SME1912 Week 3 Linear n Force

7/28/2019 SME1912 Week 3 Linear n Force

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SME1912

EXPERIMENTAL

TECHNIQUES

Chapter 3 Part 1

Measurement of Linear Dimensions

& Forces


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WHAT WILL BE

DELIVERED?

Methods of measurement for

1. Linear dimensions,

2. Mass, weight and forces,

3. Pressure and

4. Temperature


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WHAT SHOULD STUDENTLEARN?

a. concept,

b. design,

c. application andd. limitation

of various apparatus for the measurement forlinear dimensions, forces, pressure and

temperature


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PART 1

MEASUREMENT OFLINEAR DIMENSIONS


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DEFINITION

What is linear?


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UNITS OF MEASUREMENTOF LINEAR DIMENSION?

Meter

Foot

Metric system


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TOOLS TO MEASURELINEAR DIMENSIONS

Ruler Caliper

Vernier calipers Screw gauges

Dial gauges


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RULERS

Normal - straight Folding ruler


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ACCURACY OF RULER


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CALIPERS

outside caliper inside caliper

Used together with rulers


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INTERESTING USE OF ACALIPER


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VERNIER CALIPERS


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Digital vernier caliper


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Measuringoutside diameter

Measuring inside

diameter

Measuring depth


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HOW TO READ A VERNIER


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Scale reading = 3.70cm

Vernier reading = (0.1/10) x 4 = 0.04

Caliper reading = 3.74cm


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SCREW GAUGE


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HOW TO READ SCREW

GAUGES

Main scale = 3.00mmThimble reading = (0.5/50)x46 = 0.46

Gauge reading = 3.46

0.5


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DIAL GAUGES(for thickness)


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IMPORTANT NOTES

For small displacement Mechanism based on plunger and rack and

pinion system


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THICKNESS GAUGES

RANGE & ACCURACY


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PART 2

MEASUREMENT OF MASS,WEIGHT & FORCES


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What is the concept of mass, weight and force

THE CONCEPT OF MASS &FORCE


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Mass defined

Mass (symbolized m) is a dimensionless quantity

representing the amount of matter in a particle or

object.

Mass is measured by determining the extent to which a

particle or object resists a change in its direction or

speed when a force is applied. Isaac Newton statedThe standard unit of mass in the International System

(SI) is the kilogram (kg).


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EQUIPMENT TO MEASURE

MASS


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EQUIPMENT TO MEASURE

MASS


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Precision balances


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Mass Measurement PrincipleIn its conventional form, this

class of measuringinstrument compares the

sample, placed in a weighing

pan (weighing basin) and

suspended from one end of a

beam with a standard mass or

combination of standard

masses in a scale pan (scale

basin) suspended from the

other end


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To weigh an object in the measuring pan, standard

weights are added to the scale pan until the beam isin equilibrium as closely as possible. Then a slider

weight usually present is moved along a scale on or

parallel to the beam (and attached to it) until finebalance is achieved. The slider position gives a fine

correction to the mass value

Standard weight

Slider weight


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The arm from which the object to be weighed (the load) is hung is

short and is located close to the pivot point. The other arm is longer,

is graduated and incorporates a counterweight which can be moved

along the arm until the two arms are balanced about the pivot, atwhich time the weight of the load is indicated by the position of the

counterweight

Steelyard Balance

The steelyard comprises a balance

beam which is suspended from a

pivot (or fulcrum) which is very

close to one end of the beam. The

two parts of the beam which flank

the pivot are the arms.


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Special notesPrecise measurements are achieved by

ensuring that the fulcrum of the beam is friction-free (a knife edge is the traditional solution),

by attaching a pointer to the beam which amplifiesany deviation from a balance position; and finally

by using the lever principle, which allowsfractional weights to be applied by movement of asmall weight along the measuring arm of the beam

allowing for the buoyancy in air, whose effectdepends on the densities of the weights and thesample.


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Weight & Force defined

Weight (symbolized w) is a quantity representingthe force exerted on a particle or object by anacceleration field, particularly the gravitationalfield of the Earth at the surface. In the

International System of Units (SI), weight can beexpressed in terms of the force, in newtons,exerted on a mass in a gravitational field.

The weight of an object is the force of gravity onthe object and may be defined as the mass timesthe acceleration of gravity, w = mg.


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use a spring with a known spring constant

(Hooke's law) and measure the

displacement of the spring to produce anestimate of the gravitational force applied

by the object.

typically measure force

cannot be used for commercial

applications unless their springs aretemperature compensated or used at a

fairly constant temperature

Spring Scale/Balance


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A proving ring consists

of an elastic ring inwhich the deflection ofthe ring when loaded

along a diameter ismeasured by means of amicrometer screw and a

vibrating reed.

The Proving Ring


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The proving ringconsists of two main

elements, the ringitself and thediameter-measuringsystem,

Forces are applied tothe ring through theexternal bosses.

The resulting change

in diameter, referredto as the deflection ofthe ring, is measuredwith a micrometerscrew and thevibrating reedmounteddiametrically withinthe ring.


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Force TransducerA device that provides an output quantity having a

determined relationship to the force

some materials change their electrical resistance

when mechanically deformed and can thus be

used as force transducers provided there is a

means available to measure the resistance

change


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Strain Gauges For Force

Transducers

gluing such a component to the side of the metal

block in the above example effectively scales-down

the 'squeezing' force applied to it - allowing it to

survive much higher forces


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A load cell is an electronic device (transducer) that is

used to convert a force into an electrical signal.

It happens in two stages; (1)mechanical force is sensed

is using a strain gauge, (2) the strain gauge converts the

deformation (strain) to electrical signals.A load cell consists 1 - 4 strain gauges in a wheatstone

bridge configuration, The output of the transducer is

plugged into an algorithm to calculate the force appliedto the transducer.

Load Cells


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Torque Measurement

Torque is measured by either sensing theactual shaft deflection caused by a twistingforce, or by detecting the effects of thisdeflection. The surface of a shaft under torquewill experience compression and tension, asshown.


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Torque Measurement By

Strain Gauges

To measure torque, strain gage elements usuallyare mounted in pairs on the shaft, one gaugemeasuring the increase in length (in the direction inwhich the surface is under tension), the other

measuring the decrease in length in the otherdirection.


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Excitation voltage for the strain gage is

inductively coupled, and the strain gageoutput is converted to a modulated pulsefrequency. Maximum speed of such anarrangement is 15,000 rpm.

The torquesensor can beconnected toits power

source andsignalconditioningelectronics via

a slip ring


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Proximity and displacement sensors also candetect torque by measuring the angulardisplacement between a shaft's two ends. Byfixing two identical toothed wheels to the

shaft at some distance apart, the angulardisplacement caused by the torque can bemeasured. Proximity sensors or photocellslocated at each toothed wheel produce

output voltages whose phase differenceincreases as the torque twists the shaft

Torque Measurement By

Angular Displacement


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DYNAMOMETERS TOMEASURE ENGINE POWER

Consists of an absorption unit, plus a means to measure

torque and rotational speed. It has some type of rotor in a

housing. The rotor is coupled to the engine under test and is

free to rotate. Some means is provided to develop a braking

torque between dynamometer's rotor and housing. The

means for developing torque can be frictional, hydraulic,

electromagnetic etc. according to the type of

absorption/driver unit


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Dynamometer Setup


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Types of Dynamometers

Absorption Driving

Transmission


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Absorption Dynamometer

Absorption dynamometers produce the torque that they

measure by creating a constant restraint to the turning of ashaft by either mechanical friction, fluid friction, or

electromagnetic induction. A Prony brake develops

mechanical friction on the periphery of a rotating pulleyby means of brake blocks that are squeezed against the

wheel by tightening the bolts...


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The Prony Brake


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Driving Dynamometer

A dynamometer can also be used to determine the torque

and power required to operate a driven machine such as apump. In that case, a motoringordrivingdynamometer is

used


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Transmission Dynamometer

A dynamometer in which power is measured, without

being absorbed or used up, during transmission

The transmission dynamometer transmits the force

while measuring the elastic twist of the output shaft.

Also called torsion meter as it measure the torque on a

shaft, and hence the horse power of an engine, esp. of a

marine engine of high power, by measuring the amount of

twist of a given length of the shaft


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Types of AbsorptionDynamometers

Mechanical Electrical

Hydraulic


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mg

BHP = Tx = Tx2N

but T = mgL

BHP = mgLx2N

L

Mechanical Absorption Dyno. -The Prony Brake

Electrical Dynamometer


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Electrical Dynamometer


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Hydraulic/water Brake Dyno.The housing

attempts to

rotate inresponse to the

torque

produced but

is restrained by

the scale or

torque

metering cellwhich

measures the

torque.

SME1912 Week 3 Linear n Force

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