7. Mechanisms

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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.

Class 1	Class 2	Class 3
The fulcrum is between the force and the resistance.	The resistance is between the fulcrum and the force.	The force is between the fulcrum and the resistance.
The effect of the force applied is increased or decreased.	The effect of the force applied is always increased (d > r).	The effect of the force applied is always decreased ( d < r).

Pulleys

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

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

 

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):

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

7.2. Rotary transmission

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Wheels and belts

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

Friction wheels

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

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:

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

Interlocking gears

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

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:

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

7.3. Transformation of motion

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Some mechanisms transform rotary motion into linear motion or viceversa.

Winch and crank mechanism

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:

Uses: Cranes, coffee grinders, etc.

Rack and pinion mechanism

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

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.

 

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