More Related Content Similar to MAE 490 Project - Google Docs Similar to MAE 490 Project - Google Docs (20) 2. Table of Contents
Page
· Abstract 4
· Introduction 4
· Design Parts and Drawings 8
· Simulations 10
I. Static 11
II. Buckling 11
· Design C.A.M. 11
· Conclusion and Recommendations 14
· Appendices
I. References 19
II. GCode 20
a.) Walking Leg Base 21
b.) Sprinting Leg Base 22
c.) Running Leg Base 22
3. Abstract
This project is the culmination of several different designs for an athletic
prosthetic leg. Having said this, however, much of the inspiration for this project came from the
design that is utilized by a company called Ossur that was recommended by Nike. The reason
this group chose to focus on this for our project is because we each take interest in the working
collisions between the engineering world, and that of biological applications. For this project, the
most difficult part in its design was the construction of the leg housing. Just as no 2 people are
exactly alike, no 2 legs will be the same either. In order to compensate for this, we decided to set
standards so as to scale our design upon the average attributes of athletes. By assuming the
average height of a human male to be 5 feet 10 inches tall, we were able to make further
assumptions based on this fact. Coincidentally (In all honesty it was purely coincidental), two of
our group members, Samuel Lopez and Sebastian Calderon happen to be this height. This
facilitated the design somewhat, as we were able to have a partial visual representation of what
this would look like. This design took multiple attempts to draw, as it must share unique
geometrical designs of the lower leg so as to function properly; however, the overall design and
attachment to the “leg” piece was simple.
By conducting research on prosthetic limbs, we were able to verify the optimal material
to use in its design. Ideally, the “leg” portion that is attached to the housing should be made of
carbon fiber. Since the version of SolidWorks we are using has only a few options, using
Titanium Alloy Commercially Pure CPTi UNS R50400 (SS) since it has the highest yield
strength, making it the logical choice because of the design. The housing material was also
questionable because the material had to be strong enough to support a 5’ 10” human being, but
4. also be comfortable enough to wear for long periods of time. For this, the suggested material was
Plastic PP Homopolymer. The final item to design were the bolts and washers, which attach the
housing to the “leg” portion. This is important as the bolts that are used are attached
perpendicular to the housing and “leg” portion, meaning there will be a large concentration of
shear stress on the bolts. In order to counteract this, the material for the bolts and washers was
chosen to be Aluminum Alloy 6061T6 (SS), which can withstand large amounts of shear
without deforming or failing, making it optimal for this type of project.
Introduction
Prosthetic limbs are an ideal choice for amputees who want to participate in activities
they enjoy. As the hands and feet are both parts of the body that are used very often in everyday
life, there exists a challenge in replicating their function. Since we chose to focus on the design
of a prosthetic leg, it is important to think of the unique functions of that of a normal leg so as to
be able to replicate its functionality in the design of a prosthetic.
For starters, the legs provide support for the human body, which (At the assumed height
of 5’ 10”) ranges between 149183 lbs (1). This, being a primary function of this device,
becomes important since both the material as well as the geometry play an important role in
designing this part. As mentioned before, this part will endure fairly large amounts of weight,
and must also be as lightweight as possible so as to not be a burden to the user. Carbon fiber is an
optimal choice because of its respective material properties. The excerpt below, taken from
http://zoltek.com/carbonfiber/ lists several properties of carbon fiber that would be very
advantageous for this design. “Carbon fibers are classified by the tensile modulus of the fiber.
The English unit of measurement is pounds of force per square inch of crosssectional area, or
5. psi. Carbon fibers classified as “low modulus” have a tensile modulus below 34.8 million psi
(240 million kPa). Other classifications, in ascending order of tensile modulus, include “standard
modulus,” “intermediate modulus,” “high modulus,” and “ultrahigh modulus.” Ultrahigh
modulus carbon fibers have a tensile modulus of 72.5 145.0 million psi (500 million1.0 billion
kPa). As a comparison, steel has a tensile modulus of about 29 million psi (200 million kPa).
Thus, the strongest carbon fibers are ten times stronger than steel and eight times that of
aluminum, not to mention much lighter than both materials, 5 and 1.5 times, respectively.
Additionally, their fatigue properties are superior to all known metallic structures, and they are
one of the most corrosionresistant materials available, when coupled with the proper resins.”
(2). In real life, this excerpt shows precisely why carbon fiber is the ideal choice for this part. For
the purposes of modeling, however, the Solidworks software does not have sufficient material
properties to run the simulations. Because of this, we had to change the material in our design to
Titanium. Titanium, while it is a metal, is very durable and very lightweight as compared to steel
or aluminum alloys. For the purposes of this simulation, titanium is as perfect a fit as we could
hope to find.
While the foot plays an important role in providing support, another important aspect in
the design of a prosthetic leg is the leg housing, also known as the socket (3). Since prostheses
need to be as lightweight as possible, this part is generally constructed from plastics. While
several websites suggest the most common material for this is polypropylene, which is a
thermoplastic, we did find an article which suggests it is reinforced with materials such as
fiberglass, nylon, Dacron, carbon, and Kevlar in order to provide strength (4). While this makes
sense in realistic terms and in real world applications, in order to simplify the design and
8. designed use as well. When comparing to Ossurs products and our studies, we concluded that for
walking, we will compare with Flex Foot Cheetah, sprinting, we will look at Flex Foot Xtend,
and running, we will compare to the Flex Foot Cheetah Xtreme. The modeling and the parts that
are utilized in the design of an athletic leg prostheses are shown in the following section.
Figure 3: Socket Base Model
Figure 4: Ossur Base Models
Each act, walking, sprinting, and running, all have their individual designs and specifications.
Looking at Figure 4 to the design for walking (Flex Foot Cheetah), we see it has a straight pylon,
is rounded in the back, and has a slight lift in the foot, very front of the design (similar a lifted
shoe toe tip). The design specified for sprinting (Flex Foot Cheetah Xtend), has a steeper angle
9. from the pylon, and no lift in the tip of the foot. The last model for running (Flex Foot Cheetah
Xtreme), also has a steep angle from the pylon, as well as a lift in the front of the foot. The
socket of the prosthetic leg was modeled as close as possible to Figure 3 above.
Based off of the designs of Ossur, we came to reason their specifications. Each type of
step has its angle and force applied. We look closer to taking a relaxed walking step, to a full
speed sprint, to a fluid jogging or running step.
There are different foot angles when walking, sprinting, and running. A walking step is
compared to the “MidStance” step in Figure 5. A round angle from ankle to heel of the foot,
with a lift angle in the front toe of the foot. The “Take Off” step relates to the sprinting design. A
steeper angle from the heel to the ground, with a lot of pressure at the front ball of the foot
leaving the front of the foot flat. Lastly the running step is seen in both the “MidSwing” and
“Terminal Swing”. The midswing step starts at a lower angle from heel to ground, with the
terminal swing landing and stepping in a fluid motion lifting the angle from toe to ground.
These parameters were taken into account when designing our models for each type of
step and prosthetic leg purpose. The modeling and the parts that are utilized in the design of an
athletic leg prostheses are shown in the following section.
