Design of Machine Elements - I Assignmnet No I

Assignments No – I

Topic-Basic Machine Design, Design of Shaft, Keys & Coupling

Date of Issue: 8^th August, 2013                                                    Date of Submission: 14^th August, 2013

1.      Explain the different stages in the design of machine elements.

2.      A armature shaft of a 40 kW, 720 rpm electric motor, mounted on two bearings A and B as shown in figure. Total magnetic pull on the armature is 7 kN and it can be assumed to be uniformly distributed over a length of 700 mm midway between the bearings. The shaft is made of steel with a ultimate tensile strength of 770 N/mm² and yield strength of 580 N/mm².  Determine the diameter of shaft if K_m =1.5 and K_t=1.0, assume that the pulley is keyed to the shaft.

3.      The problem is shown in the given figure. A pulley drive is transmitting power to a pinion, which in turn is transmitting power to some other machine element. Pulley and pinion diameters are 400mm and 200mm respectively. Shaft has to be designed for minor to heavy shock

ASME code for shaft design is suitable in this case as no other specifications are provided.  In absence of any  data  for material  property,  the  allowable  shear  for  commercial  steel  shaft may  be  taken  as 40 MPa, where keyway is present in the shaft.  For the given condition of shock, let us consider K_m = 2.0 and K_t = 1.5.

4.      What is key? How are the keys classified? Draw neat sketches of different types of keys and state their applications.

5.      Discuss the function of couplings and effect of key way cut into shaft. Explain the difference between rigid and flexible coupling.

6.      Design a cast iron protective flange coupling to connect two shafts in order to transmit 7.5 kW at 720 rpm. The following permissible stresses may be used:

Permissible shear stress for shaft, bolt and key material = 33 MPa

Permissible crushing stress for bolt and key material = 60MPa

Permissible shear stress for the cat iron = 15 MPa

7.      Design  a  bushed  pin  type  flexible  coupling  for  connecting  a  motor  shaft  to  a  pump  shaft  for  the following service conditions:

Power to be transmitted = 40 kW; speed of the motor shaft = 1000 r.p.m.; diameter of the motor shaft = 50 mm; diameter of the pump shaft = 45 mm. The bearing pressure in the rubber bush and allowable stress in the pins are to be limited to 0.45 N/mm2 and 25 MPa respectively.

Note: On the last date of submission of your assignment, a minor test of 20 marks will be conducted