The Design and Evaluation of a Universal Clamp for a Wheelchair Lay-Tray System (University of Pittsburgh)

Genevieve M. Jerome, Joshua Telson, & Mahender Mandala

 ABSTRACT

Manual wheelchair users have reported “discomfort” and “reduced productivity” from performing tasks such as school work and writing on areas not designed to fit around their chairs. Bending over and reaching to write on unconventional desks and tables give wheelchair users poor posture that can lead to pain and health risks. This design effort was to develop a system that would allow manual wheelchair users to perform office and school work on a lap-tray that is positioned properly for better posture and productivity. A lightweight, portable, and low cost clamp that attaches to a wheelchair’s backrest was developed to meet this need. The clamp device allowed for five degrees of freedom and enabled a 15lb load when attached to the lap-tray.

INTRODUCTION/BACKGROUND

In a study where the goal was to “examine the difficulties manual wheelchair users experience in office activities and ascertain whether such problems may be due to poor relationships between the equipment and the users”, it was found that writing was one of the top two tasks reported to cause the most discomfort for manual wheelchair users (1). The pains reported by the subjects in this study were in the “lumbar region of the back, shoulders, upper back, and neck” (1). Desk heights being too high or low in relation to the wheelchair user, the angle of the writing area, and the distance between the writing area and the chair affect the comfort and amount of pain experienced by the wheelchair user (1). Similar results were found in a study of student wheelchair users at a university in Colombia (3).

Even though legislation in the U.S. has made working areas more accessible, there is still a need to further improve the working conditions for manual wheelchair users by taking into account their anthropometrics and seating needs. Troy, Cooper et al. (1997) express that “stability, mobility, postural support, skin integrity, function, and comfort” should be considered in the design AT. These needs were among the motivations behind the design of the ergonomic portable lap-tray system. Other motivations were the need to keep the final product economically feasible to be sold in developing countries such as the Republic of Colombia.

In order to meet user needs, this design effort focused on developing an adjustable clamp that can be attached securely to wheelchairs with varying styles and dimensions. The clamp was also to be designed to facilitate the end of a specially fabricated wheelchair lap-tray and yet be easily detachable. With minor modifications the final design incorporated with the lap-tray should promote comfort, better health, and increased productivity for manual wheelchair users in office and educational settings (2).

PROBLEM STATEMENT

The Colombian based company SenconTM is a “provider of products and services for people with disabilities to promote labor and education inclusion” (2). They are interested in manufacturing an inexpensive collapsible lap-tray device to be purchased by manual wheelchair users for a variety of different chair types. The goal for the universal clamp design effort was to work with SenconTM to develop a system that would attach their lap-tray securely to a variety of manual wheelchairs and allow for an easy disassembly performed by the wheelchair user. A secondary goal was to make this system versatile enough to be able to attach other devices beyond the lap-tray to a wheelchair. The usability of wheelchair users with different function levels was also kept in mind during the entire design process.

METHODS/APPROACH/SOLUTIONS CONSIDERED

Initial Prototype of Lap-tray

Once the University of Pittsburgh team received the lap-tray prototype model from SenconTM, modifications were made to improve the design. The amount of material used to build the tray was reduced by removing excess ribs. Its rigidity was increased by adding more circular supports and extending the area where the pipe connecting the tray to the clamp goes. A pin/spring mechanism was also added so that the tray could open and shut with ease.

Figure 2 - Modified Lap-tray design by University of Pittsburgh Team

Clamp Design Requirements

During the benchmarking phase of the design process, many lap-tray patents and devices were examined. It was found that the majority of existing lap-tray systems incorporate the wheelchair arms as a main element to secure the device. The design team felt that having a device secured to the arms would negatively affect propulsion performance and functionality. Also, many wheelchairs users do not use armrests in Colombia and many other countries (3). It was decided that attaching the lap-tray to the backrest frame would allow the aforementioned criteria described in the study by Troy, Cooper et al. (1997) to be met. A list of clamp design needs can be seen as follows:

• User can attach to various locations of the wheelchair

• Clamp is easy to install/uninstall by wheelchair user (e.g. no overly complicated parts)

• Can be adapted to various postural support devices (e.g.: armrest, arm orthotics, lap-tray, head support)

• Angles and distances measured with respect to the users seat are adjustable for user comfort

• 5 degrees of freedom (3 rotation and 2 displacement)

• Low cost and increased marketability for developing countries

• Manufactured using methods readily available in Columbia

Solutions Considered

Multiple alternative designs were considered and altered before the final concept was finalized. Early concepts were rejected because they did not incorporate all the needs listed as important for the overall lap-tray system. Adjustability was the main aspect of these designs that the final version evolved from. Even though the final design weighed the most it met the design requirements.

Figure 3 - Alternative Clamp Design 1 – Clamp Needed a Larger Surface Area

Figure 4 - Alternative Clamp Design 2 – Clamp Interfered with Propulsion and Needed Strengthening

RESULTS/RESOLUTION

Description of Clamp Design

The clamp mechanism can be broken into three main sections; the clamp hook, the slotted pipe, and the pair of circular pinch clamps. The clamp hook is an aluminum billet that hooks around the wheelchair pipe and is secured by tightening another small block against the pipe. One screw compresses the aluminum billet and block around the wheelchair pipe and distributes the force along the entire surface area so that no damage to the wheelchair is sustained. The slotted pipe is a small section of tubing which mates the clamp hook to the pair of circular pinch clamps. One end of the slotted pipe fits in a hole on the clamp hook and the other is secured by one of the circular pinch clamps. The hole of the clamp hook has a bolt running across which allows the slotted pipe to rest on. When the slotted pipe is turned such that the notch is aligned with the bolt, the slotted pipe drops further down and locks itself from rotating about its axis. The other end becomes a tube with which the circular pinch clamp can permanently attach to. Once attached, the slotted pipe becomes a quick method for tucking away or bringing out the clamp. The pair of pinch clamps provides adjustability. Two bolts that run along two parallel slots join them together. This allows them to be rotated in either direction and then secured. Each pinch clamp resembles a C shape that tightens around the pipe in question when the desired angle has been achieved. The pipe in question can be the slotted pipe or the mating pipe being used, for example, a tap-tray.

Figure 5 - Universal Clamp Attached to Backrest

Figure 6 - Universal Clamp with Lap-tray

Testing

To test the effectiveness of the universal wheelchair clamp, static loading was performed with it attached to the lap-tray on a manual wheelchair similar to the ones typically used in Colombia. The ANSI-RESNA standard equation used for testing the resistance to downward static forces on arm supports was used to test the system. The team’s passing criteria for testing was for the clamp/lap-tray system to withstand a 50 kg (110.23 lb.) load. This weight was chosen because it takes into account the weight of a book or a laptop and a percentage of the user’s weight leaning on the lap-tray. A force gauge was applied to the center of mass of the lap-tray. The system took a 7 kg (15 lb.) load before failure occurred where the two circular pinch clamps where bolted together. The bolts could not resist the torque applied by loading and therefore a new design is required in order to meet the 50 kg loading criteria. The next design iteration would be to replace the pinch clamps with a single hollow tube connecting the slotted pipe to the lap-tray pipe. Inside the hollow pipe would be another pipe connected to the tray, allowing it to rotate up and down.

F=(S*Md*g)/(2*cos15) (5)

where: g = gravity, 9.807 m/s2

Md= mass applied, 50 kg (110.23lbs)

S = factor of safety, 1.5

F = force applied in Newtons

Figure 7 - Force Gauge Applied to COM of Lap-Tray

Figure 8: Clamp Bending Failure

DISCUSSION/OUTCOMES/PERFORMANCE

In developing nations and in the U.S., AT like lap-trays and clamps are not usually covered by health insurance and are bought out-of-pocket. The client wanted the team to develop a low cost system that could clamp onto one of many round tubular bars that are part of a manual wheelchair (MWC) for use with a lap-tray system. The clamp designed by the team does indeed meet this goal; however a few minor modifications are required for the device to perform at its full capacity. The clamp can be mounted on most locations of a MWC and depending on its orientation; it can completely fit in a single plane. This allows the end-user to have an uninterrupted propulsion cycle.

The material used (AL 6061-T6) makes fabrication of the clamp in mass quantities, at an affordable production cost, easier for a startup company like SenconTM. A simple aluminum extrusion process, common in the US and many nations with manufacturing, can accomplish the complex shape required for the functioning of the clamp. The clamp also requires a few machining steps (tapped holes or clearance holes). Also, the clamp can potentially be a product in its own right. It can serve as an “adapter” that allows other accessories to be attached to a wheelchair. These could include umbrellas, fishing poles, medical monitoring devices, etc.

COST/IMPLICATIONS

Manufacturability and market potential

The goal for SenconTM was to work with the Human Engineering Research Laboratories (HERL) to expand the market of the entire clamp and lay-tray system as a luxury product in Latin American and a commodity product in USA. Access to high quality wheelchairs and funding is limited for individuals with disabilities in Colombia and the rest of Latin America. The gross national income per capita converted to U.S. dollars in Colombia is only $8,430 (5). Keeping production cost in mind during the universal clamp and lap-tray design process was a high priority.

The clamp was comprised of parts that can all be manufactured as extruded pieces. A few parts were also designed symmetric to allow for interchangeability and using extruded parts will reduce the cost of manufacturing versus using individual custom made pieces. The clamp parts were also designed so that standard metric hardware can be used. Sencon’s goal was for the clamp system to cost $10. The estimated cost of the final design is $17

ACKNOWLEDGEMENTS

The material used for this study was funded by SenconTM, Colciencias, and the Human Engineering Research Laboratories. Thank you to Rory A. Cooper, PhD and Maria L. Toro Hernandez for their guidance throughout the development of this design.  Thank you to David L. Hurtado M. for his role as client liaison and for his support in research and development during the early phases of the lap-tray design process.

The contents do not represent the views of the Department of Veterans Affairs or the United States Government.

REFERENCES

1. Troy, Cooper et al. An analysis of working postures of manual wheelchair users in the office environment. J Res Dev, 1997

2. Hurtado, D. (2012, January 10). Lecture presented in HRS Wheelchair Design & Evaluation. University of Pittsburgh, Pittsburgh, PA. Ergonomic and Portable Wheelchair Tray. [PowerPoint slides].

3. Hurtado D &Ágredo W. Postural analysis of eight university student wheelchair users when performing written exercises in their classroom: a case study in Santiago de Cali, Colombia. Journal Work (in press)

4. Section 8: Requirements and test methods for static, impact and fatigue strengths In (2009). R. A. Cooper (Ed.), American National Standard for Wheelchairs Volume 1: Requirements and Test Methods for Wheelchairs (including Scooters) (pp. 230-231).

5. World health organization: Colombia. (2012). Retrieved from http://www.who.int/countries/col/en/

First Author Contact Information:

Genevieve M. Jerome

Human Engineering Research Laboratories

VA Pittsburgh Healthcare System

6425 Penn Avenue, Suite 400

Pittsburgh, PA 15206

gmj11@pitt.edu