ppt mech 5sem dom subject

DYNAMICS OF MACHINERYU5MEA19

Prepared by Mr.Shaik Shabbeer Mr.Vennishmuthu.VAssistant Professor, Mechanical DepartmentVelTech Dr.RR & Dr.SR Technical University

UNIT I : FORCE ANALYSIS Rigid Body dynamics in general plane motion Equations of motion - Dynamic force analysis - Inertia force and Inertia torque DAlemberts principle - The principle of superposition - Dynamic Analysis in Reciprocating Engines Gas Forces - Equivalent masses - Bearing loads - Crank shaft Torque - Turning moment diagrams - Fly wheels Engine shaking Forces - Cam dynamics - Unbalance, Spring, Surge and Windup.

Static force analysis.If components of a machine accelerate, inertia is produced due to their masses. However, the magnitudes of these forces are small compares to the externally applied loads. Hence inertia effect due to masses are neglected. Such an analysis is known as static force analysisWhat is inertia?The property of matter offering resistance to any change of its state of rest or of uniform motion in a straight line is known as inertia.

conditions for a body to be in static and dynamic equilibrium?Necessary and sufficient conditions for static and dynamic equilibrium areVector sum of all forces acting on a body is zeroThe vector sum of the moments of all forces acting about any arbitrary point or axis is zero.

Static force analysis and dynamic force analysis.If components of a machine accelerate, inertia forces are produced due to their masses. If the magnitude of these forces are small compared to the externally applied loads, they can be neglected while analysing the mechanism. Such an analysis is known as static force analysis.If the inertia effect due to the mass of the component is also considered, it is called dynamic force analysis.

DAlemberts principle.DAlemberts principle states that the inertia forces and torques, and the external forces and torques acting on a body together result in statical equilibrium.In other words, the vector sum of all external forces and inertia forces acting upon a system of rigid bodies is zero. The vector sum of all external moments and inertia torques acting upon a system of rigid bodies is also separately zero.

The principle of super position states that for linear systems the individual responses to several disturbances or driving functions can be superposed on each other to obtain the total response of the system.The velocity and acceleration of various parts of reciprocating mechanism can be determined , both analytically and graphically.

Dynamic Analysis in Reciprocating Engines-Gas Forces Piston efforts (Fp): Net force applied on the piston , along the line of stroke In horizontal reciprocating engines.It is also known as effective driving force (or) net load on the gudgeon pin.crank-pin effort.The component of FQ perpendicular to the crank is known as crank-pin effort.crank effort or turning movement on the crank shaft?It is the product of the crank-pin effort (FT)and crank pin radius(r).

Forces acting on the connecting rodInertia force of the reciprocating parts (F1) acting along the line of stroke.The side thrust between the cross head and the guide bars acting at right angles to line of stroke.Weight of the connecting rod.Inertia force of the connecting rod (FC)The radial force (FR) parallel to crank andThe tangential force (FT) acting perpendicular to crank

Determination of Equivalent Dynamical System of Two Masses by Graphical MethodConsider a body of mass m, acting at G asshown in fig 15.15. This mass m, may be replacedby two masses m1 and m2 so that the system becomes dynamical equivalent. The position of mass m1 may be fixed arbitrarily at A. Now draw perpendicular CG at G, equal in length of the radius of gyration of the body, kG .Then join AC and draw CB perpendicular to AC intersecting AG produced inB. The point B now fixes the position of the secondmass m2. The triangles ACG and BCG are similar. Therefore,

Turning movement diagram or crank effort diagram?It is the graphical representation of the turning movement or crank effort for various position of the crank.In turning moment diagram, the turning movement is taken as the ordinate (Y-axis) and crank angle as abscissa (X axis).

UNIT II : BALANCING Static and dynamic balancing - Balancing of rotating masses Balancing reciprocating masses- Balancing a single cylinder Engine - Balancing Multi-cylinder Engines, Balancing V-engines, - Partial balancing in locomotive Engines-Balancing machines.

STATIC AND DYNAMIC BALANCING When man invented the wheel, he very quickly learnt that if it wasnt completely round and if it didnt rotate evenly about its central axis, then he had a problem!What the problem he had?The wheel would vibrate causing damage to itself and its support mechanism and in severe cases, is unusable.A method had to be found to minimize the problem. The mass had to be evenly distributed about the rotating centerline so that the resultant vibration was at a minimum.

UNBALANCE:The condition which exists in a rotor when vibratory force or motion is imparted to its bearings as a result of centrifugal forces is called unbalance or the uneven distribution of mass about a rotors rotating centreline.

BALANCING:Balancing is the technique of correcting or eliminating unwanted inertia forces or moments in rotating or reciprocating masses and is achieved by changing the location of the mass centres.The objectives of balancing an engine are to ensure:1. That the centre of gravity of the system remains stationery during a complete revolution of the crank shaft and2. That the couples involved in acceleration of the different moving parts balance each other.

Types of balancing:a) Static Balancing:i) Static balancing is a balance of forces due to action of gravity.ii) A body is said to be in static balance when its centre of gravity is in the axis of rotation.b) Dynamic balancing:i) Dynamic balance is a balance due to the action of inertia forces.ii) A body is said to be in dynamic balance when the resultant moments or couples, which involved in the acceleration of different moving parts is equal to zero.iii) The conditions of dynamic balance are met, the conditions of static balance are also met.

BALANCING OF ROTATING MASSESWhen a mass moves along a circular path, it experiences a centripetal acceleration and a force is required to produce it. An equal and opposite force called centrifugal force acts radially outwards and is a disturbing force on the axis of rotation. The magnitude of this remains constant but the direction changes with the rotation of the mass.

In a revolving rotor, the centrifugal force remains balanced as long as the centre of the mass of rotor lies on the axis of rotation of the shaft. When this does not happen, there is an eccentricity and an unbalance force is produced. This type of unbalance is common in steam turbine rotors, engine crankshafts, rotors of compressors, centrifugal pumps etc.

The unbalance forces exerted on machine members are time varying, impart vibratory motion and noise, there are human discomfort, performance of the machine deteriorate and detrimental effect on the structural integrity of the machine foundation.Balancing involves redistributing the mass which may be carried out by addition or removal of mass from various machine members. Balancing of rotating masses can be of1. Balancing of a single rotating mass by a single mass rotating in the same plane.2. Balancing of a single rotating mass by two masses rotating in different planes.3. Balancing of several masses rotating in the same plane4. Balancing of several masses rotating in different planes

BALANCING OF A SINGLE ROTATING MASS BY A SINGLEMASS ROTATING IN THE SAME PLANEConsider a disturbing mass m1 which is attached to a shaft rotating at rad/s.

r = radius of rotation of the mass m

The centrifugal force exerted by mass m1 on the shaft is given by,F = m r c 1 1

This force acts radially outwards and produces bending moment on the shaft. In order to counteract the effect of this force Fc1 , a balancing mass m2 may be attached in the same plane of rotation of the disturbing mass m1 such that the centrifugal forces due to the two masses are equal and opposite.

BALANCING OF A SINGLE ROTATING MASS BY TWO MASSES ROTATINGThere are two possibilities while attaching two balancing masses:1. The plane of the disturbing mass may be in between the planes of the two balancing masses.2. The plane of the disturbing mass may be on the left or right side of two planes containing the balancing masses.In order to balance a single rotating mass by two masses rotating in different planes which are parallel to the plane of rotation of the disturbing mass i) the net dynamic force acting on the shaft must be equal to zero, i.e. the centre of the masses of the system must lie on the axis of rotation and this is the condition for static balancing ii) the net couple due to the dynamic forces acting on the shaft must be equal to zero, i.e. the algebraic sum of the moments about any point in the plane must be zero. The conditions i) and ii) together give dynamic balancing.

Balancing Multi-cylinder Engines, Balancing V-engines

Problem 1.Four masses A, B, C and D are attached to a shaft and revolve in the same plane. The masses are 12 kg, 10 kg, 18 kg and 15 kg respectively and their radii of rotations are 40 mm, 50 mm, 60 mm and 30 mm. The angular position of the masses B, C and D are 60 ,135 and 270 from mass A. Find the magnitude and position of the balancing mass at a radius of 100 mm.Problem 2:The four masses A, B, C and D are 100 kg, 150 kg, 120 kg and 130 kg attached to a shaft and revolve in the same plane. The corresponding radii of rotations are 22.5 cm, 17.5 cm, 25 cm and 30 cm and the angles measured from A are 45, 120 and 255. Find the position and magnitude of the balancing mass, if the radius of rotation is 60 cm.

UNIT III : FREE VIBRATIONBasic features of vibratory systems - idealized models - Basic elements and lumping of parameters - Degrees of freedom - Single degree of freedom - Free vibration - Equations of motion - natural frequency - Types of Damping - Damped vibration critical speeds of simple shaft - Torsional systems; Natural frequency of two and three rotor systems

INTRODUCTION19 - *Mechanical vibration is the motion of a particle or body which oscillates about a position of equilibrium. Most vibrations in machines and structures are undesirable due to increased stresses and energy losses.Time interval required for a system to complete a full cycle of the motion is the period of the vibration.Number of cycles per unit time defines the frequency of the vibrations.Maximum displacement of the system from the equilibrium position is the amplitude of the vibration.When the motion is maintained by the restoring forces only, the vibration is described as free vibration. When a periodic force is applied to the system, the motion is described as forced vibration.When the frictional dissipation of energy is neglected, the motion is said to be undamped. Actually, all vibrations are damped to some degree.

FREE VIBRATIONS OF PARTICLES. SIMPLE HARMONIC MOTION19 - *x is a periodic function and wn is the natural circular frequency of the motion.

FREE VIBRATIONS OF PARTICLES. SIMPLE HARMONIC MOTION19 - *

SIMPLE PENDULUM (APPROXIMATE SOLUTION)19 - *Results obtained for the spring-mass system can be applied whenever the resultant force on a particle is proportional to the displacement and directed towards the equilibrium position.

SIMPLE PENDULUM (EXACT SOLUTION)19 - *

SAMPLE PROBLEM 19 - *A 50-kg block moves between vertical guides as shown. The block is pulled 40mm down from its equilibrium position and released.For each spring arrangement, determine a) the period of the vibration, b) the maximum velocity of the block, and c) the maximum acceleration of the block.

For each spring arrangement, determine the spring constant for a single equivalent spring.Apply the approximate relations for the harmonic motion of a spring-mass system.

SAMPLE PROBLEM 19 - *

Springs in parallel:determine the spring constant for equivalent spring

SAMPLE PROBLEM 19 - *Springs in series:determine the spring constant for equivalent spring

FREE VIBRATIONS OF RIGID BODIES19 - *Analysis objective is to determine wn.

SAMPLE PROBLEM 19 - *A cylinder of weight W is suspended as shown.Determine the period and natural frequency of vibrations of the cylinder.

From the kinematics of the system, relate the linear displacement and acceleration to the rotation of the cylinder.Based on a free-body-diagram equation for the equivalence of the external and effective forces, write the equation of motion.Substitute the kinematic relations to arrive at an equation involving only the angular displacement and acceleration.

SAMPLE PROBLEM 19 - *The disk and gear undergo torsional vibration with the periods shown. Assume that the moment exerted by the wire is proportional to the twist angle.Determine a) the wire torsional spring constant, b) the centroidal moment of inertia of the gear, and c) the maximum angular velocity of the gear if rotated through 90o and released.

Using the free-body-diagram equation for the equivalence of the external and effective moments, write the equation of motion for the disk/gear and wire.With the natural frequency and moment of inertia for the disk known, calculate the torsional spring constant.With natural frequency and spring constant known, calculate the moment of inertia for the gear.Apply the relations for simple harmonic motion to calculate the maximum gear velocity.

PRINCIPLE OF CONSERVATION OF ENERGY19 - *

SAMPLE PROBLEM 19 - *Determine the period of small oscillations of a cylinder which rolls without slipping inside a curved surface.

Apply the principle of conservation of energy between the positions of maximum and minimum potential energy. Solve the energy equation for the natural frequency of the oscillations.

SAMPLE PROBLEM19 - *

FORCED VIBRATIONS19 - *

SAMPLE PROBLEM 19 - *A motor weighing 350 lb is supported by four springs, each having a constant 750 lb/in. The unbalance of the motor is equivalent to a weight of 1 oz located 6 in. from the axis of rotation. Determine a) speed in rpm at which resonance will occur, and b) amplitude of the vibration at 1200 rpm.

The resonant frequency is equal to the natural frequency of the system.Evaluate the magnitude of the periodic force due to the motor unbalance. Determine the vibration amplitude from the frequency ratio at 1200 rpm.


The resonant frequency is equal to the natural frequency of the system.W = 350 lbk = 4(350 lb/in)Resonance speed = 549 rpm

SAMPLE PROBLEM 19 - *W = 350 lbk = 4(350 lb/in)Evaluate the magnitude of the periodic force due to the motor unbalance. Determine the vibration amplitude from the frequency ratio at 1200 rpm.xm = 0.001352 in. (out of phase)

DAMPED FREE VIBRATIONS19 - *All vibrations are damped to some degree by forces due to dry friction, fluid friction, or internal friction.

DAMPED FREE VIBRATIONS19 - *

DAMPED FORCED VIBRATIONS19 - *

ELECTRICAL ANALOGUES19 - *Oscillations of the electrical system are analogous to damped forced vibrations of a mechanical system.

ELECTRICAL ANALOGUES19 - *The analogy between electrical and mechanical systems also applies to transient as well as steady-state oscillations.With a charge q = q0 on the capacitor, closing the switch is analogous to releasing the mass of the mechanical system with no initial velocity at x = x0.If the circuit includes a battery with constant voltage E, closing the switch is analogous to suddenly applying a force of constant magnitude P to the mass of the mechanical system.

ELECTRICAL ANALOGUES19 - *The electrical system analogy provides a means of experimentally determining the characteristics of a given mechanical system.The governing equations are equivalent. The characteristics of the vibrations of the mechanical system may be inferred from the oscillations of the electrical system.

UNIT IV : FORCED VIBRATIONResponse to periodic forcing - Harmonic Forcing - Forcing caused by unbalance - Support motion Force transmissibility and amplitude transmissibility - Vibration isolation.

DAMPINGa process whereby energy is taken from the vibrating system and is being absorbed by the surroundings.Examples of damping forces:internal forces of a spring,viscous force in a fluid,electromagnetic damping in galvanometers,shock absorber in a car.

DAMPED VIBRATION (1)The oscillating system is opposed by dissipative forces.The system does positive work on the surroundings.Examples:a mass oscillates under wateroscillation of a metal plate in the magnetic field

DAMPED VIBRATION (2)Total energy of the oscillator decreases with timeThe rate of loss of energy depends on the instantaneous velocityResistive force instantaneous velocityi.e. F = -bv where b = damping coefficientFrequency of damped vibration < Frequency of undamped vibration

TYPES OF DAMPED OSCILLATIONS (1)Slight damping (underdamping)Characteristics:- oscillations with reducing amplitudes- amplitude decays exponentially with time- period is slightly longer - Figure -

TYPES OF DAMPED OSCILLATIONS (2)Critical dampingNo real oscillationTime taken for the displacement to become effective zero is a minimum

TYPES OF DAMPED OSCILLATIONS (3)Heavy damping (Overdamping)Resistive forces exceed those of critical dampingThe system returns very slowly to the equilibrium position

EXAMPLE: MOVING COIL GALVANOMETER the deflection of the pointer is critically damped

EXAMPLE: MOVING COIL GALVANOMETERDamping is due to induced currents flowing in the metal frameThe opposing couple setting up causes the coil to come to rest quickly

FORCED OSCILLATIONThe system is made to oscillate by periodic impulses from an external driving agentExperimental setup:

CHARACTERISTICS OF FORCED OSCILLATIONSame frequency as the driver systemConstant amplitudeTransient oscillations at the beginning which eventually settle down to vibrate with a constant amplitude (steady state)

CHARACTERISTICS OF FORCED OSCILLATIONIn steady state, the system vibrates at the frequency of the driving force

ENERGYAmplitude of vibration is fixed for a specific driving frequencyDriving force does work on the system at the same rate as the system loses energy by doing work against dissipative forcesPower of the driver is controlled by damping

AMPLITUDEAmplitude of vibration depends onthe relative values of the natural frequency of free oscillationthe frequency of the driving forcethe extent to which the system is damped

EFFECTS OF DAMPINGDriving frequency for maximum amplitude becomes slightly less than the natural frequencyReduces the response of the forced system

PHASE (1)The forced vibration takes on the frequency of the driving force with its phase lagging behindIf F = F0 cos t, then x = A cos (t - )where is the phase lag of x behind F

PHASE (2)Figure1. As f 0, 02. As f , 3. As f f0, /2ExplanationWhen x = 0, it has no tendency to move. maximum force should be applied to the oscillator

PHASE (3)When oscillator moves away from the centre, the driving force should be reduced gradually so that the oscillator can decelerate under its own restoring forceAt the maximum displacement, the driving force becomes zero so that the oscillator is not pushed any furtherThereafter, F reverses in direction so that the oscillator is pushed back to the centre

PHASE (4)On reaching the centre, F is a maximum in the opposite directionHence, if F is applied 1/4 cycle earlier than x, energy is supplied to the oscillator at the correct moment. The oscillator then responds with maximum amplitude.

FORCED VIBRATION Adjust the position of the load on the driving pendulum so that it oscillates exactly at a frequency of 1 HzCouple the oscillator to the driving pendulum by the given elastic cordSet the driving pendulum going and note the response of the blade

FORCED VIBRATION In steady state, measure the amplitude of forced vibrationMeasure the time taken for the blade to perform 10 free oscillationsAdjust the position of the tuning mass to change the natural frequency of free vibration and repeat the experiment

FORCED VIBRATION Plot a graph of the amplitude of vibration at different natural frequencies of the oscillatorChange the magnitude of damping by rotating the card through different anglesPlot a series of resonance curves

RESONANCE (1)Resonance occurs when an oscillator is acted upon by a second driving oscillator whose frequency equals the natural frequency of the systemThe amplitude of reaches a maximumThe energy of the system becomes a maximumThe phase of the displacement of the driver leads that of the oscillator by 90

RESONANCE (2)ExamplesMechanics:Oscillations of a childs swingDestruction of the Tacoma BridgeSound:An opera singer shatters a wine glassResonance tubeKundts tube

RESONANCE ElectricityRadio tuningLightMaximum absorption of infrared waves by a NaCl crystal

RESONANT SYSTEMThere is only one value of the driving frequency for resonance, e.g. spring-mass systemThere are several driving frequencies which give resonance, e.g. resonance tube

RESONANCE: UNDESIRABLEThe body of an aircraft should not resonate with the propellerThe springs supporting the body of a car should not resonate with the engine

DEMONSTRATION OF RESONANCE Resonance tubePlace a vibrating tuning fork above the mouth of the measuring cylinderVary the length of the air column by pouring water into the cylinder until a loud sound is heardThe resonant frequency of the air column is then equal to the frequency of the tuning fork

DEMONSTRATION OF RESONANCE SonometerPress the stem of a vibrating tuning fork against the bridge of a sonometer wireAdjust the length of the wire until a strong vibration is set up in itThe vibration is great enough to throw off paper riders mounted along its length

Oscillation of a metal plate in the magnetic field

SLIGHT DAMPING

CRITICAL DAMPING

HEAVY DAMPING

AMPLITUDE

BARTONS PENDULUM

DAMPED VIBRATION

RESONANCE CURVES

RESONANCE TUBEA glass tube has a variable water level and a speaker at its upper end

UNIT V : GOVERNORS AND GYROSCOPES Governors - Types - Centrifugal governors - Gravity controlled and spring controlled centrifugal governors Characteristics - Effect of friction - Controlling Force .Gyroscopes - Gyroscopic forces and Torques - Gyroscopic stabilization - Gyroscopic effects in Automobiles, ships and airplanes

GOVERNORSEngine Speed control This presentation is from Virginia Tech and has not been edited by Georgia Curriculum Office.

GOVERNORSGovernors serve three basic purposes:Maintain a speed selected by the operator which is within the range of the governor.Prevent over-speed which may cause engine damage.Limit both high and low speeds.

GOVERNORSGenerally governors are used to maintain a fixed speed not readily adjustable by the operator or to maintain a speed selected by means of a throttle control lever.

In either case, the governor protects against overspeeding.

HOW DOES IT WORK?If the load is removed on an operating engine, the governor immediately closes the throttle.If the engine load is increased, the throttle will be opened to prevent engine speed form being reduced.

EXAMPLEThe governor on your lawnmower maintains the selected engine speed even when you mow through a clump of high grass or when you mow over no grass at all.

PNEUMATIC GOVERNORSSometimes called air-vane governors, they are operated by the stream of air flow created by the cooling fins of the flywheel.

AIR-VANE GOVERNORWhen the engine experiences sudden increases in load, the flywheel slows causing the governor to open the throttle to maintain the desired speed.The same is true when the engine experiences a decrease in load. The governor compensates and closes the throttle to prevent overspeeding.

CENTRIFUGAL GOVERNORSometimes referred to as a mechanical governor, it uses pivoted flyweights that are attached to a revolving shaft or gear driven by the engine.

MECHANICAL GOVERNORWith this system, governor rpm is always directly proportional to engine rpm.

MECHANICAL GOVERNORIf the engine is subjected to a sudden load that reduces rpm, the reduction in speed lessens centrifugal force on the flyweights.The weights move inward and lower the spool and governor lever, thus opening the throttle to maintain engine speed.

VACUUM GOVERNORSLocated between the carburetor and the intake manifold.It senses changes in intake manifold pressure (vacuum).

VACUUM GOVERNORSAs engine speed increases or decreases the governor closes or opens the throttle respectively to control engine speed.

HUNTINGHunting is a condition whereby the engine speed fluctuate or is erratic usually when first started. The engine speeds up and slows down over and over as the governor tries to regulate the engine speed.This is usually caused by an improperly adjusted carburetor.

STABILITYStability is the ability to maintain a desired engine speed without fluctuating.Instability results in hunting or oscillating due to over correction.Excessive stability results in a dead-beat governor or one that does not correct sufficiently for load changes.

SENSITIVITYSensitivity is the percent of speed change required to produce a corrective movement of the fuel control mechanism.

High governor sensitivity will help keep the engine operating at a constant speed.

SUMMARYSmall engine governors are used to:

Maintain selected engine speed.Prevent over-speeding.Limit high and low speeds.

SUMMARYGovernors are usually of the following types:Air-vane (pneumatic)Mechanical (centrifugal)Vacuum

SUMMARYThe governor must have stability and sensitivity in order to regulate speeds properly. This will prevent hunting or erratic engine speed changes depending upon load changes.

GyroscopeA gyroscope consists of a rotor mounted in the inner gimbal. The inner gimbal is mounted in the outer gimbal which itself is mounted on a fixed frame as shown in Fig. When the rotor spins about X-axis with angular velocity rad/s and the inner gimbal precesses (rotates) about Y-axis, the spatial mechanism is forced to turn about Z-axis other than its own axis of rotation, and the gyroscopic effect is thus setup. The resistance to this motion is called gyroscopic effect.

GYROSCOPIC COUPLEConsider a rotary body of mass m having radius of gyration k mounted on the shaft supported at two bearings. Let the rotor spins (rotates) about X-axis with constant angular velocity rad/s. The X-axis is, therefore, called spin axis, Y-axis, precession axis and Z-axis, the couple or torque axis .

GYROSCOPIC EFFECT ON SHIP

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ppt mech 5sem dom subject

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