H7110 DESIGN OF MECHANISMS AND MACHINES

Q1. The hook of a crane, shown in Figure Q1, is frequently used to lift loads of up to 10,000 kg. All dimensions in the figure are in mm.

a) What are the most likely failure modes of the bolt? [5 marks]

b) Assuming that the contact between the bolt and the hook body extends over an angle equal to 180o and taking the contact stress, under loading as constant and equal to 26 MPa, calculate the minimum diameter of the bolt. [4 marks]

c) What material from Table Q1 would you chose, if shear yielding does not occur and a factor of safety of 5 is needed? Consider that the shear yield strength is 0.53Su. Justify your answer. [5 marks]

d) What is the loading on section AB of the hook, shown in Figure Q1? Calculate the direct stress on section AB at inner radii Ri and Ro. The cross section of the hook at this plane has dimensions shown in detail C (50X30mm). [6 marks]

Q2. A 25 mm diameter torsion bar, made out of cold rolled steel with elastic shear modulus of 80 GPa, is actuated by a lever via a split-ring clamp, as shown in Figure Q2. The split ring clamp is secured to the shaft by friction generated by tightening of two M14 bolts. The bolts are made of SAE4.8 steel and a safety coefficient of 2.5 is chosen for the loading in the bolts. The coefficient of friction between the split ring and the shaft is 0.18. Material and dimensional data for screws are found in Tables Q2 and Q3. All dimensions in Figure Q2 are in mm.

a) Find the maximum load W that can be applied to the lever such that the split-ring clamp does not slip against the shaft. [6 marks]

b) The lever has a rectangular cross section with uniform thickness of 8 mm. From Table Q4 choose a material for the lever and find the area of the cross section such that the yield stress in the lever is not reached. A factor of safety of 2.5 must be employed. [5 marks]

c) If the length of the torsion bar is 300 mm what is the maximum deflection at the point of the application of force W? You can assume that the cross-section area of the torsion bar is uniform and take the Young’s modulus of aluminium 200 GPa. [5 marks]

d) Describe the phenomena which may happen, at the interface with the bar if the splitring was to slide under loading. [4 marks]

Q3. The pinion of a gear set is to transmit 8kW power at a rotational speed of 450 rev/min. The shaft transmits power to the pinion through a flat, rectangular key.

a) Draw a sketch of the pinion/shaft subassembly. Comment on your design. The dimensions do not need to be at scale. [4 marks]

b) Choose a material from Table Q5 and comment on your choice. Calculate the minimum diameter of the shaft. Consider steady torsional loading only, with a factor of safety of 3 for this loading. Take the shear yield strength as 0.53Su. [6 marks]

c) If the transmission ratio of the gears is 2.6, the distance between axes is 162 mm and the gear has 78 teeth choose the appropriate material, from Table Q7, and calculate the module and the minimum face width for the pinion. [7 marks] d) From Table Q8 choose a key and calculate its length. [3 marks]