2 What are they?A mechanism is simply a device which takes an input of motion or force and outputs a different motion or force. The point of a mechanism is to make the job easier. The mechanisms most commonly used in mechanical systems are levers, linkages, cams, gears and pulleys.
3 Input motionInput forceMechanismOutput motionOutput force
4 Levers Mechanical advantage Linkages used to change the direction or type of motion, in combination with levers also could change the size or magnitude of a force.Cams change the type of motion from circular to rectilinear motion.Gear, gear trains and compound gear trains gear ratio and output speed.Worm gear, bevel gear, rack and pinionPulley systems and hoist velocity ratio, output speed, output torqueChain and sprocketCrank, link and slider.
7 Mechanical Advantage and Equilibrium Point M.A. =LoadEffortd2d1EffortL·d1 = E·d2LoadFulcrum
8 In summary,A lever is the simplest kind of mechanism. There are three different types of lever. Common examples of each type are the crowbar, the wheelbarrow and the pair of tweezers.All levers are one of three types, usually called classes. The class of a lever depends on the relative position of the load, effort and fulcrum:The load is the object you are trying to move.The effort is the force applied to move the load.The fulcrum (or pivot) is the point where the load is pivoted.
9 Linkages Linkage type Description Diagram Reverse Motion L. A reverse-motion linkage changes the direction of motionreverseLinkageParallel Motion L.A parallel-motion linkage creates an identical parallel motion.paralleLinkageBell-Crank L.A bell-crank linkage changes the direction of movement through 90°bellCrankCrank-Slider L.Converts rotary motion to linear motionCrankSliderTreadle L.The rotary motion is converted in to an oscillating motiontreadle
10 LinkagesLinkages are mechanisms which allow force or motion to be directed where it is needed. Linkages can be used to change:The direction of motionThe type of motionThe size of a forceA linkage consists of a system of rods or other rigid materials connected by joints or pivots. The ability of each rod to move will be limited by moving and fixed pivots. The input at one end of the mechanical linkages will be different from the output, in place, speed, direction and other ways.
11 CamsA cam is a shaped piece of metal or plastic fixed to a rotating shaft. A cam mechanism has three parts: cam, slide and follower.Pear-shaped camEccentric camDrop camcamSystems
12 GearGears consist of toothed wheels fixed to shafts. The teeth interlock with each other, and as the first shaft (the driver shaft) rotates, the motion is transmitted to the second or driven shaft.A number of gears connected together are called a gear train.
13 Gear ratioWhere there are two gears of different sizes, the smaller gear will rotate faster than the larger gear. The difference between these two speeds is called the velocity ratio, or the gear ratio, and can be calculated using the number of teeth.V1·Z1 = V2·Z2G.R. = Z2 / Z1V1: input speed (driven shaft)V2: output speed (driver shaft)Z1: number of teeth of driven gearZ2: number of teeth of driver gear
14 Compound Gear TrainWhere very large speed reductions are required, several pairs of gears can be used in a compound gear train. A small gear drives a large gear. The large gear has a smaller gear on the same shaft. This smaller gear drives a large gear. With each transfer, the speed is significantly reduced.gearSystems
15 Worm gearsAnother method of making large speed reductions is to use a worm gear.Each time the shaft spins one revolution, the gear turns forward by only one tooth.The worm can drive the worm gear round, but the worm gear cannot drive the worm. This means that worm gears are good to use in hoists, the load will not fall back when the motor stops.Worm gears are a good option when you wish to alter direction or rotary motion through 90° and reduce the speed.WormAndWormwheel
16 Bevel gearsBevel gears, like worm gears, change the axis of rotation through 90°. The teeth have been specially cut so the gears will mesh at right-angles to each other.bevelGears
17 Rack and pinionA pinion is a round cog and the rack is a flat bar with teeth.
18 Pulley systemsPulleys are used to change the speed, direction of rotation, or turning force or torque.A pulley system consists of two pulley wheels each on a shaft, connected by a belt. This transmits rotary motion and force from the input, or driver shaft, to the output, or driven shaft.V1·D1 = V2·D2
19 Velocity RatioIf the pulley wheels are different sizes, the smaller one will spin faster than the larger one. The difference in speed is called the velocity ratio.V1·D1 = V2·D2V.R. = D2 / D1V1: input speed (driven shaft)V2: output speed (driver shaft)D1: Diameter of driven pulleyD2: Diameter of driver pulleyBeltAndPulley
20 Chain and sprocketCrank, link and sliderCrank shaftRatchet and pawl
21 TorqueThe velocity ratio of a pulley system also determines the amount of turning force or torque transmitted from the driver pulley to the driven pulley.