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Cork Institute of Technology
Bachelor of Engineering in Building Services Engineering -Stage 2
(EBSEN_7_Y2)
Summer 2008
Drawing & Design – Mechanical Design
(Time: 3 Hours)
Instructions Answer FIVE questions. All questions carry equal marks.
Examiners: Mr. T. A. Farrell Mr. D. Leonard Dr. M. Smyth
Q1 (a) Explain carefully, with the aid of sketches, the following terms in connection with belt drives: (i) Belt pitch length. (ii) Sheave pitch diameter. (iii) Belt contact angle. (8 marks)
(b) A compressor in a refrigeration plant is driven by a 20kW AC squirrel cage electric motor, running at 1450 RPM. The compressor runs for 24 hrs. per day, and the required compressor speed is 950 RPM approximately. Design a suitable drive consisting of classical V-belts and sheaves. The design should include: (i) The sheave diameters, and actual centre distance. (3 marks) (ii) Belt cross-section, type, number, and pitch length. (3 marks) (iii) The required number of belts. (3 marks) (iv) The actual compressor speed. (3 marks) Assume an initial centre distance estimate of C = D + 1.5d, where D and d are the diameters of the large and small sheaves respectively.
Q2. (a) Explain carefully the meaning of ‘Fatigue failure’ when applied to engineering materials, and explain the main characteristics of such failure, when compared with ‘Static failure’. (6 marks) (b) Explain, with the aid of a sketch, what is an ‘S-N’, or an S-Log N’ curve, and show the main difference between such a curve for a ferrous material, and for a non-ferrous material. (6 marks) (c) Sketch a schematic diagram of a ‘Rotating Beam Test Machine’, and explain briefly how it could be used to derive the Fatigue Strength of a ferrous engineering material. (8 marks)
Q3. The main fan on an air conditioning plant is as shown in Fig. 1.1. below. It is mounted on two bearings A and B, on a fabricated steel foundation and the load due to the impeller is 600N acting through the contreline of the impeller, which is 350 mm from the face of bearing B. The intended shaft material is steel, having a yield strength in tension Syt = 650MPa. (a) Considering only the bending of the shaft between B and C, between the face of the bearing and the impeller contreline, calculate the required shaft diameter ‘d’, allowing a factor of safety on yielding of 0.5. (6 marks)
(b) If the torque being transmitted by the drive motor, in shaft section AB is 500N-m, calculate the shaft diameter ‘d’ required to withstand static yielding due to torsional shear stress in this section. Assume a yield stress in shear for the material = 0.575 Syt, and a factor of safety of 0.5. (6 marks)
(c) Calculate the shaft diameter ‘d’ required to withstand yielding due to the combined induced stresses in shaft section BC, due to the torque of 500N-m, and the bending due to the impeller load of 600N., Explain any assumptions made. (8 marks)
Q5. The shaft of a fan in the air-conditioning plant of a shopping mall is connected to the motor drive-shaft by a rigid cast-iron coupling. The fan requires 7.5kW for operation, and rotates at 1500 RPM. (a) Calculate the minimum safe fan shaft diameter, if it is to be machined from mild steel. Assume a factor of safety on material strength of 3, and state any assumptions made to allow for the presence of the keyway. (5 marks) (b) Design a suitable key to connect the cast-iron coupling to the shaft, giving: (i) Length, (ii) Breadth, (iii) Height of the key, and choose a suitable key material. Assume a factor of safety of 3. (15 marks)
Q6. A mild steel flat tie-bar is required to carry a steady tensile load of 100kN. Length considerations mean that the bar must be joined in the middle. (a) Select a suitable bolted connection for this application. (10 marks) (b) Select a suitable welded joint for the same application. (10 marks)
