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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
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