MECHANISMS THAT CONTROL MOTION

DIRECTION CONTROL:RATCHETS

A ratchet is a mechanisms that controls the direction of motion. It allows motion in one direction, but not in the other, as you can see the  picture.

Some ratchets are reversible, so they can turn and loc in one direction to another.

 

SPEED REDUCTION:BRAKES

Brakes use friction to reduce speed. They are activated by certain levers. The lever transmits forcé to an output receptor, which puts pressure on the Wheel. This produces friction, which slows down the Wheel.

There are various types of brake systems according to where the friction is produced:

Ø  Disc brakes: A disc is connected to an axle. Brake pads apply pressure to the disc.

Ø  Brand brakes: A drum is connected to an axle. A fexible band applies pressure to the outside of the drum. These brake were used in carriages and they depended on the sttrength of the driver.

Ø  Drum brakes: A drum is connected to the axle. A pair of brake shoes apply pressure to the inside of the drum.

RECIPROCATING ROTARY-LINEAR TRANSFORMATION

Some mechanisms work in the opposite way, transforming rotary motion into reciprocating linear motion.

Crank and rod mechanism

 

The piston moves a rod forwards and backwards. This rod turns the first Wheel. The second Wheel  turns because it is connected to the first Wheel by another rod.

Crankshaft mechannism

We can connect multiple rods to one shaft. The rods are connected to cranks, and the cranks are connected to the crankshaft.

 https://www.youtube.com/watch?v=ZJuZ92zvgfk, this is an animation of crankshaft mechanism.

A crankshaft mechanism can synchronise the movements of various parts.

Cam mechanisms

A cam is an irregulary shaped device that rotates on a shaft. When the cam rotates, it pushes a special bar called a follower. The follower can move other parts ori t cab turn a switch on and off. In some mechanisms, a spring pushes the follower back to its original position.

 

We can put multiple cams on one shaft, called a camshaft. We can use a camshaft to synchronise the movements of various parts, such as the valves of an internal combustion motor. A clockwork music box also has a camshaft mechanism.

There is a metal roller with many tiiny bumps. When the roller turns, the bumps at like cams, moving a series of metal teeth that play musical notes.

TRANSFORMATION OF MOTION

Some mechanisms transform linear motion into rotary motion. Most of these mechanisms are reversible. They also transform rotary motion into linear motion

The linear motion can be unidirectional or reciprocating. Reciprocating motions alternate from one side to the other.

ROTARY LINEAR TRANSFORMATION

Wheel

With a rotation, a Wheel moves forward a distance that is equal to its circumference. As a result, we need less force to move vehicles with larger wheels and they move more quickly.

Rack and pinion mechanism

A racka and pinion mechanism has two parts. The rack is a bar with many teeth and the pinion is gear with teeth that interlock with the rack. When the pinion rotates, the rack moves in a linear direction. If the mechanism is reversible, the pinion also rotates when the rack mooves.  Like a Wheel, this mechanism transforms rotary motion into linear motion

https://www.youtube.com/watch?v=F1Vy2Iwg5Nw

Nut and bolt mechanism

A nut and bolt mechanism transforms rotary motion into linear motion. It has two parts:  a bolt or shaft with a spiral Groove and a nut that turns around it. We can turn and tighten the nut i order to hold thing together. If the nut is held in place, we can turn the bolt to make it move forward. In this way, we can use a nut and bolt mechanism to lift loads because it uunctions as a reducing system.

Winch and crank mechanism

A winch is a cylinder that rotates around a horizontal axis. We attach a rope to the winch and to a load. Then we turn the crank to rotate the winch. The rope rolls up around the winch abd lifts the load. The crank increases the force and the winch transforms rotary motion into linear motion.

The increase in force is proportinal to the ratio between the radius of the crank and the radius of the winch. These ratios obey the Law of the Lever.

                                                               F×d=R×r

We use winch and crank mechanism to lift our pull heavy loads. These mechanisms should also include a braking system to avoid accidents.

WORM DRIVE

A worm drive reduces the speed of a rotary system very effectively. A worm drive has two parts: a worm shaft and worm gear. The shaft has two, three or even more grooves. Each Groove interlocks with one tooth of the worm gear.

We use worm drives for tuning the strings of a guitar, for elevator mechanisms and for speed reducing system

BELT DRIVES AND GEAR TRAINS

A belt drive is a system of pulleys connected by belts. Each belt connects a apir of pulleys, so they turn together.

To calculate the ratio of transmission between the first Wheel and the last Wheel of a belt drive, we must multiply the ratios of transmission of the first pair or wheels and the second pair of wheels:

(N4/N1=(D1*D3/D2*D4))

N is the speed of rotation and D is the diameter of Wheel.

We can also find the radius ® or the circumference os each Wheel and use those measurements to calculate tje ratio of transmission. For gear trains, we make these calculations using the number  of cogs or teeth (Z)

CHANGES IN DIRECTION AND ROTATION

We can use various systems to change the direction of rotation of the axis of rotation in a belt drive. We can also vary the distance between the wheels.

With the belts, we can change the direction of rotation and the axis of rotation quite easily. However, gear drives require special parts to make these changes. We use different types of gears when two axes are parallel, perpendicular or crossed.

 In some gear mechanisms, several cogs or teeth interlock at the same time. These mechanisms are more precise and they transmit more rotary force, or torque.

ROTARY TRANSMISSION

Rotary transmission system put two rotating elements into contact.These mechanisms have two purposes:

v  Transferring rotary force from an input location to another location.

v  Changing the rotary speed by using rotating elements of different sizes.

      This images represented: Friction wheels, pulleys with belts, interlocking gears and sprockes with chains.All of these mechanisms keep the same ratios, but each one offers a diferent advantage.

 CHANGES IN SPEED

If we want to increase the speed of a rotary system, we must transmit motion from a larger element to a smaller element. However, when we increase the speed we also decrease the rotary  force,or  torque.

The opposite is true if we want to decrease the speed of a rotary system. We mst transmit motion from a smaller element to a larger element. At the same time, we can also increase the torque. If the input and output elements are the same size, the rotary speed remains constant. The rotary force will also remain constant.

SPEED RATIOS

The relationship between the speeds of the two wheels is inversely proportional to their sizes.

 (N2/N1=D1/D2)

This relationship is called the ratio of the tansmission, where N is the speed of rotation and D is the diameter of the Wheel.

If we want  to calculate the size ratio of wheels or  pulleys, we can compare their diameter, radius or circumference. In the case of gears, we compare the number of cogs or teth that each gear has.

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PULLEYS AND COMPOUND PULLEYS SYSTEMS

In a system of pulleys, the equilibrium between the forces depends on the parth that the rope follows.

PULLEYS

A pulley is a Wheel that rotates around an axis and has a Groove. If we pull rope, belts or chains through pulleys,we can lift objects with less effort. We can divide pulleys into two basic types:

 

COMPOUND PULLEY SYSTEMS

A compound pulley system is a combination of fixed and moveable pulley. It is also called a block and tackle system.The more pulleys there are, the less forcé we need to lift the load. We can combine the pulleys in various ways.