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      • AA

      º

      • Actividades
      s

      • A
        A

      © Oxford University Press

      º

            7. Mechanisms

            Talking book

            Structures bear forces statically, that is, without moving. Mechanisms, on the other hand, allow objects to move. They transmit and transform force and motion from a motor to a receptor. They make work easier and more comfortable for human beings.

            7.1. Linear transmission of motion

            Talking book

            Linear transmission mechanisms transmit motion and force through a motor to another point.

            Levers

            A lever is a rigid bar that turns around a point called a fulcrum. At one end of the bar, force (F) is applied to move a resistance or load (R) at the other end of the bar.

            There are three types of levers: Class 1, Class 2 and Class 3.

            The chain and gears on your bicycle, the gears of an old clock, a seesaw, the pulley of a well are all examples of simple mechanisms and form parts of many objects.
            We say the lever is in equilibrium when the forces acting on opposite ends of a lever are equal, that is: the result of the force (F) multiplied by the distance (d) from the fulcrum is the same as the result of the load (R) multiplied by the distance (r) from the fulcrum. We can express this mathematically as the Law of the Lever:


             

            F x d = R x r

             

            Class 1 Class 2 Class 3

            The fulcrum is between the force and the resistance.

            image

            The resistance is between the fulcrum and the force.

            image

            The force is between the fulcrum and the resistance.

            image

            The effect of the force applied is increased or decreased.

            image

            The effect of the force applied is always increased (d > r).

            image

            The effect of the force applied is always decreased ( d < r).

            image

            Pulleys






            image
            Fixed pulley

            A pulley is a wheel with a groove that turns on an axis. It is fixed to a solid surface. There is a rope or a chain inside the groove that allows you to lift objects easily by apply a force (F) against a resistance (R).

            The pulley is balanced when the force we apply F equals the load R.

            F = R

            It allows us to lift and lower loads easily and is used in wells, simple cranes and gym equipment.

             

            Movable pulley






            image
            Movable pulley

            This is a set of two pulleys, one fixed and another with linear movement. It is balanced when:

             
            image
             

            With this system, we only need have the force to lift the same weight as with a fixed pulley.

            Compound pulley systems

            This is a special combination of fixed and movable pulleys.

            The more pulleys we have, the more complex the mechanism, but the easier it is to lift the load.

            Look at these two compound pulley systems (n is the number of movable pulleys):

            image

            Movable and compound pulleys can be used for lifts and cranes.

            7.2. Rotary transmission

            Talking book

            Wheels and belts

            These are systems where two or more wheels are in contact, directly or with a belt.

            Friction wheels

             





            image

            Two wheels are in direct conduct: one of the wheels is called the driver, because as it moves, it drives the other wheel which is called the driven. Each wheel moves in a different direction.

            Uses: Friction wheels are very common in industry, for example in making and transferring sheets of metal and rolls of paper.

            Pulleys with belts






            image

            Two wheels are at a certain distance from each other. Their axes are parallel to each other and they turn together with a belt.

            They turn in the same direction because the belt, which is fixed to both wheels, transfers the turn to the wheels.

            Uses: Industrial machines, washing machines and drills.

            The relationship between the speeds of the two wheels depends on the relative size of the wheels, expressed through the following equation:

            image

            D1 and D2 are the diameters; N1 the speed of the driver; and N2 is the speed of the driven.

            Interlocking gears






            image

            These are sets of wheels with teeth that match each other so that one wheel moves another. The teeth must all have the same shape and size. The rotary movement of axis 1 transmits to axis 2 through the gears 1 and 2 on the axes. Each gear turns in a different direction.

            Uses: Industrial and car engines, home appliances such as drills, electric blenders and toys.

            Sprockets with chains






            image

            These are sets of two wheels with teeth on parallel axes, at a distance from each other. They turn at the same time with a chain or belt with teeth which is fixed to the wheels. The chain transmits the rotary motion of sprocket 1 to sprocket 2. They both turn in the same direction.

            Uses: Industrial machines, engines and motorbikes.

            The relationship between the speeds depends on the relative size of the wheels, expressed through this equation:

            image

            Z1 and Z2 are the number of teeth, and N1 and N2, are the speeds.

            7.3. Transformation of motion

            Talking book

            Some mechanisms transform rotary motion into linear motion or viceversa.

            Winch and crank mechanism






            image

            A crank is a bar that turns an axis. You need less force to turn the winch with a crank that to turn the winch directly. The mechanism uses a winch, which is a cylinder that turns on an axis, to pull an object. A winch is balanced when:

            F ∙ d = R ∙ r

            So the load equals:

            image

            Uses: Cranes, coffee grinders, etc.

            Rack and pinion mechanism

            image

            A pinion is a wheel with teeth that interlock with a rack, which is a bar with teeth. When the wheel turns the bar moves in a linear direction. This mechanism transfers the rotary movement of the wheel into the linear motion of the bar. The mechanism can also be reversed.

            Uses: Corkscrews, steering wheels, etc.

            Crank and rod mechanism






            image
            Corkscrew

            The crank turns, which moves the rod. The rod is fixed to the crank at one end, and at the other end to something that makes a reciprocating movement. As the wheel turns, the crank transforms rotary movement to the rod, which moves back and forth.

            This system also works the other way around. That is, the linear motion of the crank transforms into a rotary movement.

            image
            image
            Internal combustion engine

             

            Uses: This mechanism was important when making the first steam engines; nowadays we use it in internal combustion engines, windscreen wipers and power tools.

            1

            Activity 39
            Say what kind of lever each of the following technological objects is:

            2

            Activity 40
            Look at the images of a fixed pulley and a movable pulley. What are these mechanisms used for?

            3

            Activity 41
            Complete the following sentence in your notebook:

             
            Mechanisms
            Structures
            Without background sound
            Logo

            Activity 39

            0/6
            Say what kind of lever each of the following technological objects is:

            Objeto

            Tipo de palanca

            Esquema

            Wheelbarrow

             

             

            Balance

             

             

            Ice bucket tong

             

             

            Nutcracker

             

             

            Pliers

            Broom

             

              3 attempts
            Done
            Mechanisms
            Structures
            Without background sound
            Logo

            Activity 40

            0/4

            Look at the images of a fixed pulley and a movable pulley. What are these mechanisms used for?

             

            Type of pulley

            Uses

            Applications

             

             

             

             

            Lift and download charges easily

             

             

             

             

            Lift and download charges easily

              3 attempts
            Done
            Mechanisms
            Structures
            Without background sound
            Logo

            Activity 41

            0/6
            Complete the following sentence in your notebook:

            A system of more than two pulleys is called a  pulley system. It is made up of groups of pulleys: and . The pulleys we have, the it is to lift the load although the system is more complex.

              movable easier compound two fixed more

              3 attempts
            Done