- Transferring rotary force from an input location to another location.
- Changing the rotary speed by using rotating elements of different sizes.
All the mechanisms keep the same ratios, but each one offers a different advantage.
Uses: Friction wheels and pulleys are often used in toys and other devices with moving parts, such as industrial rollers or conveyor belt systems. Gears are used in cloks, while sprockets and chains are commmon in home appliances.
3.1 Changes in speed
If we want to increase the speed of a rotary systems, we must transmit motion from a large element to a smaller (output) 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 systems. we must transmit motion from from a smaller (input) element to alarger (output) element. At the same time, we also increase the torque. If the input and and output elements are the same size, the rotary speed remais constant. The rotary force will also remain constant.
Pulley : Increase speed systems D1>D2 N2>N1
Pulley 2: Constant speed systems D1=D2 N2=N1
Pulley 3: Decreasing speed systems D1<D2 N2<N1
3.2 Speed ratios
The relationship between the speeds of the two wheels is inversely proportional to their sizes.
N2/N1 = D1/D2
If we want to calculate the size ratio of wheels or pulleys, we can compare their diametre, radius, or circunfrence.
3.3. Belt drives and gear trains
A belt drive is a sysstem of pulleys connected by belts. each belt connects a pair of pulleys, so they turn together.
to understand how a belt drive works, we can analyse the example adove:
- wheel 1 turns wheel 2, which moves faster because it is smaller. The size ratio between the wheels is D1/ D2 = 1.5.If wheel 1 makes 1 rotation, wheel 2 makes 1.5 rotations.
- Wheel 2 and wheel 3 are connected to the same axis, so they turn together. If wheel 2 makes 1.5 rotations, wheel 3 also makes 1.5 rotations.
- Whell 3 turns wheel 4, which moves faster because it is maller. The size ratio between the wheels is D3 / D4 = ". if wheel 3 1.5 rotations, wheel 4 makes 1.5x2=3 rotations
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 the first pair of wheels and the second pair of wheels:
N4/N1 = D1xD3 / D2/D4
N is the speed of rotation and D is the diameter of the wheel.
3.5. Worm drive
A worm drive reduces the speed of rotary system very effectively. A worm drive has two parts: a worm shalt and a worm gear. The shaft has two, three or even more grooves. Each groove interlocks whit one tooth of the worm gear.
Uses: we use worm drives for tuning of a guitar, for elevator mechanisms and for speed reducing systems.
4. 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.
4.1 Rotary-linear transformation
Wheel
Wheels are essencial parts of bicycles. They let us move more easily because they reduce our contact with the ground and decrease friction. However, if there isn't enough friction, the wheels can slide out of control.
We need less force to move vehicles with larger wheels and they more more quickly.
Rack and pinion mechanism
A rack and pinion mechanism has two parts. The Rack is a bar with many teeth and the pinion is a 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 moves.
Uses: We use rack and pinion mechanism for sliding doors, conveyor belts and other devices that require precise movements.
Nut andbolt mechanism
A nut and bolt mechanism transforms rotary motion into linear motion. It has two parts: a bolt or shaft whit a spiral groove and a nut that turns around it. We can turn and tighten the nut in the order to hold things together.
Uses: We use nut and bolt mechanisms to hold things together. We also find them in scissor jacks for lifting cars, water tap mechanisms and screw-top bottles.
Which and crank mechanism
Which 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 and lifts the load. The crank increases the force and the winch transforms rotary motion into linear motion.
The increase in force is proportional to the ratio betweeen the radius of the crank and the radius of the winch . These ratios obey the law of the lever.
Fxd = Rx r
Uses: we use winches for lifting loads. We find them in construction cranes and in the mechanism that raises window blinds in our homes.
4.2. Reciprocating rotary-linear transformation
The pedal mechanism of a bicycle transform the reciprocating movements of our legs into continuous rotary motion. In a similar way, the pistons of a car engine produce a reciprocating linear motion that turns the wheels.
Crank and rod mechanism
In the picture, you can see the parts of a crank and rod mechanism. The piston moves a rod forwards and backwards. this rod turns the firs wheel. The second wheel turns because it is connected to the first wheel by another rod.
uses: This mechanism was important for the first steam engines. Today we find cranks and rods in internal combution engines, as well as windscreen wiper mechanisms.
Crankshaft mechanism
We can connect multiple rods to one shaft. The rods are connected to cracks, and the cranks are connected to the crankshaft.
In the case of a bicycle, our legs act like connecting rods that turn the crank mechanism of the pedals.
Uses: we use crankshafts for combustion motor that use pistons. we also use them for sewing machines.
Cam mechanism
A cam is an irregulary shaped device that rotates on a shaft. When the cams rotates, it pushes a special bar called a follower. The follower can move other parts or it can turn a switch on and off.
A clockwork music box also has a camshaft mechanism.
There is a metal roller with many tiny bumps. When the roller turns, the bumps act like cams, moving a series of metal teeth that play musical notes.
uses: We can find camshafts in toys, automatic tools and combustion motors.
Some cams are circular, but with an axis of rotation that is off-centre. These are called eccentric cams because they rotate in an irregular or eccentric way.
Uses: There are often eccentric cams in sewing machines and other devices that transform rotary motion to linear motion.
5. MECHANISM THAT CONTROL MOTION
5.1 Direction control: ratchests
A ratches is a mechanism that controls the direction of motion. It allows motion in one direction, but not in the olther, as you can see in the picture.
Uses: We find ratches in watches, cable-tensor and elevator brake systems.
5.2. Speed reduction: brakes
brakes: Use friction to reduce speed. They are activated by certain levers. The lever transmits force 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 produce:
Disc brakes: A disc is connected to an axle. Brake pads apply pressure to the disc.
Band brakes: A drum is connected to an axle. A flexible band applies pressure to the outside of the drum. These brakes were used in carriages and they depended on the strength of the driver.
Drum brakes: A drum is connected to the axle. A pair of brake shoes apply pressure to the inside of the drum.
