THEORY EXAMINATION (SEM–VI) 2016-17 MACHINE DESIGN- II

MACHINE DESIGN – II (EME602)

 SECTION – A

(Attempt All | 10 × 2 = 20 Marks)

(a) Wedge film and squeeze film journal bearings

Wedge film bearing: Load is supported by a pressure wedge formed due to relative motion between journal and bearing.

Squeeze film bearing: Load is supported by pressure developed when lubricant film thickness decreases due to squeezing action, even without rotation.

(b) Journal bearing terms

(i) Bearing characteristic number (Sommerfeld number):

S=μNp(rc)2S=\frac{\mu N}{p}\left(\frac{r}{c}\right)^2S=pμN​(cr​)2

It indicates performance of a hydrodynamic bearing.

(ii) Bearing modulus:

μNp\frac{\mu N}{p}pμN​

It relates viscosity, speed, and bearing pressure.

(c) Bearing life and median life

Bearing life is expressed as the number of revolutions or operating hours until fatigue failure.
Median life (L₅₀): Life at which 50% of bearings survive under given conditions.

(d) Gear tooth failures and remedies

Failures include wear, pitting, scoring, tooth breakage, and corrosion.
Remedies: proper lubrication, correct material selection, surface hardening, accurate alignment, and controlled loading.

(e) Herringbone gear

A herringbone gear is a double helical gear with opposite helix angles on either side.
Use: heavy-duty applications such as marine gearboxes due to zero axial thrust.

(f) Forces acting on worm and worm gear

Tangential force

Axial thrust

Radial (separating) force

These depend on transmitted power, lead angle, and friction.

(g) Types of cylinder liners

Dry liner: Not in direct contact with coolant

Wet liner: Directly in contact with coolant

Semi-dry liner: Partial contact with coolant

(h) Forces induced in connecting rod

Gas pressure force                                          Inertia force due to reciprocating mass

Bending force                                                 Buckling force

(i) Function of connecting rod

It transmits motion and power from the piston to the crankshaft and converts reciprocating motion into rotary motion.

(j) Helical gear terms

Helix angle: Angle between tooth direction and gear axis

Normal pitch: Pitch measured normal to the tooth direction

SECTION – B

(Attempt Any Five | 5 × 10 = 50 Marks)

(a) Journal bearing – maximum safe speed

Given data substituted in bearing power loss = heat dissipation capacity condition.
Using Petroff’s equation, maximum safe speed is calculated by equating frictional heat generated to heat dissipated.

(b) Power absorbed by collar bearing

Assuming uniform pressure, frictional torque:

T=23μW(Ro3−Ri3)(Ro2−Ri2)T = \frac{2}{3}\mu W\frac{(R_o^3 - R_i^3)}{(R_o^2 - R_i^2)}T=32​μW(Ro2​−Ri2​)(Ro3​−Ri3​)​

Power absorbed:

P=2πNT60P = \frac{2\pi NT}{60}P=602πNT​ 

(c) Dynamic load rating of ball bearing

Equivalent dynamic load:

Peq=(P1pt1+P2pt2+… )1/pP_{eq} = (P_1^p t_1 + P_2^p t_2 + \dots)^{1/p}Peq​=(P1p​t1​+P2p​t2​+…)1/p

Using life equation:

L=(CPeq)3L = \left(\frac{C}{P_{eq}}\right)^3L=(Peq​C​)3

Dynamic load rating C is calculated.

(d) Power transmitted by spur gears (strength basis)

Using Lewis equation:

WT=σbypcW_T = \sigma b y p_cWT​=σbypc​

Allowable stress of weaker member (bronze pinion) governs design.
Power transmitted is obtained from tangential load.

(e) Helical gear design

Normal module and virtual number of teeth calculated.
Face width = 10m to 14m.                         Gears checked for:

Static strength                                            Wear strength
(using given σₑₛ)

(f) Worm gear calculations                          Tangential force

Axial thrust                                                  Separating force

Efficiency:

η=tan⁡γtan⁡(γ+ϕ)\eta = \frac{\tan\gamma}{\tan(\gamma+\phi)}η=tan(γ+ϕ)tanγ​ 

(g) Diesel engine cylinder design                Using indicated power relation:

IP=pmLAN2IP = \frac{p_m L A N}{2}IP=2pm​LAN​

Bore and stroke calculated.
Cylinder head thickness and stud size obtained using strength equations.

(h) Variable load on rolling contact bearing

Life equation derived using:

L=(CPeq)pL = \left(\frac{C}{P_{eq}}\right)^pL=(Peq​C​)p

where equivalent load considers load variation over duty cycle.

SECTION – C

(Attempt Any Two | 2 × 15 = 30 Marks)

3) Full journal bearing – cooling and oil flow

Heat generated:                                           Hg=fWVH_g = f W VHg​=fWV

Heat carried away by oil:                             Hd=mcpΔTH_d = m c_p \Delta THd​=mcp​ΔT

Artificial cooling = Hg−HdH_g - H_dHg​−Hd​
Oil flow rate obtained from energy balance.

4) Bevel gear design

Steps:                                                           Determine module using Lewis equation

Face width = 1/3 pitch cone length             Pitch diameters from velocity ratio

Pinion shaft diameter from torsion equation:

T=π16τd3T = \frac{\pi}{16}\tau d^3T=16π​τd3 

5) Design of cast iron piston

Design includes:                                          Piston head thickness

Piston rings (number and dimensions)       Piston skirt length

Piston pin dimensions

Thermal and strength considerations applied using standard design relations.