DuaLock: A Simultaneously Locking Wheelchair Lock (Tulane University)

Figure 1. Team with DuaLock assembled on client's wheelchair.

Matthew Giannetti, Atman Dave, Kristina Farragut

ABSTRACT

Many wheelchair users worldwide have limited or no mobility in one arm. Common manual wheelchair locking systems require force application to both locks on either side of the wheelchair. If an individual is unable to lock one side due to lack of mobility in his or her arm, the wheelchair is either unstable when locked or requires the user to ask for the help of an attendant.

Our client, a stroke victim, has unilateral arm strength. Our project’s goal was to develop a locking system that would lock both wheels of our client’s wheelchair from one side. The final design of our wheelchair locking system allows for the simultaneous locking and unlocking of both wheelchair locks from a single side. This system is side-independent meaning that no matter which side is engaged or disengaged, both wheels simultaneously lock or unlock respectively.

BACKGROUND

Many disabled individuals have use of only one arm, or have limited mobility in one arm. Although electric wheelchairs are available for these individuals, this is not always an option for patients due to cost. Common manual wheel locking systems require application of a lock on each side. If an individual is unable to use one side, the wheelchair is either unstable, or requires the help of an attendant.

Current available single-hand wheel locks for manual wheelchairs use a single lever attached to a single cable to move the locks in the same direction (1). In these cases, a spring depresses the lock on the side without a lever when the lever is disengaged. Additionally, brakes may be used where tension upon a cable is applied to brake pads, which brake the wheel. Furthermore, a rigid bar linkage between the two becomes problematic when folding the wheelchair is necessary (2).

None of the previous options are ideal. Use of a single lever on one side must be customized for each patient and the spring will need to be replaced over time. Use of brake pads may slow the wheelchair, but they must remain engaged in order for a parking stop. This is not a practical option for a parking lock.

A manual wheelchair lock is needed such that a lever on each side of the chair can engage the wheel locks on both sides.

DESIGN GOALS

1. Lock both wheels from a single point of application.
2. Unlock both wheels from a single point of application.
3. Maintain the ability for the wheelchair to fold for storage or transport.
4. Allow for locking/unlocking from either side of wheelchair, regardless of prior changes of state.
5. Visually indicate the current state of the wheelchair locks (locked or unlocked).
6. Maintain intuitive locking motion for user.

DESIGN

The DuaLock functions by taking advantage of the pivot point in a standard wheelchair lock. Two cables transmit the force from the handles by attaching to opposing sides of the pivot. When either handle is pushed:

• The lock of that side is engaged
• The cable attached to the top of the pivot also pulls at the bottom of the opposite handle, providing the same force to lock both sides simultaneously.

The same happens when a handle is pulled to unlock, but the process is reversed.

Figure 2. Schematic of DuaLock: A Simultaneously Locking Wheelchair Lock

Figure 3. DuaLock assembled upon client's wheelchair

A cable linkage between the handles was chosen to couple the locking motion from one side to the other. Bowden cables allow for the transfer of force from one object to another through tension applied to the cable. The design utilizes a two-cable system to allow for both locking and unlocking linkage.

Figure 4. Components of DuaLock: A. Custom handle and cable attachments B. Adjusting Barrel C. Looped cable termination D. Coarse adjustment L-bracket

DEVICE

Figure 5. DuaLock advanced prototype with labeled components

The locks used were standard-issue Invacare locks that come on the Tracer EX2 wheelchair. These locks were modified by changing the handle to a custom handle to accommodate the cable attachments, shown in FIGURE 4A. The handle was machined out of 3/16” thick stainless steel and tapped to allow for insertion of shoulder bolts.

Cables A and B are threaded steel cables that attach to the handle. Cables A and B both are looped and crimped per FIGURE 4C to articulate around shoulder bolts.

Both cables are housed by a glide-lined cable housing typically used for bicycle braking systems. The housing inserts into adjusting barrels, FIGURE 4B, which fit into machined and tapped L-brackets. The adjusting barrels serve as a fine adjustment for the tension in the cables. This setup is shown from the side in FIGURE 4D. It is attached to the underside of the frame of the wheelchair on both sides. They are held in place by adjustable hose clamps. The hose clamps and L-bracket together serve as a coarse adjustment during the installation of the device.

The tension in the cables can be optimized by using both the coarse and fine adjustment of the cable system.

RESULTS AND CONCLUSION

Upon installation on the client’s chair, DuaLock met all six of the stated design goals. Locking and unlocking of both wheels is available from either side. Wheelchair folding capability is maintained and lock function is maintained regardless of condition (folded/unfolded). Wheelchair can be locked from either side and subsequently unlocked from either side regardless of locking side. When locks are engaged, the handles of the locks extend forward; when disengaged, handles extend vertically to visually indicate state. Locking motion of standard push-to-lock wheelchair locks maintained in device. DuaLock allows the client to lock and unlock both wheels of her wheelchair with her “good” left hand, elimating the need for an attendant to secure her wheelchair. Any user of a manual wheelchair would benefit from the addition of DuaLock, as it reduces the locking maneuver from a two step process to a single motion.

EXTERNAL MEDIA

Brochure

Poster

REFERENCES

1. Tanksley, K.A., et al. (2005) U.S. Patent No. 6,929,100 Washington, D.C.: U.S. Patent and Trademark Office.

2. Leggett, G. M., et al. (1989). U.S. Patent No. 4,809,818 Washington, D.C.: U.S. Patent and Trademark Office.

ACKNOWLEDGEMENTS

The team thanks Dr. David Rice and Dr. Lars Gilbertson; their insturction guided the team and was invaluable.

We would also like to thank our mentor Dr. Walter Murfee, whose design experience allowed the team to streamline its thoughts.

Thanks are also extended to Dr. Ronald Anderson and Dr. Michael Moore, both of whom served as on the faculty panel for the team.

Additionally, Laminar Flow would like to thank John Sullivan, whose technical assistance was crucial in the progression of the project.