Machine Design Flashcards
the creation of new and better machines and improving the existing ones. A new or better machine is one which is more economical in the overall cost of production and operation.
Machine Design
Classification of New Design
- Rational design.
- Empirical design.
- Industrial design.
- Adaptive design
- Development design
- New design.
- Optimum design.
- System design.
- Element design.
- Computer aided design.
General Consideration in Machine Design
- Type of load and stresses caused by the load.
- Motion of the parts or kinematics of the machine.
- Selection of materials
- Form and size of the parts.
- Frictional resistance and lubrication.
- Convenient and economical features.
7.Use of standard parts. - Safety of operation.
- Workshop facilities.
- Number of machines to be manufactured.
- Cost of construction.
- Assembling.
GENERAL PROCEDURE IN MACHINE DESIGN
- Recognition of need
- Synthesis (Mechanisms).
- Analysis of forces.
- Material selection.
- Design of elements (Size and Stresses).
- Modification.
- Detailed drawing.
- Production.
refers to selecting the correct materials for the application in which the engineered part is being used.
This selection process includes choosing the material, different classifications of engineering materials, properties of engineering materials, mechanical properties of materials and applications of engineering materials.
Engineering Materials
high resistance to corrosion
Non-Ferrous
Non-Metal Eningeering Materials
Organic and Inorganic
Metal Engineering Materials
Ferrous and Non-Ferrous
Properties of Engineering Materials
Mechanical properties
Physical properties
Electrical properties
Chemical properties
Thermal properties
Magnetic properties
materials are the properties which describe the behavior of the material under the action of external forces.
mechanical properties
defined as the ability of the material to resist, without rupture, external forces causing various types of stresses.
Strength
defined as the ability of the material to regain its original shape and size after the deformation, when the external forces are removed.
Elasticity
defined as the ability of the material to retain the deformation produced under the load on permanent basis.
In this case, the external forces deform the metal to such an extent that it cannot fully recover its original dimensions.
Plasticity
BONUS:
Elasticity is the ability of metal to regain its original shape after temporary deformation under external force. Plasticity is the ability to retain the deformation permanently even after the load is removed.
The amount of elastic deformation is very small while plastic deformation is relatively the same.
During the elastic deformation, atoms of metals are temporarily displaced from their original position but return back when the load is removed. During the plastic deformation, atoms of metal are permanently displaced from their original positions and take up new positions.
For majority if materials, the stress-strain relationship is linear in elastic range and non-linear is plastic strange.
Elasticity is an important consideration in machine tool components while plasticity is desirable for components made by press working operations.
BONUS
This is theproportionality limit, which represents the maximum value of stress at which the stress-strain curve is linear.
P
represents the maximum value of stress at which there is no permanent set.
E: This is theelastic limit,
represents the value of stress above which the strain will begin to increase rapidly.
Y: This is theyield point,
which is the maximum value of stress on the stress-strain diagram.
U: This point corresponds to theultimate strength,Stu,
which is the point at which the material fails and separates into two pieces
F: This is thefracture pointor thebreak point,
commonly needed when analyzing an engineered component.
Stress-strain curves
or rigidity is defined as the ability of the material to resist deformation under the action of external load
Stiffness
defined as the ability of material to absorb energy when deformed elastically and to release their energy when unloaded.
Resilience
defined as the ability of the material to absorb energy before fracture takes place. In other words, _______ is the energy for failure by fracture.
Toughness
BONUS:
Resilience is the ability of the material to absorb energy within elastic range.
Toughness is the ability to absorb energy within elastic and plastic range.
Modulus of resilience is the area below stress-strain curve in tension test up to yield point.
Modulus of toughness is the total area below stress-strain curve.
defined as the ability of material to deform to a greater extent before signs of crack appear, when it is subjected to compressive force.
Malleability
defined as the ability of the material to deform to a greater extent before the sign of crack, when it is subjected to tensile force.
Ductility
All ductile materials are also malleable; however, the converse is not always true. (True or False)
True
is that property of the material which shows negligible platform deformation before fracture takes place. __________ is opposite to ductility.
Brittleness
Ductile materials deform to a greater extent before fracture in tension test.
Brittle materials shows negligible plastic deformation prior to fracture.
True or False?
True
defined as the resistance of the materials to penetration or permanent deformation.
Hardness
the property of the material which indicates the ease with which two similar or dissimilar metals join together. It is the ability of a material to get welded.
Weldability
This is the measure of the ease with which a material can be machined or finished.
Machinability
This is a form of failure that occurs in components subjected to dynamic and fluctuating loads.
Fatigue
This is the slow plastic deformation of metal under constant loads. Usually at high temperatures.
Creep
required to estimate the quality and condition of material without any external force.
Physical Properties
the ratio of mass to the volume of the material
Bulk density
gives the volume of the material occupied by pores
Porosity
– the property of a material to withstand against the combined action of atmospheric and other factors
Durability
ratio of mass of material to its volume in homogeneous state.
Density
– ratio of bulk density of material to its density
Density index
ratio of mass of given substance of water at 4oC for the equal volumes.
Specific gravity
– the ability to withstand against fire without changing its shape and other properties
Fire resistance
the ability of material to resist freezing or thawing
Frost resistance
the property of a material to withstand against all atmospheric actions without losing its strength and shape
Weathering resistance
abilityof amaterial to undergo certain numberof cycles ofsharptemperature variations without failing
Spalling resistance
The property of materials against the chemical actions or chemical combinations
Chemical Properties
ability to resist the effects by chemicals like acids, salts, and alkalis.
Chemical Resistance
formation of rust (iron oxide) in metals, when they are subjected to atmosphere is called corrosion. So, the metals should be corrosive resistant.
Corrosion Resistance
the property of material to absorb heat and it is required to design proper ventilation.
Thermal Capacity
the amount of heat transferred through unit area of specimen with unit thickness in unit time the amount of heat transferred through unit area of specimen with unit thickness in unit time
Thermal conductivity Thermal conductivity
is the ability to resist heat conduction, the reciprocal of thermal conductivity.
Thermal Resistivity
is the quantity of heat required to heat 1N of material by 1oC
Specific Heat
The properties of material to conduct or to resist electricity through them.
Electrical Properties
material like permeability, hysteresis etc. are required in the case pf generators etc. Iron is magnetic material and aluminum is non-magnetic material.
Magnetic Properties
processes used for the preliminary shaping of the machine component
Primary shaping processes.
processes used for giving final shape to the machine component, according to planned dimensions
Machining processes.
processes used to provide a good surface finish for the machine component
Surface finishing processes.
processes used for joining machine components
Joining processes.
processes are used to impart certain specific properties to the machine components so as to make them suitable for particular operations or uses
Processes effecting change in properties.
one of the most important manufacturing process used in Mechanical Engineering. The _________ are obtained by remelting of ingots* in a cupola or some other foundry furnace and then pouring this molten metal into metal or sand molds.
Casting
casting produced by pouring molten metal in sand mold
Sand Mold Process
It is a special application of permanent metal mold casting. This method is used for production of hollow castings without the use of cores.
Slush Casting
casting produced by forcing molten metal under pressure into a permanent metal mold (known as die)
Die casting.
Advantages of Die Casting
a) The production rate is high, ranging up to 700 castings per hour.
(b) It gives better surface smoothness.
(c) The dimensions may be obtained within tolerances.
(d) The die retains its trueness and life for longer periods. For example, the life of a die for zinc base castings is up to one million castings, for copper base alloys up to 75 000 castings and for aluminum base alloys up to 500 000 castings.
(e) It requires less floor area for equivalent production by other casting methods.
(f) By die casting, thin and complex shapes can be easily produced.
(g) The holes up to 0.8 mm can be cast.
Disadvantages of Die Casting
a) The die casting units are costly.
(b) Only non-ferrous alloys are casted more economically.
(c) It requires special skill for maintenance and operation of a die casting machine.
casting produced by a process in which molten metal is poured and allowed to solidify while the mold is kept revolving
Centrifugal casting.
process of heating a metal to a desired temperature in order to acquiresufficient plasticity, followed by operations like hammering, bending and pressing, etc. to give it a desired shape.
Forging
Type of forging that is done by means of hand tools and it is usually employed for small jobs.
smith or hand forging
the forging is done by means of power hammers
power forging
done by means of forging machines
machine forging
carried out with the help of drop hammers and is particularly suitable for mass production of identical parts.
drop forging
working of metals above the *recrystallization temperature
hot working
Advantages of Mechanical Working of Metals
The porosity of the metal is largely eliminated.
The grain structure of the metal is refined.
The impurities like slag are squeezed into fibers and distributed throughout the metal.
The mechanical properties such as toughness, ductility, percentage elongation, percentage reduction in area, and resistance to shock and vibration are improved due to the refinement of grains.
process is the most rapid method of converting large sections into desired shapes. It consists of passing the hot ingot through two rolls rotating in opposite directions at the same speed.
Hot rolling.
It consists of heating the metal to forging temperature and then forming it into the desired shape on a spinning lathe.
Hot spinning.
It consists of pressing a metal inside a chamber to force it out by high pressure through an orifice which is shaped to provide the desired form of the finished part.
Hot extrusion.
It is mostly used for the production of thick walled seamless tubes and cylinders. It is usually performed in two stages.
Hot drawing or cupping.
This process is used for the manufacture of seamless tubes. In its operation, the heated cylindrical billets of steel are passed between two conical shaped rolls operating in the same direction.
Hot piercing.
working of metals below their recrystallization temperature is known as ________
Cold Working
. The increase in hardness due to cold working is called
work-hardening.
It is generally employed for bars of all shapes, rods, sheets and strips, in order to provide a smooth and bright surface finish. It is also used to finish the hot rolled components to close tolerances and improve their toughness and hardness.
Cold rolling.
This process of cleaning the articles is known as
pickling.
. During this method of cold working, the metal is allowed to flow in some pre-determined shape according to the design of dies, by a compressive force or impact. It is widely used in forming ductile metals.
cold forging
The cold forging is also called
swaging.
It is the simplest form of cold forging. It is the operation of slightly compressing a forging, casting or steel assembly to obtain close tolerance and a flat surface.
Sizing.
It is the simplest form of cold forging. It is the operation of slightly compressing a forging, casting or steel assembly to obtain close tolerance and a flat surface.
Sizing.
This process is extensively used for making bolts, rivets and other similar headed parts.
Cold heading.
This method is used for reducing the diameter of round bars and tubes by rotating dies which open and close rapidly on the work.
Rotary swaging.
is similar to hot spinning except that the metal is worked at room temperature
Cold spinning.
The bars, wires, tubes, structural shapes and sheet metal may be bent to many shapes in cold condition through dies.
Cold bending.
This process is used to improve the fatigue resistance of the metal by setting up compressive stresses in its surface.
Cold peening.
The Machine Parts are subjected to Various Forces
Energy transmitted,
Weight of machine,
Frictional resistances,
Inertia of reciprocating parts,
Change of temperature, and
Lack of balance of moving parts.
It is defined as any external force acting upon a machine part.
Load
The following four types of the load
Dead or steady load.
Live or variable load.
Suddenly applied or shock loads
Impact load.
When some external system of forces or loads act on a body, the internal forces (equal and opposite) are set up at various sections of the body, which resist the external forces.
Stress
When a body is subjected to two equal and opposite axial pulls P
tensile stress
The ratio of the increase in length to the original length
tensile strain
When a body is subjected to two equal and opposite axial pushes P (also called compressive load)
compressive stress
. The ratio of the decrease in length to the original length
compressive strain
_________ * states that when a material is loaded within elastic limit, the stress is directly proportional to strain
Hooke’s law*
E is a constant of proportionality known as
Young’s modulus or modulus of elasticity.
it is measured by the angular deformation accompanying the shear stress. The shear stress and shear strain are denoted by the Greek letters tau (τ) and phi (φ) respectively. Mathematically,
shear strain
localized compressive stress at the surface of contact between two members of a machine part that are relatively at rest
bearing stress or crushing stress
When designing machine parts, it is desirable to keep the stress lower than the maximum or ultimate stress at which failure of the material takes place. This stress is known as
working stress or design stress
defined, in general, as the ratio of the maximum stress to the working stress.
factor of safety=maximum stress ÷working or design stress
In case of ductile materials e.g. mild steel, where the yield point is clearly defined, the factor of safety is based upon the yield point stress
factor of safety=yield point stress ÷working or design stress
the factor of safety for brittle materials is based on ultimate stress.
factor of safety=ultimate stress ÷working or design stress
Stresses due to Change in Temperature
thermal stresses.
every direct stress is accompanied by a strain in its own direction which is known as
linear strain
an opposite kind of strain in every direction, at right angles to it, is known
lateral strain.
Poisson’s Ratio
lateral strain/linear strain
The ratio of the change in volume to the original volume is known as
volumetric strain.
the ratio of the direct stress to the corresponding volumetric strain is known as
bulk modulus. (K)
The stress produced in the member due to the falling load
impact stress.
The strain energy stored in a body due to external loading, within elastic limit
Resilience
defined as a combination of rigid and resistant bodies having definite motion and capable of performing some useful work.
Machine
is a simplified model, frequently in the form of a line diagram, which will reproduce exactly the same motion that takes place in actual machine. The fundamental objective in case of mechanism is to achieve a desired motion.
Mechanism
is also a combination of rigid and resistant bodies, but there is no relative motion between its various parts.
Structure
may consist of other parts, which are manufactured separately.
Machine element
Machine Element Basic Requirement
STRENGTH
RIGIDITY
WEAR RESISTANCE
MINIMUM DIMENSIONS AND WEIGHT
MANUFACTURABILITY
SAFETY
CONFORMANCE TO STANDARDS
RELIABILITY
MAINTAINABILITY
MINIMUM LIFE CYCLE COST
STEPS OF DESIGNING OF MACHINE ELEMENTS
- SPECIFICATION OF FUNCTION
- DETERMINATION OF FORCE
- SELECTION OF MATERIAL
- FAILURE CRITERION
- DETERMINATION OF DIMENSIONS
- DESIGN MODIFICATIONS
- WORKING DRAWING
Uses of Mechanisms
Force amplification eg. That given by levers
Change of speed eg. That driven by gears
Transfer of rotation about one axis to rotation about another eg. A timing belt
Particular types of motion, eg. That given by quick-return mechanism
Study of motion without regard to forces.
Kinematics
Types of Motion
Translational
Rotational
Complex motion
are the number of components of motions that are required in order to generate the motion
Degrees of freedom
is needed for each degree of freedom that is to be prevented from occurring.
Constraints
Each part of a mechanisms which has motion relative to some other part is called
link
attachment to the links
Nodes
connection between two or more links at their nodes
Joint
Series of joints and links
Kinematic Chains
Consists of four links connected to give four joints about which turning can occur
The Four Bar Chain
mechanisms are used to obtain large force amplification in such applications as sheet metal punching and formingmachines.
Toggle
body which rotates or oscillates and in doing so imparts a reciprocating or oscillatory motion to a second body called the follower
Cams
can be used for transmission of mechanical power between two rotating shafts.
DRIVES
three types of drives
belt drives, chain drives and gear drives.
transfer the torque between two shafts (drive/driven).
Flat Belt Drives
Types of belt drives
Light drives – use to transmit smaller power, 10 m/s as in agricultural machines and small machines
Medium drives – use to transmit medium power, over 10m/s but up to 22m/s as in machine tools
Heavy drives – used to transmit large powers, above 22m/s as in compressors and generators.
Types of Belts
Flat Belts - mostly used in factories and workshops where a moderate amount of power is to be transmitted, not more that 8 meters apart
V-belt – mostly used in factories and workshops where great power is to be transmitted, when two pulleys are very near
Circular belt/rope – mostly used in factories and workshop where great power is to be transmitted, when two pulleys are more than 8 meters apart
Advantages of V-belt drive over Flat Belt
V-belt drive gives compactness due to small distance between centers of pulley
The drive is positive, because the slip between the belt and pulley groove is negligible
Since v-belts are made endless and there is no joint trouble, the drive is smooth.
It provides longer life, 3 to 5 years
are widely used where a large amount of power is to be transmitted, from one pulley to another, over a considerable distance.
Ropes
Two types of ropes
Fibre ropes (60m apart)
Wire ropes (150m apart)
are mostly used to transmit motion and power from one shaft to another, when the center distance between their shafts is short such as in bicycle, motor cycles, agricultural machinery, conveyors, rolling mills, road rollers, etc.
Chains
Normally made out of the following materials: Low Carbon Steel, High Carbon and Cast Iron.
The Sprocket
An idler can be used to take up the slack or the shaft can be adjusted.
Roller chain layout
Oils to avoid
Heavy oil
Low-grade oil
impure oil or grease
Used oil
widely in use. The advantage of this belt type is the constant speed for driven wheel and drive wheel.
Positive-belt drives
is a rotating element that gives oscillating or reciprocating motion to the follower which is another element of this machine by direct contact.
cam
cam has not regular contour to transmit a specific motion to the follower.
Disk or Plate Cam:
has a groove in a cylindrical surface and the follower runs on the cylindrical surface parallel to the axis of the cylinder.
cylindrical cam
has a grooved or contoured plate and its follower oscillates in the face of the plate.
Translating cam
has an angled flat regular contour to impart a specific motion of the follower.
The wedge cam
has a half-circular or a spiral shaping grooved contour, the cam moves in reciprocating motion and the follower moves vertically to the axis of the cam.
spiral cam
This type of cam looks like an asymmetric heart.
Heart-shaped Cam
is a rotating or an oscillating element of a machine that follows the motion of cam by direct contact.
follower
This type of follower has a sharp area of contact with the cam
Knife-edge Follower:
This type of follower is mainly used in high-speed operation because it has a smooth contact with the surface.
Roller Follower:
This type of follower looks like a flat surface with an irregular cam.
Flat-faced Follower
This type of follower has a curved but regular follower as well as cam.
Spherical Follower
In this type of follower, the line of movement passes through the center of the camshaft.
Radial Follower
In this type of follower, the movement of the axis of the follower is not co-linear with the cam axis.
Offset Follower
It is the surface area of a cam where follower touches
Cam Profile
It is the smallest circle of a cam profile drawn from the center of rotation of the cam
Base Circle
By this point, we can trace the cam profile.
Trace Point
It is formed between normal to the pitch curve and line of motion of the follower
Pressure Angle
At this point the pressure angle is maximum.
Pitch Point
It is the circle which passes through the pitch point
Pitch Circle
is a rotating member usually of circular cross-section (solid or hollow), which transmits power and rotational motion.
SHAFT
rotates on rolling contact bearings or bush bearings
SHAFT
used to transmit power from the drive shaft (e.g., motor) to the driven shaft (e.g. gearbox, wheels).
Couplings
used for shafts that support rotating elements like wheel, hoisting drum or rope sheave and which is fitted to the housing by means of bearing.
AXLE
a short rotating shaft. It originated from the round tapering stick on the spinning wheel on which the thread is twisted.
SPINDLE
is a secondary shaft, which is driven by the main shaft and from the power is supplied to a machine component.
COUNTERSHAFT-
it is an auxiliary or intermediate shaft between two shafts that are used in transmission of power.
JACKSHAFT
shaft consists of a number of shafts which are connected in axial direction by means of coupling.
LINE SHAFT
standard length of the shafts
5 m, 6 m and 8 m
These shafts transmit power between the source and the machines absorbing power. The counter shafts, line shafts, overhead shafts and all factory shafts are _______
Transmission shafts.
These shafts form an integral part of the machine itself. The crank shaft is an example of ________
Machine shafts.
shafts may be designed on the basis of
- Strength, and 2. Rigidity and stiffness.
can be defined as a machine element, which is used to connect the transmission shaft to rotating machine elements like pulley, gear, sprocket or flywheel.
KEY
A recess or slot machines either in the shaft or in the hub to accommodate the key is called
Keyway
Classification of Keys
Saddle key and sunk key –fits in keyway of the hub only (no keyway on the shaft)
Square key and flat key – half of the thickness of key fits into the keyway of the shaft
Taper key or parallel key
Key with and without gib-head
Woodruff key – is a sunk key in the form of almost semicircular disk
Kennedy key -
Feather key – is a parallel key, which is fixed either to the shaft or to the hub
connects two shafts which are perfectly aligned.
Rigid coupling
connects two shafts having both lateral and angular misalignment.
Flexible coupling
a machine element which supports another moving machine element (known as journal).
bearing
the load acts perpendicular to the direction of motion of the moving element
radial bearings
the load acts along the axis of rotation
thrust bearings
the sliding takes place along the surfaces of contact between the moving element and the fix element. It is also known as plain bearings.
sliding contact bearings
the steel balls or rollers are interposed between the moving and fix elements.
rolling contact bearings
the angle of contact of the bearing with the journal is 360°
full journal bearing
the angle of contact of the bearing with the journal is 120
partial journal bearing
when a partial journal bearing has no clearance letter
fitted journal bearing
are those in which the working surfaces are completely separated from each other by the lubricant.
thick film bearings
are those in which all the lubricant is present, and the working surfaces partially contact each other at least part of the time.
thin-film bearings
operate without any lubricant present.
Zero film bearings
those which can support steady loads without any relative motion between the journal and the bearing.
hydrostatic or externally pressurized lubricated bearings
Properties of lubricants
Viscosity
Oiliness
Density
viscosity index
flashpoint
fire point
freezing point
the simplest form of journal bearing it is simply a block of cast iron with the whole for a shaft providing running fit.
Solid journal bearing
is an improvement of solid bearing in which a bush of brass or gunmetal is provided.
Bushed bearing
It is used for shafts running at high speeds and carrying heavy loads.
Split bearing or plummer block
it is used to guide or support the shaft which is subjected to a load along the axis of the shaft. Such types of bearings are mainly used in turbines and propeller shafts.
Thrust bearing
suitable for slow running and lightly loaded shaft.
Footstep or pivot bearings
the shafts continue through the bearing
Collar Bearing
the contact between the bearing surfaces is rolling instead of sliding as in sliding contact bearings.
Rolling Contact Bearing
is a kind of machine element in which teeth are cut around cylindrical or cone shaped surfaces with equal spacing.
they are used to transmit rotations and forces from the driving shaft to the driven shaft.
GEAR
types of gears
spur gears, helical gears, bevel gears, worm gears, gear rack, etc.
belong to the parallel shaft gear group and are cylindrical gears with a tooth line which is straight and parallel to the shaft. _________ are the most widely used gears
Spur Gear
are used with parallel shafts similar to spur gears and are cylindrical gears with winding tooth lines. They have better teeth meshing than spur gears and have superior quietness and can transmit higher loads
Helical gears
Same sized and shaped teeth cut at equal distances along a flat surface or a straight rod
Gear Rack
have a cone shaped appearance and are used to transmit force between two shafts which intersect at one point (intersecting shafts).
Bevel Gear
are bevel gears with curved tooth lines.
Spiral bevel gears
are a pair of same hand helical gears with the twist angle of 45° on non-parallel, non-intersecting shafts.
Screw Gear
are bevel gears with a speed ratio of 1.
Miter gears
A screw shape cut on a shaft is the worm, the mating gear is the worm wheel, and together on non-intersecting shafts
Worm Gear
have teeth cut on the inside of cylinders or cones and are paired with external gears.
Internal gear
a gear meshes with another gear while a sprocket meshes with a chain and is not a gear. Aside from a sprocket, an item that looks somewhat like a gear is a ratchet, but its motion is limited to one direction. T or F
T
Kinds of tooth shape
involute tooth shape (most commonly used), cycloid tooth shape (mostly used in clocks) and trochoid tooth shape.
Why is it difficult to obtain needed gears?
There is no standard for the gear itself
There is a diversity of gear specifications
Cannot obtain the desired gears
When only one gear is needed, the production cost is high
defined as an elastic body, whose function is to distort when loaded and to recover its original shape when the load is removed.
Spring
It is made up of a wire coiled in the form of a helix and is primarily intended for compressive or tensile loads.
Helical Springs
used in special applications where a telescoping spring or a spring with a spring rate that increases with the load is desired.
Conical and volute springs.
These springs may be helical o spiral.
Torsion Springs.
It consists of flat plates (known as leaves) of varying lengths held together by means of clamps and bolts. These are mostly used in automobiles
Laminated or leaf springs.
springs consist of a number of conical discs held together against slipping by a central bolt or tube.
Disc of belleville springs.
These springs are air or liquid springs, rubber springs, ring springs, etc. The fluids (air or liquid) can behave as compression springs.
Special Purpose Springs.
Spring designed to operate with a compressive load
Compression spring
Spring designed to operate with a tensile load.
Extension spring
Spring designed to operate with torque (twisting force);
Torsion spring
the starting point of every spring application problem.
load condition
is formed by cutting a continuous helical groove on a cylindrical surface.
screw thread
