(SEM VI) THEORY EXAMINATION 2021-22 MACHINE DESIGN

MACHINE DESIGN (KME602)

Section-wise Detailed Answers – B.Tech Semester VI

SECTION A

(Attempt all questions – brief but explanatory answers)

(a) Preferred numbers

Preferred numbers are standardized series of numbers selected according to geometric progression to reduce the variety of sizes used in engineering design. These numbers follow Renard series such as R5, R10, R20, and R40, ensuring economical production, interchangeability, and ease of standardization in machine components.

(b) Endurance limit

Endurance limit is the maximum stress value below which a material can withstand an infinite number of load cycles without failure. It is mainly applicable to ferrous materials like steel and plays a crucial role in fatigue design of machine components subjected to fluctuating stresses.

(c) Types of riveted joints

Riveted joints are classified based on arrangement of plates and rivets. They may be lap joints or butt joints. Butt joints may further be single-strap or double-strap joints. Based on rivet arrangement, joints may be chain riveted or zig-zag riveted.

(d) Types of keys

Keys are machine elements used to connect rotating shafts to hubs. Common types include sunk keys, saddle keys, tangent keys, feather keys, and splines. They transmit torque while preventing relative rotation between shaft and hub.

(e) Pressure angle in gears

Pressure angle is the angle between the line of action and the tangent to the pitch circle. It affects force transmission between gear teeth. Standard pressure angles such as 14.5° and 20° are commonly used. A higher pressure angle provides stronger teeth but increases bearing load.

(f) Comparison between spur gear and helical gear

Spur gears have teeth parallel to the axis and are simple in construction but generate noise at high speeds. Helical gears have inclined teeth, resulting in smoother operation and higher load capacity, but they produce axial thrust and are costlier.

(g) Bearing modulus

Bearing modulus is defined as the product of bearing pressure and surface velocity. It is used to evaluate heat generation and performance of journal bearings and helps in selecting suitable lubricant and bearing material.

(h) Sliding contact vs rolling contact bearings

Sliding contact bearings operate with relative sliding motion and rely on lubrication to prevent wear. Rolling contact bearings use rolling elements such as balls or rollers, resulting in lower friction and higher efficiency. Rolling bearings are more precise but costlier.

(i) Types of cylinder liners

Cylinder liners are classified as dry liners and wet liners. Dry liners are pressed into the cylinder block and do not come in contact with coolant. Wet liners are directly exposed to coolant, providing better heat transfer but requiring effective sealing.

(j) Coupling vs clutch

A coupling permanently connects two shafts to transmit power, whereas a clutch allows engagement and disengagement of power transmission. Couplings do not permit relative motion during operation, while clutches control motion and speed.

SECTION B

(Attempt any three – detailed solutions)

(a) Preferred number selection for tractor models

Given a power range from 7.5 kW to 75 kW and five models, preferred numbers are selected using a geometric progression. The ratio is calculated so that power increases uniformly. Using standard preferred number series ensures smooth gradation. For expansion to nine models, intermediate preferred numbers are selected to satisfy farmers’ requirements while maintaining standardization.

(b) Design of double transverse fillet weld

The tensile load carried by the plates is equated to the strength of weld. Allowable stress for both plate and weld is limited to 110 N/mm². The throat area of weld is calculated based on plate thickness. Required weld length is obtained by equating plate strength to weld strength, ensuring joint efficiency is equal to one.

(c) (i) Lewis equation for gear tooth strength

Lewis equation is derived by considering the gear tooth as a cantilever beam subjected to tangential load at the pitch circle. The equation expresses tooth strength as a function of module, face width, allowable stress, and Lewis form factor. It is used for static strength analysis of gear teeth.

(ii) Formative number of teeth in helical gears

The formative number of teeth accounts for the helical nature of gears. It is obtained by dividing actual number of teeth by the cube of cosine of helix angle. This equivalent number is used to apply spur gear equations to helical gears.

(d) Bearing load definitions

Basic static load capacity is the load causing permanent deformation. Basic dynamic load capacity is the load under which bearing survives a specified life. Equivalent loads represent combined radial and axial loads. Rating life is the number of revolutions a bearing completes before failure with a certain reliability.

(e) Design of cylinder, cylinder head, and studs

Cylinder thickness is calculated using thin cylinder theory based on maximum gas pressure and allowable stress. Cylinder head thickness is designed to resist bending stresses. Studs are designed based on tensile stress due to gas pressure, ensuring factor of safety is satisfied. Material properties of cast iron and steel are used for safe design.

SECTION C

(Attempt any one part – long numerical-based explanation)

(3a) Design of hacksaw frame

The hacksaw frame is subjected to bending due to blade tension. Bending moment is calculated using applied load and frame geometry. Allowable stress is obtained using yield strength and factor of safety. Using bending equation, dimensions of rectangular cross-section are determined while maintaining the given depth-to-width ratio.

(3b) Design of flat plate under tensile load

The tensile force is divided by allowable tensile stress to determine the required cross-sectional area. Using material strength of grey cast iron and factor of safety, the minimum plate thickness is calculated to prevent failure.

(4a) Rivet diameter for bracket connection

Shear load on rivets is calculated from applied load. Using allowable shear stress, rivet cross-sectional area is determined. The rivet diameter is obtained ensuring safe load transmission without failure.

(4b) Shaft subjected to gear and pulley loads

Forces due to gear and belt tensions are resolved. Reactions at bearings are calculated. Maximum bending moment is obtained from bending moment diagram. Shaft diameter is determined using maximum shear stress theory considering allowable stress.

(5a) Gear design for compressor drive

Power transmission, speed ratio, and center distance are used to calculate number of teeth. Module is selected based on strength and wear considerations. Face width is taken as ten times the module. Material properties and service factors are included in design.

(6a) Journal bearing design

Bearing dimensions are calculated based on load, speed, oil viscosity, and temperature. Bearing pressure, clearance, and heat dissipation are checked to ensure safe and stable operation.

(7a) Piston crown thickness

Thickness is calculated based on heat transfer and bending stress criteria. Heat flow through crown is equated to conduction equation. Allowable temperature difference and thermal conductivity of aluminum alloy are used to determine thickness.

(7b) Single plate clutch design

Torque transmitted is equated to frictional torque. Plate dimensions are calculated using uniform pressure theory. Axial force required is obtained and distributed among springs. Initial compression is determined using spring stiffness.