UNIVERSITY OF PORTLAND
DONALD P. SHILEY SCHOOL OF ENGINEERING
MEMORANDUM
DATE:
TO:
FROM:
SUBJECT:
October 25, 2013
Dr. Lulay, Instructor
Dr. Khan, Faculty Advisor
Mr. Kammeyer, Industrial Advisor
Ben Bruns, Stephen Christensen, Cody Fast
Demonstration Memo, Adjustable Rugby Wheelchair
The purpose of this project is to design and construct a rugby wheelchair that incorporates both
seat width and the dump angle adjustability. To that end, the purpose of this report is to describe
the construction and testing of an initial prototype and to outline the conclusions and adjustments
made to the design based on this prototype. An initial prototype was constructed out of LEGO
Technics and was used to evaluate the various joints that would be used for making adjustments.
Based off of the observed member behavior of the prototype, it was determined that a different
variety of joint would be needed at both the front and back members of the chair in order to
control the inner/outer tilt of the main cross members. Work on the prototype was performed at
the team members’ home.
The primary goal of this project is to develop a device capable of being adjusted in two key
dimensions; seat width and the dump angle. In order to adjust these two dimensions, a design
was created which included six joints. The adjustment of the front and back sliding members
controls the seat width and the main front-to-back members controls dump angle. Illustrations of
these adjustments can be found in Attachment 1 as Figures 1 and 2. The initial design, prior to
experimenting with the prototype, was to use different varieties of joints on the front-to-back
member connection and the front and back sliding members. For the front-to-back members, a
simple pivot joint would be used to control the angle between the members and a shaft collar
would be used to make length adjustments. For the front and back sliding members, an
articulating joint would be used to allow the front-to-back members to rotate while translated
along their respective horizontal members. The prototype was constructed from LEGO Technics
allowing for rapid production and ease of adjustment, but forced the use of overly complex
assemblies to represent simple joints.
From the wheelchair prototype, various design aspects were verified and a few problems
concerning joint constraint were revealed. The adjustment capabilities of the dump angle and
seat width were incorporated into the prototype, and are illustrated in Attachment 1, specifically
in Figures 1 and 2. Likewise, the prototype introduced a previously unforeseen problem
concerning the constraint of adjustable members. As seen in Attachment 1, Figure 3, the current
design allows for rotational movement at the joints on the front member. As a result of these
joints not being fully constrained, the front-to-back members are able to freely rotate inward
when weight is placed on the seat. In addition, the unconstrained members do not posses the
rigidity required for the fine adjustments and the data collection needed for acquiring relevant
sizing data.
Based on the analysis of the prototype and further design work, a few solutions were proposed
for constraining the front member joints. One design possibility includes using a fastener and a
rubber washer. This design would rely largely on friction to constraint rotational movement and
will require further testing to verify that the resulting frictional force will be sufficient to counter
the torque applied through the chair’s members. Alternate designs included using meshing teeth
located on the ends of two members and held together with a fastener to prevent relative motion.
However, due to issues with manufacturability, the washer solution was chosen for the next
iteration. Further information about the proposed joint adjustments can be found in the Design
Decision Document in Attachment 2.
If you have any questions or concerns pertaining to the current design and the revisions
discussed do not hesitate to contact Ben Bruns, [email protected], Stephen Christensen,
[email protected], or Cody Fast, [email protected].
Attachment 1: Prototype Adjustability
a)
b)
c)
Figure 1. Seat width adjustments: a) narrow front and back, b) wide front, c) wide back
a)
b)
c)
d)
Figure 2. Dump angle adjustments: a) baseline, b) front members lengthened,
c) rear members lengthened, d) rear members shortened
a)
b)
c)
Figure 3. Side members freely pivoting: a) narrow, b) normal, c) wide
Attachment 2: Design Decision Document
UNIVERSITY OF PORTLAND
DONALD P. SHILEY SCHOOL OF ENGINEERING
DESIGN DECISION DOCUMENT
DATE:
TO:
FROM:
October 25, 2013
Dr. Lulay, Instructor
Dr. Khan, Faculty Advisor
Mr. Kammeyer, Industrial Advisor
Ben Bruns, Stephen Christensen, Cody Fast
Adjustable Wheelchair capstone project team
Purpose - Select a joint.
Given - LEGO Technic prototype.
Assumptions - Friction will be enough to hold joints in place.
Solution - Sketch and describe the proposed joint.
Conclusion - Joint must be tested; consider toothed design if friction is not sufficient.
Problem: While constructing a prototype to validate design ideas for the team’s adjustable rugby
wheelchair, a flaw was discovered. The wheelchair’s side members are not fully constrained,
and as a result they pivot in and out freely. Although the members are relatively rigid, this
would likely become a problem when a person is seated in the chair and their weight is supported
by those members. The team needed to find a method of securing the joints so that they will not
pivot when someone is seated in the chair. The new joint design must not be overly complex,
expensive, or difficult to machine.
Given: In the LEGO prototype, the joints are held in place by friction; however, in the final
design the joints must be capable of locking in place to allow sizing data to be collected.
Assume: Since friction keeps the joint from pivoting under the weight of the members in the
prototype, sufficient friction in a full size joint should serve the same purpose.
Solution: The proposed joint revision incorporates a bolt, wing nut, and rubber washer to
increase friction in the joint as shown in Figure 4 below. The wing nut allows the assembly to be
loosened or tightened by hand as the wheelchair is adjusted to fit a new player. These new joints
would be fitted to the side bars at both the front and back of the chair.
Figure 4. Diagram of proposed new joint assembly
Conclusion: The above solution meets all the criteria and utilizes mostly common hardware, but
requires testing in order to verify its feasibility. If friction is not sufficient to hold the side bars
in place, an alternative is to use mating teeth in place of the rubber washer. In this case, the wing
nut could be loosened, the joint rotated incrementally, and the nut tightened down to hold the
joint in place. This solution likely incorporates complex machining and would be more difficult
to adjust while someone is sitting in the chair.