Mechanical Engineers Data Handbook Episode 8 pps

170 MECHANICAL ENGINEER’S DATA HANDBOOK 4.7.5 Impulse Pelton water turbine This is a water turbine in which the pressure energy of the water is converted wholly to kinetic energy in on

FLUID MECHANICS 165 Drag coefficients for various bodies (continued)

A centrifugal pump consists of an impeller with vanes

rotating in a suitably shaped casing which has an inlet

at the centre and usually a spiral ‘volute’ terminating

in an outlet branch of circular cross-section to suit a

166

Some pumps have a ring of fixed (diffuser) vanes

into which the impeller discharges These reduce the

velocity and convert a proportion of the kinetic energy

into pressure energy

Symbols used:

D , =mean inlet diameter of impeller

D , =outlet diameter of impeller

b , =mean inlet width of impeller

b , = outlet width of impeller

t = vane thickness at outlet

b1 =vane inlet angle

bz =vane outlet angle

N = impeller rotational speed

1 refers to impeller inlet

2 refers to impeller outlet

3 refers to diffuser outlet

P =power

Vt = tangential velocity

Vw = whirl velocity

V, =flow velocity

V, = velocity relative to vane

V = absolute velocity of fluid

qh = hydraulic efficiency

q, =volumetric efficiency

q,, = mechanical efficiency

qo = overall efficiency

a =diffuser inlet angle

d,=diffuser inlet width

d , =diffuser outlet width

a, =diffuser inlet area = bd,

a, =diffuser outlet area = bd,

V, =diffuser outlet velocity

p = pressure rise in pump

b = diffuser breadth (constant)

MECHANICAL ENGINEER'S DATA HANDBOOK

Head

Refemng to velocity triangles

Theoretical head Hth =

It is usually assumed that V,, is zero, Le there is no

'whirl' at inlet The outlet whirl velocity V,,, is reduced

by a whirl factor K to KVw,(K < I) Then:

Pump volute

Q

Velocity in volute V, = -

A4 where: A4=maximum area Then:

A4 Pump outlet velocity V, = V, -

Static pressure=po=pgHo Total pressum=Pp,=pgHt

Static efficiency = ‘I, =o P Q P Total efficiency = q1 = P ~ Q - P

Total head HI= no+- v:

Pump characteristics are plotted to a base of flow rate for a fixed pump speed Head (or pressure), power and efficiency are plotted for dl&rent speeds to give a

family of curves For a given speed the point at which maximum efficiency is attained is called the ‘best

etficiency point’ (B.E.P.) If the curves are plotted nondimensionally a single curve is obtained which is

also the same for all geometrically similar pumps

168

Head (H), power ( P ) and efficiency ( q ) are plotted

against flow at various speeds (N) and the B.E.P can

be determined from these

MECHANICAL ENGINEER’S DATA HANDBOOK

vapour pressure at the operating temperature and also

on the ‘specific speed’

Symbols used:

p=fluid density

pa = atmospheric pressure

p , = vapour pressure of liquid at working

V, =suction pipe velocity

h, = friction head loss in suction pipe plus any other losses

Ha =pump head

u, =cavitation constant which depends on vane

Minimum safe suction head

To give single curves for any speed the following

non-dimensional quantities, (parameters) are plotted

If the suction pressure of a pump falls to a very low

value, the fluid may boil at a low pressure region (e.g

at the vane inlet) A formula is given for the minimum

suction head, which depends on the fluid density and

Range of 6,:

Safe region u, >0.0005Nf.37, where N,=specific speed

Dangerous region u, < O.OOO~~N:.~’

A ‘doubtful zone’ exists between the two values

4.7.4 Centrifugal fans

The theory for centrifugal fans is basically the same as that for centrifugal pumps but there are differences in construction since fans are used for gases and pumps for liquids They are usually constructed from sheet metal and efficiency is sacrificed for simplicity The three types are: the radial blade fan (paddle wheel fan); the backward-curved vane fan, which is similar in design to the centrifugal pump; and the forward- curved vane fan which has a wide impeller and a large number of vanes Typical proportions for impellers, maximum efficiencies and static pressures are given together with the outlet-velocity diagram for the impeller

Max Static efficiency No of pressure Velocity Type and application Arrangement blD (%I vanes (cm H,O) triangle

170 MECHANICAL ENGINEER’S DATA HANDBOOK 4.7.5 Impulse (Pelton) water turbine

This is a water turbine in which the pressure energy of

the water is converted wholly to kinetic energy in one

or more jets which impinge on buckets disposed

around the periphery of a wheel The jet is almost

completely reversed in direction by the buckets and a

high efficiency is attained Formulae are given for the

optimum pipe size to give maximum power, and for

the jet size for maximum power (one jet)

Mean bucket speed U = nDN

nd2 V Flow through jet Q=- 4

Hydraulic efficiency qh = 2r( 1 - I ) ( 1 + k cos 0) where: r = -, 0 U =bucket angle (4-7”),

V

k =friction coefficient (about 0.9)

(1 + k cos 8)

2 Maximum efficiency (at r = 0.5): qh(max) = Overall efficiency qo = qhqm

Maximum power when H r = - = L H t O I 4ftv2 Hence:

3 29Dp

Optimum size of supply pipe D,= F -

(approximately)

Jet size for maximum power d = (z)’ -

4.7.6 Reaction (Francis) water turbine

The head of water is partially converted to kinetic energy in stationary guide vanes and the rest is converted into mechanical energy in the ‘runner’ The water first enters a spiral casing or volute and then into the guide vanes and a set of adjustable vanes which are used to control the flow and hence the power The water then enters the runner and finally leaves via the

‘draft tube’ at low velocity The draft tube tapers to reduce the final velocity to a minimum

FLUID MECHANICS 171

Velocity triangles

Radial velocities: V,, =Q/nb,D, (inlet)

V,, = Q/nbzD, (outlet)

Tangential velocities: VI, = x D , N (inlet)

Whirl velocities: V,, =gHqh/Vl, (inlet, usually)

VI, = nD,N (outlet)

Vw2 =O (outlet, usually) Guide vane velocity: V, =

vanes

0

Vane and blade angles

Guide vanes: a=tan-'V,,/V,,

Blade inlet: B1 =tan- Vrl/( Vll - V,,)

Blade outlet: & =tan- V,,/V,,

Overall efficiency q,, = qmqh

Shaft power = pgHQq,

Available head H = HI,, - H , - Vf/2g

where: V,=draft tube outlet velocity

Specific speed of pumps and turbines

It is useful to compare design parameters and charac- teristics of fluid machines for different sizes This is done by introducing the concept of 'specific speed', which is a constant for geometrically similar machines

4.7.7 Specific speed of pumps and turbines

~ Manufacturing technology

5.1 Metal processes

Metals can be processed in a variety of ways These can

be classified roughly into casting, forming and machin-

The following table gives characteristics of different

processes for metals, although some may also apply to non-metallic materials such as plastics and compos-

General characteristics of metal processes

Minimum

3000 cm3 500mm diameter

-

-

3 mm/6 m diameter 6mm/4.5m diameter 6-l00mm diameter 80g to 4kg

MANUFACTURING TECHNOLOGY 173

~~

5.2 Turning

5.2 I

In metal cutting, a wedge-shaped tool i s used to

remove material from the workpiece in the form of a

‘chip’ Two motions are required: the ‘primary

motion’, e.g the rotation of the workpiece in a lathe;

and the ‘secondary motion’,e.g the feed ofa lathe tool

Single-point tools are used for turning, shaping,

planing, etc., and multi-point tools are used for

milling, etc It is necessary to understand the forces

acting on the tool and their d e c t s on power require-

ment, tool life and production cost

In the following tables of tool forces and formulae

specific power consumption, metal removal rate, tool

life, etc., are given A graph shows the tool life plotted

against cutting speed for high-speed steel, carbide and

ceramic tools

Single point metal cutting

5.2.2 Cutting tool forces

Tool forces vary with cutting speed, feed rate, depth of

cut and rake angle Force may be measured experi-

mentally by using a ‘cutting tool dynamometer’ in

which the tool is mounted on a flexible steel diaphragm

and its deflections in three planes measured by three

electrical transducers Three meters indicate the force,

typically of 25 N up to, say, 2000 N Graphs show

typical characteristics

Symbols used:

F , =cutting force (in newtons)

F , = radial force (in newtons)

F,=feed force (in newtons)

Resultant force on tool in horizontal plane

(an3 min-’)

n(D-d)d f N

lo00 Metal removal rate Q = where: f=feed rate (mm rev-’)

Specific power consumption P , = - (wattscrr-3 min- P

Q

174 MECHANICAL ENGINEER’S DATA HANDBOOK Typkd values of P

Material

Specific power consumption, P

Plain carbon steel 34

Force versus cutting speed

F, is constant over normal range of cutting speed

F, increases slowly with cutting speed

Force versus depth of cut

F, increases with depth of cut

F, increases at decreasing rate with depth of cut

1200 lo00

i- 400

200

0 0.5 1.0 1.5 2.0 2.5 Wdcut d(mm)

Force versus rake angle

F, and F, fall slowly with rake angle

MANUFACTURING TECHNOLOGY 175

700

9 2400 ~ e ,t 300 200-

F , increases linearly with feed rate

Brass, free cutting

90-120 90-120 50-80 45-60 50-90 110-150 18-25 45-60 7-12 12-18 45-60

> 200 170-250 70-150

60

70-150 85-135 25-45 70-120 20-35 30-50 70-120

> 350 350-500 150-250 9CL120 100-300 50-80 85-135

-

100-200

176 MECHANICAL ENGINEER’S DATA HANDBOOK 5.2.7 Turning of plastics

Turning of plastics - depth of cut, feed, and cutting sped

Cutting speed (m min- ’)

Material

Throw-

Condition (mm) (mm rev- ’) HSS carbide tip

There are many types of lathe tool, the principal ones

being: bar turning; turning and facing; parting-off

facing; boring; and screw cutting Some are made from

a bar of tool steel, others with high-speed steel tips

welded to carbon steel shanks and some with tungsten

carbide tips brazed to a steel shank A tool holder with

interchangeable tips can also be used

5.2.9 Lathe-tool nomenclature and Tool features

setting

For cutting to take place the tool must have a ‘front clearance angle’ which must not be so large that the tool is weakened There must also be a ‘top rake angle’

to increase the effectiveness of cutting The value of this angle depends on the material being cut Typical values are given in the following table

Rake angle for Merent workpiece materials

Tensile Tool rake strength angle Workpiece material (Nmm-2) (")

I

Nickel-chrome steel loo(r1150 -5

Another feature is the 'chip breaker' which breaks

long, dangerous and inconvenient streamers of 'swarf'

into chips

Plan approach angle

To reduce the load on the tool for a given depth of cut the cutting edge can be angled to increase its length Note the direction of chip flow - if the angle is too large there is a danger of chatter

Symbols used:

4-top rake angle

a =front clearance angle /3 = wedge angle

S =plan relief or trail angle

E = plan approach angle

8 =true rake angle

y = true wedge angle

1 =side clearance angle

$ =side rake angle

Single-point tool Chip breaker

178 MECHANICAL ENGINEER’S DATA HANDBOOK

Tool setting

The tool must not be set too high or too low, or

inclined at an angle The effects are shown in the figure

\

Inclined downwards Inclined upwards

TOOL SEl-rING

Above centre: tool tends to rub

Below centre: work tends to climb over tool

Inclined upwards: tool rubs

Inclined downwards: work tends to drag tool in

5.2 I O Parting-off tool

This is used for ‘parting-off the workpiece from bar

stock held in a chuck Note that there is ‘body

clearance’ on both sides as well as ‘side clea’rance’ The

tool is weak and must be used with care It must be set

on or slightly above centre If set even slightly below

centre the work will climb onto the tool before

parting-off

Side clearance Body clearance

* 3

PARTING-OFF TOOL

5.3 Drilling and reaming

A twist drill is a manually or machine rotated tool with

cutting edges to produce circular holes in metals,

plastics, wood, etc It consists of a hardened steel bar

with usually two helical grooves or ‘flutes’ ending in

two angled cutting edges The flutes permit many

regrinds and assist in removal of cuttings

Drills vary in size from a fraction of a millimetre to over 1Ocm As with a lathe turning tool, the cutting edges must have top rake and clearance Grinding is best done on a special drill grinding machine

MANUFACTURING TECHNOLOGY 179 5.3 I

The helix angle is usually a standard size but 'quick'

and 'slow' helix angles are used for particular ma-

terials It is sometimes necessary, e.g for brass and thin

material, to grind a short length of straight flute, as

shown It is also sometimes necessary to thin down the

web or core

The point angle was traditionally about 120" (in-

cluded angle), but other angles are now used to suit the

material The lip clearance also varies (see table)

Helix and point angles

5.3.2 Core drills

Core drills have three or four flutes and are used for

opening out existing holes, e.g core holes in castings

: Reduction

IJ Clearance Taper shank

on the clearance face otherwise the size will be lost Flutes may be straight or helical (usually left handed)

A hand reamer requires a long slow taper, but machine reamers have a short 45" Lead The hole is drilled only slightly smaller than the reamer diameter, the allow- ance is about 0.015 mm per millimetre, but depends on the material Taper reamers are used for finishing holes for taper pins

-

-@ I

Rake Reamer

5.3.4 Drilling parameters

The tables below give drilling feeds and speeds includ-

ing information on drilling plastics Cutting lubricants

for drilling, reaming and tapping are also given and tapping drill sizes for metric coarse threads A table of suggested angles for drills is given

180 MECHANICAL ENGINEER'S DATA HANDBOOK Drilling feeds

Feed (mm rev.- ')

diameter (mm) materials* materials?

*Steels above 0.3 % C and alloy steels

tGrey cast iron, steels below 0.3 %C, brass, bronze,

aluminium alloys, etc

Drilling plastics, cutting speeds and feeds

Medium carbon steel 0.2-0.3

*Speed = nDN/60 OOO m s- ' , where D = diameter (mm), N = number of revolutions per minute

Cutting Feed (mmrev.-') for nominal hole diameter (mm) of: speed

Material Condition (mmin-') 1.5 3.0 6.0 12.0 20.0 25.0 30.0 50.0

or cast Extruded, moulded

or cast Moulded Moulded or extruded Cast, moulded

or filled moulded

or filled Cast,

MANUFACTURING TECHNOLOGY 181

Mild steel (hot and

cold rolled)

Tool steel (carbon

and high speed)

Mineral lard oil Lard oil Lard oil Soluble oil Soluble oil Mineral lard oil Mineral lard oil, Soluble oil sulphur base oil Dry

Soluble oil, lard oil Sulphur base oil, mineral lard oil Sulphur base oil, mineral lard oil Soluble oil, lard oil Soluble oil, lard oil Soluble oil, mineral Mineral lard oil, Soluble oil Dry, lard oil for

lard oil sulphur base oil

nickel cast iron

Tapping tirill &xis for metric coarse threads

1 .o 1.25

1 s o 1.75 2.00 2.00

1.20 1.60 2.05 2.50 2.90 3.30 4.20 5.30 6.80 8.50 10.20 12.00 14.00

20.0 24.0 30.0 36.0 42.0 48.0 56.0 64.0 72.0 80.0 90.0 100.0

2.50

3 .O

3.50 4.00 4.50 5.00 5.50 6.00

6 00 6.00 6.00 6.00

17.5 21.0 26.5 32.0 37.5 43.0 50.5 58.0 66.0 74.0 84.0 94.0