design of transmission systems by a.vinoth jebaraj

Design of Transmission Systems

Prepared by Dr.A.Vinoth Jebaraj

To avoid the slipping

Exact velocity ratio Transmit large power Used for small centre

distances High efficiency Reliable service Compact layout

Require special tools and equipments to produce Improper cutting of teethproduce vibration and noise Lubrication is must

TERMINOLOGIES USED IN GEARS

Driver pinion

Driven gear wheel

Arc of contact: Path traced by a point on the pitch circle from

the beginning to the end of the engagement of a given pair of

teeth. It consists of two parts.

Arc of approach: Portion of the path of contact from the

beginning of engagement to the pitch point.

Arc of recess: Portion of the path of contact from the pitch point

to the end of the engagement of a pair of teeth.

Line of Action

Arc of approach Arc of recess

Spur gear Helical gear

Double helical or Herringbone gear Cross helical gear

Straight bevel gear Spiral bevel gear

Worm and worm wheelRack & Pinion

Pressure Angle or Angle of Obliquity:

Angle between common normal to two gear teeth at the point

of contact (line of contact) and the common tangent at the

pitch point. Standard values include 14.5, 20 and 25degrees.

Backlash:

It is the difference between tooth space and the tooth thickness as measured

along pitch circle. Theoretically backlash should be zero. But in actual practice

some backlash must be allowed to prevent jamming of the teeth due to the tooth

errors and thermal expansion.

Module (m): Pitch diameter divided by number of teeth. The

pitch diameter is usually specified in inches or millimeters;

It is a measure of the tooth strength. Higher the module

bigger the size of the gear. More important, higher the

module, wider the tooth at the base and larger the height of

the tooth.

Main parameters to be designed:

Center Distance

Module

Face width

Let Mt be the torque transmitted by the pinion

Normal force on the tooth Fn = Torque / lever arm

Radius of the base circle = ½ d1 cos α

1

Diameter d1 can be expressed in terms of center distance ‘a’ as

Substituting the value of d1 in Eq. 1

Therefore, Fn is inversely proportional to the centre

distance.

As center distance decreases the normal force will

increase and hence the surface compressive stress

increases.

Therefore the centre distance is limited by the

permissible surface compressive stress of the

material of the pinion.

IMPORTANT POINTS TO BE NOTED:

Minimum centre distance depends upon the surface compressive strength

of the material

So induced surface compressive stress < Design surface compressive

strength of the material

Minimum module depends upon the bending strength

So induced bending stress < Design bending strength of the material

Design surface compressive strength [σc]

Surface strength is proportional to the hardness of thesurface.

σc α HB or RC

Therefore,

σC = CB × HB N/cm2

= CR × RC N/cm2

CB and CR are constants depending on the materialand heat treatment.

Also the design compressive strength depends on load conditions.Hence correction factor is introduced.

Design surface compressive strength

[σC ] = CB HB Kcl in N/cm2

[σC] = CR HRC Kcl in N/cm2

Where Kcl is the life factor for surface compressive strength.

Kcl = ퟏퟎퟕ

푵Where N – number of fatigue cycles the pinion teeth has undergonein its life period of T hours.

Number of fatigue cycles per hour = 60 n

Number of cycles in life period N = 60 n T

Design Bending Stress [σb]

It depends on endurance limit, stress concentration factor at the root and lifefactor for bending.

[σb] = 흈 ퟏ 풌풃풍풏풌흈

For gears having both directions of rotation

[σb] = 흈 ퟏ 풌풃풍풏풌흈

× 1.4 For gears having one direction of rotation only

흈 ퟏ = endurance limit stress in bending

풌풃풍 = life factor in bending

풌흈 = stress concentration of fillet at the root

n = factor of safety

Gear materials

Commonly used materials cast iron and steel

For large power transmission and reduction in size Alloy steel ofNickel, chromium & vanadium (with proper heat treatment to obtainsufficient surface strength)

For corrosive environment Brass and bronze

Non metallic materials Laminated fabric, Bakelite and mica (to reducenoise)

Gear Failures

Teeth breakage: due to fatigue

Pitting: hard and smooth working surfaces of the teeth reduce the dangerof pitting

Surface abrasion: due to sliding of the teeth

Seizure: surface of the teeth mesh so tightly together causes particles ofsofter material to break away from the teeth surface and groove it.

Law of gearing

The common normal to the tooth profile at the point of contact should

always pass through a fixed point, in order to obtain a constant

velocity ratio.

Only involute and cycloidal curves satisfies the fundamental law of

gearing.

Involute Profile Cycloidal Profile

Helical Gears and Herringbone Gears

They have teeth cut in the form of helix on their pitch cylinders. Teeth are not

parallel to the axis of rotation.

More than one pair of teeth are in engagement. Runs smoothly because of the

gradual engagement of teeth. Higher peripheral speeds are permissible in

helical gears.

Limitation: Axial thrust

By providing another helical gear of opposite hand, the axial thrust can be

balanced. They are called as double helical or herringbone gears.

Helix angle is helical gears are in between 8° and 25°

Axial pitch = π m , where m is the axial module

Normal pitch = π mn , where mn is the normal module

Cos β = π mn / π m

Therefore,

Cos β = mn / m

Centre distance =

Forces acting on a Helical gear tooth

Design of Bevel Gear

Straight Bevel Gear Spiral Bevel Gear

Direction of shaft’s rotation can be changed by means of bevel

gears. Usually two shafts are arranged at an angle of 90°. But the

other angles are also possible.

Bevel gears in differential

AXES MUST INTERSECT EACH OTHER AND MUST LIE IN THE SAME PLANE IN BEVEL GEAR ARRANGMENT

Bevel Gear - Nomenclature

Terminology of bevel gears

Pitch cone: Imaginary cone that the surface of which contains the pitch lines of all

teeth in the bevel gear.

Cone center: The apex of the pitch cone is called cone center.

Cone distance: Length of the pitch cone element also called as pitch cone radius.

Pitch angle: Angle that the pitch line makes with the axis of the gear is called the

pitch angle.

Addendum angle: Angle subtended by the addendum at the cone center.

Face angle: Angle subtended by the face of the tooth at the cone center.

Transverse module mt : It is based on the pitch circle diameter at the outer portion.

Average module mav : It is based on pitch circle diameter at the centers of the teeth.

Miter Gear

When two identical gears are mounted on

shafts, that are intersecting at right angles,

then they are called as Miter gears.

Pitch angles of pinion and gear of Miter

gears are same and each is equal to 45°.

Pinion and gear of Miter gears rotate at

same speed.

Crown Gear

In a pair of bevel gear, when one of the gear has a pitch

angle of 90°, then that gear is called as crown gear.

They are intersecting at an angle that is more than 90°.

Internal Bevel gear

When the teeth of a bevel gear are cut inside the pitch cone,

then it is called as internal bevel gear.

In this case the pitch angle of the internal gear is more than 90°

and the apex point is on the backside of the teeth on the gear.

Skew bevel gear: When two straight bevel gears are mounted on

shafts, which are non parallel and non intersecting, then they are

called as skew bevel gears

Hypoid Bevel Gear

They are similar to spiral bevel gears that are mounted on

shafts, which are non-parallel and non-intersecting.

Face gears: They consists of a spur or helical pinion mating with

a pair gear of disk form.

Force Analysis on Bevel gear

tooth

Design of worm and worm wheel

Worm(Driver)

Worm wheel(Driven)

Worm always drives the worm wheel. It is a self locking drive. Reversible

direction of power transmission is not possible.

Higher speed reduction and more torque at the output

is possible through

worm drive.

Consider a single start worm and a 20 teeth worm gear will reduce the speed by the ratio of 20:1.

The gear ratio of a worm gear is

i = 푵풐 풐풇 풕풆풆풕풉 풐풏 풘풐풓풎 풘풉풆풆풍푵풐 풐풇 풔풕풂풓풕 풐풏 풘풐풓풎

The worm acts as a single toothed gear so the ratio is;i = ퟐퟎ

ퟏ

Gear Ratio = 20:1(Rotary velocity is reduced by 20:1)

If this speed reduction is achieved by spur gears, then a gear of 12 teeth (the

smallest size permissible) would have to be matched with a 240 tooth gear to

achieve the same ratio of 20:1.

Therefore according to the physical size of the 240 tooth gear to that of the 20

tooth gear, the worm arrangement is considerably smaller in volume.

Applications

Self-locking Worm Gear The worm always acts as a driving gear and the spur gear as a driven gear- vice

versa is not possible. If you try to run it in opposite direction, it will lockautomatically.

A worm and gear will be self-locking depends on the lead angle, the pressure angle,and the coefficient of friction;

If the tangent of the lead angle of the worm gear is less than the coefficient offriction between the worm and the gear, then the worm gear train should be a self-locking type.

The self-locking worm gear USED for the

applications where loading against the

gravitational force is required.

This is because the angle on the worm is soshallow that when the gear tries to spin it,the friction between the gear and the wormholds the worm in place.

Losses in worm gears are high, they need costly materials like

bronze and the manufacturing cost is high.

Power transmission between worm and worm wheel happens

through sliding. Therefore, the materials used should have low

coefficient of friction.

Seizure and wear are the two major failures in worm gear drive.

Proper lubrication and cooling surfaces should be provided to limit

the operating temperature between 60° and 70°C.

Bearing : Machine element used tosupport a rotating member with thevery minimum frictional power loss.

Types:

Rolling contact bearings

Sliding contact bearings

Anti-friction bearing due to its low friction characteristics used forradial load, thrust load and combination of thrust and radial load relatively lower price maintenance free friction increases at highspeeds noisy while running Types : Ball bearing and Roller bearing

Single row deep groove ball bearing Radial load but it can also take up considerable amount of axial load.

Functions of bearings :

Ensure free rotation withminimum friction

Act as a support for shaft andaxle and holds in correct position

Takes up the forces acting on theshaft and axle and transmits themto the frame or foundation

Advantages

1. Low starting and running friction except at

very high speeds.

2. Accuracy of shaft alignment.

3. Low cost of maintenance, as no lubrication is

required while in service.

4. Small overall dimensions.

5. Reliability of service.

6. Easy to mount and erect & Cleanliness.

Disadvantages

1. More noisy at very high speeds.

2. Low resistance to shock loading.

3. More initial cost.

4. Design of bearing housing complicated.

Applications of rolling contact bearings:

Machine tool spindles automobile front and rear axles gear boxes small size electric motors rope sheaves, crane hooks and hoisting drums

Single row Angular Contact Ball Bearing

Used for radial loads and heavy axial loads

Double Row Angular Contact Bearing

Has two rows of balls. Axial displacement of the shaft can be kept very small even for axial loads of varying magnitude

Single thrust ball bearing

Used for unidirectional axial load

Taper Roller Bearing

Used for simultaneous heavy radial load and

heavy axial load

Roller bearings has more contact area than aball bearing, therefore, they are generallyused for heavier loads than the ball bearings

Spherical Roller Bearing Cylindrical Roller Bearing

For heavy radial load and high speed use, cylindrical

roller bearings

It is mainly used for heavy axial loads. However, considerable amount of

loads in either direction can also be applied

Self aligning principle

Static load carrying capacity: The static load which corresponds to a total

permanent deformation of balls and races, at the most heavily stressed

point of contact, equal to 0.0001 of the ball diameter. [Load acting on the

bearing when the shaft is stationary]

STIBECK’S EQUATION

Static load CO = (k.d2.z) / 5Where k = factor depends upon the radii of curvature at the point of contactd = ball diameter z = number of balls

Dynamic load carrying capacity: (fatigue life of the bearing)

Life of an individual bearing is defined as the number of revolutions

which the bearing runs before the first evidence of fatigue crack in

balls and races.

The dynamic load carrying capacity of a bearing is defined as the

radial load in radial bearings that can be carried for a minimum life

of one million revolutions.

The minimum life in this definition is the L10 life, which 90% of the

bearings will reach or exceed before fatigue failure.

Equivalent bearing load [P]:

Two components of load acting on the bearing single hypothetical load

The equivalent dynamic load is defined as the constant radial load in radial

bearings (or thrust load in thrust bearings), which if applied to the bearing would

give same life as that which the bearing will attain under actual condition of forces.

Where

Fr radial load

Fa axial load

X and Y radial and thrust factors from manufacturer’s catalogues

V Race factor

P = X .V. Fr + Y . Fa

Load factor in bearings:

Load factors are used in applications involving gear, chain and belt drives.

Gear drives additional dynamic load due to inaccuracies of the toothprofile and the elastic deformation of the teeth.

Chain and belt drives additional dynamic load due to vibrations

Bearing failure – causes and remedies

breakage of parts like races and cages

crushing of balls due to misalignment leads to overload

failure of a cage due to centrifugal force acting on balls

surface wear abrasive wear, corrosive wear, pitting, scoring (breakagelubrication film leads to excessive heat in the contact surfaces)

Journal Bearing ( Hydrodynamic bearing)

Journal bearing is a sliding contact bearing working on hydrodynamic lubrication

and which supports the load in radial direction. The portion of the shaft inside the

bearing is called journal and hence the name ‘Journal bearing’

Since the pressure is created within

the system due to rotation of the

shaft, this type of bearing is known

as ‘self acting bearing’.

Can take load in any radial direction[many industrial applications)

Can take load in only one radial direction[rail road cars]

Hydrostatic bearing: In a system of lubrication, load supporting fluid film,

separating the two surfaces is created by an external source, like pump,

supplying sufficient fluid under pressure. This is also called as externally

pressurized bearings.

Advantages: High load carrying capacity even at low speeds, no starting

friction, no rubbing action at any operating speed or load

Bearing which operates without any lubricant Zero film bearings

Two surfaces of the bearing in relative motion are completely separated by

a lubricant Thick film bearings

Lubricant film is relatively thin and there is partial metal to metal contact

Thin film bearings

The factor ZN / p is termed as bearing characteristic number andis a dimensionless number

Between Q and R Partial metal to metal contact

(The viscosity (Z) or the speed (N) are so low, or the pressure ( p) is sogreat that their combination ZN / p will reduce the film thickness)

Between R and S Thin film or boundary lubrication or imperfectlubrication

(This is the region where the viscosity of the lubricant ceases to be ameasure of friction characteristics but the oiliness of the lubricant iseffective in preventing complete metal to metal contact and seizure of theparts)

Between P and Q Stable operating conditions(Since from any point of stability, a decrease in viscosity (Z) will reduce ZN/ p. This will result in a decrease in coefficient of friction (μ) followed by alowering of bearing temperature that will raise the viscosity (Z ))

Bearing should not be operated at ‘K’ (Bearing modulus).

Because, a slight decrease in speed or slight increase in pressure willbreak the oil film and make the journal to operate with metal to metalcontact. This will result in high friction, wear and heating.

In order to prevent such conditions, the bearing should be designed fora value of ZN / p at least three times the minimum value of bearingmodulus (K). If the bearing is subjected to large fluctuations of load andheavy impacts, the value of ZN / p = 15 K may be used.

On the other hand, when the value of ZN / p is less than K, then the oilfilm will rupture and there is a metal to metal contact.

Critical pressure of the journal bearing

The pressure at which the oil film breaks down so that metal tometal contact begins, is known as critical pressure or theminimum operating pressure of the bearing.

Clutch is a mechanical device which

transmits power from the driving shaft to the

driven shaft when it is engaged and cuts the

power when it is disengaged.

Example: Engine to road wheels , Drilling

machine motor to spindle

Clutch Engaged position Clutch disengaged position

Multiplate Clutch

Single plate clutch

Method of Analysis

The torque transmitted by a clutch is a function of

Geometry

The magnitude of the actuating force applied

The condition of contact prevailing between themembers

Uniform Pressure Theory

If the applied force keep the frictional surfaces together with a

uniform pressure all over its contact area , then the analysis is

based on uniform pressure condition .

Uniform Wear Theory

However, as the time progresses some wear takes place between

the contacting members and this may alter or vary the contact

pressure appropriately and uniform pressure condition may no

longer prevail. Hence the analysis here is based on uniform wear

condition. [Wear α contact pressure and sliding velocity]

Cone Clutches

Consider a small ring of radius “r” and thickness “dr”

“dl” is the length of ring of the friction surface = dl = dr cosec α

Area of ring = 2π r. dl = 2π r.dr cosec α

1. Considering uniform pressure

The normal force acting on the ring

δWn = Normal pressure × Area of ring = pn × 2π r.dr cosec α

The axial force acting on the ring

δW = Horizontal component of δWn (i.e. in the direction of W)

δWn × sin α = pn × 2π r.dr cosec α × sin α = 2π × pn.r.dr

Total axial load transmitted to the clutch or the axial springforce required

Frictional force on the ring acting tangentially at radius r

Fr= μ.pn × 2πr.dr cosec α

Frictional torque acting on the ringTr = Fr × r = μ.pn × 2πr.dr cosec α × r

Tr = 2π μ.pn cosec α.r2 dr

Centrifugal clutch

centrifugal force > spring force(Outward) (Inward)

Increase of speed causes the shoe to press harder the rim

inner surface and enables more torque to be transmitted.

Design of a centrifugal clutchMass of the shoes

Consider one shoe of a centrifugal clutch m = Mass of each shoe,

n = Number of shoes

r = Distance of centre of gravity of the shoe

from the centre of the spider,

R = Inside radius of the pulley rim,

N = Running speed of the pulley in r.p.m.,

ω = Angular running speed of the pulley in

rad / s = 2 π N / 60 rad/s,

ω1 = Angular speed at which the

engagement begins to take place, and

μ = Coefficient of friction between the shoe

and rim.

Centrifugal force acting on each shoe at the running speed

Since the speed at which the engagement begins to take place is generally

taken as 3/4th of the running speed, therefore the inward force on each

shoe exerted by the spring is given by

Therefore, Net outward radial force (i.e. centrifugal force) with which theshoe presses against the rim at the running speed

The frictional force acting tangentially on each shoe

Frictional torque acting on each shoe

Total frictional torque transmitted

Size of the shoes

l = Contact length of the shoes

b = Width of the shoes

R = Contact radius of the shoes. It is same as the inside radius of the rim

of the pulley

θ = Angle subtended by the shoes at the centre of the spider in radians

p = Intensity of pressure exerted on the shoe. In order to ensure

reasonable life, it may be taken as 0.1 N/mm2.

Area of contact of the shoe = l.b The force with which the shoe presses against the rim = p×A = p.l.b

Since the force with which the shoe presses against the rim at therunning speed is (Pc – Ps), therefore

Dimensions of the spring obtained from the relation below

Classification of Mechanical drives

Friction drives (Belt and Rope drives)

Toothed drives (Gears and chain drives)

According to physical

condition

According to method of

linking

Direct contact drives (Gear drives)

Drives with intermediate link

(Belt, rope and chain drives)

Flat belt joints

Cemented joints

Laced joints

Hinged joints

Open belt drives Cross belt drives

No crossing between belts,

Pulleys are rotating in same direction,

Pulleys are rotating in opposite direction

due to crossing,More angle of

contact

Vibration due to long centre distance, slip due to low

frictional grip

Belts rubs during crossing leads to wear, bending in two

different planes

Friction between the belt and the pulley is responsible for transmitting

power from one pulley to the other. Due to the presence of friction between

the pulley and the belt surfaces, tensions on both the sides of the belt are

not equal.

Relationship between belt tensions

Free body diagram of a belt segment

The length of the belt segment

frictional force N

Centrifugal force due to the motion of the belt

Important terms

The motion of the belt and pulley assuming a firm frictional grip between the belts

and pulleys. Sometimes, the frictional grip becomes insufficient and may cause

forward motion of a pulley without carrying the belt. This is called slip of the belt.

When the belt passes from the slack side to the tight side, a certain portion

of the belt extends and it contracts again when the belt passes from the tight

side to slack side. Due to these changes of length, there is a relative motion

between the belt and the pulley surfaces. This relative motion is termed as

creep.

Classification of Belt Drives

Based on power transmission

Light duty drivesAbout 5 kW power, velocity up to 10m/sExample: Pumps

medium duty drives5 kW to 20 kW power, velocity up to 20

m/sExample: punch and printing

machinery

Heavy duty drivesMore than 20 kW power, more than 20

m/sExample: Turbines

Belt materials

Leather(Oak tanned or chrome tanned)

fabrics(Canvas or woven cotton ducks)

rubber(Canvas or cotton duckimpregnated with rubber, Forgreater tensile strength, therubber belts are reinforced withsteel cords or nylon cords)

plastics(Thin plastic sheets with rubberlayers )

Based on centre distance

Flat belts

V belts ( single V belt, multiple V belt, ribbed belt)

Toothed or timing belt

Round belt

Based on cross section

For long distance about 5m to 20m Flat belts

For short distance less than 5m V belts, toothed belts etc.

Factors considered for selection of belt drives

Based on wear resistance, durability, strength, flexibility

& coefficient of friction

Power to be transmitted Space

availability for installation

Speed of the machinery

shaft

Velocity ratio

Center distance

Service conditions

Advantages: long distance power transmission, withstand shock and vibration, adjusting misalignment between driving and driven machine, simple in design, low cost

Disadvantages: large space, belt slipping, exert heavy load on the shaft and bearings,power loss due to friction, shorter life

Flat belt drive Applications

Packing Industry

Baggage Handling

Coal Industry

Applications of belt drives

The belt thickness can be built up with a number of layers. The number of layers is

known as ply.

Typical Belt drive specifications Material No. of ply and Thickness Maximum

belt stress per unit width Coefficient of friction of the belt material Density of

Belt material

Centrifugal Tension in the Belt

When a belt runs over a pulley, some centrifugal force is caused,

whose effect is to increase tension on both tight side and slack

side of the drive. This tension caused by centrifugal force is

known as centrifugal tension.

At high speed greater than 10 m/s,

effect of centrifugal force is

considerable .

If the effect of centrifugal tension is considered,

Then, the total tension in the tight side Tmax = T1 + TC

Total tension in the slack side Tmin = T2 + TC

Where,

T1 = Tension in the tight side of the belt

T2 = Tension in the slack side of the belt

TC = Centrifugal tension

Therefore, centrifugal tension has no effect on the power transmission.

Condition for Maximum Power Transmission

when the power transmitted ismaximum, 1/3rd of the maximumtension (T) is absorbed ascentrifugal tension (TC) .

Crowning of Pulley

Pulleys are provided with a slight conical shape or convex shape in their

outer rim surface to prevent the belt from running off the pulley due to

centrifugal force. This is known as crowing of pulley.

Usually crowning height may be 1/96 th of the pulley width.

Sag in the belt drive

In horizontal belt drive, loose side is usually kept on the top. On the

upper side, the sag of the belt due to its own weight slightly increases the

arc of contact with the pulleys and increases the efficiency of the drive.

If the lower side is slack side, then sag will reduce the angle of contact

with the pulleys. This has to be avoided to gain the power transmission.

In case of vertical belt drive, due to gravitational force on the belt, it will

try to fall away from the lower surface of the lower pulley. This causes

slip and reduces the efficiency of the drive. To run such a drive, the belt

has to run with excessive tension with consequent increase in bearing

reactions and reduced belt life.

Timing belt or Ribbed belt

Timing belt has toothed shape in

their inner surface. Their

engagement with toothed pulley

will provide positive drive without

any belt slip where as in the case of

ordinary V – belts chances for slip

are more.

Hence toothed shape belts ( i.e.

timing belts) are always superior

than V – belts.Initial tension not required – reduces the bearing action – high strength to

weight ratio – costlier than the V and flat belts – more sensitive to

misalignment

Timing belt or Ribbed belt

Timing belt or Ribbed belt

Round BeltsRound belts are made of leather, canvas and rubber. The diameter ofthe round belts are usually 3 to 12 mm.

They are suitable for , 90° twist, reverse bending or serpentineapplications.

Round belts are limited to light duties dish washer drives, sewingmachines, vacuum cleaner, light duty textile machinery.

Trapezoidal Half roundgroove groove

Quarter turn Belt Drive

The quarter turn belt drive (also known as right angle belt drive) as is used withshafts arranged at right angles and rotating in one definite direction.

In order to prevent the belt from leaving the pulley, the width of the face of thepulley should be greater or equal to 1.4 b, where b is width of belt.

when the reversible motion is desired, then a quarter turn belt drive with a guidepulley, may be used.

Design of V – Belt Drive In case of V – belt drive, power is transmitted by the wedging action between

the belt and the v – groove in the pulley or sheave.

A clearance should be provided at the bottom of the groove to preventtouching of the bottom as it becomes narrower from wear.

To increase the power transmission, multiple V – belts can be operated sideby side. All the belts should stretch at the same rate so that the load is equallyshared between them.

When one of the set of belts break, the entire set should be replaced at thesame time. If only one belt is replaced, the new unworn and unstretched beltwill be more tightly stretched and will move with different velocity.

Forces acting on an element of V – Belt

The force components T, T+ dT and Centrifugal force are same as like flat

belt element. But the normal reaction which act on the sides of the V – belt.

Different failures in belt drives

Wire rope is a type of rope which consists of several strands of metal wire

twisted into a helix. Lighter in weight silent operation withstand shock

loads do not fail suddenly more reliable

Applications: Elevators - mine hoists – cranes – conveyors - hauling devices -

suspension bridges

Right-hand Lang's lay (RHLL) wire rope Strands are twisted into a right hand side

Left-hand Lang's lay (RHLL) wire rope Strands are twisted into a left hand side

Design of Wire rope

When a large amount of power is to be transmitted over long distances from

one pulley to another (i.e. when the pulleys are upto 150 meters apart), then

wire ropes are used.

Rope construction Wire diameter dw

where d = rope diameter

Area of cross section (Approx.

6 x 7 0.106 d 0.38 d2

6 x 19 0.063 d 0.38 d2

6 x 37 0.045 d 0.38 d2

8 x 19 0.05 d 0.38 d2

Cross or regular lay ropes direction of twist of wires in the strands is

opposite to the direction of twist of the stands

Parallel or lang lay ropes direction of twist of the wires in the strands is

same as that of strands in the rope

Composite or reverse laid ropes wires in the two adjacent strands are

twisted in the opposite direction

Wire rope applications

Stresses in Wire Ropes

Direct stress due to axial load lifted and weight of the rope

Bending stress when the rope winds round the sheave or drum

The approximate value of the bending stress in the wire as proposed by Reuleaux

Equivalent bending load on the rope

Load on the whole rope due to bending

Impact load and stress during starting

Effective stresses in the wire rope at different situations

Impact Loading: The load which is rapidly applied to the machinecomponent is known as impact load.

Impact stress = Twice the stress produced by gradual load

Special offset thimble with clips

Regular thimble with clips

Three bolt wire clamps

Thimble with four or five wire tucks

Wire rope socket with zinc

Design of Chain Drives

Positive drive – No slip – No Creep –

high temperature service – Easier to

install – compact than belt drives

Classification of Chains

Hoisting and hauling chains Conveyor chains

Chain with oval links

Chain with square links

Detachable or hook joint type chain

Closed joint type chain

Used for suspending, raising or lowering loads in material handling equipments

Used for carrying materials continuously in conveyors by sliding

Roller chain

Power transmitting chains

Used for transmittingpower from one shaft toanother shaft

Types of roller chain

Silent chainInverted tooth chain – formed by laminated steel plates – each plate has two teeth

with space to accommodate tooth of the sprocket – for high speed applications –

silent operation

Breaking load: The maximum tensile

load which if applied will result in chain

failure is known as breaking load.

Chain sag: catenary effect

Over tensioned chain will wear faster due to high pressureloading between the roller and pin and high pressurebetween the roller and sprocket. Over tensioning will alsoresult in higher bearing and shaft loads.

Under tensioning can result in the chain ratcheting

1. Drop lubrication

2. Oil bath lubrication

3. Forced feed lubrication

1 2

3