TECHNOLOGY MECHANISMS 3º ESO IES CHAN DO MONTE URTAZA 1
WHAT IS A MECHANISM? Mechanism are devices that have been designed to make jobs easier. They all have certain things in common: They involve some kind of motion They involve some kind of force They must have some kind of input to make them work They produce some kind of output WHAT IS A MACHINE? If we connect mechanisms together we can build mechanical systems called machines. 2
TYPES OF MOTION ROTARY MOTION Rotary Oscillating Linear Reciprocating Motion in a circle is called rotary motion. The number of complete revolutions made per minute (rpm), is called rotary velocity. n = r.p.m. ω= 2 π n 60 OSCILLATING MOTION Oscillating motion is motion backwards and forwards in a circular arc. E.g. Playground swings and clock pendulums. 3
TYPES OF MOTION LINEAR MOTION Is a motion in a straight line. Steady linear motion is know as velocit ( Uniform motion in a straight line). An example of linear motion is the cutting arm of a paper guillotine a it travels one side of the machine to the other. V = l/t V= ω. r RECIPROCATING MOTION Is linear motion backwards and forwards in straight line. Sewing machines make use of this type of motion. Jigsaws is other example of reciprocating motion. 4
TRANSMITTING ROTARY MOTION MECHANISMS Pulley systems and Belts Gear Systems Sprockets and Chain Worm and Worm wheel 5
PULLEY SYSTEMS AND BELTS It is a method to transmitting force and rotary motion from one shaft, c driven pulley, to another called driven pulley. Using pulleys and belts Clic here Pulleys wheels are grooved so that the belt cannot slip off Most pulley wheels have a central shaft on which they rotate 6
Pulley systems and Belts SPEED CHANGES can be made by using different size pulleys. By comparing the the diameters of the two pulley wheels it is posible to calculate the velocity ratio: 1 When they have the same diameter, No modifications or variations o speed. 2. If the driver pulley is larger than the driven pulley, is a speed multiplier mechanism. 3. If the driven pulley is larger than the driver pulley a speed reducer mechanism. Clic here Notice the shaded areas in both pulleys. In the time the small one makes one turn, the bigger just makes a half turn. 7
Pulley systems and Belts TRANSMISSION RATIO ( I ) Is the relantionship between the number of revolutions of the driven pulley ( n2 ) and the number of revolutions of the driver pulley ( n1 ). It can also be expressed in term of the diameter. I = n2/n1 = d1/d2 REVERSING ROTATION Sometimes it is necessary to reverse the rotation of the driven pulley wheel in relation to the driver pulley. This is achieved by twisting the belt as shown in the diagram above Clic here 8
Pulley systems and Belts STEPPED CONE PULLEYS Speed setting are changed by altering the position of the Vee belt. This i done manually with the machine isolated ( diconnected) from the mains electricity supply. COMPOUND PULLEYS SYSTEM When we have a transmission system with more than two shafts, say that it's COMPOUND TRANSMISSION SYSTEM We have two transmissions ratios: 9
GEAR SYSTEMS Spur Gears are toothed wheels fixed to rotating shafts. The teeth of each gear mesh together to transmit rotary motion. The gears shown below are called spur gears because they mesh together. Gear A is called the driver because this is turned by a motor. As gear A turns it meshes with gear B and it begins to turn as well. Gear B is called the driven gear Depending on the arrangement of the shafts and the shape of your teet can be: * Spur Gears: the axes are parallel * Bevel gears: the axes are perpendicular Bevel gears like worm and worm wheels transmit torque and rotary motion through 90º * Helical gears. axes can be parallel or perpendicular Helical gears are quieter and more efficient than normal spur gears. They have their teeth cut at an angle across the gearwheel. Each tooth is slightly curved so that it forms part of a spiral or helix. Helical gears are found in car and motorcycle gear boxes (photo) where smooth, quiet, high speed transfer of power is required. 10
Gear Systems SIMPLE GEAR TRAIN Use two gears, which may be of different sizes. If one of these gears is attached to a motor is called the driver gear. The gear that is turned by the driver gear is called the driven gear. In a simple gear trains the driver and driven gears will rotate in opposite directions. GEAR RATIO (velocity ratio) Is equal to the number of teeth on the driver gear divided by the number of teeth on the driven gear. I = n2/n1 = Z1/Z2 IDLER GEAR A third gear, called an idler gear, can be inserted between the driver gear and driven gear make them rotate in the same direction. An idler gear does not affect the gear ratio (veloc ratio) between the driver gear and the driven gear. 11
Gear Systems COMPOUND GEAR TRAIN Compound gear trains involve several pairs of meshing gears, often have two or more gears mounted on the same shaft. They are used where large speed changes are required or to get different outputs moving at different speeds Clic here Gear ratios are calculated using the same principle as for simple gear trains, number of teeth on the driver gear divided by the number of te on the driven gear. 12
Sprockets and Chain Sprockets and chains are used when no slip, is essential. E.g. on a bicycle or motor bike Sprockets are toothed wheels attached to the driver and driven shafts. Clic here Chains consist of many loosely jointed links which engage with the sprocket teeth The Velocity Ratio of a sprocket and chain system is determined by comparing the number of teeth on each sprocket. I = n2/n1 = Z1/Z2 A low velocity ratio allows the rider to climb hills slowly, but with the minimum of effort. High velocity ratios mean that more effort is required but greater speeds can be achieved without having to pedal very fast. 13
WORM AND WORM WHEEL Worm gears are used when large speed reductions are required. The worm, which is attached to the driver shaft, has one tooth and takes the form of a screw thread. A worm and wormwheel transmits rotary motion through 90º. Gear ratio I = n2/n1 =* e1/z2 *e1 = Number of teeth on worm 14
Transforming mechanisms of rotary motion : IN LINEAR MOTION : * RACK AND PINION * SCREW THREADS RECIPROCATING MOTION CRANK AND CONNECTING ROD CAMS AND ECCENTRICS CAMS 15
RACK AND PINION Rack and Pinion is used to transform rotary motion into linear motion and vice versa, it s more usual to find applications of the first type. It is composed of two gears. The pinion is the normal round gear and the rack is straight or flat. The rack has teeth cut in it and they mesh with the teeth of the pinion gear. Train has to go up a steep bank or hil Direction of car: When the steering wheel turns, it makes a piñón rotate which causes a rack to move. to raise and lower the platform, the pinion is operated by turning the handle Sliding Door 16
SCREW THREADS Screw threads make use of the inclined plane principle. If you wrap an inclined plane around a cylinder you get the same helix form as on a screw thread. Make use of parallel linkages and square screw threads in order to raise the chassis of a motor car when changing a wheel. BENCH VISE SCISSORS SCREW JACKS 17
CRANK AND CONNECTING ROD A crank and slider mechanism consists of a rotating crank connected to a slider by a connecting rod. This type of mechanism can either convert rotary motion into reciprocating motion or vice versa. Clic here Clic here Above, the crankshaft( in red), sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation 18
CAMS AND ECCENTRICS CAMS A cam has two parts, the Cam Profile and the Follower: 1. Cam is a specially shaped piece of metal or hard wearing plastic, which is usually fixed to a rotating shaft. PEAR CIRCULAR OR * HEART DROP ECCENTRIC Clic here 2. A follower is held against the cam, either by its own weight or by a spring. As the cam rotates the follower moves up and down in a reciprocating motion. Clic here 19
CIRCULAR CAM OR ECCENTRIC CAM A circular cam is also known as an 'eccentric' cam. The centre of rotation of the cam is offset from the geometric centre of the circle. Clic here 20
CAMS AND ECCENTRICS CAMS Four stroke engine Clic here 21
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Adjuntos Pulleys.doc Pulleys 1.doc Reversing rotation.doc compound gear.doc Sprockets and Chain.doc water pump.doc Crankshaft and Piston.doc CAM.doc Eccentric Cam.doc Four stroke engine.doc Animations with cams and eccentrics.doc