Answers to a Question

Condition Monitoring

a)What are the 8 main steps in the generic procedure for implementing a condition monitoring programme which are outlined in BS ISO 17359:2011?

b)With the aid of a block diagrams, outline the main elements of any two of the following systems:

·Active condition monitoring system.

·Vibration analysis system

·PC based acoustic emission monitoring system

c)Answer the following questions about condition monitoring techniques, according to your experience during the module. (Each is worth 2 marks)

1)Name two maintenance strategies that apply condition monitoring techniques.

2)Name three vibration analysis methods used to analysis vibration signals.

3)Name two Acoustic Emission (AE) monitoring features that are used in AE monitoring; based on the signal types.

4)Name two approaches used in oil debris analysis.

5)For the given accelerometer response (shown in the Figure 1.c-5):

i.What units should the sensitivity be expressed in?

ii.At what frequencies (or a range of frenquencies) does attenuation occur?

iii.What frequencies (or a range of frenquencies) does resonance occur?

6)For the bearing shown in Figure 1.c-6, which position (A, B, C or D) is suitable to monitor the condition of the shaft and bearing? Why?

7)Name three common mounting methods used to mount Accelerometers.

8)Based on the BS ISO 13373-3, ‘condition monitoring and diagnostics of machines’, name the four expected defect frequencies of a typical cylindrical roller bearing.

Structural Health Monitoring

a)Structural health monitoring for aircraft fulfils two main functions: operational monitoring and damage monitoring. Describe the features of each of these types of monitoring and identify the main benefits they provide for operating and maintaining aircraft. (5 marks)

b)Describe three types of SHM systems capable of detecting damage in structures. For each system identify

1.The principle of operation.

2.The main system components.

3.The advantages and disadvantages of the technique.

c)Shows a typical signal from an acoustic emission sensor. This is the kind of signal generated by a growing crack in a metal structure. Why is it important that the time of arrival of the signal at the sensor (indicated by the arrow) can be accurately determined?

Typical acoustic emission signal generated by a crack growth in a metal structure.

1.What method can be used to determine that time of arrival? (1 mark)

2.For the signal shown in  what is the main factor that can affect the accuracy of the time of arrival measurement and how does it affect the accuracy?

Shows an aluminium stiffener in an aircraft structure with 4 fastener positions. The stiffener has 2 acoustic emission sensors, one at each end of the stiffener. A crack at one of the fastener locations generates acoustic emissions which are detected by the sensors as shown in Figure 2.d-2. An example single emission detected by sensors 1 and 2 is shown in

Section 2

By estimating information from these sensor signals in Figure 2.d-2 deduce which fastener position (1, 2, 3, or 4) contains the crack that generated the acoustic emission.

Acoustic emission from a crack at a fastener position in a spar stiffener.

In answering Question 2 use the following information:

For a 1-dimensional location calculation using 2 AE sensors Xs = ½ (D - V Δt)

Where: Xs = distance from sensor 1 to source of acoustic emission D = distance between sensor 1 and sensor 2 = 2.5 m

V= velocity of sound in aluminium = 3100 m/s

Δt = difference in time of arrival of acoustic emission signal between sensor 1 and sensor 2.

Qualitative Systems Reliability Analysis

FMEA and HAZOP are two key concepts. The first concept has been invented by Heap in 1960’s to prevent “planes” falling down the sky while the second was developed at the same time in the chemistry industry to improve safety.

a)Identify the acronyms FMEA and HAZOP.

b)State the main steps and advantages of an FMEA process.

c)Consider ONE of the following cases:

-Wind turbine braking system

-Pipe flow system with control arrangements (valves, tanks, etc.)

-Aircraft flap hydraulic system

Perform an indicative 3-level system break down describing your system, identify ONLY 4 failure modes (at least two different modes for one component) and perform FMEA to prioritise risks.

Quantitative System Reliability Analysis

The brakes are an important part of any bicycle. Figure Q4 below shows some rim brakes. When the brake lever on the handle bar is squeezed the cable is pulled up and the brake pads close on the rim of the bicycle wheel. Imagine a bicycle that has a braking system consisting of two braking sub-systems (one on the front wheel and one on the back).

2. Cable 0.94

3. Clamp nut 0.97

4. Pivot 0.99

5. Brake shoe nut 0.96

6. Brake shoe nut 0.96

7. Brake pad 0.90

8. Brake pad 0.90

Important note!

Assume all other parts have a reliability of 1 and can be ignored

Bicycle brake lever and rim brakes with table of part reliabilities.

a) Assuming that only one brake subsystem (either front or back) is required for safe operation. In order to make the bicycle more reliable, Designer A is suggesting for redundancy at the system-level (or high-level), while Designer B is suggesting for redundancy at the component-level (or low-level). Draw the reliability block diagrams (RBD) for the whole brake system according to these two Designers.

b)Using the two reliability block diagrams drawn in part a, calculate the reliability of the braking system, respectively. And recommend the better design.