The following alterations were necessary to fulfill customer requirements:
To fulfill our client’s needs the wheelbase of the chair had to be reduced from 16 inches to 14 inches. The actual width of the chair is dependent on the seat mounts labeled in the Figure 4.1. These mounts serve two purposes: connecting the two vertical halves of the chair and supporting the seat. The connection of the two halves of the chair is done by placing a 5/8 inch galvanized grade 5 bolt through the end of each set of mounts.
Each set of mounts was reduced by 1 inch to produce the final width of 14inches. After the length of the mounts was adjusted, new end holes and holes for the crossbars were drilled. The placement of the holes for the crossbars was determined by extending the crossbars along the mount until both the appropriate fit and largest possible angle theta was found.
This was done for added stability and to ensure no modifications to the top of the crossbars would be necessary. After the modifications to the mounts were completed the wheelchair was re-assembled to ensure the correct width had been obtained. A temporary seat was cut and mounted to the wheelchair for stability during testing. At this point it was discovered an additional modification to the seat mounts would be needed to allow the seat to fold.
Originally,
it was believed only one modification would be necessary to allow the chair
to fold easily. The wheelchair was already equipped with a soft chair
back and rotating seat mounts. This suggested the easiest means of
folding the chair would be to mount one half of the seat to the seat mounts
and place a clamp on the other half of the seat, which would fit snugly
over the side member of the wheel chair. This would lock the seat
in its horizontal position. However, shortening the lengths of the
seat mounts made it necessary for a second modification. 2” slots
from the original crossbar holes to the new crossbar holes were cut to
allow the chair to fold. The slots were cut using the CNC machine
located in the Advanced Design and Manufacturing Laboratory (ADML) building.
The addition of the slots allowed the chair to fold easily. However,
two concerns arose following the placements of the slots.
A finite element mesh/analysis of the seat mount was created in ANSYS. The results are included in the Appendix. The results indicated the force due to the weight of a passenger would not jepardize the structural integrity of the seat mounts. In fact, the finite element analysis (FEA) was conducted using a distributive load higher than the specified weight of the client and showed no sign of reaching the yield strength of steel. Once satisfied with the seat mount structure, it was important to ensure no movement was occurring along the slots during the use of the wheelchair.
It is important
to understand how the wheelchair is connected in order to understand the
importance of constraining the movement of the crossbars.
The crossbars directly affect the position or width at the base of the
wheelchair. The rotation of the seat mounts about the frame of the
wheelchair allows for the outward rotation of the base of the chair if
a force is placed on the side of the handgrips.
The rotation of the base was constrained in two ways:
1. The seat was clamped over one side member constraining the motion of the opposite base member.
2. A removable pin was installed in the crossbars locking their position at the outer edge of the slots.
Several types
of constraints were considered including the incorporation of hinges, a
brace which rotated over one rear member latching to the same member on
the other side, and the pin. The rotation of the sides of the chair
can be completely constrained if the movement of the crossbars is constrained.
Using either the latch or the brace would have indirectly constrained the
movement of the crossbars, however the pin directly constrains the movement
of the crossbars thus providing stability within the frame of the wheelchair.
The location of the pin is illustrated in Figure 4.4.
The following table contains the specifications of the
custom ordered single action pull pin.
| Specifications | |
| Material | Pin Corrosion Resistant Steel |
| Handle | Aluminum |
| Diameter | 5/16 inch |
| Grip Length | 2 inches |
| Total Length | inches |
Several steps were taken in the installation of the pull pin in the crossbars. The pin is actually installed into a 1/4 inch inner diameter thick walled shaft. This shaft was bored out to a 5/8 inch inner diameter and silver soldered into each crossbar at the intersect point of the crossbars. The purpose of the shaft is to distribute the force applied to the crossbar by the bolt over the area of the shaft. Had the shaft not been placed in the hole, a concentrated force would have been applied to the edges of the hole.
The customer specifications included the reduction of the height of the armrests to approximately the same height as the wheel. We determined the armrest must be lowered approximately 3inches. This dimension accounts for the thickness of the armrests. Three-inch sections were cut from the vertical members of the armrests of the chair. The vertical member was chosen such that a minimal amount of cutting and welding would be needed for the re-assembly of the armrests. Personnel in the J. M. Patterson machine shop did both the machining and the welding of the armrests. Figure 4.5illustrations the portion of the wheelchair armrest that was removed. A carbon steel shaft was placed within the cut armrests to provide added support for the silver soldered welds that reattached the shortened section of the chair.
**Note: this figure is not meant for accurancy but to illustrate
to the reader the where the cuts were made. Illustrating the cuts were
not at the same height on both members of the sides of the wheelchair.
The exact modification to the height of the armrests
is illustrated in the engineering drawings provided in the Appendix B.
In addition to the modifications made to meet the specifications, general
and cosmetic modifications were also made.