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日期：2021-10-05 10:47

MECH2400 9400 Assignment B

Swivelling Cable Tether System Design

Due Friday the 8th of October (Week 8) 11:59pm through CANVAS

Problem Summary:

There is an identified need to acquire reliable but cost effective low to medium altitude meteorological data

(atmospheric pressure, temperature, humidity and wind speed) on Western Australia’s north western coastline

(Figure 1.0). The region of interest is bounded by Carnarvon through to just north of Port Hedland (a distance

of approximately 1000kms) on Australia’s west coast. The coastal region has the highest wind rating and has

been classified as Region D – severe cyclonic (Figure 2.0). The data is required to serve as part of an early an

early warning system for incoming cyclones.

Scope:

It is planned to install ten, Tethered Weather Balloons (TWB) separated by a distance of 100km from each

other as the methodology to obtain the necessary data. The TWB will carry a payload of approximately 20kg

which is comprised of; sensors to measure; air pressure, wind speed, temperature and humidity.

A solenoid valve that is connected to each TWB to enable to release of Helium gas for lowering the TWB (as

a safety mechanism when wind speed reaches 25.0m/s or for servicing).

Additional electronic components include; high capacity Li-Po batteries, a GPS system to track the TWB in

the event of it disconnecting from its tether, a 3 axis accelerometer to measure speed and acceleration, an

inertial measurement unit to measure attitude, a small on-board CPU with RAM and an SSD, a cable harness

and a long range Wi-Fi antenna. The components will be housed in an industrial shock proof case.

The TWBs will have the ability to collaboratively send and receive data to each other as a method to enable

the sending of the required data to the main meteorological bases at Carnarvon and Port Hedland on a regular

hourly basis (to minimise battery usage) or on a per minute basis if an urgent need arises.

The TWBs are to operate at a fixed altitude of 1000m. The operation of the TWB system is designed to be

unattended with a projected life of 10 to 20 years. Six monthly service intervals to replenish the Helium gas,

replace Li-Po batteries and inspect and replace any faulty electronics or mechanical components is part of the

maintenance schedule.

Data:

Balloon Data:

Balloon diameter (DB) = 25.0m (assume the balloon is perfectly spherical in shape at all times)

Balloon volume (VB) = (1.333 * π * (0.5 * DB)3)

Balloon altitude (HB) = 1000.0m

Balloon payload (Bp) = 20kg (assume the mass of balloon is negligible)

Density of Helium gas (ρHe) = 0.174kg/m3

Density of air (ρair) = 1.29kg/m3

Acceleration due to gravity (ag) = 9.81m/s2

Helium Balloon Lifting Force (FL) = VB *(ρair - ρHe) * ag

Wind Speed Drag Force Data:

Maximum allowable wind speed velocity (vmax) = 25.0m/s

Balloon area (AB) = 0.25 * π * DB2

Drag Coefficient of Sphere (cd) = 0.5

Drag Force (Fd) = cd * 0.5 * ρair * v2max * AB

Cable Data:

A galvanised wire cable with a 6 x 24 construction (as shown in Figure 3.0) available from Bullivants (order

code 110176) has been selected as the preferred choice as it has higher resistance to corrosion. The cable is

assumed to be in a taunt (non-sagging) state at all times.

Figure 3.0

Diameter of cable = 26mm

Mean Breaking Force (MBF) = 304kN

Approximate mass per linear length (kg/100m) = 215kg

End connections: End connections are to be of a, ‘closed swage socket’ style as defined in AS2759 – 2004

and illustrated in Figure 4.0 in Third Angle Projection below. No exact dimensions are available for the end

connection but may be scaled to suit based on the wire cable diameter.

Your Task:

With reference to the Problem Summary, Scope and Data provided, your main requirement for Assignment B

focuses on the mechanical design and stress analysis of a Swivelling Cable Tether System (SCTS) to provide

an anchored support for the TWB that differs in design from the ‘Cable Tensioning System’ presented in

Assignment 1B and illustrated in Figures 5.0 and 5.1. Under the effects of wind, the SCTS is to be able to

rotate on at least two axis (similar to a mechanism).

Cable Tensioning System (Courtesy of Dr. Thai Nguyen)

Concrete Support Data:

A concrete foundation approximately 3.0m in length, 3.0m in width and 3.0m in depth has been secured deeply

below ground level with protruding studs to ensure that your SCTS has a secure mounting point. A sample

image of a typical concrete slab that connects to a mechanical post is shown in Figure 6.0 below.

In a report format which includes; title page, contents page, conclusions/insights and references to APA 7th or

Harvard, consider the following three criteria:

1. Conceptualisation / Creativity (5%)

Using methods of Creativity, conceptualise at least two distinct, original designs for your SCTS. Discuss in

detail which Creativity methods (or combinations of) you used (approximately 300 words including images)

to come up with your designs.

Develop a Trade-off table to analyse which of these two potential designs is the most feasible. You may wish

to discuss these two designs with other students in terms of arriving at weighting values. Explain your

reasoning for; selecting certain criteria, arriving at weightings and the scores you allocated for each of the two

designs (approximately 300 words).

2. Basic Stress Analysis (15%)

The stress analysis is to be carried out on your final selection of SCTS from Point 1 above. There are two

cases that need to be analysed:

Case 1: The case of no wind any direction acting on the TWB i.e. the TWB is direct above your final SCTS.

Case 2: The case of when the wind velocity is acting along only one axis of your final SCTS in its swivelled

position.

Perform stress analysis as outlined below on the main components of your final SCTS design to ensure that

the TWB is safely secured in place.

1. Carry out basic stress analysis of the major load carrying members including the fasteners used. Any

mild or stainless steel may be used and fasteners of any diameter, grade or style may be used.

2. You will also need to define the threaded mounting studs shown in Figure 6.0 that connect the concrete

base to your SCTS design (in terms of: quantity, arrangement, diameter, grade, length and nuts plus

washers required). You are not required to design the grade of concrete needed or how the threaded

studs are connected to the concrete.

3. A factor of safety (F of S) of 2.0 is to be used on all stressed components of your SCTS design.

4. Include all FBDs, SF Diagrams, BM Diagrams and deflection in key points. Use standard equations,

do not derive equations from first principles.

5. Your SCTS must be able to be easily pulled apart. Consequently, welding is not be used as a method

of joining major components together.

6. No Finite Element Analysis (FEA) is to be used in this assignment.

3. Engineering Drawings (5%)

Draw an Assembly Drawing of your final assembled SCTS with some basic overall dimensions and a parts

list. Use drawing sheets of size A2, A1 or A0 size. You are not required to produce detail drawings for

your design in this assignment.

Submit your Assignment B into the CANVAS portal as a single PDF file named using your student number

e.g. 496394428.pdf.

THIS ASSIGNMENT SHOULD TAKE AN AVERAGE STUDENT APPROXIMATELY 20

HOURS TO COMPLETE

The author would like to extend thanks to Prof. Steve Armfield and A/Prof. KC Wong for their

advice and support in the drafting of this assignment.