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Cork Institute of Technology
Higher Certificate in Engineering in Building Services Engineering – Award
(NFQ – Level 7)
Summer 2006
Drawing & Design – Mechanical Design
(Time: 3 Hours)
Answer FIVE questions. All questions carry equal marks.
Examiners: Mr. M. Brady Dr. N. J. Hewitt Mr. D. Leonard
Q1. The rotating impeller section of a ventilation fan is mounted on a shaft as shown in Fig. below. The shaft is supported on two bearings at A and B, and the fan impeller centreline is 320 mm from the face of bearing B. The load due to the impeller weight is 260 N. the material of the shaft is steel, having a yield strength in tension, S (^) yt = 825 N/mm^2. (a) If the drive motor generates a torque of 35 N-m in shaft section AB, between the bearings, calculate the shaft diameter ‘d’ required to withstand static yielding due to torsional shear stress in this section. Assume a yield strength in shear for the material, τy = 0.57Syt , and a factor of safety of 2.0. (6 marks) (b) Considering bending only, in the shaft section BC between the shaft support point in bearing B, and the impeller centreline, calculate the shaft diameter ‘d’ allowing a factor of safety on static yielding of 2.0. (6 marks) (c) Considering the combined induced stresses in shaft section BC, due to the torque of 35 N-m, and the impeller load of 260 N, calculate the shaft diameter ‘d’ required to withstand yielding. Assume the combined induced stress effect, σ′i is given by:
σ′ 1 = σ i^2 + 3 τ i^2 with a factor of safety of 2.0. (8 marks)
Note: Refer to Fig.1 below, and Table 1.1 attached.
Q2. (a) Explain the terms ‘Derating Factors’, and ‘Safety Factors’, referring to the reasons for the use of each, to the conditions under which each would be used, and to their values. (6 marks) (b) Part of the operating mechanism of a louver damper is shown in Fig. 2.1 below. The material of the part is steel, having an ultimate tensile strength of 640 N/mm^2 , and is hot rolled and then machined. A reliability of 99% is required with the part operating at 800 C. The loading is axial, with medium impact. (i) Calculate the static stress on section A, with an axial load of 80 kN, and also calculate the Factor of Safety which this would give on ultimate tensile strength. ( marks) (ii) Calculate the actual fatigue strength, Se , of the part, for fully reversed loading. ( marks) Note: Refer to Fig.2.1 below, and to Figs.2.2 to 2.4, and Tables 2.1 to 2.4 attached.
Q3 (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. ( marks)
(i) The sheave diameters, and actual centre distance. ( marks) (ii) Belt cross-section, type number, and pitch length. ( marks) (iii) The required number of belts. ( marks) (iv) The actual compressor speed. ( 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. Note: Use Figures 5.1 to 5.3, and Tables 5.1 to 5.5 attached. Q6 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 EN8 Carbon Steel. Assume a factor of safety on material strength of 5, and a derating factor of 0. on shear strength, to allow for the presence of the keyway. ( marks) (b) Design a suitable key to connect the cast-iron coupling to the EN8 Carbon Steel shaft, giving: (i) Length, (ii) Breadth, (iii) Height of the key, and choose a suitable key material. Assume a factor of safety of 5 for shear, and crushing strengths. ( marks) Note: Use Tables 6.1.1 to 6.2 attached.
Q7 (a) Compare and contrast the use of (i) Rigid Couplings, and (ii) Flexible Couplings, for connecting rotating shafts, giving advantages, and disadvantages of each, illustrating your answer with a sketch of each of the two types of coupling. ( marks) (b) The main fan of an Air Conditioning Unit for a large office block is to be connected directly to an electric drive motor through a pin-type flexible coupling. The nominal power required by the fan is 25kW, at a speed of 1500RPM.and the electric motor power
is to be 27.5 kW. The average period of service of the air conditioning unit is 10 hrs. per day, the frequency of starts is up to one per hour, and ambient temperature is 20o^ C. If the coupling pins are made from EN1A steel of 8mm diameter, and are located on a P.C.D., of 80mm, calculate the number of pins required in the coupling, allowing a factor of safety of 10 on material strength. (10marks) Note: Use Tables 7.1 - 7.3, and Table 6.1.2. attached.
