Evaluating durability of bushings and bearings for wheelchair use in adverse environments

Download PDF

Evaluating durability of bushings and bearings for wheelchair use in adverse environments

John J. Fried1, Austin J. Kasmer1, Andre C. Kuminkoski1, John R. Larson1, Jon L. Pearlman1,2, David E. Schmidt1, Anand A. Mhatre1,2

1University of Pittsburgh, 2 International Society of Wheelchair Professionals



According to the World Health Organization (WHO), an estimated 75 million people with spinal cord injury, cerebral palsy and similar diagnoses around the world need wheelchairs for mobility, independence and social participation. [1] While wheelchairs play a significant role in the lives of people with disabilities, they are known to breakdown frequently. More than 50% of wheelchairs suffer a breakdown every ~6 months in urban settings and every ~3 months in the adverse environments in rural areas and low- and middle-income countries (LMICs). [2-4] One-third of these breakdowns result in consequences for wheelchair users including injuries, depression, and loss of access to work and/or school. [2] Without a functional wheelchair, the user may have to stay in bed and is vulnerable to development of pressure sores as found in a recent study. [4] Pressure sores are associated with an increased risk of mortality. [5,6]

Of all failures reported for wheelchair parts, nearly one-third are caster failures. [7-9] The pressure to control selling price often leads to selection of low-cost, substandard casters that experience different failure modes including locked and missing bearings, damaged bolts, fractured wheels and forks, worn-out tires and missing fasteners. [9] Stem and axle bearings are subjected to rapid fatigue and stress as wheelchairs are exposed to corrosion, shocks, high temperatures and dirt during use. Bearing fractures can lead to a cascade of high-risk failures with caster stems bolts and forks as they experience stresses higher than the ultimate tensile strength of their materials. Recent wheelchair failure data collection studies have found that caster stem bearings fracture within 2 years of use in adverse environments. [10] Similar bearing models have failed with the caster durability testing protocol developed by the International Society of Wheelchair Professionals (ISWP) that requires casters to complete a minimum of 2 years of field-equivalent testing exposure. [11] These congruent findings from the community and laboratory-based testing studies motivated the authors to investigate the durability of bearings. 

The development of a more dependable, low-cost stem and axle bearing for wheelchair casters is crucial to reduce bearing failures and ensure user safety. The WHO Guidelines on provision of manual wheelchairs recommend improvements in wheelchair design for better outdoor reliability and accordingly, the authors extended their investigation in this study to other rolling elements such as bushings that can be a low-cost alternative to bearings. [12,13] The goal of this study is to conduct comparative testing with bearings and thermoplastic bushings using the ISWP caster durability testing protocol and evaluate their durability and cost-effectiveness. 



Figure 1.  Standard Caster

A standard 8” caster model (Figure 1), widely used on wheelchairs in LMICs, was chosen for the testing study. Four samples of the model underwent the ISWP testing protocol, each with a different bearing/bushing model for the stem and axle as shown in Table 1. The bushing materials were selected based on recommendations from wheelchair experts from the ISWP Standards Working Group. Prior to this study, extensive preliminary testing was carried out with bushing materials sponsored by IGUS. [14] Both bushing materials in Table 1 were selected because of their lower wear rate, resistance to corrosion and low humidity absorption. IGUS and GG Bushings provided samples of the materials that were machined to the bearing dimensions with press-fit tolerances recommended by the manufacturers. [14,15]

ISWP Caster Durability Testing

Figure 2. ISWP Chakra (left) and salt-fog chamber (right)

The ISWP Caster Durability Testing Protocol, proposed as ISO/AWI 7176-32, includes corrosion testing in a salt fog chamber (as per ASTM B117) followed by shock and abrasion testing on ISWP Chakra. [16,17] Table 2 shows the durability testing exposure. The testing was limited to an exposure equivalent to 6 years of wheelchair use as majority of the bearings fail within the time period in adverse environments and need replacement as per our data collection studies. Durability was determined by the number of shock testing cycles completed and cost-effectiveness was calculated as a test cycles-to-dollar cost ratio. The cost included combined cost of stem and axle bearing. Testing was discontinued following a bearing/bushing failure. 

Table 2. ISWP durability testing protocol simulating 2 years of outdoor use



Figures 3 and 4 show the durability testing conducted with the caster models in this study. Results of the testing study are shown in Figure 5 below. All of the axle bearings survived the test without a failure. Minor material wear was noticed after testing the bushings, but it had no noticeable effect on their function. Figure 6 shows the stem fracture observed with the manufacturer’s bottom stem bearing. These fracture failures with the bearing model are typical and have been observed during previous testing studies.

Figure 3. ISWP Corrosion Testing

Figure 4. ISWP Shock and Abrasion Testing

Figure 5. Comparative Bearing and Bushing Testing Results

Figure 6. Standard  caster bearing failure


Results from the comparative bearing and bushing testing study show that bushings can be a cost-effective alternative to bearings used on wheelchair casters. We found additionally that the polymer bushings were resistant to corrosion as compared to the metallic bearings which suffered from obstruction to rolling.

Based our findings, bushings may offer a low-cost alternative because they can be mass produced using injection molding which reduces the total product cost and improves part availability especially in LMICs where access to wheelchair parts is challenging. Additionally, bushings can be manufactured using 3D printing technology that is gaining traction in the global assistive technology sector and will further facilitate easy access. 

In this study, the standard caster stem bearings failed to meet the minimum test cycle requirements. These bearings may be suitable for wheelchairs used in institutional settings and for lessactive users. To ensure user safety and avoid breakdowns, routine servicing and replacement of such bearing models is necessary. The high-quality bearing products are used on certain wheelchairs in high-income settings and they certainly exceed the strength requirements of a typical manual wheelchair caster bearing.

The ISWP caster testing protocol has been instrumental in generating design improvements and guidelines for the wheelchair manufacturers. [11] The study results indicate a positive impact with employment of bushings however, there is additional work required to reinforce the findings. Testing with additional bearing and bushing samples is in progress to improve the power of our research study and building further evidence. There are additional environmental conditions that must be examined to fully evaluate bearing and bushing performance. Tests for dirt and dust contamination will be conducted soon to evaluate the efficiency of bearings and bushings.



Wheelchair caster breakdowns due to bearing failures are commonplace in adverse environments. This comparative testing study demonstrated that bushings could serve as a durable, cost-effective alternative to bearings. With ongoing studies at the University of Pittsburgh, we are developing additional evidence with bushing and bearing evaluations to inform bushing and bearing selection based on the application of use and thereby, reduce bearing failures and wheelchair breakdowns in the community. 



This study was performed by a University of Pittsburgh Mechanical Engineering Senior Design team and would not be possible without the sponsorship and guidance of University of Pittsburgh’s Department of Rehabilitation Science and Technology. We would like to thank IGUS and GG Bearings for the bushing material samples. We do not intend to endorse any of the manufacturers listed on this publication. 



[1] World Health Organization, “Assistive technology,” 2018. [Online]. Available: https://www.who.int/news-room/fact-sheets/detail/assistive-technology.

[2] M. Toro, L. Worobey, M. L. Boninger, R. A. Cooper, and J. Pearlman, “Type and Frequency of Reported Wheelchair Repairs and Related Adverse Consequences Among People With Spinal Cord Injury,” Arch. Phys. Med. Rehabil., vol. 97, no. 10, pp. 1753–1760, Oct. 2016.

[3] K. Rispin, K. Riseling, and J. Wee, “A longitudinal study assessing the maintenance condition of cadres of four types of wheelchairs provided in low-resource areas.,” Disabil. Rehabil. Assist. Technol., vol. 13, no. 2, pp. 146–156, Feb. 2018.

[4] N. S. Hogaboom, L. A. Worobey, B. V Houlihan, A. W. Heinemann, and M. L. Boninger, “Wheelchair breakdowns are associated with pain, pressure injuries, rehospitalization, and self-perceived health in full-time wheelchair users with spinal cord injury,” Arch. Phys. Med. Rehabil., 2018.

[5] J. Pearlman et al., “Lower-limb prostheses and wheelchairs in low-income countries.,” IEEE Eng. Med. Biol. Mag., vol. 27, no. 2, pp. 12–22, Jan. 2008.

[6] Jenny Kim, Susan J. Mulholland, J. Kim, and S. J. Mulholland, “Seating/wheelchair technology in the developing world: need for a closer look,” Technology and Disability, vol. 11, no. 1,2. IOS Press, pp. 21– 27, Jan-1999.

[7] R. P. Gaal, N. Rebholtz, R. D. Hotchkiss, and P. F. Pfaelzer, “Wheelchair rider injuries: causes and consequences for wheelchair design and selection.,” J. Rehabil. Res. Dev., vol. 34, no. 1, pp. 58–71, Jan. 1997.

[8] C. Mair, “Applied internet-of things technology in the management of wheelchair maintenance at NHS WestMARC: A retrospective,” in European Seating Symposium, 2018.

[9] A. Mhatre, N. Reese, and J. Pearlman, “Design and evaluation of a laboratory-based wheelchair castor testing protocol using community data,” PLoS One, vol. 15, no. 1, p. e0226621, Jan. 2020.

[10] A. Mhatre, J. L. Pearlman, and S. Lachell, “Development, reliability, and piloting of a wheelchair caster failure inspection tool (C-FIT),” Disabil. Rehabil. Assist. Technol., 2018.

[11] A. Mhatre, “Development and validation of a wheelchair caster testing protocol,” 2018.

[12] J. Borg and C. Khasnabis, “Guidelines on the provision of manual wheelchairs in less-resourced settings,” World Health Organization, 2008.

[13] A. Mhatre et al., “Developing product quality standards for wheelchairs used in less-resourced environments,” African J. Disabil., vol. 6, no. 0, p. 15 pages, Jan. 2017.

[14] igus, “iglide® bearings - maintenance-free, made of plastic,” 2019. [Online]. Available: https://www.igus.com/iglide/plain-bearing. [Accessed: 13-Feb-2020].

[15] GGB, “Plain Bearings, Self-lubricating Bushings & Polymer Coatings | GGB,” 2020. [Online]. Available: https://www.ggbearings.com/en. [Accessed: 13-Feb-2020].

[16] International Organization for Standardization, “ISO/AWI 7176-32 - Weelchair -- Part 32: Standard Practice for Wheelchair Castor Durability Testing,” 2019. [Online]. Available: https://www.iso.org/standard/77566.html. [Accessed: 23-Apr-2019].

[17] ASTM International, “ASTM B117,” 2016.