Slide 1

GEARS

A toothed machine part, such as a wheel , that meshes with another toothed part to transmit motion or to change speed or direction

Gears operate in pairs Smaller one called pinion Larger one called gearNormally the pinion drives the gear

What is a gear?GEARSGears are versatile mechanical components capable of performing many different kinds of power transmission or motion controlTheir general purpose is as follows : Changing rotational speed

Changing rotational direction

Changing the angular orientation of rotational motion

Multiplication or division of torque or magnitude of rotation

Converting rotational to linear motion and its reverse

Offsetting or changing the location of rotating motionGEAR TEMINOLOGY

Addendum : the distance from the top of a tooth to the pitch circle Dedendum : the distance from the pitch circle to the root circle. It equals the addendum + the working clearancePitch Circle : the imaginary circle that comes in contact with the imaginary circle of another gear when the two are in meshPitch diameter : The diameter of the pitch circle, the imaginary circle that rolls without slipping with the pitch circle of the mating gear, measured in inches or millimetersClearance: The radial distance between the bottom land and the clearance circleCircular pitch: The distance along the pitch circle from a point on onetooth to a corresponding point on an adjacent toothGEAR TOOTH PROFILESPUR GEARS

Spur gears have their teeth parallel to the axis Used for transmitting power between two parallel shaftsApplications : Clocks, household gadgets, motor cycles, automobilesThey offer constant velocity ratio Highly reliableSimplest, hence easiest to design and manufacture A spur gear is more efficient if you compare it with helical gear of same size Spur gear teeth are parallel to its axis Hence, spur gear train does not produce axial thrust. So the gear shafts can be mounted easily using ball bearings They can be used to transmit large amount of power (of the order of 50,000 kW)Spur gear are slow-speed gearsThus, gear teeth experience a large amount of stressThey cannot transfer power between non-parallel shaftsThey cannot be used for long distance power transmissionSpur gears produce a lot of noise when operating at high speedsNot as strong as other gears

HELICAL GEARSUsed for transmitting torque in non-parallel shaftsWhen two helical gears are engaged the helix angle has to be the same on each gear, but one gear must have a right-hand helix and the other a left-hand helixApplications : Blowers , feeders, sugar industry , rolling millsThe angled teeth engage more gradually than spur gear teeth causing them to run more smoothly and quietly Helical gears are highly durable and are ideal for high load applicationsAt any given time their load is distributed over several teeth, resulting in less wear Can transmit motion and power between either parallel or right angleHelical gears cause losses due to the unique geometry along the axis of the helical gears shaftEfficiency of helical gear is less because helical gear trains have sliding contacts between the teeth which in turns produce axial thrust of gear shafts and generate more heatSo, more power loss and less efficiency

BEVEL GEARSUsed when the direction of a shaft's rotation needs to be changedUsually mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as wellApplications: Used in hand drills ,differential drives

The angled teeth engage more gradually than spur gear teeth causing them to run more smoothly and quietly

Helical gears are highly durable and are ideal for high load applications

At any given time their load is distributed over several teeth, resulting in less wear

Can transmit motion and power between either parallel or right angleHelical gears cause losses due to the unique geometry along the axis of the helical gears shaft

Efficiency of helical gear is less because helical gear trains have sliding contacts between the teeth which in turns produce axial thrust of gear shafts and generate more heat.

So, more power loss and less efficiency

WORM GEARSThese gearsare used when large gear reductions are neededIt is common for worm gears to have reductions of 20:1, and even up to 300:1 or greaterAre used for transmitting motion between non parallel and non transmitting shafts Applications : Turning instruments like guitar, elevators, conveyor belts

Worm gear drives operate silently and smoothly. They are self-locking. They occupy less space. They have good meshing effectiveness. They can be used for reducing speed and increasing torque. High velocity ratio of the order of 100 can be obtained in a single stepWorm gear materials are expensive They have high power losses A disadvantage is the potential for considerable sliding action, leading to low efficiency They produce a lot of heat

Worm Worm Gear

PLANETARY GEAR

Sun gear 1Sun gear 2First gear stepSecond gear stepRing gear planet gear A planetary transmission system (or Epicyclic system), consists normally of a centrally pivoted sun gear, a ring gear and several planet gears which rotate between these

The advantage of a planetary transmission is determined by load distribution over multiple planet gears. It is thereby possible to transfer high torques utilizing a compact design

Used to achieve large speed reductions in compact space

Can achieve different reduction ratios by holding different combinations of gears fixed

Applications :Used in automatic transmissions of carsTurbine enginesGearhead motorsCar mirrors GEAR MODULE"Module" is the unit of size that indicates how big or small a gear is

d reference diameter Z Number of teeth

Reference pitch

p reference pitch Z Number of teethLAW OF GEARINGThe fundamental law of gearing : the angular velocity ratio between the gears of a gear set must remain constant throughout the mesh

This amounts to the following relationship

PinionGearNumber of teeth (Z)Z1Z2Diameters (d)(mm)d1d2Speed(rpm)n1n2Speed(rad/s)12

GEAR RATIOS Thegear ratioof agear train, also known as itsspeed ratio, is the ratio of the angular velocity of the input gear to the angular velocity of the output gearThe gear ratio can be calculated directly from the numbers of teeth on the gears in the gear trainMany machines use gears. A very good example is a bicycle which has gears that make it easier to cycle, especially up hills Bicycles normally have a large gear wheel which has a pedal attached and a selection of gear wheels of different sizes, on the back wheel When the pedal is revolved the chain pulls round the gear wheels at the back

SPROCKET PEDAL GEAR CONCEPT OF GEAR RATIOS The concept of gear ratios can be explained using the bicycle gears

The reason bicycles are easier to cycle up a hill when the gears are changed is due to what is called Gear Ratio

The ratio is determined by the number of teeth on each gear wheel

SPROCKET 30 TeethPEDAL GEAR 60 TeethDRIVEN GEAR WHEEL DRIVER GEAR WHEEL

This means The DRIVEN gear makes TWO rotations for every ONE rotation of the Driving GearCOMPOUND GEAR RATIOSWhen faced with three gears the question can be broken down into two parts First work on Gears A and BWhen this has been solved work on gears B and CGear A revolves at 60 revs/min in a clockwise direction What is the output in revolutions per minute at Gear C? In what direction does Gear C revolve ?

GEAR AGEAR BGEAR C20 TEETH 60 TEETH 10 TEETHWe first calculate for GEAR A and B

COMPOUND GEAR RATIOSSince we are going from a SMALLER gear to a LARGER gear we DIVIDE the rpm

Now we calculate for GEAR B and C

This means for every ONE rotation of gear B, gear C makes SIX rotationsFinally, the output of GEAR C is

GEAR TRAINS

15PROBLEMS WITH GEAR DRIVESBacklash

It is the amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the pitch circles

Backlash may be determined in the transverse, normal, or axial-planes, and either in the direction of the pitch circles or on the line of action

PROBLEMS WITH GEAR DRIVESInterference

BEARINGSA bearing is a mechanical device that supports the moving parts of a machineIt is a device that is used to enable rotational or linear movement, while reducing friction and handling stressBearings are made to support radial loads, thrust loads, or combined radial-thrust loads They may be categorized into two general classes, each with two sub-types 1. Plain bearings (a) Cylindrical (b) Thrust 2. Anti Friction Bearings (a) Ball bearing (b) Roller bearingsHOW BEARINGS WORK Thrust bearings are designed to manage thrust (axial) loads and provide high-shock-load resistance in a variety of applications

Bearings make use of a relatively simple structure: a ball with internal and external smooth metal surfaces, to aid in rolling The ball itself carries the weight of the loadthe force of the loads weight is what drives the bearings rotationHowever, not all loads put force on a bearing in the same manner There are two different kinds of loading : Radial and Thrust

A radial load, as in a pulley, simply puts weight on the bearing in a manner that causes the bearing to roll or rotate as a result of tension

Radial loading

Thrust loading

TYPES OF BEARINGS

Thrust bearingRadial bearingCylindrical roller bearingFlanged bearing

COUPLINGSAcouplingis a device used to connect two shafts together at their ends for torque transmission

Couplingsdo not normally allow disconnection of shafts during operation, however there are torque limiting couplingswhich can slip or disconnect when some torque limit is exceeded

Couplers are available in two major styles: 1. Rigid Rigid couplers must be strong enough to hold the shafts ends together as if they were one shaft 2. Flexible Flexible couplers allow for misalignment and are used where the two shafts are already running in their own bearings, but might be slightly out of alignment

TYPES OF COUPLINGS

Rigid coupling

Flexible sleeve coupling

Rubber flexible coupling

Clamped Rigid couplingJOINTSMechanical joints are parts of a machine which are used to join one mechanical part to another

Knuckle Joint A knuckle joint is used to connect the two rods which are under the tensile load, when there is requirement of small amount of flexibility or angular moment is necessary

There is always axial or linear line of action of load

UNIVERSAL JOINTIs a joint or a coupling in a rigid rod that allows the rod to bend in any direction, and Is commonly used in shafts that transmit rotary motion

It consists of pair of hinges located close together , oriented at 90 to each other, connected by a cross shaft

The universal joint is not a constant velocity joint

KINEMATIC JOINTSKinematic pair A joint which is formed by the contact between two bodies and allows relative motion between themTYPES OF KINEMATIC JOINTS

Revolute pair (Pin joint)Prismatic Pair(Slider Joint)Cylindrical PairScrew Pair(Helical) Spherical Pair(Globular) Planar Pair(Flat) D.O.F : 1Relative motion : CircularD.O.F : 1Relative motion : LinearD.O.F : 2Relative motion : CylindricalD.O.F : 1Relative motion : HelicalD.O.F : 3Relative motion : SphericalD.O.F : 3Relative motion : PlanarLEAD SCREWIt is also known as a power screw or translation screw

Used as a linkage in a machine to translate turning motion into linear motion

BALL SCREWBallscrews use recirculatingballs to reduce friction and gain higher efficiency than conventionalLeadscrews

Used as a linkage in a machine to translate turning motion into linear motion

In the ballscrew driven mechanism ,the motor turns the ballscrew, and its rotary motion is translated into the linear motion that moves the carriage and load by the stages bolt nut

The bearing ways act as linear guidesBallscrew driveBallscrew-driven single-axis slide mechanism