dme concepts

DESIGN CONCEPT CLASS FOR MECHANICAL ENGINEERING STUDENTS

By

K.S.K.Sasikumar

Assistant professor

Department of mechanical engineering

Kongu engineering college

Stresses in machine elements

• Stresses are developed in machine elements due to applied load and machine design involves ensuring that the elements can sustain the induced stresses without yielding.

Tensile stress

Compressive stress

stresses

Lever arms AB and AC - Bending stresses Hinge pin - Shear and bearing stresses. Spring - Shear stress.

Bearing stress When a body is pressed against another, the compressive stress developed is termed bearing stress.

Shear stress

Bending stresses

Bending stress

Torsion of circular members

Buckling

Compound stresses in machine parts

Superposition of stresses due to axial force and bending moment

Example

Stress Concentration

Any such discontinuity in a member affects the stress distribution in the neighbourhood and the discontinuity acts as a stress raiser.

Methods of reducing stress concentration

Flat Belt drives

Belt Material

Leather

Oak tanned or chrome tanned.

Rubber

Canvas or cotton duck impregnated with rubber. For greater

tensile strength, the rubber belts are reinforced with steel

cords or nylon cords.

Plastics

Thin plastic sheets with rubber layers

Fabric

Canvas or woven cotton ducks

Ply of belt:

The belt thickness can be built up with a number of layers.

The number of layers is known as ply.

V- Belt drives

Standard V-belt sections

couplings

Sleeve coupling

Shear plane

Sleeve coupling with taper pins

Clamp coupling

Ring compression type couplings

Flange coupling

Flexible rubber – bushed couplings

Oldham coupling

Universal joints

Design for Strength

Loading may be due to:

a) The energy transmitted by a machine element.

b) Dead weight.

c) Inertial forces.

d) Thermal loading.

e) Frictional forces.

stresses

Types of loading

Factor of safety

Stress vs strain

Theories of failure

• Maximum principal stress theory ( Rankine theory)

• Maximum principal strain theory (St. Venant’s theory)

• Maximum shear stress theory ( Tresca theory)

• Maximum strain energy theory ( Beltrami’s theory)

• Distortion energy theory( von Mises yield criterion)

Design of shoe brakes

Types of brakes

• Shoe or block brakes – braking force applied radially

• Band brakes – braking force applied tangentially.

• Disc brake – braking force applied axially

Shoe or block brake

Double block brake

Internal expanding shoe brake

External contracting shoe brake

Band brakes:

• Simple band brake:

Differential band brake:

Welded joints and their advantages

• (i) Compared to other type of joints, the welded joint has higher efficiency. An efficiency > 95 % is easily possible.

• (ii) Since the added material is minimum, the joint has lighter weight.

• (iii) Welded joints have smooth appearances.

• (iv) Due to flexibility in the welding procedure, alteration and addition are possible.

• (v) It is less expensive.

• (vi) Forming a joint in difficult locations is possible through welding.

Typically welded machine components

• Pressure vessels, steel structures.

• Flanges welded to shafts and axles.

• Crank shafts

• Heavy hydraulic turbine shafts

• Large gears, pulleys, flywheels

• Gear housing

• Machine frames and bases

• Housing and mill-stands.

Basic types of welded processes

• Liquid state (fusion) welding

where heat is added to the base metals until they melt. Added metal (filler material) may also be supplied. Upon cooling strong joint is formed.

Depending upon the method of heat addition this process can be further subdivided, namely

• Electrical heating: Arc welding

• Resistance welding Induction welding – Chemical welding: Gas welding

• Thermit welding – Laser welding

– Electron beam welding

Solid state welding:

Here mechanical force is applied until materials deform to plastic state. Bonds are then formed through molecular interaction. Solid state welding may be of various kinds, namely, Cold welding Diffusion welding Hot forging

Strength of welded joints:

strength of a welded joint gets affected mainly by the following factors.

(i) Crack initiation: it is possible that cracks form while cooling a

melted metal.

(ii) Residual stresses: due to inhomogeneous heating of the base metals, residual stresses may exist upon cooling.

(iii) Metallurgical transformation: in heat affected zone (HAZ)

metallurgical properties may change leading to weakening of the joint. (iv) Defects: of various kinds like incomplete penetration, porosity, slag

inclusion which affect the strength of a welded joint. (v) Stress concentration: abrupt change in the geometry after welding

may introduce stress concentration in the structure.

Types of welded joints:

Butt joints

Square butt joint Single V-butt joint, double V-butt joint Single U-butt joint, double U-butt joint Single J-butt joint, double J-butt joint Single bevel-butt joint, double bevel butt joint

Butt joints

Other types of joint

Weld symbols

Welding symbol

Design of a butt joint:

The main failure mechanism of welded butt joint is tensile failure

Transverse fillet joint:

Parallel fillet joint:

Circular fillet weld subjected to torsion:

Riveted Joints : Types and Uses

Mechanical joints are broadly classified into two classes viz.,

non-permanent joints

permanent joints.

Rivets and Riveting:

Riveted joints are mainly of two types

Lap joints

Butt joints

Lap joint

Butt joint

Important terms used in riveted joints

a) Pitch: This is the distance between two centers of

the consecutive rivets in a single row. (usual symbol p)

b) Back Pitch: This is the shortest distance between two successive rows in a multiple riveted joint. (usual symbol pt or pb)

c) Diagonal pitch: This is the distance between the centers of rivets in adjacent rows of zigzag riveted joint. (usual symbol pd)

d) Margin or marginal pitch: This is the distance between the centre of the rivet hole to the nearest edge of the plate. (usual symbol m)

Design of shaft

• Design based on Stiffness

In the context of shaft, design for stiffness means that the lateral deflection of the shaft and/or angle of twist of the shaft should be within some prescribed limit.

Torsional rigidity

Helical Springs • Commonly used spring materials Hard-drawn wire: • This is cold drawn, cheapest spring steel. Normally used for low stress and static

load. The material is not suitable at subzero temperatures or at temperatures above 1200C.

Oil-tempered wire: • It is a cold drawn, quenched, tempered, and general purpose spring steel.

However, it is not suitable for fatigue or sudden loads, at subzero temperatures and at temperatures above 1800C.

• When we go for highly stressed conditions then alloy steels are useful. Chrome Vanadium: • This alloy spring steel is used for high stress conditions and at high temperature up

to 2200C. It is good for fatigue resistance and long endurance for shock and impact loads.

Chrome Silicon: • This material can be used for highly stressed springs. It offers excellent service for

long life, shock loading and for temperature up to 2500C. Music wire: • This spring material is most widely used for small springs. It is the toughest and has

highest tensile strength and can withstand repeated loading at high stresses. However, it can not be used at subzero temperatures or at temperatures above 1200C.

Spring manufacturing processes

If springs are of very small diameter and the wire diameter is also small then the springs are normally manufactured by a cold drawn process through a mangle.

However, for very large springs having also large coil diameter and wire diameter one has to go for manufacture by hot processes. First one has to heat the wire and then use a proper mangle to wind the coils.

Forces in spring

Laminated leaf springs

Materials for leaf spring

Plain carbon steel,

Chromium vanadium steel,

Chromium- Nickel- Molybdenum steel,

Silicon- manganese steel,

Methods to reduce additional stresses

1. Master leaf is made of stronger material than the other leaves.

2. Master leaf is made thinner than the other leaves. This will reduce the bending stress as evident from stress equation.

3. Another common practice is to increase the radius of curvature of the master leaf than the next leaf.

Nipping of leaf spring

overview of bearings

Fluid Film bearings

In fluid film bearing the entire load of the shaft is carried by a thin film of fluid present between the rotating and non-rotating elements.

The types of fluid film bearings are as follows,

• Sliding contact type

• Journal bearing

• Thrust bearing

• Slider bearing

Rolling contact bearings

In rolling contact bearings, the rotating shaft load is carried by a series of balls or rollers placed between rotating and non-rotating elements.

The rolling contact type bearings are of two types, namely,

• Ball bearing

• Roller bearing

Journal Bearing

Materials for bearing • The common materials used for bearings are listed below.

• Lead based babbits : around 85 % Lead; rest are tin, antimony and copper (pressure rating not exceeding 14MPa)

• Tin based babbits : around 90% tin; rest are copper, antimony and lead (pressure rating not exceeding 14MPa)

• Phosphor bronze : major composition copper; rest is tin, lead, phosphorus (pressure rating not exceeding 14MPa)

• Gun metal : major composition copper; rest is tin and zinc (pressure rating not exceeding 10MPa)

• Cast iron : pressure rating not exceeding 3.5 MPa

• Other materials commonly used are, silver, carbon-graphite, teflon etc.

Rolling contact bearings • Rolling contact bearings are also called anti-

friction bearing due to its low friction characteristics. These bearings are used for radial load, thrust load and combination of thrust and radial load. These bearings are extensively used due to its relatively lower price, being almost maintenance free and for its operational ease. However, friction increases at high speeds for rolling contact bearings and it may be noisy while running. These bearings are of two types,

• Ball bearing and Roller bearing

Ball bearing

A typical arrangement for housing a

bearing

Single row Angular Contact Ball Bearing

It is mostly used for radial loads and heavy axial loads.

Double Row Angular Contact Bearing

has two rows of balls. Axial displacement of the shaft can be kept very small even for axial loads of varying magnitude.

Single thrust ball bearing

It is mostly used for unidirectional axial load.

Taper Roller Bearing

It is generally used for simultaneous heavy radial load and heavy axial load. Roller bearings has more contact area than a ball bearing, therefore, they are generally used for heavier loads than the ball bearings.

Spherical Roller Bearing

It is mainly used for heavy axial loads. However, considerable amount of loads in either direction can also be applied.

Cylindrical Roller Bearing

For heavy radial load and high speed use, cylindrical roller bearings,

Within certain limit, relative axial displacement of the shaft and the bearing housing is permitted for this type of bearings.

Threaded Fasteners

• Bolts: They are basically threaded fasteners normally used with nuts.

• Screws: They engage either with a preformed or a self made internal threads.

• Studs: They are externally threaded headless fasteners. One end usually meets

Types of screw heads

Types of bolt heads

Set screws

Thread forms