels.

F* 280 N-M (200 lbf-ft)-- Moment on Rear Axle due to E.

G* 1,140 N (250 lbf)[C6.8]-- Lifting force on Push Handle when pulling chair up stairs with one hand.

H 1,000 N (225 lbf)[C6.5]-- Pulling force on Handgrip during G © difficult to test. Use a good plastic-to-metal glue and skip testing.

I* 280 N-M (200 lbf-ft)-- Moment on Folding Mechanism or Frame Cross Members during E.

J* 610 N (135 lbf)-- Pulling force of Seat Back fabric on top of Seat Back Tubes during E.

K* 1,140 N (250 lbf)[C6.2]-- Rider's hand force at an angle on armrest during pressure relief "push up", or boosting up to sit on the armrest to reach high.

L* 2,670 N (600 lbf)[C7.4]-- Distributed force along the seat during wheelie off curb in E.

M* 180 N-M (125 lbf-ft)-- Twisting moment on frame cross members when only two diagonal wheels are supporting the wheelchair on rough ground. Test with rider seated, tester holds one footrest down while lifting the opposite side-frame with force of 355 N (80 lbf).

N 440 N (100 lbf)-- Upward force on non-folding Footrest. Folding or removable Footrest must fold or remove at less than 44 N (10 lbf) applied at the free end.

O 440 N (100 lbf)-- Upward force on Armrest. If the Armrest swings away or is removable, this must occur at less than 44N (10 lbf).

Drop Test 40cm (16in), empty -- Impact during handling. Drop from the height on to each wheel when opened and folded.

Results of employing the tests

The current Whirlwind II design wheelchair passes these tests. One such chair was tested to, and passed, the ANSI/RESNA Standard tests, including the minimum 200,000 cycles on the double-drum durability tests [C8.2]. In addition, thou-sands of Whirlwind chairs have been built to established specifications, and are withstanding rigorous riding throughout the world. This attests to these tests being appropriate.

Therefore, passing the above static strength and impact tests is a good indicator of a chair's ability to also pass the ANSI/RESNA durability tests.

Discussion

It is recognized that passing the above strength and impact tests does not directly test fatigue failure of the structural materials or joints. For that reason it is recommended that chairs be built of materials that don't become brittle with repeated flexing. For example, the Whirlwind chair uses malleable (mild or low-carbon) steel for the frame and other mechanical parts. This steel experiences minimal work-hardening when flexed well below its elastic stress limit. Also, when parts are inadvertently over loaded, they will bend rather than break --avoiding a catastrophic safety hazard (16mm(5/8") diameter axle bolts, for example). Also, it is generally braze-welded (gas-welded with brass) because the steel at the joints has less tendency to become brittle, and a broken weld can be easily and reliably welded by local bicycle repair person.

The Whirlwind chairs have the design features needed by a large segment of the wheelchair-riding public in developing counties. That is,they are designed for countries where the chairs will be used in both city and village environments where rough riding conditions exist. Also, the materials and techniques used to manufacture and repair the Whirlwinds are appropriate for these countries.

There are many good wheelchair designs, besides the Whirlwind, being manufactured. Also, shop strained to manufacture the Whirlwind often modify the design to meet requirements important to them or their customers. It is recommended that all current and future designs and modifications be strength and impact tested as suggested in this paper. Doing so should help improve the quality of wheelchairs so that they better meet the needs of riders, and keep good manufacturers in business.

References

1. ANSI/RESNA Wheelchair Standards, RESNA, 1700 N. Moore Street, Suite 1540 , Arlington, VA 22209.

2. Peter Axelson, MSME, A Guide to Wheelchair Selection, How to use the ANSI/RESNA Wheelchair Standards to Buy a Wheelchair, Beneficial Designs Inc., 585 Empire Grade, Santa Cruz, CA 95060-9603

3. Peter Axelson, MSME, List of Wheelchair Testing Facilities (an informal, unpublished list of addresses and test capabilities), Beneficial Designs Inc., 5858 Empire Grade, Santa Cruz, CA 95060-9603.

Acknowledgements

The information contained in this paper is primarily the results of tireless efforts and experience of Ralf Hotchkiss on behalf of people who need wheelchairs throughout the world. He is Technical Director of the Wheeled Mobility Center, SFSU, 1600 Holloway Ave., San Francisco, CA 94132, (415)338-7734.

The author is also grateful to Peter Axelson,MSME, Beneficial Designs, Inc.; and to Rory Cooper, Ph D., and Brad Lawrence, B.S., Human Engineering Research Laboratories, University of Pittsburgh, for critiquing draft descriptions of these tests, and the latter for testing Whirlwind chairs to the ANSI Standards.

Dwight O. Johnson, MSME, VIVCO Rehab. Engineering Services 6670 Landerwood Ln, San Jose, CA 95120 Tel/Fax (408)268-1214