Simplified Strength Testing of Manual Wheelchairs

February 9, 2011

2008 Kien Tuong Testing Matt IMG_8073

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 ISO International Standard for Wheelchairs.

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, a standard for evaluating “Western” wheelchairs has been established. It is the ISO International Standard for Wheelchairs, dated 1997 (1,2). (Previously, it was the similar ANSI/RESNA Wheelchair Standards.) 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 ISO Standard 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 ISO Standard, 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 ISO 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 are in addition to those in the ISO International Standard for Wheelchairs, Part 8, or that have more demanding test values, are marked with an “*”. The rest are similar, but not necessarily identical, to those in the ISO Standard. Some test load values are followed by an “e” to indicate they are estimates of dynamic values. The related Paragraphs in the ISO Standard are in brackets.

A description of each test follows its minimum test value.

All the tests can be performed using a tape measure, large square (e.g., a wood or metal sheet with a square corner), 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.

The lettered arrows (vectors) 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 permanent 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). Better, back the chair against a flat wall. Use a large square and tape to measure perpendicularity of each rear wheel to the wall.

– Rear wheel-to-rear wheel at the top and bottom (camber). Better, set the chair on a flat surface and use the square and tape to measure the perpendicularity of each rear wheel.

– Caster axle to a rear-most point on the bottom of the each side frame. Measure when the casters are in the rolling-straight ahead position.

– 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
The order in which the tests are generally listed is determined by the clockwise progression of the lettered force vectors around the diagram. Please note that this is not necessarily the order in which the tests should be performed. 

* – Indicates tests which are either in addition to or have more demanding test values than those in the ISO wheelchair standards, part 8.
e – Indicates estimated values of dynamic forces.
[Px.x] – Indicates related ISO standards paragraph number.


Test A, Downward Force on Footrests
1,000 N (225 lbf)[P8.5] — Force of rider’s leg pushing, or a “hitchhiker” standing, on Footrest. Test by holding the chair so the Footrest is parallel to the ground. Stand on it. Test both footrests if there are two.

Test A

Test B*, Footrest Rolling Impact
1,800e N (400e lbf)[P9.6] — Impact force on one footrest or front of chair running into a solid object. Test by hitting a solid step at 1.5 m/sec (4.8 ft/sec) — a typical rolling speed, at about 90 degrees and 45 degrees. Test both sides.

Test B

Test C*, Caster Wheel Rolling Impact
1,800e N (400e lbf)[P9.5] — Peak force when one Caster Wheel with a soft rubber or pneumatic tire impacts a solid step. Test as for B except a 7 cm (3 inch) thick block is against the curb. This tests both the caster fork and casterwheel. Impact each caster wheel at about
90 degrees and 45 degrees to the block.

Test C

Test D*, Caster Force Bending Strength
195e N-M (140e lbf-ft) — Moment on a Caster Fork due to C. Component test using a vise and lever-tube over the stem. About 40 mm (1.5 in) of the fork should extend above the vise.

Test D

Test E*, Wheelie Drop Off Inclined Curb
950e N (200e lbf) — Approximate sideways force on a rear wheel when doing a wheelie off a 18 cm (7 in) step, with the chair tipped sideways 5degrees, pneumatic tires on rear wheels, or rolling off a level step at an angle so that the rear wheels don’t hit simultaneously. Test by doing a wheelie off an inclined plane. It is also, the approximate force that a well-spoked 24 inch rear wheel should withstand without collapsing. 

Test E

Test F*, Rear Axle and Mount Bending Strength
280e N-M (200e lbf-ft) — Moment on Rear Axle due to E. Component test using a lever-tube over the axle while the side frame is held securely against a vertical reference surface.

Test F

Test G*, Upward Force on Push Handles
1,140 N (250 lbf)[P8.10] — Lifting force on Push Handle when pulling chair up a 18 cm (7 in) stair with one hand. Test by doing so.

Test G

Test H, Handgrip Attachment
1,000 N (225 lbf)[P8.7] — Rearward pulling force on Handgrip during G – difficult to test. Use a good rubber/plastic-to-metal-to-glue and skip testing.

Test I*, Inward Force on Seat and Seatback Tubes
280e N-M (200e lbf-ft) — Moment on Folding Mechanism or Frame Cross Members when doing a wheelie off a 18 cm (7 in) step. It is best to test a complete loaded wheelchair by doing a wheelie off a step as in tests E and L. 

For a frame assembly component test lay the chair on its side and squeeze the push handles together with the force necessary to cause the specified moment about the axle position.

Test I

Test J*, Seat Back Tube Bending Strength
610e N (135e lbf) — Pulling force of Seat Back fabric on top of Seat Back Tubes during test I. Do not test since this is redundant with Test I.

Test J

Test K*, Downward Force on Armrests
1,140 N (250 lbf)[P8.4] — Rider’s hand force on armrest during a pressure relief “push up”, or when boosting up to sit on the armrest to reach high. Test by applying load to armrest.

Test K

Test L*, Wheelie Drop Off Level Curb
2,670e N (600e lbf) — Distributed force on the seat and seatback during wheelie off a 18 cm (7 in) step so that both rear wheels hit simultaneously. 

Test L

Test M*, Frame Twisting Strength
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).

Test M

Test N, Upward Force on Footrests
440 N (100 lbf) [P8.9] — 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.

Test N

Test O, Upward Force on Armrests
895 N (200 lbf) [P8.8] — Upward force on Armrest. If the armrest swings away or is removable, this must occur at less than 90N (20 lbf).

Test O

Test P, Handling Drop Test
1m (39in), empty — Impact during handling. Drop from the height on to each wheel when opened and folded.

Test P

Results of employing the tests
The Whirlwind II design wheelchair passes these tests. Two such chairs were tested to, and passed, the ISO Standard tests, including the minimum 200,000 cycles on the two-drum durability tests [P10.4], and 6,666 cycle horizontal 50 mm (2 inch) drop test [P10.5]. In addition, thousands of Whirlwind chairs have been built to the 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 ISO Standard 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, and any welds or brazed joints be of high quality.

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, thus, avoiding a catastrophic safety hazard. (For example, 16mm (5/8″) diameter low-carbon steel axle bolts are used.) 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 countries. 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, shops trained 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.

References 
1. “ISO International Standards for Wheelchairs (Final Draft), ISO/FDIS 7176-8, Part 8, Requirements and test methods for static, impact and fatigue strengths”, Copyright ISO 1997.

Available from:

American National Standards Institute
13th Floor
11 West 42nd St
New York, NY 10036
Attn: Customer Service
Ph:212/642-4900
Telfax: 212/398-0023
e-mail: info@ansi.org,
WWW: http://www.ansi.org

or

RESNA
1700 N. Moore Street,
Suite 1540
Arlington, VA 22209
Ph 703/524-6686
WWW: http/www.resna.org (Contact Persons/Publication)

2. Peter Axelson, MSME, A Guide to Wheelchair Selection, How to
use the ANSI/RESNA Wheelchair Standards to Buy a Wheelchair,
Beneficial Designs Inc.
2240 Meridian Blvd, Suite C
Minden, NV 89423-8628
775.783.8822 ph
775.783.8823 fax
mail at beneficialdesigns.com

3. Peter Axelson, MSME, List of Wheelchair Testing Facilities (an
informal, unpublished list of addresses and test
capabilities)

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:
Whirlwind Wheelchair International
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 ISO Standards.

Note: This paper first appeared in RESNA 1996 Proceedings, Pages 414-416. This revision includes corrections, comments, and changes that reflect the current testing practice of Whirlwind Wheelchair International (formally, Wheeled Mobility Center). We intend to publish a simplified version for dissemination to wheelchair builders worldwide. It will use diagrams and require minimal English comprehension.