MECHANISM GUNJAN VED VENUGOPAL

A mechanism is a device designed to transform input forces and movement into a desired set of output forces and movement . Mechanisms generally consist of moving components such as gears and gear trains, belt and chain drives, cam and follower mechanisms, and linkages . Primary function of a mechanism is to transmit or to modify motion

TYPES OF MECHANISM Gear and gear train mechanism Cam and follower machanism linkages

Types of gear and gear train Spur gear: gear with radial teeth parallel to its axis

Rack & Pinion a toothed wheel (pinion) engages a notched bar (rack) to convert rotary motion into linear motion. Worm gear :A worm is used to reduce speed. For each complete turn of the worm shaft the gear shaft advances only one tooth of gear.

Helical gear: a gear that has the teeth cut at an angle to the center line of the gear

Nomenclature of gear Pitch circle: A circle the radius of which is equal to the distance from the gear axis to the pitch point. „ Addendum circle: A circle bounding the ends of the teeth, in a right section of the gear. „ Root (or dedendum) circle: The circle bounding the spaces between the teeth, in a right section of the gear. „ Addendum: The radial distance between the pitch circle and the addendum circle. „ Dedendum: The radial distance between the pitch circle and the root circle. „ Clearance: The difference between the dedendum of one gear and the addendum of the mating gear

Gear trains Combination of two or more gears when they made to mesh each other to transmit power from one shaft to another. TYPES OF GEAR TRAIN Simple gear train Compound gear train Reverted gear train Epicylic gear train

Simple gear train : When there is only one gear on each shaft. The only function of the idler gear is to change the direction of rotation. It has no affect on the gear ratio.

Compound gear train : When there are more than one gear on a shaft,it is called a compound gear train.

Reverted gear train : When the axes of the first driver and the last driven are co-axial, then the gear train is known as reverted gear train. In a reverted gear train, the motion of the first gear and the last gear is same

Epicyclic gear train : Epicyclic means one gear revolving upon and around another. The design involves planet and sun gears as one orbits the other like a planet around the sun. A small gear at the center called the sun, several medium sized gears called the planets and a large external gear called the ring gear.

Advantages Simple gear train : To obtain desired direction of motion of the driven gear ( CW or CCW) and to obtain high speed ratio. Compound gear train : A much larger speed reduction from the first shaft to the last shaft can be obtained with small gear. If a simple gear trains used to give a large speed reduction, the last gear has to be very large. Reverted gear train : The reverted gear trains are used in automotive transmissions, lathe back gears, industrial speed reducers, and in clocks (where the minute and hour hand shafts are co-axial).

The simplest and basic is four bar chain mechanism linkages The function of a link mechanism is to produce rotating, oscillating, or reciprocating motion from the rotation of a crank or vice versa The simplest and basic is four bar chain mechanism

Some important concepts in link mechanisms are: Crank: A side link which revolves relative to the frame is called a crank Rocker: Any link which does not revolve is called a rocker. Crank-rocker mechanism: In a four bar linkage, if the shorter side link revolves and the other one rocks (i.e., oscillates), it is called a crank-rocker mechanism. Double-crank mechanism: In a four bar linkage, if both of the side links revolve, it is called a double-crank mechanism. Double-rocker mechanism: In a four bar linkage, if both of the side links rock, it is called a double-rocker mechanism.

s = length of shortest bar l = length of longest bar p, q = lengths of intermediate bar Grashof's theorem states that a four-bar mechanism has at least one revolving link if s + l <= p + q and all three mobile links will rock if s + l > p + q

CAM AND FOLLOWER A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel (e.g. an eccentric wheel) or shaft that strikes a lever at one or more points on its circular path

The cam can be seen as a device that translates from circular to reciprocating motion.  A common example is the camshaft of an automobile, which takes the rotary motion of the engine and translates it into the reciprocating motion necessary to operate the intake and exhaust valves of the cylinders.

Types of cam and follower                                                           Figure 6-2 Classification of cam mechanisms Types of cam and follower Knife edge follower Roller follower Flat faced follower Spherical faced follower Offset follower Rotating follower

CAM NOMENCLATURE

Trace point: It is used to generate the pitch curve. Pitch curve: The path generated by the trace point at the follower is rotated about a stationary cam. Pitch circle: A circle from the cam center through the pitch point. The pitch circle radius is used to calculate a cam of minimum size for a given pressure angle. Prime circle (reference circle): The smallest circle from the cam center through the pitch curve.

Base circle: The smallest circle from the cam center through the cam profile curve. Stroke :The greatest distance or angle through which the follower moves or rotates. Pressure angle: The angle at any point between the normal to the pitch curve and the instantaneous direction of the follower motion.

BASED ON THE MOTION OF FOLLOWERS SIMPLE HARMONIC MOTION CONSTANT ACEELERATION & DECCELERTION CONSTANT VELOCITY CYCLOIDAL MOTION

APPLICANTIONS OF CAMS AUTOMATIC MACHINES INTERNAL COMBUSTION ENGINES PRINTING CONTROL MECHANICS MACHINE TOOLS