UNIT 5

UNIT 5

1. MECHANISMS

   • Machines and mechanisms
     • A machine is a set of elements or parts that work together and use energy to perform a task and obtain a result
       • Driver element
         • Feeds energy to the machine
       • Driven element
         • Recieves the force produced in the machine
       • Converting and/or transmitting element
         • Transmits and/or converts the motion from the driver element to the driven one
     • Devices that transmit and/or convert motion
       • Types of motion that can be converted by mechanisms are:
         • Linear motion
           • The object moves in a straight line
         • Rotary motion
           • The object moves in circles
         • Oscillating motion
           • The object moves backwads and forwards tracing a curve
         • Reciprocating motion
           • The object moves forwards and backwards in a straight line
       • According to their function, we can divide them into:
         • Motion transmission mechanisms
           • The mechanism is fed energy from the driver element and simply transfers it to the driven element
             • Linear transmission mechanisms
               • Pulleys
                 • Wheel with a grooved rim used to lift heavy weights
                   • Fixed pulleys
                     • The axis of rotation of the pulley is fixed by attaching it to a support that prevents it from moving
                     • We still have to put in the same effort to lift the weight, but the make the task easier y changing the direction of the effort
                     • Formula: F=R
                   • Moveable pulleys
                     • They move vertically as well as turning when we pull on the rope. They reduce the effort by half
                       • Formula: F=R/2
                     • The set of moveable and fixed pulleys is called hoist. They reduce the effort depending of the number of pulleys
                       • Formula: F= R/number of pulleys
               • Levers
                 • Rigid bar that can pivot on a point of support and lift weight by applying a force. It has 3 important parts (left side) and 3 different types (right side)
                   • Effort: applied force
                   • Fulcrum: point of support
                   • Resistance: load to lift
                   • The law of the lever describes the forces of a lever in equilibrium. It is the same formula for all the types
                     • E*Ae=R*Ar
                   • Class 1
                     • The fulcrum is located between E and R to overcome and the point where the force is applied
                   • Class 2
                     • The fulcrum is at one end of the lever and the effort is at the other end. The resistance to overcome is located between the fulcrum and E.
                   • Class 3
                     • The E is located between the fulcrum and the R.
             • Rotary transmission mechanisms
               • Worm gear
                 • It consists of a worm that is fitted to the driver shaft and a toothed wheel that is fitted to the dirven shaft
                   • The worm has to rotate as many times as the number of teeth on the wheel
                   • It is an excellent speed reducer
                   • The motion isn't reversible
                   • It is compact and quiet
               • Friction wheels
                 • Mechanism consisting of two wheels located one next to the other. Motion is transmitted between them by the friction
                   • Formula: i= w1/w2=d1/d2
                   • The smallest wheel have to turn faster than the biggest one
                   • The driven wheel always rotates in the opposite direction to the driver wheel
                   • They can slip
                   • They can be subject to wear and tear when they are used over long periods
               • Belt and pulleys
                 • This system is made up of two pulley wheels joined by a belt that slots into the grooved rim of the wheels
                   • Formula: i= w1/w2=d1/d2
                   • They transmit rotary motion between shafts located at different points
                   • Their operation is silent
               • Gears
                 • Mechanism consisting of two toothed wheels
                   • Formula: i= w1/w2=z1/z2
                   • The 2 wheels rotate in opposite directions
                   • They can be used for large power transmission
                   • It is necessary to grease between the wheels
                   • They are noisy
               • Chain and sprockets
                 • Two toothed wheels separated by a distance and connected by a chain that meshes with the teeth of the two wheels
                   • They can be used for large power transmission
                   • They need greasing
                   • They are noisy
         • Motion conversion mechanisms
           • The mechanism converts the movement from the driver element into a movement that the driven element can use
             • Cam
               • An elliptical component that rotates to drive an element called a follower
                 • They convert rotary motion into linear reciprocating one
             • Rack and pinion
               • A pinion that meshes with a rack. The rotary motion is converted into linear one
                 • It is very smooth and precise and it can be also reversible
                   • Formula: L= p*z*w
             • Screw and nut
               • It consists of a screw and a nut. The rotary motion is converted to forward linear motion
                 • It is a good speed reducer and reversible
             • Crank and slider
               • A rigid bar connected at one end to a crank. The rotary motion is converted into reciprocating one
                 • It is reversible
             • Eccentric
               • It is a mechanism similar to a regular cam. It consists of a wheel where the axis of rotation is not the geometrical centre. It converts rotary motion into linear reciprocating one

2. STRUCTURES

   • Definition and types
     • A structure is a collection of elements arranged to support a body on an object
       • The structure of an object is the distrbution of its essential parts
     • Frame
       • They are formed by columns and beams that act as supporting elements
         • The structure of a building
     • Truss
       • They are composed of bars that form triangles
         • Electricity plyons
     • Suspended
       • They are held in place by cables and suspenders attached to sturdy supports
         • Tents
     • Mass
       • They are solid, heavy and sturdy
         • Like a wall
   • Forces, stresses and strenght
     • A force is an influence that can deform a body or change its movement, or produce motion in a body at rest
     • Stresses are physical demands that an object withstands when one or more forces are applied to it
       • Compression
         • Caused by forces acting on a body that tend to fletten it or reduce its length or thickness
       • Tension
         • Forces acting on a body that tend to stretch it
       • Bending
         • Forces acting on an element that can make it curve or bend
       • Torsion
         • Forces acting on a body that make it twists
       • Shear
         • Two equal forces applied in opposite directions that act on lines of action to close each other
     • The strenght of a material is its ability to withstandexternal forces without breaking
       • So, it is essential to a structure to be strong, but it has some conditions:
         • Rigidity
           • The material doesn't change shape when force is applied to it
             • Elements that increase it:
               • Trussing
                 • It is a method for making weak structures more rigid. This works because of the triangle, the only figure that cannot be deformed.
                   • It can also work if we do it with a quadrilateral structure, but only if we add a diagonal bar, since we divide it like this into two triangles
               • Brace
                 • A part that is positioned diagonally between two bars to form a right angle
               • Stay cables
                 • Cables that help support tensile stress
               • Sections
                 • Bars with different cross-sections
                   • Close sections
                     • Square section
                     • Triangular section
                     • Round section
                   • Open sections
                     • L-shaped
                     • T-shaped
                     • U-shaped
                     • I-shaped
         • Equilibrium
           • When a body's structure remains stationary
             • Stable
             • Unstable
               • When a force is applied to the body, it makes it move
         • Stability
           • When a structure remains in equilibrium even when a force is applied to it
             • It depends on where the centre of gravity is placed, the point where the weight of an object is consdered to be concentrated
               • By widening the base of the structure
               • By locating the centre of gravity at a low point
               • By supporting the structure with stay cables
               • By making a good foundation for the structure
       • It depends on...
         • Types of forces applstructure and the points where they actied to the
         • The estructure's shape and the elements that form it

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