Mechanical Engineers Data Handbook Episode 12 doc

I .4 Vernier calliper gauge This is used for internal and external measurement.. I .6 Gauge blocks slip gauges These are hardened, ground and lapped rectangular blocks of steel made i

ENGINEERING MATERIALS 265

C o e E i t s of linear expansion a( x IO6 "C- ') at normal temperature (unless otherwise stated)

9.9 ( l W 2 0 0 " C ) 11.9 (200-300 "C)

Gold Granite Graphite Gunmetal Ice Iron: cast Wrought Lead Magnesium Nickel Phosphor bronze Plaster Platinum Porcelain Quartz

17

8.9

4 8-14

18 (0-1oOo"C)

11 (cr8Oo0C)

Rubber: natural, soft natural, hard nitrile silicone Sandstone Silver Slate Solder (2 lead: 1 tin) Steel: hardened mild stainless Tin

Titanium Tungsten

Vanadium Zinc

10

25 12.4

11 10.4

21

9 4.5 (20°C)

Ammonium Crushed ice or

nitrate snow in water Temperature

Calcium Crushed ice or Temperature

chloride snow in water ("C)

6.2 I 4 CoeiRcients of cubical expansion

Solid carbon dioxide with alcohol - 72

Solid carbon dioxide with - 77

chloroform or ether

266 MECHANICAL ENGINEER’S DATA HANDBOOK 6.2 I 6 Anti-freeze mixtures

Freezing point (“C)

R Engineering measurements

These are made from hardened and tempered steel

marked off with high accuracy in lengths from about

10-3Ocm with folding rules up to 60cm They are used

for marking off, setting callipers and dividers, etc

When used directly, the accuracy is &0.25mm, and

when used to set a scribing block the accuracy is

f 0.125 mm

These consist of a number of thin blades of spring steel

of exact, various thicknesses They are used for measuring small gaps between parts

variety of sizes, the most popular being 25mm in

0.01-mm steps It has a fixed ‘barrel’ graduated in

Outside micrometer

268 MECHANICAL ENGINEER’S DATA HANDBOOK

Micrometer head

Large outside miaumeter with extension rod

Inside micrometer

1-mm and 0.5-mm divisions screwed with a 0.5mm

pitch thread and a ‘thimble’ graduated around its

circumference with 5W.01 mm divisions

An ‘inside micrometer’ has the fixed anvil projecting

from the thimble; extensions may be attached A

‘micrometer head’ is available consisting of the barrel

and thimble assembly for use in any precision measur-

7 I 4 Vernier calliper gauge

This is used for internal and external measurement It has a long flat scale with a fixed jaw and a sliding jaw, with a scale, or cursor, sliding along the fixed scale and read in conjunction with it Two scales are provided to allow measurement inside or outside of the jaws

Part of vernier Vernier calliper guage

Reading a vernier calliper gauge

Reading shown:

Reading on main scale=43.5 mm Reading on cursor = 0.18 mm Total reading = 43.68 mm

Main scale

Cursor

The linear movement of a spring-loaded plunger is

magnified by gears and displayed on a dial Various sensitivities are available and a smaller scale shows complete revolutions of the main pointer A typical indicator has a scale with 1OO-O.01 mm divisions and a small dial reading up to 25 revolutions of the pointer,

Le a total range of 25 mm

ENGINEERING MEASUREMENTS 269

7 I 6 Gauge blocks (slip gauges)

These are hardened, ground and lapped rectangular

blocks of steel made in various thicknesses of extreme

accuracy and with a high degree of surface finish so

that they will ‘wring’ together with a slight twist and

pressure and remain firmly attached to one another

They are made in a number of sets; BS 888 recom-

mends metric sets, two of which are given in the table

The following table gives the accuracy of different

methods of linear measurement

7 I 7 Measurement of large bores

The size of very large bores may be measured by means

of a gauge rod of known length slightly less than the bore The rod is placed in the bore and the ‘rock’ noted The bore can be determined from the amount of

rock and the rod length

a’

Bore diameter D = L + -

8L

where: L = gauge length, a = ‘rock’

To set a scribing block Vernier calipers External k 0.03

25-mm micrometer Directly f 0.007

Preset to gauge blocks k 0.005

Dial gauge Over complete range f 0.003-0.03

Dial gauge As comparator over small range fO.ooO1-0.0025

k0.125

270 MECHANICAL ENGINEER'S DATA HANDBOOK

7.2 Angle measurement

1.2 I Combination angle slip gauges

Precision angle blocks are available with faces inclined

to one another at a particular angle accurate to one

second of arc The puges may be wrung together as

with slip gauges, and angles may be added or subtrac-

ted to give the required angle Details of a 13-block set

bores

The method of measuring the angle of internal and

external bore tapers is shown using precision balls,

rollers and slip gauges

External taper (using rollers and slip gauges)

This is used to measure the angle of one surface relative

to another It consistsof a precision bar with rollers, a precise distance apart The angle of tilt is determined from the size of slip gauge used

(9

Angle of surface 6 =sin-

where: L = distance between rollers, h = height of slip gauges

ENGINEERING MEASUREMENTS 271

In carrying out strength tests on materials it is

necessary to measure the strain This is defined as the

extension divided by the original length In the case of

mechanical extensometers, the original length is a

‘gauge length’ marked on the specimen A typical gauge length is 2 cm and the magnification is up to

2000

7.3 I Extensometer

A typical extensometer (the Huggenberger) is shown

The knife edges A and B are held on to the specimen by

a clamp with gauge length L There are pivots at C and

D and knife edges E and F are held in contact by a

tension spring The magnified increase in L is indicated

by a pointer H on a scale J

JA

Y-

7.3.2 Strain gauges

The commonest type of strain gauge is the electrical

resistance strain gauge (‘strain gauge’ for short) These

are devices which produce an electrical signal propor-

tional to the mechanical strain of the surface to which

they are bonded They can be made extremely small

and can be attached to components ofany shape which

may be moving, e.g an engine con-rod

The gauge consists of a grid of resistance wire or,

more usually, foil mounted on an insulating backing

cemented to the component Leads are connected to a

bridge circuit and the strain is measured by a gal- vanometer or calibrated resistor Dynamic strains may

be indicated on an oscilloscope or suitable recorder It

is usually necessary to use ‘dummy’ gauges mounted

on an unstressed surface at the same temperature to compensate for temperature effects

Electrical resistance strain gauge

The sensitivity of a strain gauge is given by the

‘gauge factor’, i.e the ratio of change in resistance to gauge resistance divided by the strain Various ar- rangements are used, depending on the type of stress being measured, e.g tension, compression, bending and torsion For two-dimensional stress situations a

‘strain gauge rosette’ consisting of three gauges at different angles is used The principal stresses and their direction can be calculated from the three strains

272 MECHANICAL ENGINEER’S DATA HANDBOOK

Tension or compression (one active gauge, one

ENGINEERING MEASUREMENTS 273

Torque measurement

Two gauges are mounted on a shaft at 45" to its axis

and perpendicular to one another Under torsion one

gauge is under tension and the other under compres-

sion, the stresses being numerically equal to the shear

stress The gauges are connected in a bridge circuit, as

for bending To eliminate bending effects four gauges

may be used, two being on the opposite side of the

shaft In this case:

P= 2F,e V

7.3.4 Strain gauge rosette

In the case of two-dimensional stress, it is necessary to

use three gauges If the gauges are at 45" to one

another, then the principal stresses may be found as

("C- I) Remarks Material F , R , (0)

Advance 2 .o 100 0.1 1 x 10-4 F , constant over wide range of

(57%Cu, 43%Ni) strain; low-temperature ( < 250°C)

use Platinum alloys 4.0 50 0.22 x 1 0 - 2 For high-temperature ( > 500 "C)

Silicon - 1 0 0 to 200 0.09 Brittle, but high F, Not suitable

use semiconductor + 100 for large strains

214 MECHANICAL ENGINEER'S DATA HANDBOOK

7.4 I Liquid-in-glass thermometers

Mercury

The commonest type of thermometer uses mercury

which has a freezing point of - 39 "C and a boiling

point of 357"C, although it can be used up to 500°C

since the thermometer may contain an inert gas under

pressure

The advantages of this thermometer are: good

visibility; linear scale; non-wetting; good conductor of

heat; and pure mercury is easily available

The disadvantages are: it is fragile; slow cooling of

glass; long response time; and errors arise due to

non-uniform bore and incorrect positioning

Alcohol

Alcohol can be used down to - 113 "C, but its boiling

point is only 78 "C The alcohol needs colouring It is

cheaper than mercury, and its low-temperature oper-

ation is an advantage in a number of applications

Mercury in steel

This thermometer employs a mercury filled capillary

tube connected to a Bourdon-type pressure gauge

which deflects as the mercury expands with tempera-

ture It is extremely robust and can give a remote

indication

thermocouples connected in series, known as a 'ther- mopile', gives an e.m.f proportional to the number of

thermocouples Practical thermocouples are protected

by a metal sheath with ceramic beads as insulation The advantages of thermocouples are: they are simple in construction, compact, robust and relatively cheap; they are suitable for remote control, automatic systems and recorders since they have a short response time

The disadvantages are that they suffer from errors due to voltage drop in the leads, variation in cold- junction e.m.f and stray thermoelectric effects in leads

7.4.3 Thermocouple circuits

Basic thermocouple circuit

V = Constant x Temperature (usually) Galvanometer e.m.f Y = Vh - Vc

where: Vh=e.m.f for 'hot' junction, Vc=e.m.f for 'cold' junction

Metal a

Thermocouple circuit with ice bath

7.4.2 Thermocouples

When a junction is made of two dissimilar metals (or

semi-conductors) a small voltage, known as a 'thermal

electromotive force (e.m.f.)' exists across it, which

increases, usually linearly, with temperature The basic

circuit includes a 'cold junction' and a sensitive

measuring device, e.g a galvanometer, which indicates

the e.m.f The cold junction must be maintained at a

known temperature as a reference, e.g by an ice bath

or a thermostatically controlled oven If two cold

junctions are used then the galvanometer may be

connected by ordinary copper leads A number of

A bath of melting ice is used for the cold junction Temperature is given relative to 0 "C

G =galvanometer, C =cold junction, H = hot junction

ENGINEERING MEASUREMENTS 275

Thermocouple circuit with extension leads

Two cold junctions at the same temperature are used

and copper extension leads to the measuring instru-

ment

Practical thermocouple

The wires pass through ceramic beads inside a protec-

tive metal sheath

Thermopile

This consists of a number of thermocouples connected

in series to give a higher e.m.f

Bismuth Constantan Nickel Potassium Sodium Platinum Mercury Carbon Aluminium Lead Tantalum Rhodium

- 72

- 35

- 15 -9 -2

0

0.6

3 3.5

4

4.5

6

Silver Gold Tungsten Cadmium Iron Nichrome Antimony Germanium Silicon Tellurium Selenium

Copper

6.5

6.5 6.5

7.5 7.5

Copper/constantan (57%cu, 43%Ni) -250 400 Flue gases, food processes, sub-zero Iron/constantan - 200 850 Paper pulp mills, chemical reactors, Chrome1 (90%Ni, lO%Cr)/Alumel 0 1100 Blast-furnace gas, brick kilns,

Platinum/platinum rhodium 0 1400 Special applications

Tungsten/mol ybdenum 1250 2600 Special applications

temperatures low-temperature furnaces

glass manufacture (94%Ni, 3%Mn, 2%A1, l%Si)

276 MECHANICAL ENGINEER'S DATA HANDBOOK

7.4.6 Thermal e.m.f for thermocouple

combinations

Thermal e.m.f, for common thermocouple combinations (reference junction at 0 "C)

E.m.f (mV) Temperature

Copper/ Chromel/ Iron/ Chromel/ Platinum

- 3.380

- 2.559

- 1.654 -0.670 0.389 1.517 2.71 1 3.967 5.280 6.647 8.064 9.525

1 1.030 12.575 15.773 19.100

3 1.09 40.06 49.04 62.30 70.90

- 5.76 -4.68

- 3.49

- 2.22 -0.89 0.05 1.94 3.41 4.91 6.42 7.94 9.48

1 1.03 12.57 14.12 17.18 20.26 23.32 29.52 36.01

- 4.29

- 3.52 -2.65

- 1.70 -0.68 0.04 1.52 2.66 3.82 4.97 6.09 7.20 8.31 9.43 10.57 12.86 15.18 17.53 22.26 26.98 33.93 38.43 44.91 54.92

1.4.7 Electronic thermocouple

thermometer

This has a robust sheathed thermocouple connected to

a voltmeter which gives a digital or analogue readout

of temperature It avoids many of the usual disadvan- tages of thermocouples

1.4.8 Resistance thermometers

Resistance thermometers are based on the fact that the electrical resistance of a metal wire varies with tem- perature The metals most used are platinum and nickel, for which the resistance increases with tempera- ture in a linear manner

The construction of a typical resistance thermom-

eter is shown in the figure It consists of a small

resistance coil enclosed in a metal sheath with ceramic

insulation beads The temperature range is 100 "C to

300 "C for nickel and 200 "C to 800 "C for platinum

Dummy leads beads

With other metals it is possible to reach 1500 "C The small resistance change is measured by means of a Wheatstone bridge and dummy leads eliminate tem- perature effects on the element leads

The resistance thermometer is used for heat treat- ment and annealing furnaces and for calibration of other thermometers

The main disadvantages are fragility and slow response

7.4.9 Thermistors

278 MECHANICAL ENGINEER'S DATA HANDBOOK

F P Thermistors

Most metals have a positive temperature coefficient of

resistance, i.e resistance increases with temperature

Semi-conductors may have a very large negative

coefficient which is non-linear A 'thermistor' is a bead

of such material, e.g oxides of copper, manganese and

cobalt, with leads connected to a measuring circuit

They are extremely sensitive; for example, a change

from 4OOQ at 0°C to l00Q at 140°C They are

inexpensive and suitable for very small changes in

temperature The graph shows curves of resistivity for

three thermistor materials compared with platinum

onto the filament the brightness of which is varied by means of a calibrated variable resistor until the filament appears to vanish A red filter protects the eye

7.4 I I Bimetallic thermometer

The deflection of a bimetallic strip or coil may be used

to indicate temperature This type is not very accurate but is simple and cheap These thermometers are used for alarms and temperature controllers when connec- ted to a mechanical system

Heat Bimetallic thermometei 7.4 I O Pyrometers

7.4 I 2 Temperature-sensitive paints

Total radiation pyrometer

At very high temperatures where thermometers and

thermocouples are unsuitable, temperature can be

deduced from the measurement of radiant energy from

a hot source The radiation is passed down a tube and

focused, using a mirror, onto a thermocouple or

thermopile which is shielded from direct radiation

Disappearing-filament pyrometer

The brightness and colour of a hot body varies with

temperature and in the case of the disappearing

filament pyrometer it is compared with the appearance

of a heated lamp filament The radiation is focused

tures The range is from about 30 "C to 700 "C, with an

accuracy of about 5% Several paints are required to

cover the range Crayons are the easiest to use The method is suitable for inaccessible places

7.4 I 3 Fixed-point temperatures

The table below gives fixed-point temperatures known

to a high degree of accuracy from which instruments can be calibrated

Temperature ("C) Boiling point of liquid oxygen Melting point of ice

Triple point of water Boiling point of water Freezing point of zinc Boiling point of liquid sulphur Freezing point of liquid antimony Melting point of silver

Melting point of gold

- 182.97 0.00

0.01

100.00 419.505 444.60

630.50

960.80

1063 OO