What is creep

What is creep ?

Answer :

Creep is defined as slow and progressive deformation of the material with time under a constant stress.

What is notch sensitivity

What is Notch sensitivity ?

Answer :

• A measure of a reduction in strength of metal caused by presence of stress or a notch.

Tensile test

A tensile test is one of the simplest and basic tests and determines the value of a number of parameters concerned with mechanical properties of materials like strength, ductility, toughness.

Some other information can be obtained by this test are following below :

• Proportional limit
• Elastic limit
• Modulus of elasticity
• Yield strength
• Ultimate tensile strength
• Modulus of resilience
• Modulus of toughness
• Percentage elongation
• Percentage reduction in area

The specimen used in a tensile test is illustrated in the figure below :

The shape and dimensions of this specimen are standardized and confirm to 

IS 1608:1972. 

The cross-section of the specimen can be circular, rectangular or square.

The standard gauge length is given by l₀.

Procedure :

In the tensile test, the specimen is subjected to axial tensile force, which continues increasing and corresponding to that deformation is measured.

The specimen is mounted on the machine and gripped in the jaws. It is subjected to tensile stress which is increased by increments. After each increment, the amount by which the gauge length l₀ increases and deformation of gauge length are measured by an extensometer.

This procedure of measuring the tensile force and corresponding deformation continued till fracture. 

This results of a tension test are expressed by means of a stress strain diagram.

Uniaxial tensile testing is the most commonly used for obtaining the mechanical characteristics of isotropic materials. For anisotropic materials, such as composite materials and textiles, biaxial tensile testing is required.

Stress strain diagram with explanation

Very useful information concerning the behaviour of material for engineering applications can be obtained by making a tension test and plotting a curve showing the variation of stress with respect to strain. Therefore, the results of tension test are expressed by means of this curve. A stress-strain diagram for ductile material like mild steel is shown in the figure below.

Proportional Limit :

It is observed from the diagram that the stress strain relationship is linear from the point O to A. After A curve begins to deviate from the straight line. 

Hooke's law states that stress is directly proportional to strain. 

The term proportional limit is defined as the stress at which the stress-strain curve begins to deviate from the stress line so point A indicates the proportional limit.

Modulus of Elasticity :

Modulus of elasticity is the ratio of stress to strain up to point P. 
It is given by the slope of the line OP 

E = tanƟ = AP/OP = Stress / Strain

Where, 

Stress = Vertical line perpendicular to point A on the X-axis point named P = AP

Strain = Horizontal distance from O to P = OP

Elastic Limit :

When the specimen is stressed beyond point A and up-to point B. It will regain its initial size and shape when the load is removed. This indicates that the material is an elastic stage up to the point E. So the E is called the elastic limit.

The Elastic Limit of the material is defined as the maximum stress without any permanent deformation.

Proportional limit and Elastic limit are very close to each other.

Yield Strength :

When the specimen is stress beyond point B, plastic deformation occurs and the material starts yielding. It is seen from the diagram that beyond point B, the strain increases at a faster rate up to a certain point then a small reduction in load and the curve drops down point C. So B is called upper yield stress point and C is called lower yield stress point.

The Yield Strength is defined as the maximum stress at which a marked increase in elongation occurs without an increase in the load.

Ultimate Tensile Strength :

After the point C plastic deformation of the specimen increases. The material becomes stronger due to strain hardening, and higher and higher load required to deform the material. Finally, the load increase so stress reach a maximum value, as given by the point D. The stress corresponding to the point D is called ultimate stress point. 

The Ultimate Tensile Strength is the maximum stress that can be reached in the tension test.

Breaking or Rupture point :

For ductile material, the diameter of the specimen begins to decrease rapidly beyond the ultimate stress point D. There is a start reduction in cross-sectional area is called necking.

As the tensile stress progress and load increases the fracture takes place. This is shown by point E. So E is called breaking or rupture point.

Therefore, ultimate tensile strength is considered as failure criterion in brittle materials.

Clamp coupling

What is clamp coupling?

Answer :

• A device for uniting the ends of a shaft by means of conical binding-sleeves, which by longitudinal motion wedge themselves between the shaft ends and an outer cylinder, thus binding the whole together.

Clamp coupling is a coupling method that can be used when joining two piping units or hoses.

The clamp coupling is also called compression coupling or split muff coupling.

Clamp coupling is a rigid type of coupling. In this coupling, the sleeve is made of two halves, which are split along a plane passing through the axes of shafts. The two halves of the sleeve are clamped together by means of bolts and a small clearance is provided in the parting plane between two halves. Therefore, when the bolts are tightened, a force is exerted between the sleeve halves and the shaft.

Applications of clamp coupling :

The main application of clamp coupling is for line shaft in power transmission.

Clamp coupling is designed on the basis of standard proportions for sleeve halves and clamping bolts.

For sleeve halves :

D = 2.5 d 
L = 3.5 d

Where,
D = Outer diameter of sleeve halves ( mm )
L = Length of sleeve ( mm )
d = Diameter of shaft ( mm )

For clamping bolts :

d₁ = 0.2d + 10 mm

When d < 55 mm

and d₁ = 0.15d + 10 mm

When d > 55 mm 

Where, 

d₁ = Diameter of clamping bolt ( mm )

Muff coupling application

Applications of muff coupling are as follows :

• Piece couplings
• Grooved muff couplings for vertical applications
• Through bore muff couplings
• Stepped bore muff couplings
• Ribbed muff couplings
• 3 piece muff couplings

All of above can be made in different material like cast iron, carbon steel aluminium and stainless steel.

Muff coupling

What is muff coupling?

Answer :

• A coupling in which a hollow cylinder or muff is used to connect the abutting ends of two shafts.

In muff coupling, the muff is made into two halves and they are joined together by means of bolts. Where halves are made by cast iron and bolts are by mild steels. 

Muff coupling is also called sleeve coupling or box coupling.

Muff coupling is fall in the category of rigid coupling.

Features of muff coupling :

• Easy to manufacture.
• Components are easy to assemble and Dismantling.
• Their high torque capabilities make them suitable for higher RPM power transmission applications.
• Due to rigid connection lubrication is not required.
• Maintenance is minimal.
• Low operational cost due to no maintenance and no lubrication
• No moving part hence smooth and quiet operation.

Muff coupling is usually designed on shop floors by assuming standard proportions for the dimensions of the sleeve.

For the sleeve muff coupling the standard proportions used are as follows :

D = ( 2d + 13 ) mm 

L = 3.5 d

Where 

D = Outer diameter of the sleeve ( mm )

L = Axial length of the sleeve ( mm )

d = Diameter of the shaft ( mm )

What is coupling

This article is all about couplings and different types of coupling.

Coupling :

What is coupling?

Answer :

• A coupling can be defined as a mechanical device that permanently joins two rotating shafts to each other.

In other words, the coupling is a device for connecting parts of machinery.

The most common application of coupling is joining of shafts of two separately built or purchased units so that new machine can be formed.

Clutch and coupling doing work for the same purpose but the main difference between them is coupling is permanent connection while the clutch can connect or disconnect two shafts at the will of the operator.

The shafts to be connected by the coupling may have col-linear axes, intersecting axis or parallel axis with a small distance in between depending upon that different coupling are used.

Oldham coupling is used to connect two parallel shafts when they are at a small distance apart.

Hooke's coupling is used to connect two shafts having intersecting axes.

The axis is co-linear or in the same line, rigid or flexible coupling are used.

Types of keys in machine design

There are different types of keys are available and there are a number of standards. 
Many ways to classify the keys. so let we check it out types of keys below :

• Saddle key and Sunk key
• Square key and Flat key
• Taper key and Parallel key
• Key with and without Glib-head

There are some special types of keys are also available. some of the special types of keys are given below :

• Woodruff key
• Kennedy key or Feather key 

This special types of keys are used in the following applications depending upon the following factors :

• Power to be transmitted
• Tightness of fit
• Stability of connection
• Cost 

Saddle key :

A saddle key is a key which fits in the key ways of the hub only there are no keyways on the shaft.

Saddle keys are suitable for light duty only. They tend to work loose rock on the shaft under heavy duty.

There are two types of saddle keys :

• Hollow saddle key
• Flat saddle key  

Hollow saddle key :

Hollow saddle keys is a taper key which fits in key ways in the hub and the bottom of the key is a shaped to fit the curved surface of the shaft. 

It is held on by friction, therefore suitable for light loads.

Flat saddle key :

Flat saddle keys is a taper key which fits in key ways in the hub and is flat on the shaft.

It is likely to slip round the shaft under load, therefore suitable for comparatively light loads.  

In both types of saddle keys friction between the shaft, key and hub prevent the relative motion between the shaft and the hub.

In case of the flat key, the resistance of slip is more than that of the hollow key, therefore flat saddle key is slightly superior to hollow shaft key as far as the power transmitting capacity is concerned.

Sunk key :

A sunk key is a key in which half the thickness of the key fits into the keyways on the shaft and the remaining half in the keyways on the hub.

In these keys, keyways are required both on the shaft as well as the hub of the mating element.

This is the standard form of the key.

In sunk key, power is transmitted due to shear resistance of the key.

The sunk key is suitable for the heavy-duty applications because there is no possibility of the key to slip around the shaft.

Square key and flat key :

Sunk keys are used in two ways :

• Square 
• Rectangular cross-section

A sunk key with rectangular cross-section is called a flat key.

The flat key is more stability as compared with the square key.

Square keys are used in general industrial machinery while flat keys are more suitable for machine tool applications.

Taper key and Parallel key :

A parallel key which is uniform in width as well as height throughout the length of the key.

A tapper key is also uniform in width but tapper in height.

Standard tapper is 1 in 100.

Tapper is provided due to the following reason :

• Due to tapper, it is easy to remove the key and dismantle the joint easily.
• Tapper insures tightness of joint in operating conditions and prevent loosening of the parts.

Glib-head key :

Glib keys are tapered and notched machine keys that are used on power transmission keyed shafts to hold pulleys and gears tightly on the shaft.

Taper keys are often provided with glib-head to facilitate removal.

Glib head key has following advantages as compare with parallel key, taper key :

• The taper surface results in wedge action and increases frictional force and the tightness of the joint.
• The taper surface facilitates easy removal of the key, particularly with glib-head keys.

Feather key :

A feather key is a parallel key which is fixed either to the shaft or to the hub and which permits relative axial movement between them. 

The feather key is a particular type of sunk key with uniform width and height.

Feather key used where the parts mounted on the shaft are required to slide along the shaft such as clutches or gear shifting devices. 

Feather key is an alternative to splined connection.

Woodruff key :

A woodruff key is a sunk key in the form of an almost semicircular disk of uniform thickness.

Woodruff keys are used on tapered shaft in machine tools and automobiles.

Advantages of woodruff key :

• It can be used on tapered shaft because it can align by slight rotation in the seat.
• The extra depth of key in the shaft prevents its tendency to slip over the shaft.

Disadvantages of woodruff key :

• The extra depth of key ways in the shaft increases stress concentration and reduces its strength.
• The key does not permit axial movement between the shaft and the hub.

Keys in machine design

Keys :

What is the key ?

• A key can be defined as a machine element which is used to connect the transmission shaft to rotating machine elements like pulleys, gears, sprockets or flywheels.

Key joints consisting of shaft, hub and key.

Keys may be made of plain carbon steels like 45C8 or 50C8.

Functions of key :

• To transmit the torque from the shaft to the hub of the mating element and vice versa.
• To prevent relative motion between the shaft and the joined machine element like gear or pulley.
• In some cases, it is also used to prevent axial motion between two elements in case of father key or splined connection.

A drawback of the key joint :

• The main drawback of a key joint is due to keyway stress concentration in the shaft and the part becomes weak.

There are different types of keys are available and there is a number of standards.

Castigliano's theorem

Italian engineer Alberto Castigliano developed a method of determining the deflection of structures by strain energy method.

He is known for his two theorems :

1. Castigliano's first theorem - for forces in an elastic structure
2. Castigliano's second theorem - for displacements in a linearly elastic structure.

We can discuss the second theorem below :

Castigliano's theorem statement : 

It is mainly used in machine design while designing shafts, keys and coupling.

• When a body is elastically deflected by any combination of forces or moments, the deflection at any point and any direction are equal to the partial derivative of total strain energy of the body with respect to the force located at that point and acting in that direction.

Castigliano's theorem is one of the important techniques for determining the deflection of a complex structure.

Castigliano's theorem is applicable only in the elastic range of the materials.

Procedure :

Steps :

1. Write an expression for each of the internal actions (axial force, bending moment, shear force, and torque) in each member of the structure in terms of external loads. 
2. Take derivatives of strain energy to get deflections and/or rotations.

For example :

Consider an elastic body subjected to a system of forces P₁, P₂, P₃ etc and U is total strain energy of the body. ∂1,∂2,∂3 are deflection at point of application and in direction of P₁, P₂, P₃ etc. then according to the theorem,

∂₁ = ∂U / ∂P₁

∂₂ = ∂U / ∂P₂

∂₃ = ∂U / ∂P₃

∂_I = ∂U / ∂P_i

Ergonomic consideration in design

First of all, before we go to the point where the ergonomic used in the design of the machine. We should know first what is ergonomic?

Ergonomics :

Ergonomics is defined as the relationship between man and machine and the application of anatomical and physiological to solve the problems arising from the man-machine relationship.

The word ergonomics comes from the two Greek words :

Ergon means work and Nomos means Natural laws.

So we see above Ergonomics means the natural laws of work.

In the following design concentration ergonomics study are important :

• Design of hand levers and handwheels.
• The layout of instrument dials and displays panels for accurate perception by the operators.
• Anatomical factors in the design of a driver's seat.
• Energy expenditure in hand and foot operations.
• Lighting, noise and climatic conditions in the machine environment.
• The ergonomics applied during the design phase of this door assures the easy and correct use, without any harms to the driver.

We have seen above how ergonomics must be present at all stages of development of a project. 

In short, we must emphasize the understanding and the contribution of ergonomics to the management of the design. You have considered ergonomics as part of the whole design process.

Use of standards in machine design

The following standards are used to all mechanical engineer while design a machine components.

• Standards for materials, their chemical compositions, mechanical properties and heat treatment.
• Standards for shapes and dimensions of commonly used machine elements.
• Standards for fits, tolerance and surface finish of components.
• Standards for testing of products.
• Standards for engineering drawing of components.

Mainly there are three types of standards used in design.

• Company standards

They are used in particular company or group of company.

• National standards

National standards are :

IS ( Bureau of Indian Standards ) 

DIN ( German )

AISI or SAE ( USA )

BS ( UK )

• International standard

These are prepared by the International Standards Organization ( ISO )

What is hardness

What is hardness :

Answer 

• Hardness if defined as the resistance of the material to penetration or permanent deformation.

Hardness of the material depends upon the resistance of plastic deformation.

What is brittleness

What is brittleness :

Answer 

• Brittleness is the properties of a material which shows negligible plastic deformation before fracture takes place.

Brittleness is the opposite to ductility.

What is ductility

What is ductility?

Answer 

• Ductility is defined as the ability of a material to deform to a greater extent before the sign of a crack when it is subjected to a tensile force.

Ductility is a permanent strain that accompanies fracture in a tension test.

Ductility is characteristic of substances with metals.

You can also check it out: 

• Property of ductility. 

What is malleability

What is malleability?

Answer :

• Malleability is defined as the ability of a material to deform to a greater extent before the sign of a crack, when it is subjected to a compressive force.

Malleability comes from a word from a hammer.

What is toughness

What is toughness?

Answer :

• Toughness is defined as the ability of the material to absorb energy before fracture takes place.

Toughness is the energy for failure by fracture.

Toughness is important for machine components which are required to withstand impact loads.

Toughness is measured by a quantity called modulus of toughness.

What is resilience

What is resilience?

Answer :

• Resilience is defined as the ability of the material to absorb energy when deformed elastically and to release this energy when unloaded.

A resilient material absorbs energy within elastic range without any permanent deformation.

Resilience is measured by a quantity called modulus of resilience.

What is stiffness

What is stiffness :

Answer

• Stiffness is defined as the ability of the material to resist deformation under the action of eternal load.

All materials are deform when they are stressed, to a more or less extent.

Stiffness is also called like rigidity.