Mechanisms
1º Mechanism
Mechamsms are devices that transmit and convert forces and motion from a dnvmg force or input element to an output element. They allow us to carry out certam tasks more easily and more effioently
transporting people from A to B
telling the time
entertaining children in the park
liftmg and carrying objects
Types of levers
2.1. Levers
2º Linear motion mechanisms
A lever is a rigid bar that is supported by a fulcrum.
The following equation indicates a balanced lever:
F·d = f·r It's called the law of the lever
Mechanisms that transnut motion and force in a straight line from one point to another. Examples include levers and fixed, mobile and compound pulleys.
There are three types of levers: class l, class 2 and class 3. These classilications are based on the relative position of the lever, the effort applied and the resistance.
2.2. Fixed pulley
A fixed pulley is a wheel that has a groove around lt into which a rope, cham or belt fits It rotates around an axle that is fixed to an immobile surface
A fixed pulley is balanced when the effort F is equal to the resistance of the load, R: F = R
2.3. Movable pulley
A moveable pulley ts a set of two pulleys one ts fixed while the other can move tn a linear direction.
A moveable pulley is balanced when it satisfies this equation:
F = R/2
2.4. Compound pulley
This ts a system of fixed and moveable pulleys, often called block and tackle
On rhe left you can see two different types of block and tackle pulleys (n = the number of movable pulleys).
3.1. Friction drives
3º Rotary motion mechanisms
Fnctton dnves are made up of two or more wheels that are in contact. The first wheel is called the primary drive wheeL When it moves, it turns or drives the second or output wheel, causing it to move as well.
The ratio between the rotation veloaty of the wheels or pulleys depends on the relative size of the wheels.
N1 · D1 = N2 · D2 =} D1/D2 = N2/N1
3.2. Pulleys with belt
They consist of two pulleys or wheels that are a certain distance apart. Their axles are parallel and they rotate simultaneously due to the effect of the belt.
The rotation of one axle is transmitted to the other through the connected pulleys
3.3. Gear mechanisms and cogwheels
Cogwheels are sets of wheels that have teeth called cogs. The cogs fit into the spaces between the cogs of another wheel, so that one wheel moves the other.
The ratio between the rotation veloaues of the wheels depends on the number of teeth on each wheel. It's expressed by this equation:
N1 · Z1 = N2 · Z2 =} Z1/Z2 = N2/N1
3.4. Worm gear
This is a screw that moves a helical cogwheel that is set perpendicular to the screw. Each time the screw rotates, the gear moves forward as many teeth as there are grooves in the screw, usually a small number I, 2 or 3
A worm screw fulfils this equation:
Nwheel = Nscrew· Zgrooves/ Zwheel
3.5. Gear mechanisms with a chain
These are two cogwheels with parallel axles that are a certain distance apart, they rotate simultaneously by means of a metal chain or a toothed belt stretched over both wheels.
The relation between the rotation veloaties of the wheels depends on the number ofteeth on each gear.
N1 · Z1 = N2 · Z2 =} Z1/Z2 = N21/N1
3.6. Gear train
This is a system of more than two gears, connected together as shown in the diagram The rotary motion of the first wheel ( I) doves the second wheel (2), and so on
The gear ratio between the drive wheel (I) and the dnven wheel (4) depends on the number ol geared teeth m the system:
N4/N1 = Z1 · Z3/ Z2 · Z4
3.7. Pulley trains with belts
he gear rauo between the dnve pulley (I) and the dnven pulley l4) depends on the relauve sizeof the pulleys in the system. It's expressed as a function of their diameters
N4/N1=D1·D3/D2·D3
Rack and pinion syste
4.1. From rotary into linear
4º Mechanisms that transform motion
Crank-link-slider
4.2. From rotary into reciprocating motion
The ratio between the number of rotations of the pinion and the veloaty of the movement of the rack is expressed by this equation:
L = P· Z·N
Nut and bolt system
This consists of a bolt or threaded bar and a nut that has the same intenor diameter as the diameter of the bol
Winch and crank hand
A winch is balanced when it satisfies this equation
F·d=R·r=} F=R· R·r/d
This is composed of a crank and a rod called a connecting rod or link. This rod has articulated Ioints at each end — one is connected to the crank and the other to the slider. The slider produces a reciprocating motion.
Crankshaft
This is a set of connecting rods attached to a jointed axle. Each of the points of the axle acts as a crank.
Cam
This is basically a rotating objectthat pushes a follower as it moves A cern transforms rotary motion into reciprocating motion in the follower or bar
Eccentric cam
This consists of a wheel with an off centre rotation axle that doesn't coincide with the centre of its circumference
5.1. Mechanisms for controlling and directing motion
5º Other mechanism
Disc brakes consist of pads and a disc anached to the oblect that we want to slow down
Band brakes consist of a metal band that puts pressure on a drum attached to the axle of the oblect we want to slow down.
Drum brakes include one or two brake shoes that come into contact with the drum.
5.2 Mechanisms that store energy
Springs are devices that absorb energy This energy can be released later, little by little or all at once
5.3. Connecting mechanisms
Clutches are mechanisms that allow axles or shafts to be connected or separated
Fixed connections are used to make permanent connections
between axles and shafts
Mo v able connections are used to connect shafts that can move
along the axle or at an angle to each other. There are two main types: Oldham iomts and Cardan joints.
5.4. Supports: bushings and bearings
In bushings the axle or the shaft is inserted in a plain circular piece that is placed inside a housing to provide a bearing surface.
Bearings are made up of two concentric nngs with balls or rollers
between them
