Manual Wheelchair Materials

The majority of manual wheelchairs are made of either aluminum or steel. High-end, meaning ultralight wheelchairs or sport wheelchairs, are typically being constructed of exotic materials such as high performance aluminum, titanium and advanced composites. The choice of one material over another depends on the material’s strengths and weaknesses and how that chair will be used.

If you watch what goes on in the aerospace industry, you see those trends gliding into the manual wheelchair market. This happens because the two industries deal with similar issues (e.g., strength versus weight, cost, and reliability). This can be seen in the progression from steel alloys, to aluminum, titanium and more recently advanced composite materials.

Material Type

The types of common alloy steel typically used in manual wheelchairs are mild steel (AISI 1040 or 1060), chromium-molybdenum alloy steel (AISI 4130) or chromium-nickel-molybdenum alloy steel (4340 and 8620).

• The AISI 1040 and 1060 are typically used in wheelchair frames are mild steel and chromium-molybdenum alloy steel. They are inexpensive, easy to work, readily available, and versatile. However, they have a low strength-to-weight ratio relative to other materials. This material is used in standard wheelchairs, such as the Invacare Tracer.
• AISI 4130 is widely used because of its strength, its ability to be easily welded, and ease in fabrication. It can be treated for higher strength and to resist abrasion. It is used in the frames of ultralight wheelchairs such as the Sunrise Medical Quickie ST/DT and the Invacare Top End Terminator.
  
  
  Invacare Top End Terminator Everyday Chair

The types of aluminum used in manual wheelchairs are SAE 2024, SAE 6061, and SAE 7075, though SAE 2024 is not used as often as the other two.

• SAE 6061 (also known as aircraft grade aluminum) is an inexpensive and versatile structural aluminum alloy that offers good mechanical properties and corrosion resistance. It can be welded using most common methods. Most aluminum wheelchair frames are made of this alloy. There are numerous examples of the use of this material in ultralight wheelchairs and sports wheelchairs. These include the Colours Boing (ultralight with a four wheel independent suspension) and the Invacare Top End Eliminator (racing).
• SAE 7075 (also known as high performance aluminum) is one of the highest-strength aluminum alloys, which is ideally suited for high-stress parts. It is not recommended for welded parts. This material is used in the Sunrise Medical Quickie A Frame Racer and the Quickie T-Tube Racer. An increasing number of high-end wheelchairs, such as ultralight and sport, are made of titanium. Titanium is the most exotic and therefore the most expensive metal used in manual wheelchair production. Titanium is used because of its availability, appearance, corrosion resistance, and high strength-to-weight ratio. Cost is not the only drawback to using titanium. It requires a highly skilled welder to make suitable welds, since titanium can only be welded using special inert gases (TIG welding). It also requires special tools as well as a highly skilled and experienced machinist when manufacturing a wheelchair. If not, the titanium may become worn or flawed, leading to a rapid failure of the material. Examples of wheelchairs that use titanium are the frame of the Sunrise Medical Quickie Ti Titanium, the TiLite Cross-Sport and portions of the Everest and Jennings Barracuda.
  
  
  
  TiLite Sport Cross-Sport

Advanced composites also have been making the transition to wheelchair design from aerospace and industrial applications. Composites are carbon fiber, fiberglass, and Kevlar^®. Composites come as cloth, tape or yarn and are woven to form special layers that are held together by resin (i.e. often epoxy-based). To achieve greatest strength a minimum amount of epoxy must be used while wetting all of the fibers. To increase the strength and stiffness of structural components, a foam (e.g., Styrofoam, urethane, or PVC) core is used to separate the cloth layers.

Composites also can be molded into elaborate shapes, which open a multitude of possibilities for wheelchair design. Molding, specifically for curved pieces, allows for the design of wheelchairs with fewer joints. This is important since the majority of frame failures occur at joints (bolted or welded). Also, complex shapes provide the greatest benefit to individuals who use a wheelchair because:

• the dampening (reduction of vibrations felt by the individual) characteristics of the material can be most effectively utilized;
• the wheelchair can be made with less material and therefore be less conspicuous and,
• the wheelchair can better conform to the features of the body, providing a more comfortable fit.

On the other hand, the ability to produce complex shapes can be considered a downside of composites; these shapes may require intensive manual labor, which is not cost effective. However, new technologies in the production of composite components and frames are being introduced which should reduce the manufacturing costs. Composites are currently used in order to reduce the weight of the casters and the rear wheels, while improving the aerodynamics of the rear wheels, which is important for racers. Also, the Everest and Jennings Barracuda uses composites in the frame.

Function

The different types of material are used depending on the desired functionality of the wheelchair.

• Mild steel is often used for depot or standard wheelchairs, where the cost of materials and production is the primary consideration. A depot or standard wheelchair is meant for temporary use by more than one person, typically in an institutional setting, and is often attendant-propelled. Therefore, the weight of the wheelchair, long-term comfort, maneuverability, and durability on rough terrain are not a major concern.
• For a ,lightweight or ultralight wheelchair, intended for individual use as a long-term independent mobility aid, its comfort, durability and maneuverability are primary concerns. Therefore, chromium-molybdenum alloy steel, aluminum, titanium and advanced composite materials are employed in the manufacture of the frame and components. Material which reduces the amount of vibrations an individual feels (i.e. good dampening properties) (typically composites) can be used to improve rider comfort.

Material with a high strength-to-weight ratio (e.g., aluminum, titanium, or composite) may allow for an inconspicuous wheelchair, so the individual is seen before the wheelchair in a social setting. An inconspicuous wheelchair also means a lighter wheelchair, allowing for easier propulsion and maneuverability. Reduction in weight may also lead to a reduction in secondary injuries such as repetitive stress injuries. These injuries may be a result of wheelchair propulsion or lifting the wheelchair during car/van transfers.

Though cost may not be a primary concern to the individual, it automatically becomes one as soon as a third-party payer gets involved. Obtaining a lighter, more durable, and consequently more expensive wheelchair with exotic materials (e.g. titanium and composites) may be justified by a reduction in the dollars per life cycle and in the costs incurred while treating a repetitive stress injury. Therefore, the selection of the material used in manual wheelchairs is a function of the durability, aesthetics, function, ride comfort, and cost of the wheelchair, as well as the wheelchair purchaser.

Once the required material characteristics are known, the most appropriate material and associated manufacturing process can be selected to meet the overall needs of the consumer.

Bibliography

Cooper, R. A., Rehabilitation Engineering: Applied to Mobility and Manipulation, 1995, Institute of Physics Publishing, Ltd., Philadelphia, ch. 7, pp. 255 – 290.

Cooper, R. A., Wheelchair Selection and Configuration, 1998, Demos Medical Publishing, Inc., New York, ch. 3, pp. 43 – 88.

June, R. R., "Aerospace Materials" in The Engineering Handbook, Dorf, R. C., ed., 1995, CRC Press, Inc., Boca Raton, FL, ch. 175, pp. 1849 – 1855.

Cooper, R. A., "A Perspective on the Ultralight Wheelchair Revolution", Technology and Disability, v. 5, pp. 383-392, 1996.

Cooper, R. A., Robertson, R.N., Lawrence, B., Heil, T., Albright, S. J., VanSickle, D. P., and Gonzalez, J., "Life-Cycle Analysis of Depot versus Rehabilitation Manual Wheelchairs", Journal of Rehabilitation Research and Development, v. 31, pp. 45-55, 1996.

Cooper, R. A., Gonzalez, J., Lawrence, B., Rentschler, A., Boninger, M. L., and VanSickle, D. P., "Performance of Selected Lightweight Wheelchairs on ANSI/RESNA Tests", Archives of Physical Medicine and Rehabilitation, v. 78, pp. 1138-1144, 1997.

Cooper, R. A., DiGiovine, C. P., Rentschler, A., Lawrence, B. M., and Boninger, M. L., "Fatigue-Life of Two Manual Wheelchair Cross-Brace Designs", Archives of Physical Medicine and Rehabilitation, v. 80, pp. 1078-1081, 1999.