FINAL EXAMINATION SEMESTER II SESSION 2011/2012

COURSE NAME COURSE CODE PROGRAMME EXAMINATION DATE DURATION INSTRUCTION

: : : : : :

MECHANICS OF MACHINES BDA20303 / BDA2033 2 BDD JUNE 2012 3 HOURS PART A: ANSWER ALL QUESTIONS PART B: ANSWER ONE (1) QUESTION ONLY

THIS PAPER CONTAINS SEVEN (7) PAGES

BDA20303 / BDA2033

PART A (COMPULSORY) Q1 (a) Indicate ‘True’ or ‘False’ for the following statements: The radial distance between pitch circle and root circle is called addendum. The common point of contact of the pitch circles of two meshing gears is called pitch point. (iii) Straight bevel gears are used to connect two intersecting co-planer shafts. (iv) In a reverted gear train, the axes of the first gear and last gear are parallel. (v) Train value of a gear train is equal to reciprocal of speed ratio of gear train. (vi) The angle of contact in cross belt drive is more than that for open belt drive. (vii) The ratio of tension of tight side to slack side in case of V-belt drive is e µθ sec β . (viii) In crossed belt drive, the directions of rotation of two pulleys connected by it are opposite. (ix) Because of slip of the belt, the velocity ratio of the belt drive is decreases. (x) If the initial tension in the belt is increased, the power transmitted by the belt increases. (10 marks) (b) A horizontal shaft running at 400 rpm is to drive a parallel shaft at 500 rpm. The diameter of the pulley on the driving shaft is 80 cm. If the belt is 8 mm thick, calculate the diameter of the driven pulley : (i) (ii) (iii) Neglecting belt thickness Taking belt thickness into account Assuming in the latter case a slip of 5% Given: N B DA = , N A DB (i) (ii)

N B DA + t = , N A DB + t

N B DA + t s = × 1 − N A DB + t 100 (6 marks)

(c)

What is velocity ratio? What is slip? Discuss the effect of belt thickness and slip on velocity ratio. (9 marks)

2

BDA20303 / BDA2033

Q2

(a)

FIQURE Q2 shows a system with three weights on rotating shaft. W1 = 40 N∠90° at a 101.6 mm radius, W2 = 40 N∠225° at 152.4 mm radius, and W3 = 26.7 N∠315° at 254 mm radius. The balance weight in planes 4 and 5 are placed at a 76.2 mm radius. To dynamically balance the system, determine using the graphical method: (i) (ii) The magnitudes of the balance weights in Newton (N) The angles of the balance weights, measured counter clockwise from reference angle. (10 marks)

(b)

Base on theory of dry friction, discuss about static friction force and kinetic friction force. You may use some diagram to support your explanation. (5 marks)

(c)

A vertical screw threads of mean diameter 50 cm and 12 cm pitch. Given that the coefficient of friction between nut and screw is 0.15 Find; (i) (ii) (iii) Torque required on the screw to raise a load of 25kN Find the percentage efficiency. If a Vee thread of β = 10° is used to replace the present screw, what is the torque required on the screw and its efficiency? Given: tan α = p' Wp' η% = Fπ d πd tan α + tan φ F =W 1 − tan α tan φ T = Fr FV = W tan φ = µ

(cos β tan α + tan φ ) (cos β − tan α tan φ )

(10 marks)

3

BDA20303 / BDA2033

Q3

(a)

What is inversion of mechanism in engineering words? List out three (3) examples of inversion mechanism. (4 marks)

(b)

In a slider crank mechanism shown in FIGURE Q3, the crank OA rotates clockwise about ‘o’ at 120 rpm. (i) (ii) (iii) (iv) Find the linear velocity of slider B. Find the linear velocity of point P located at a distance of 8 cm on the connecting rod extended. Determine the velocity of rubbing at the pin of the crank shaft OA , if the diameter of it pins is 5 mm. What would happen to the system if the rpm of crank OA is increased to 200rpm? Please briefly explain and verify your answer with calculation. Given:

ω=

2πN 60

V = ω× r (21 marks)

PART B (OPTIONAL), ANSWER ONE(1) QUESTION ONLY Q4

A kinematics of system is consists a cylinder of mass m , spring and...