1. MEng Mechanical Engineering
Designing a Mobility Walker for Adult Dwarfs
Alexander PARFITT
May 2015
Dr J. Rowson
Thesis submitted to the University of Sheffield in partial
fulfilment of the requirements for the degree of
Master of Engineering
Department
Of
Mechanical
Engineering
2. ii
SUMMARY
This project finds a suitable mobility walker solution for adult dwarfs. The
specialised user cohort requires a mobility walker with adjustable height and
width, and specifically designed with adult dwarfs in mind; the likes of which
cannot be found in the current mobility walker market.
After reviewing regulatory and anthropometric literature, a complete redesign of
the mobility walker was undertaken. The design concept was visualised using
Computer Aided Design, which in turn allowed the use of Finite Element Analysis.
Finite Element Analysis identified the stresses within suggested designs when
under loading. The analysis of these stresses resulted in a mechanically tested and
validated concept. The project outcome is a mobility walker with a rated load of 75
kg, a total mass of 3.920 kg, an adjustable width of 225-525 mm, and an adjustable
handle height of 550-750 mm, compliant with British Standards design and stability
regulations.
The creation of a mobility walker concept paves the way for future development
within this field, and has further implications for dwarfism research and overall
mobility walker development. The project not only highlights the needs of adult
dwarfs, but also the possible demographic neglect of many other medical
conditions whose needs remain unfulfilled.
3. iii
GLOSSARY
Achondroplasia A common cause of disproportionate dwarfism.
Anthropometry The scientific study of the proportions of the human body.
Average size A person without dwarfism.
Centre of mass …the point in a body or system of bodies at which the whole
mass may be considered as concentrated… (1)
Computer Aided
Design (CAD)
A computer program which is used for 3D-visualisation and
analysis of designs.
Disproportionate
Dwarfism
Dwarfisms where the limbs are abnormally short in
comparison to the rest of the body.
Dwarf A person with dwarfism.
Dwarfism A range of medical or genetic conditions where a person has
an abnormally low stature or small size, usually with a height
of below 150 cm.
Factor of Safety The ratio of the yield strength of a material to the maximum
stress experienced by a part made from that material.
Finite Element
Analysis (FEA)
A computer program which uses the finite element method to
determine the mechanical properties of a design.
Proportionate
Dwarfism
Dwarfisms where the limbs and abdomen are in proportion,
with the entire body abnormally small.
Rollator Another name for a wheeled mobility walker.
von Mises stress A criterion used to determine when an isotropic material will
yield when subjected to loading.
5. v
5.2 Forward Stability ....................................................................................................19
5.3 Backward Stability .................................................................................................19
5.4 Sideways Stability...................................................................................................19
5.5 Rollator Length .......................................................................................................20
5.6 Design Concepts....................................................................................................20
6 Frame Design.............................................................................................................22
6.1 Finite Element Analysis of First Iteration ....................................................22
6.2 Design Improvements (Second Iteration) .................................................25
6.3 Second Iteration FEA ...........................................................................................25
6.4 Whole Frame FEA .................................................................................................. 27
7 Wheel Design .............................................................................................................32
7.1 Finite Element Analysis of First Iteration ....................................................33
7.2 Second Wheel Design Iteration.......................................................................34
7.3 Second Iteration FEA ...........................................................................................34
8 Wheel Attachment Design....................................................................................36
8.1 Anterior Attachment............................................................................................36
8.2 Anterior Attachment FEA ................................................................................... 37
8.3 Posterior Attachment.......................................................................................... 37
8.4 Posterior Attachment FEA.................................................................................38
9 Mesh Independence Study..................................................................................40
10 Additional Parts .........................................................................................................41
10.1 Walker Handle..........................................................................................................41
10.2 Brake Handle ............................................................................................................41
10.3 Brakes.........................................................................................................................42
11 Final Assembly...........................................................................................................43
12 Discussion ...................................................................................................................45
13 Conclusion..................................................................................................................48
14 References..................................................................................................................49
Appendix I............................................................................................................................53
Appendix II ..........................................................................................................................54
Appendix III .........................................................................................................................55
Appendix IV.........................................................................................................................56
Appendix V.......................................................................................................................... 57
6. vi
ACKNOWLEDGEMENTS
I would like to make known my gratitude towards my project supervisor, Dr
Jennifer Rowson, for her guidance and unwavering support throughout this
project.
I offer my sincere appreciation to the Little People of America, to the Disability and
Dyslexia Support Service of the University of Sheffield, and to Sheffield Teaching
Hospitals for aiding me during my research.
Thanks are due to my parents, Dan and Justin for their advice and instruction, and
to Christina and Joceline for giving me inspiration.
7. 1
1 INTRODUCTION
This project calls for the design of a mobility walker (e.g. Figure 1.1), or rollator, to
be used by adult dwarfs. All anthropometric research for this project with regards
to dwarfism centres around achondroplastic dwarfism as this is the most common
manifestation of dwarfism; studying achondroplasia exclusively provides a definite
research direction. A new product is necessary for the specified user group as the
current walkers provided are designed for either children or adults and are
therefore too weak or too large respectively, as well as lacking other important
features. A strong yet small rollator is the desired project outcome, filling a precise
gap in the current rollator product range. The product could also fill an additional
product range position as a rollator for overweight and obese children who may
have similar needs to those of adult dwarfs.
Figure 1.1 – An example of an anterior rollator (2).
1.1 Aim
The aim of the project is to design an assistive mobility device for achondroplastic
dwarfs and people with similar mobility issues who have more specialised needs
when compared with people of average height.
8. 2
1.2 Objectives
In order to achieve the aim the following objectives were put in place at the
beginning of the project:
The most appropriate design methodology will be selected to provide an
appropriate project model.
The differences between people with dwarfism and people of average
height will be analysed.
A primary product design specification will be created to satisfy the
structural and engineering aspects of the project.
Opinions and suggestions from those who would be using the mobility
device will be obtained in the form of a questionnaire and take an important
advisory role in the design process.
Current solutions for mobility issues will be researched and analysed.
An updated specification will be created upon the receipt of information
from a questionnaire and from anthropometric research.
National guidelines on mobility devices will be taken into account to create a
final marketable product; there is little use creating a product with no
commercial capability.
The overall design concept will be visualised, and an initial structural
assessment will be made.
9. 3
2 BACKGROUND RESEARCH
2.1 British Standards
For this project, British Standards (BS) documents dictate the health and safety
aspects, and therefore legality, of the design of the product. All standards used are
also Euro Norms (EN) – the European product standards – so compliance with
both British Standards and Euro Norms results in Europe-wide acceptance of the
safety of any design. Health and safety limitations will form the basis of the
specification. The following standards were analysed and used in the formation of
the Product Design Specification (PDS):
Assistive products for persons with disability – BS EN 12182:2012 (3);
Safety of machinery – BS EN 614-1:2006 (4).
Walking aids – BS EN 1985:1999 (5);
Walking aids manipulated by both arms – BS EN 11199-2:2005 (6);
Wheeled child conveyances – BS EN 1888:2012 (7);
2.1.1 Overall Dimensions
BS EN 1985:1999 dictates that a rollator should have a width of 650 mm or less for
indoor use.
BS EN 614-1:2009 adds that the dimensions should fit the needs of the user.
BS EN 12182:2012 states that the handles should be between 900 mm and 1200 mm
from the floor.
BS EN 614-1:2009 however allows a lower handle height if more appropriate for the
specific user. This last point is especially relevant when designing for people of
short stature.
10. 4
2.1.2 Wheels
BS EN 1985:1999 dictates that front wheel diameter must be no less than 180 mm
for outdoor use and that the wheels must be wider than 28 mm. There is no
information for rear wheel diameter.
2.1.3 Brakes
BS EN 1985:1999 requires brakes on two wheels which can operate whilst in
motion.
BS EN 11199-2:2005 requires a maximum brake to grip distance of 75 mm, though
BS EN 614-1:2006 allows for specific user needs. The specific needs of the user in
this case will abide by BS EN 11199-2:2005 due to below average finger length.
BS EN 1888:2012 dictates that the parking brakes must be released using at least 50
N of force, or by multiple actions.
BS EN 1985:1999 requires the parking brakes to be two wheeled.
BS EN 11199-2:2005 requires no more than 10 mm/min of movement with full
parking brakes engaged.
2.1.4 Load
All standards state that the load limit must be clearly marked.
BS EN 11199-2:2005 states that for testing “a force of 12.0 N per kilogram of user
mass … shall be applied” (e.g. 75 kg = 900 N).
BS EN 12182:2012 supersedes the above standards due to it being a more recent
publication, instead dictating that the model should be dynamically tested at 105%
of the maximum rated load, with an additional Factor of Safety (FoS) of 1.5.
2.1.5 General Safety
BS EN 12182:2012 requires a rollator of more than 10 kg to include at least two
carrying handles and has several requirements for moving parts to avoid trapping
of body parts.
11. 5
BS EN 11199-2:2005 covers stability requirements, with the rollator being required
to tilt 15o
forward, 7o
backward, and 3.5o
sideways whilst remaining stable. The
standard states that the rollator should be tested with a vertical weight of 250 N
acting upon the midpoint between the centres of the two handgrips, rather than
the rated load.
BS EN 11199-2:2005 also states that handgrips should have a width of between 20
mm and 50 mm, although this can be contested using BS EN 614-1:2006 which
allows for specific user needs.
BS EN 1888:2012 states that all edges should be rounded or chamfered, where
possible.
2.2 Design Approaches
The design approach was designated early in the process in order to effectively
plan further research and development. Five different approaches were analysed
and compared.
2.2.1 Top-Down Approach
This creates a product which is suited to the most specialised individual within the
targeted user cohort, with the product further designed to include more of the
population with less specific needs. A product developed using the Top-Down
approach may be too specialised for use by the general population. This approach
is suitable for a small user cohort with a single specialisation (8).
2.2.2 Bottom-Up Approach
The approach starts with a design for the most general user and then is further
developed to include members of the population with more specific needs. The
result of this approach would most likely exclude the users with the most
specialised needs. This approach is suitable for a large user cohort, with the range
of user specialisations not having great bearing (8).
12. 6
2.2.3 User-Centred Approach
This involves users in the research process to gather information with which the
product will be designed. User input would come in the form of physical and
cognitive activities as well as user opinion. This approach is possibly the most
appropriate for advanced human interaction studies (9).
2.2.4 Activity-Centred Approach
This approach stems from Activity Theory, where users are observed attempting to
reach a goal. The user interaction data provides a design foundation, though with
less user opinion than the User-Centred approach. The Activity-Centred approach
lends itself to studying how humans interact with patterns and systems (9).
2.2.5 Goal-Directed Approach
This is a complex approach involving many user and medical professional
interviews and competitor and literature reviews to obtain research data. This is
perhaps the most comprehensive approach however it is noticeably time and
resource consuming (9).
2.2.6 Approach Selection
All approaches take the form of Inclusive Design (8) where potential users
between the percentiles 5 and 95 with regards to height are considered. In order
to select an approach, project conditions were taken into account.
A strict time duration was given for the project so there was no overreliance on
volunteers. This effectively omitted the User-Centred, Activity-Centred, and Goal-
Directed approaches from selection.
The user cohort is relatively small when compared with the total population and
the users all exhibit dwarfism as a single specialisation. This condition definitively
separated the Top-Down and Bottom-Up approaches.
After taking into account the conditions of the project, the most suitable approach
as described above was the Top-Down approach.
13. 7
2.3 Existing Solutions
Research into currently available rollator solutions allowed the formation of the
gap analysis in Figure 2.1. Gap Analyses are used to compare existing solutions with
market constraints in order to discover a suitable gap by which a new product can
enter the market. In this case the gap analysis shows where the dwarf walker is
specifically needed and can help to further develop the PDS.
In Figure 2.1, current load and height limits for marketed rollators are defined by
the child (blue) and adult (green) regions. The striped overlay represents the adult
dwarf height limits (360-750 mm) and a provisional maximum load of 75 kg.
Justification for these parameters can be found in Section 3. The graph shows that
the height and load requirements of adult dwarfs coincide with those of children
however the width of users is not taken into account. A rollator for an adult dwarf
may differ considerably from one made for a child or small adult of the same
height due to body shape (height and width) and weight considerations, providing
possible areas of further development.
Figure 2.1 - Gap Analysis for current market rollator solutions (10) (11) (12). Blue
regions represent marketed rollators for children. The green region represents
marketed rollators for adults. The red striped region represents the range of dwarf
requirements: a height range of 360-750 mm and a maximum load of 75 kg.
0
25
50
75
100
125
150
300 400 500 600 700 800 900 1000
MaximumLoad(kg)
Handle Height (mm)
14. 8
3 ANTHROPOMETRY
To design the rollator it was necessary to research body dimension and weight
values for adult dwarfs. Dwarfism can be divided between proportionate and
disproportionate dwarfism (13). Those with disproportionate dwarfism have
disproportionately short limbs in comparison to their head and abdomen. People
who have proportionate dwarfism and who require a mobility walker are within
the user cohort of mobility walkers designed for children or small adults. A
mobility walker specifically designed for disproportionate dwarfs requires the
morphing, not shrinking, of current mobility walker designs. For this project, as
stated previously, all research focuses on achondroplasia: a disproportionate type
of dwarfism. However, during the research, it was found that there was very little
information with regards to limb dimensions. As a result, much of the accrued data
is based upon the anthropometric relationship between children and adult dwarfs.
The height limits for the user have been obtained from the journal paper Genetics
of Achondroplasia (14), with 1140 mm and 1470 mm as minimum and maximum
user heights respectively. These heights are represented by black vertical lines in
Figures 3.1-3.4.
Anthropometric data from the report titled Physical Characteristics of Children
(15) provided a base upon which the graphs in Figures 3.1-3.4 were created. Dwarf
limb and head proportions provided by Sanford Research (16) allowed the
conversion of the data from Physical Characteristics of Children (15) to better
represent the dwarf population; dwarf limbs are four fifths the length of average
height limbs.
3.1 Height and Limb Dimensions
To determine the maximum and minimum handle heights of the walker, data was
generated through the combination of information from Physical Characteristics of
Children (15) and Sanford Research (16) to simulate a range of postures. For the
minimum height, Figure 3.1 visualises the distance between the hand of a child and
the floor when the arm is by the child’s side. Also, for the minimum height, Figure
15. 9
3.2 represents the distance between the crotch of a dwarf and the floor when their
legs are straight. Figures 3.1 and 3.2 were both created to find the lowest rollator
height required, with Figure 3.1 taking precedence as it resulted in a lower height.
For the maximum height, Figure 3.3 shows the distance between a dwarf’s hand
and the floor when the dwarf’s upper arm is by their side and their lower arm at
70o
to the upper arm. This stance reflects the highest probable hand height of a
mobility walker user. The series labels in Figures 3.1 – 3.4 represent height
percentiles for children and dwarfs, depending on the figure. For example, "Child
5%" represents results for children in the 5th height percentile. The child/dwarf
stances described are shown in Appendix I.
Figure 3.1 – Height vs distance between the hand and floor of children with their arms
by their sides. Data source: (15).
Figure 3.2 – Height vs distance between the crotch of a dwarf and the floor. Data
source: (15), (16).
300
350
400
450
500
550
600
1000 1100 1200 1300 1400 1500 1600 1700
ChildHandtoFloor(mm)
Height (mm)
Child
5%
Child
50%
Child
95%
300
350
400
450
500
550
600
650
700
1000 1100 1200 1300 1400 1500 1600 1700
DwarfCrotchHeight(mm)
Height (mm)
Dwarf
5%
Dwarf
50%
Dwarf
95%
16. 10
Figure 3.3 – Height vs hand to floor distance when upper arm adjacent to the body
and lower arm at 70
o
to the upper arm. Data sources: (15), (16).
The lowest and highest rollator handle heights are represented in Figures 3.1 and
3.3 respectively. The upper limit for the highest posture gives a maximum handle
height of 750 mm. The lower limit for the lowest posture gives a minimum of 360
mm.
3.2 Width
Width plays an important role in determining the specification of the mobility
walker. Due to the large dimension variations in adult dwarfs, and in keeping with
the Top-Down design approach, a mobility walker with a fully adjustable width
would be most appropriate.
Figure 3.4 represents the width of the user though only data from children was
found throughout the research stage. This source initially gave a user width of 190
mm – 260 mm.
Figure 3.4 – Height vs child lower torso breadth. Data source: (15).
500
550
600
650
700
750
800
850
900
1000 1100 1200 1300 1400 1500 1600 1700
DwarfUpperHandLmit
(mm)
Height (mm)
Dwarf
5%
Dwarf
50%
Dwarf
95%
150
175
200
225
250
275
300
325
1000 1100 1200 1300 1400 1500 1600 1700
LowerTorsoBreadth(mm)
Height (mm)
Child
5%
Child
50%
Child
95%
17. 11
The above limits are not representative of dwarfs whose width proportions do not
translate in the same way as height and limb proportions.
Figure 3.5 shows a range of rollators compared by handle height and width. The
black line represents the maximum handle height for the user cohort. After
interpolating the graph, a sensible external frame width would be 580 mm.
Figure 3.5 – Rollator handle height vs external frame width. Data sources: (10), (11),
(12).
The external frame width above suggests that a sensible maximum internal rollator
width would be 480 mm when taking into account the camber of the device. The
minimum internal width should be 250 mm to ensure all users are encompassed,
although this lower width is only a guideline.
3.3 Weight
The height against weight curves of adult male achondroplastic dwarfs are shown
in Figure 3.6, taken from the American Journal of Medical Genetics (17). In the
graph, as replicated online by Medscape (14), the maximum weight recorded is 80
kg, recorded for percentile 97.5, although the graph continues above 80 kg for
higher percentiles. 80 kg is outside the previously stated user percentiles, so the
maximum load value shall be 75 kg as this represents users up to and inclusive of
the 95th percentile.
500
540
580
620
660
700 750 800 850 900
FrameWidth(mm)
Handle Height (mm)
Adult
Rollator
Child
Rollator
18. 12
Figure 3.6 – Adult male achondroplastic dwarf heights vs weight. Central line:
percentile 50. Dashed lines: percentile 2.5 (lower) and percentile 97.5 (upper).
Data source: (17).
3.4 Walker Type
There are two main options for walker type when designing a rollator: anterior and
posterior (18). Both walker types would be suitable for the requirements of this
project although there are some key differences.
The anterior rollator is positioned in front of the user and is pushed. It is used
more by the elderly and those who are not destined to regain their independent
walking ability. This type of rollator is gripped and followed.
The posterior rollator surrounds the rear of the user and is pulled. It is used more
frequently by people who are recovering from injury and young people with
mobility issues. This type of rollator is rested-upon.
For this project it is believed that the posterior walker will give more benefits to
the user because they should be maintaining their mobility rather than declining
and in particular young users should see the benefit of a regenerative posterior
walker.
Another reason for the selection of a posterior rollator is the finger length of
dwarfs. Shorter than average fingers cause gripping issues (19) so a posterior
walker upon which the user would rest seems to be a more suitable choice.
19. 13
4 PRODUCT DESIGN SPECIFICATION
A Primary Specification was initially devised to give an overall framework to the
design. It was formed using information taken from the Research and
Anthropometry sections of the project.
4.1 Primary Specification
The rollator should take the form of a posterior rollator.
The product handles should be adjustable with a minimum handle height of 360
mm, and a maximum height of 750 mm. Clearly visible limit markings should be
present.
The maximum user rated load is set as 75 kg and should be marked as such. The
rollator should be designed with a minimum Factor of Safety of 1.5 (3). The
product should be physically tested using 785 N as this is 105% of the rated load
and should exhibit no permanent deformation or failure.
The rollator should have a minimum internal width of 250 mm and a maximum
internal width of 480 mm.
The wheels should have a minimum width of 28 mm. The front wheels should be at
least 180 mm in diameter (5). The rear wheels will use the same design as the front
wheels.
The rollator should have rear braking as a minimum and a locking parking brake.
Both types of braking system should comply with aforementioned British
Standards.
The rollator should conform to BS EN 11199-2:2005 with regards to stability.
4.2 Secondary Specification
From forums provided by the Little People of America (20), information has been
obtained with regards to user preferences. People within the user cohort stated
that the most important features of a mobility walker were ease of use, folding
capabilities, and overall walker weight.
20. 14
Designing a lightweight rollator would come at the expense of an incorporated
seat. For smaller users an incorporated seat strong enough to support a 75 kg user
would be cumbersome so the omission of a seat in favour of a lightweight design is
justified.
It was also suggested that the walker should be able to operate both indoors and
outdoors. This suggestion has already been provided for in the Primary
Specification.
Due to the vast range of user heights, it would be sensible to separate the users
into two sub-cohorts: users requiring a handle height from 360 mm to 560 mm,
and those requiring handles at a height of between 550 mm and 750 mm, with a
slight overlap to allow for user preference. This would also allow for more relevant
folding capabilities, with the smaller rollator folding much smaller than the larger
rollator, making it easier to transport.
For the purposes of this project, the design will focus on the larger of the two
rollators. The smaller rollator could be designed using the findings from this
project or by directly scaling down the final design. Issues in scaling down the final
design arise as some components such as the wheels must maintain their
diameter.
4.3 Materials
Whilst the rollator designed is not going to be prototyped during this project,
materials are needed for loading simulations and weight calculations.
4.3.1 Frame Material
To create a successful rollator, the material selected should have both high tensile
and high yield strengths. A metal alloy is the most obvious choice, such as an
aluminium alloy or steel, however fibre composites should also be considered. In
Figures 4.1 and 4.2, the aforementioned materials are compared using the program
CES Selector (21); Figure 4.1 shows Yield Strength vs Density and Figure 4.2 shows
Price vs Density.
21. 15
Figure 4.1 – Yield strength vs density for frame material selection, with suggested
materials labelled (21).
Figure 4.2 – Price vs density for frame material selection, with suggested materials
labelled (21).
22. 16
The most practical choice of material is an aluminium alloy due to its impressive
yield strength, low density, and cost effectiveness. Carbon fibre proves too
expensive and steel too dense for this application.
A sensible aluminium alloy would be Aluminium 6061-T6 due to it being readily
available and a common material used for bicycle frames. Material properties for
Aluminium 6061-T6 are shown in British Standard EN 485-2:2013 (22).
4.3.2 Wheel Material
The wheels need to be durable, cost effective, and lightweight in order to fulfil
their purpose. The most logical choice of material is a polymer. Due to the
limitations of the software chosen only a small selection of polymers are suitable
for loading simulations. As a result, the most appropriate polymer with which the
simulation should take place is Nylon 6/10.
Possible future materials for consideration are shown below in Figures 4.3 and 4.4.
Using CES Selector, Figure 4.3 plots yield strength against density, and Figure 4.4
shows hardness against fracture toughness.
Sensible choices for the wheel material include ABS, Nylon 6/10, polypropylene,
and epoxy/carbon fibre composites. ABS would provide a cost effective solution,
however its performance under loading remains to be seen.
23. 17
Figure 4.3 – Yield strength vs density for wheel material selection, with suggested
materials labelled (21).
Figure 4.4 – Hardness vs fracture toughness for wheel material selection, with
suggested materials labelled (21).
24. 18
5 INITIAL DESIGNS
For simulation, the rollator design was split up into several components: frame,
wheel, wheel attachment and other (brakes, handles, brake handles). The overall
750 mm height of the rollator is divided giving 500 mm for frame height and 250
mm for combined wheel and wheel attachment height. The wheel must maintain a
diameter of 180 mm.
The initial design stage dealt with the mechanical and physical properties of the
design. Specifications were written using anthropometric and user data however
they do not take into account the stability of the rollator, as explained in BS EN
11199-2:2005.
The rollator should feature a lightweight design, therefore instead of focusing on
the centre of mass of the rollator, the focus should be on the combined user and
rollator centre of mass or possibly the centre of mass of the user only. To achieve
this, the position of the handles in relation to the wheels/wheel axles should be
prioritised.
5.1 Software
All designs were created using the CAD software SolidWorks 2012 (23), supplied by
the University of Sheffield. The reasons for this choice are previous experience
with this software, its availability, and the loading simulation package that
accompanies the software. The simulation package allows Finite Element Analysis
(FEA) of the design. This is a valuable tool which analyses design structure and
provides design justification before the prototyping stage through meshing.
Meshing works by tessellating the surface of the design so that stresses and
displacements can be solved using algorithms.
The software processes the probable maximum von Mises stresses and
displacements within any part when under loading. The von Mises stress is a
criterion used to determine when an isotropic material will yield when subjected
to loading. It also calculates the Factor of Safety throughout the part, allowing
areas of weakness to be easily identified. The Factor of Safety in this instance is
25. 19
defined as the Yield Strength of the design material as a proportion of the
maximum von Mises stress experienced during simulation. A FoS value greater
than 1 signifies that the design should withstand a given static load.
5.2 Forward Stability
The rollator must be able to stay upright when tilted forward by 15o
whilst loaded
with 250 N (6). To achieve this, the front wheel axle of the rollator must not fall
within 15o
of the line perpendicular to the handles. The rollator has an overall
height of 750 mm, so taking into account the distance from the floor to the wheel
axle (90 mm) the relevant vertical distance is 660 mm. Therefore the horizontal
distance between the front wheel axle and the handles in the -plane should be at
least 177 mm, as shown in the left diagram of Figure 5.1.
5.3 Backward Stability
The rollator must be able to stay upright when tilted backwards by 7.5o
and loaded
with 250 N simultaneously. To achieve this, the rear wheel axle of the rollator must
not fall within 7.5o
of the line perpendicular to the handles. Using the same
calculations as in Section 5.2 the vertical distance between the handles and the
rear axle is 660 mm. Therefore the horizontal distance between the front wheel
axle and the handles in the -plane should be at least 87 mm, as shown in the
central diagram of Figure 5.1.
5.4 Sideways Stability
Testing for sideways stability is slightly different to the previous two tests. For this
test the relevant vertical distance is the overall height of the rollator of 750 mm.
The rollator must be able to stay upright when tilted sideways by 3.5o
and loaded
with 250 N simultaneously. To achieve this, the point of contact between the wheel
and the floor must not fall within 3.5o
of the line perpendicular to the handles.
Therefore the horizontal distance between the wheel and the handles in the -
plane should be at least 46 mm, as shown in the right hand diagram of Figure 5.1.
26. 20
This calculation gives a minimum stable rollator width of 92 mm therefore the
minimum width given in the Primary Specification (Section 4.1) of 250 mm would
also be stable when tested.
Figure 5.1 – Forward, backward and sideways stability calculation diagrams, left to
right respectively.
5.5 Rollator Length
Differing from the other rollator dimensions which are dictated by
anthropometrics or British Standards, the rollator has no fixed length. From the
forward and backward stability calculations a minimum rollator length can be given
as 444 mm (taking into account the full wheel diameters). Current rollator
solutions (10) (11) (12), including those for children, barely differ in their lengths
regardless of user height, all with a length of around 600 mm.
5.6 Design Concepts
Two frame design concepts shown in Figure 5.2 were created taking into account
the specifications and the stability parameters. Both vary their height using
overlapping tubing and quick-release clamps akin to those used for bicycle seats.
For the 200 mm user height range previously mentioned, 200 mm of straight inner
tubing and 200 mm of straight outer tubing above it would be required.
27. 21
Figure 5.2 – Frame design concepts.
When choosing between the two designs, factors including stability, structural
integrity and ease of manufacture were considered. Design A would be more stable
when tilted forward and backward. Any improvements to Design B would create a
design similar to Design A. The structure of Design A is more consistent with
current rollators and more versatile. Design A demonstrates the ability to change
handles, something that could be very useful for the user due to shorter than
average finger length. The modern appearance of Design B would possibly be more
desirable for the user. The manufacture process of both designs would be similar
however the curvature of Design B could cause difficulties. Using these
justifications, Design A was selected for further development.
28. 22
6 FRAME DESIGN
The first frame design iteration featured a handle system where the handle would
rotate 90o
between the vertical and horizontal, simultaneously changing the height
and width of the rollator. This feature was included on the premise that the taller
the user, the wider they are. An X-shaped rear structure (shown in Figure 5.2)
would allow the design to fold.
The design in Figure 6.1 shows half the frame with the handles both fully vertical
and fully horizontal. The handles would be removable however for the purposes of
simulation the design included handles.
Figure 6.1 – Frame design in vertical and horizontal handle configurations.
The design uses tubes with a thickness of either 1 mm or 2 mm depending on their
location. Tubing overlaps in areas where movable features would be implemented
(leg extensions, handles, handle rotating base).
The frame has a negative camber of 5o
to provide additional stability for the user.
6.1 Finite Element Analysis of First Iteration
For the FEA, a flat region was integrated into the design on top of each handle to
allow for suitable loading conditions. The frame was loaded in both configurations
with 785 N, and fixtures positioned at the base of each leg and where an X-shaped
structure would connect to the other half of the frame (see Figure 6.2). From the
material selection available, Aluminium 6061-T6 was selected (see Appendix II for
material properties). Only half of the main frame was simulated due to the
29. 23
symmetry of the rollator. Simulating only half of the design allowed more
fundamental analysis of the loading.
Figure 6.2 – Loading arrangements for 1st iteration FEA.
The loading simulation for the vertical handle configuration resulted in a minimum
Factor of Safety of 9.83; an over cautious design. The results for the vertical handle
simulation are shown in Figure 6.3. In Figure 6.3, the left hand image shows von
Mises stress and the right hand image shows displacement (highly exaggerated),
both under full loading, with their respective scales to the right of each image. All
further simulation results will follow this template.
Figure 6.3 – Simulation results for vertical handle configuration of the 1st iteration.
The horizontal handle configuration gave a FoS of only 1.36 which falls below the
British Standards requirements (3). The simulation results (Figure 6.4) show the
30. 24
high von Mises stress and large displacement that the structure experienced. The
regions in which the design had a low FoS are shown in Figure 6.5.
Figure 6.4 – Simulation results for horizontal handle configuration of the 1st iteration.
Figure 6.5 – Low FoS regions for horizontal configuration of the 1st iteration. Red
regions signify a FoS lower than 2.00.
Whilst the design could have been reinforced by thickening the tubing this would
go against the lightweight design ideology, an important factor for the user. A
support below the handle when horizontal would have been impractical to
implement. Whilst the overall rotating handle design was deemed unsuitable, the
vertical handle configuration showed advantageous characteristics and was used
as the basis of the 2nd design iteration.
31. 25
6.2 Design Improvements (Second Iteration)
Instead of rotating the handles to adjust the rollator width, tubing between the two
sides of the frame could be fully adjustable for use, rather than just adjustable for
storage. This design is visualised in Figure 6.6. Whilst the tubing connecting the two
sides of the frame would be split into three overlapping parts of different
diameters. For the purposes of simulation a single tube with constant diameter at
full extension was used. The 2nd iteration maintains the previously mentioned 5o
negative camber.
Figure 6.6 – 2nd iteration in fully extended configuration.
6.3 Second Iteration FEA
As with the previous FEA only one half of the design has been simulated. Again the
frame was vertically loaded with a force of 785 N and fixtures were placed at the
leg bases and where the horizontal tubing would connect to the other half of the
frame (see Figure 6.7).
32. 26
Figure 6.7 – Loading arrangement for the 2nd iteration FEA.
The lowest Factor of Safety of the 2nd iteration was 6.23. This FoS is much larger
than that required by British Standards however a large FoS is beneficial due to the
nature of the simulation where the design is simulated as a single part.
Figure 6.8 – Low FoS regions for the 2nd iteration. The red region signifies a FoS
lower than 8.00.
The displacement and von Mises stress simulation results are shown in Figure 6.9.
The greatest displacement experienced is 0.138 mm as opposed to 3.243 mm
experienced by the horizontal handle configuration of the 1st iteration.
33. 27
Figure 6.9 – Simulation results for the 2nd iteration.
The 2nd iteration experiences a maximum von Mises stress of around 44 MPa, with
202 MPa the maximum stress experienced by the horizontal handle configuration
of the 1st iteration. Due to these excellent results the improvements made were
taken forward in the design process.
6.4 Whole Frame FEA
The successful 2nd iteration design was then tested as a full frame in two loading
arrangements. The full frame configuration is shown in Figure 6.10.
Figure 6.10 – Full frame configuration for the 2nd iteration.
Whilst testing on the frame has so far featured a force acting vertically upon the
handle, other situations need to be considered to fully appreciate all loading
34. 28
possibilities. Instead of vertical loading, force was applied at 45o
from vertical (see
Figure 6.11). This accounted for a situation in which the user might push out on the
frame. The same 785 N load was used as before, although it was distributed evenly
between the two handles instead of 785 N upon each handle. This loading
arrangement fully complies with the primary specification set in place.
Figure 6.11 – Loading arrangement for the 2nd iteration in full frame configuration
with loads acting at 45
o
from vertical.
The lowest Factor of Safety of the full frame 2nd iteration was 3.19 (see Figure
6.12). This was deemed too low so a slight adjustment was made to the frame
before completing the simulation, creating the 3rd iteration. The 3rd iteration
design is shown in Figure 6.13. Improvements include a thickening around the
tubing in the area of weakness highlighted in Figure 6.12, and the increase in the
fillets of some angles.
Figure 6.12 – Region of lowest FoS for the 2nd iteration in a full frame configuration
with loading at 45
o
from vertical. The red region represents a FoS lower than 4.00.
This region of weakness is mirrored on the other side of the frame.
35. 29
Figure 6.13 – Adjustment to the 2nd iteration. The left image shows the 3rd iteration
with an adjustment made around the upper rear bar. The right image shows the 2nd
iteration. The adjustment is mirrored on the other side of the frame.
The 3rd iteration frame was then tested, with forces applied at both 45o
and 315o
from vertical. These simulated pushing and pulling on the frame, and also
accounted for a situation in which the user might collapse and hold on to the
rollator for support. These loading arrangements are shown in Figure 6.14.
Figure 6.14 – Loading arrangements for the 3rd iteration in full frame configuration
with loads acting at 45
o
(left) and 315
o
(right) to vertical.
The frame, when loaded at 45o
, had a FoS of 3.58, whilst the 315o
loading gave a FoS
of 3.48 (see Figure 6.15). The lowest FoS occurred in the same location for both
loading arrangements; slightly lower than the 2nd iteration (Figure 6.12).
36. 30
Figure 6.15 – Region of lowest FoS for the 3rd iteration in a full frame configuration
with loading at 315
o
from vertical. The red region represents a FoS lower than 4.00.
This region of weakness is mirrored on the other side of the frame.
The results of the simulations for 45o
and 315o
loading can be seen in Figure
6.16 and Figure 6.17 respectively.
Figure 6.16 – Simulation results for the 3rd iteration when loaded at 45
o
from vertical.
Figure 6.17 – Simulation results for the 3rd iteration when loaded at 315
o
from
vertical.
Both loading arrangements experienced similar maximum von Mises stresses and
displacements, even though the displacements occurred in opposite directions.
37. 31
The FoS experienced by the frame in its most compromising arrangement (315o
loading) of 3.48 is more than twice the FoS of 1.5 required by BS EN 12182:2012.
Having such a high FoS in CAD is advantageous as the simulations do not take into
account joints, welds, and other areas of weakness which would all most likely
reduce the overall FoS of the design.
The successful loading simulation of this frame completes the Frame Design
section. The frame fulfils all requirements of the specifications with regards to size
and function. Stability analysis involves the entire frame, shown in Section 11. To
summarise this section, a comparison of all frame iterations is shown in Table 6.1.
Table 6.1 – Frame iteration minimum FoS and maximum displacement values.
Iteration Arrangement Minimum FoS Displacement (mm)
1st Vertical half frame 9.83 0.132
1st Horizontal half frame 1.36 3.243
2nd Half frame 6.23 1.383
2nd Full frame 3.19 –
3rd Full frame 45o
3.58 3.831
3rd Full frame 315o
3.48 3.839
38. 32
7 WHEEL DESIGN
There is little variation in the design of rollator wheels, even with the most striking
designs following a standard spoke structure (24), involving between three and
nine spokes. Most wheels feature a pneumatic tyre or plastic cushioning for
comfort. In this design no form of cushioning has been included due to the
software constraints. Whilst the wheel designed can be used independently of any
form of cushioning, either a rubberised, solid plastic tyre or a non-pneumatic tyre
could be used (25). Tyre examples are found in Appendix III.
The original intention was to create a novel, artistic wheel concept with curved
spokes however this proved to be too weak when subjected to the heavy loads. A
more basic approach was taken to gauge the areas of weakness within a wheel.
The left hand image in Figure 7.1 shows the 1st wheel design iteration with a
diameter of 180 mm and a width of 30 mm in line with specification requirements.
For the FEA, the wheel was loaded in two arrangements. The first arrangement (0o
loading), shown in Figure 7.1 (centre image), had the force acting directly upon one
spoke. The second arrangement (45o
loading) had the force acting between two
spokes, as shown in the right image of Figure 7.1. The wheels were subjected 785 N
which is four times the 75 kg rated load however this was deemed necessary to
take into account all possible loading conditions, inclusive of when the entire user
weight passes through one wheel. From the material selection available, Nylon 6/10
was selected (see Appendix II for material properties). The wheel design is one
single part with no internal air pockets.
Figure 7.1 – 1st design iteration and loading arrangements.
39. 33
7.1 Finite Element Analysis of First Iteration
The design performed well at 0o
loading with a minimum FoS of 13.70, occurring at
the top of the spoke directly below the axle region. A FoS of 2.53 was recorded for
45o
loading (see Figure 7.2).
Figure 7.2 – Region of lowest FoS for the 1st iteration with 45
o
loading. The red region
represents a FoS lower than 3.00.
The greatest displacement of any region of the wheel during 0o
loading was 0.054
mm (see Figure 7.3), with 2.289 mm displacement for 45o
loading (see Figure 7.4).
The von Mises stress results were acceptable although the displacement for 45o
loading was excessive, requiring improvements to the design.
Figure 7.3 – Simulation results for the 1st iteration with 0
o
loading.
40. 34
Figure 7.4 – Simulation results for the 1st iteration with 45
o
loading.
7.2 Second Wheel Design Iteration
Due to the weaknesses of the 1st iteration when tested at 45o
, both ribs and fillets
were required to more evenly distribute the stresses. The 2nd iteration was loaded
at 45o
. Loading at 45o
puts the wheel in the most compromising situation and
should give the most accurate results for maximum stress, maximum
displacement, and minimum FoS. Both an improved, more aesthetically pleasing
wheel design (2nd iteration) and the loading arrangement are shown in Figure 7.5.
Figure 7.5 – 2nd iteration and loading arrangement for the improved wheel design.
7.3 Second Iteration FEA
The loading simulation of the 2nd iteration showed a minimum FoS of 5.24. This is a
very promising value, especially as the loading arrangement puts the most stress
possible upon the wheel, with the wheel taking the entire rated load of the rollator.
The lowest FoS occurred on the inside of the wheel rim, as shown in Figure 7.6.
41. 35
The simulation results for von Mises stress and displacement are shown in Figure
7.7.
Figure 7.6 - Region of lowest FoS for the 2nd iteration loaded at 45
o
from vertical. The
red region represents a FoS lower than 7.00.
Figure 7.7 – Simulation results for the 2nd iteration.
The design is well within the yield strength (139 MPa) when loaded and has an
acceptable maximum displacement of 0.380 mm. As stated for the final frame FEA
in Section 6.4, the 2nd wheel design iteration complies with all British Standards
and all specifications therefore is the accepted final wheel design. To summarise
this section, a comparison of all frame iterations is shown in Table 7.1.
Table 7.1 – Wheel iteration minimum FoS and maximum displacement values.
Iteration Arrangement Minimum FoS Displacement (mm)
1st 0o
arrangement 13.70 0.054
1st 45o
arrangement 2.53 2.289
2nd 45o
arrangement 5.24 0.380
42. 36
8 WHEEL ATTACHMENT DESIGN
The wheel attachments are components which connect the frame to the wheels.
They secure the axles about which the wheels rotate, and they encase the feet of
the frame. Their loading has been simulated using the same Aluminium 6061-T6 as
used for the frame. The anterior attachments also house the parking brakes. All
wheel attachments connect to the frame with a negative camber of 5o
whilst
securing the axle with zero camber.
8.1 Anterior Attachment
The anterior attachments connect the frame to the front wheels. The anterior
attachments rotate, allowing the user to steer the rollator. As previously stated, the
attachment and wheel combined have to have a height of 250 mm or lower, and
with a wheel radius of 90 mm that leaves 160 mm for the height of the anterior
attachment. Figure 8.1 shows two anterior attachment designs. Their ability to
rotate under loading has not been simulated due to software limitations.
Figure 8.1 – Anterior attachment designs.
Both designs from Figure 8.1 fulfil the specification requirements although Design B
would move the centre of mass of the walker too far backwards, resulting in
noncompliance with BS EN 11199-2:2005. Design A overcomes this issue by housing
the brakes within an enclave of the attachment.
43. 37
8.2 Anterior Attachment FEA
The loading arrangement for the selected anterior attachment is shown in Figure
8.2. The attachment was loaded with 785 N to simulate the same conditions as
described for the wheel.
Figure 8.2 – Loading arrangement for the anterior attachment.
The results of the simulation showed a minimum FoS of 2.16. This value complies
with British Standards and is a simulation for a very unlikely, highly compromising
situation. The simulation results for von Mises stress and displacement are shown
in Figure 8.3. This design would fulfil the multiple functions performed by the part.
Figure 8.3 – Simulation results for the anterior attachment design.
8.3 Posterior Attachment
A different design is required for the posterior attachments as they are not used
for steering although the same 160 mm height constraint applies. The posterior
44. 38
attachment must instead support the rear brakes which can be used whilst in
motion. The selected design and loading arrangement are shown in Figure 8.4.
Figure 8.4 – Posterior attachment design and loading arrangement.
8.4 Posterior Attachment FEA
The posterior attachment design was also loaded with 785 N to simulate the worst-
case scenario, where the entire load is transferred through one wheel. The lowest
FoS for the design was 4.61 (see Figure 8.5). The simulation results, shown in Figure
8.6, show encouraging values meaning that neither further action nor
improvements are necessary for the posterior attachment.
Figure 8.5 – Region of lowest FoS for the posterior attachment. The red region
represents a FoS lower than 5.00.
45. 39
Figure 8.6 – Simulation results for the posterior attachment design.
Table 8.1 – Wheel iteration minimum FoS and maximum displacement values.
Iteration Attachment Minimum FoS Displacement (mm)
1st Anterior 2.16 0.357
1st Posterior 4.61 0.070
46. 40
9 MESH INDEPENDENCE STUDY
For the verification of the FEA results and the justification of the software used, a
mesh independence study is required. To perform a loading simulation of a part,
triangular tessellation of the surface of that part is required. The results from all
triangles – also known as elements – are combined to produce the simulation
results, such as those shown in previous sections. More elements means a finer
mesh, but also a longer simulation time.
Although it would seem more appropriate to analyse the mesh used for the frame,
this is not possible as the meshing of the frame was only successful with the finest
possible mesh. Instead the anterior attachment design was used. The results for
the study are shown in Figure 9.1.
Figure 9.1 – Mesh independence study for the anterior attachment.
In ideal circumstances the design simulation would achieve "mesh independence".
This is where the Number of Elements no longer has a bearing upon the calculated
FoS. Unfortunately, mesh independence was not achieved due to the limitations of
the software, however as the graph has neared a plateau, the results obtained
should be deemed sufficiently accurate for this project. A suggested progression of
this design concept would be to use a specialised meshing and simulation program
for a more thorough analysis.
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
0 10000 20000 30000 40000 50000 60000 70000
FactorofSafety
Number of Elements
47. 41
10 ADDITIONAL PARTS
The rollator final assembly also included some parts which have not been
mechanically analysed. This is due to the ambiguity in terms of their loading and
placement. The sizing of all parts complies with the British Standards from Section
2 however more anthropometric information is necessary to validate the designs.
10.1 Walker Handle
No reliable information for hand and finger dimensions of disproportionate dwarfs
could be found. This means that only an example handle has been designed. The
overall design of the frame allows for handles to be interchanged, so once the hand
dimension information has been gathered, any new handle designs can be
incorporated into the current frame design. The default handle design is shown in
Figure 10.1 alongside a frame without connected handles.
Figure 10.1 – Default handle design and a handle-less frame with example handle.
10.2 Brake Handle
The anthropometric data available is insufficient to determine the correct length of
a brake handle or to determine a suitable handle tension. The brake handle is
adjustable in the vertical direction – as shown in Figure 10.2 – however further
studies are required, similar to the situation of the walker handle.
48. 42
Figure 10.2 – Brake handle connected to frame showing direction of adjustment.
10.3Brakes
The brakes are similar to those used on existing rollator solutions (26). They
operate via an internalised cable system although a hydraulic system could be used
such as those on high specification bicycles (27). This decision would alter
operational and economic factors, with hydraulic brakes around five times more
expensive than cable brakes (27). Further discussion is required to determine the
mechanical suitability of each system. Additional factors affecting the brake design
include the operational height of the rollator and the wheel/tyre material.
49. 43
11 FINAL ASSEMBLY
All components from previous sections, when combined, form the final assembly,
bringing together all findings from the project. To demonstrate the adjustment
capabilities of the concept, three configurations are demonstrated in Figure 11.1.
Figure 11.1 – Three configurations of the final concept demonstrating height and width
settings. Left image: full leg extension, maximum width. Central image: full leg
extension, minimum width. Right image: full leg retraction, minimum width.
At full leg extension, the walker stands at 750 mm from the ground. This height can
be reduced to 550 mm with the retraction of the legs. The walker has a maximum
width of 710 mm however the handle width (distance between handles) is more
relevant for the user and has a range of 225 mm – 525 mm. The walker varies in
length between 650 mm and 770 mm depending on the leg extension setting. The
height can also be altered with handle modifications. Appendix IV shows a full
dimensioning of the rollator.
The assembly can be used in all configurations although the minimum width would
most likely only be used for transport and storage. To comply with BS EN 11199-
2:2005, the rollator is required to tilt 15o
forward, 7o
backward, and 3.5o
sideways
whilst loaded with 250 N upon the handles, and remain stable throughout. The best
configuration to test is the 'full leg extension, minimum width' configuration shown
in the central image of Figure 11.1 as it is the least stable of the possible
configurations. The compliance of the design with BS EN 11199-2:2005 is shown in
Figure 11.2. Lines have been drawn from the overall centre of mass at the angles
50. 44
defined by the British Standard. The lines are required to fall inside the wheel axes
to demonstrate that the concept would be stable. As the lines fall inside the axes,
the concept also abides by the interpretation of the aforementioned standard set
out in Section 5.4.
Figure 11.2 – Centre of mass investigation. Left image: sideways stability. Right image:
forward and backward stability.
The overall mass of the rollator is 3.920 kg. Each wheel weighs 0.314 kg if Nylon
6/10 with a density of is used, giving a total wheel mass of 1.256 kg. The
remainder of the rollator was assumed to consist of Aluminium 6061-T6 with a
density of , giving a mass of 2.664 kg. This overall mass is considerably
lower than the masses of current solutions, further discussed in Section 12.
51. 45
12 DISCUSSION
This project highlights the lack of a specialised mobility walker for adult dwarfs.
Mobility walkers currently used by adult dwarfs are designed either for children or
small adults. Consequently the designs of these mobility walkers often do not take
into account adult dwarf necessities such as the possible adjustment of mobility
walker width, mobility walker handle height, brake handle distance, and handle
diameter.
Dwarfism has many types, primarily divided into proportionate and
disproportionate dwarfisms; disproportionate dwarfs have disproportionately
short limbs in comparison to their head and abdomen. People with proportionate
dwarfism who require a mobility walker fall within the user cohorts of mobility
walkers designed for children and small adults. A mobility walker specifically
designed for disproportionate dwarfs does not require shrinking, but instead
morphing of current mobility walker designs.
As there are currently no mobility walkers designed specifically for adult dwarfs,
very little data related directly to the project could be sourced. With this in mind,
the project consisted of a large research component in order to calculate
previously unrecorded data. The majority of the anthropometric data was
calculated using the relationships between average height children and adult
dwarfs. This generated data for the initial concept stage although ideally this would
not be the case; instead anthropometric data would be collected directly from
adult dwarfs.
This paper acts as the initial step in the design of a mobility walker for adult
dwarfs. Whilst a design has been created and tested computationally, the real
result of the project is determining the necessities of the adult dwarf population.
The project draws attention to the areas where further development is necessary
and provides a platform from which further discussion may take place.
Overall the results of this project imply that the adult dwarf population has specific
unmet needs, previously shown in Figure 2.1. Furthermore, this paper implies that
52. 46
the lack of anthropometric data hinders the progression of the design process
from concept to prototyping and beyond to manufacture.
The final design, and especially its FEA component, may benefit other user cohorts
with mobility issues. The rollator weighs 3.920 kg which is under 60% of the weight
of an average mobility walker (28) (29), and 40% of the 9.3 kg weight of an
advertised "Lightweight Rollator" (30). This lightweight design could benefit the
designs of paediatric walkers, walkers for shorter average height people, and
walkers for people with a low body mass. The last cohort, users with a low body
mass, may include those suffering from cerebral palsy, who may find it difficult to
move around with a walker designed for people with a body mass of up to 125 kg.
Whilst a quantitative comparison of existing mobility walker solutions is shown in
Figure 2.1, a visual comparison of the sizes of the project concept and existing
rollator solutions is shown in Appendix V.
The final design should be recognised as an interpretation of the more significant
background research and anthropometric findings of this project. It should be
assumed that if further research and development is undertaken by another party,
they will not continue with the design produced in this project. Further uses of the
final design are limited to inspiration and implementation after prototyping and
testing.
The final concept, shown in Figure 12.1, fulfils the user cohort requirements as set
out in the specification of this project. The concept includes many features
specifically designed for adult dwarfs. This sets the concept apart from existing
solutions which are occupied by adult dwarfs to only partially fulfil their needs.
Although the posterior type of walker was selected for this project, if the user so
wishes they can use the rollator in an anterior fashion. This capability is due to the
uniform fashion of the design. The removable handles allow the brakes to change
positions so fully configuring the rollator for anterior use. Depending on the
preferences of the user the front and back wheel attachments are interchangeable
so a walker could have front and/or rear steering in both anterior and posterior
configurations. Both configurations equally comply with the specifications.
53. 47
Figure 12.1 – Final Assembly at full extension.
Specialised design features include:
Varying walker handle width from 225 mm to 525 mm.
Handle height variation from 550 mm to 750 mm for the larger model.
Removable handles to adjust handle height and handle grip diameter.
Posterior design to take into account the lack of user dexterity.
Adjustable brake-handle height independent of handle height.
Lightweight design of 3.920 kg.
Compact folding method.
The final design would fulfil adult dwarf necessities and is much more appropriate
for adult dwarfs than the mobility walkers for children and small adults currently
used.
The final design has had its mechanical properties computationally justified
through the use of CAD and FEA. CAD allows designs to be visualised, removing the
need for repeat prototyping stages. The use of CAD coupled with FEA allows for
rapid design analysis and improvement.
54. 48
13 CONCLUSION
During this project a gap was identified within the dwarf demographic for which no
mobility walker was suitably designed. This led to the creation of a user cohort
consisting of people with disproportionate dwarfism.
Anthropometric research found that a new design for a mobility walker was
required which could support a load of 75 kg and be adjustable from 360 mm to
750 mm in height. The project focused upon a walker with a height range of 550
mm – 750 mm as this height range poses a more technical challenge in terms of
structure. The mobility walker width necessary for the user cohort could not be
accurately identified nonetheless a range of 250 mm to 480 mm was selected to
incorporate the majority of possible users, keeping to the selected Top-Down
Design Approach.
The mobility walker concept abides by the constraints and British Standards, with
a minimum FoS of 3.48 under standard loading and 2.16 when put in the most
compromising loading situation (full load through one wheel).
The progression of the project beyond this point would require more in-depth
user data, ideally sourced directly from user interaction through the use of surveys
and focus groups, therefore the User-Centred Design Approach might be more
applicable for further design development. The concept can then be evaluated
through multidisciplinary collaboration. The final progression of this project before
prototyping would involve a study into possible manufacturing techniques and
methods of construction of the mobility walker.
This project has identified the anthropometric characteristics of adult dwarfs and
converted this data, along with regulatory documents, into a mechanically tested
and validated concept for a mobility walker for adult dwarfs. The project not only
highlights the needs of adult dwarfs, but also the possible demographic neglect of
many other medical conditions whose needs remain unfulfilled.
55. 49
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57. 51
20. Little People of America. Discussion Group. Little People of America.
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59. 53
APPENDIX I
Posture diagrams adapted from an image from IBDT Dictionary
(http://www.schenectady.k12.ny.us/users/pattersont/IBDT%20Website/IBTerms.ht
ml) representing "Distance between the hand and floor of children with their arms by
their sides", "Distance between the crotch of a dwarf and the floor" and "Distance
from hand to floor distance when upper arm adjacent to the body and lower arm at
70
o
to the upper arm" respectively.
60. 54
APPENDIX II
Standard material properties used for FEA, from the SolidWorks material library.
Property Units Nylon 6/10 Aluminium 6061-
T6 (SS)
Elastic Modulus N/m^2 8.30E+09 6.90E+10
Poissons Ratio N/A 2.80E-01 3.30E-01
Shear Modulus N/m^2 3.20E+09 2.60E+10
Density kg/m^3 1.40E+03 2.70E+03
Tensile Strength N/m^2 1.43E+08 3.10E+08
Yield Strength N/m^2 1.39E+08 2.75E+08
Thermal Expansion
Coefficient
/K 3.00E-05 2.40E-05
Thermal Conductivity W/(mK) 5.30E-01 1.67E+02
Specific Heat J/(kgK) 1.50E+03 8.96E+02
61. 55
APPENDIX III
Tyre possibilities – clockwise from top left: high profile tyre (twindle.com.au); low
profile tyre (twindle.com.au); airless tyre honeycomb structure (airless-tire.com);
cushioning rubber ultra-low profile tyre (trustcare.se).
63. 57
APPENDIX V
A comparison of the sizes of mobility walkers with the project concept – clockwise
from top left:
small child mobility walker – 413 mm height (justwalkers.com);
medium child mobility walker – 560 mm height (amazon.com);
largest settings for project concept – 750 mm height;
mobility seat walker – 950 mm height (12voltsales.com.au);
adult rollator – 915 mm height (restorativeinnovations.com);
smallest settings for project concept – 550 mm height.
