Mechanical Devices

This section explains the mechanical devices used in Design and Technology. Mechanical devices are used to convert, control, and transmit motion in a wide variety of applications. In this revision, we will cover the different types of motion, key mechanical systems such as levers, linkages, cams and followers, gear trains, and pulleys and belts, all of which play important roles in engineering and product design.

Types of Motion

Understanding different types of motion is essential for designing mechanical systems. The main types of motion are:

Linear Motion: Movement in a straight line. This can be either uniform (constant speed) or variable (changing speed).

• Example: A drawer sliding open or a piston moving in a cylinder.

Rotary Motion: Movement around a central point or axis. Rotary motion occurs when an object turns in a circular path.

• Example: A wheel turning or a motor’s rotating shaft.

Reciprocating Motion: A back-and-forth linear motion, often in a straight line.

• Example: The piston in an engine or the motion of a sewing machine needle.

Oscillating Motion: A repetitive, back-and-forth motion about a central point, like a pendulum swinging.

• Example: A clock’s pendulum or a seesaw.

Levers

A lever is a rigid bar that rotates around a fixed point known as the fulcrum. Levers are used to lift or move loads by applying a force. There are three types of levers, defined by the position of the fulcrum, load, and effort:

First-Class Lever: The fulcrum is between the effort and the load. This type of lever can change the direction of force.

• Example: A seesaw or crowbar.

Second-Class Lever: The load is between the fulcrum and the effort. This type of lever always increases force.

• Example: A wheelbarrow or a nutcracker.

Third-Class Lever: The effort is between the fulcrum and the load. This type of lever increases speed and distance rather than force.

• Example: A pair of tongs or a fishing rod.

Linkages

Linkages are systems of levers connected by joints that transfer motion and force between different parts of a mechanism. Linkages are used to create various types of motion, including reverse motion, parallel motion, and oscillating motion.

Reverse Motion Linkage: A mechanism that changes the direction of motion, typically by 180°.

• Example: The linkage in a mechanical arm where the direction of movement is reversed.

Parallel Motion or Push/Pull Linkage: This linkage maintains the direction of motion, moving in parallel.

• Example: A slider-crank mechanism, used in machines such as pumps.

Bell Crank Linkage: This type of linkage changes the direction of force at a right angle (90°).

• Example: The bell crank linkage in a bicycle brake system.

Crank and Slider Linkage: Converts rotary motion into reciprocating motion. It is used in engines, pumps, and other systems.

• Example: The mechanism inside a steam engine or the piston in an engine.

Treadle Linkage: Converts linear motion into rotary motion, often used in foot-operated machines.

• Example: A sewing machine treadle or a pump lever.

Angles in Linkages: The angles formed between the links in a linkage determine the path of motion and how much mechanical advantage is gained.

Cams and Followers

A cam is a rotating wheel or shaft that converts rotary motion into reciprocating motion. The follower is the part that moves in response to the cam's shape. Cams and followers are used to create specific motion profiles.

Types of Cams:

Circular Cam: Produces uniform motion, with a smooth rise and fall. Used for repetitive tasks.

• Example: A cam used in a clock mechanism.

Pear Cam: Has a shape that provides a slow rise followed by a fast fall. It is often used when a controlled, slow start and fast end are needed.

• Example: A cam in a printing press.

Snail (or Drop) Cam: Causes the follower to drop quickly and then return slowly. It is used when there is a need for rapid movement at specific points in the cycle.

• Example: In automatic machinery that needs to “drop” a component quickly, like in a slot machine.

Heart-Shaped Cam: This cam shape provides uniform and constant velocity motion. It is used when smooth, consistent motion is needed throughout the cycle.

• Example: In clockwork mechanisms for even movement.

Types of Followers:

• Flat Follower: A follower with a flat surface that slides on the cam profile. It is used for simple, smooth motion transfer.
• Point (or Knife) Follower: A sharp-pointed follower that contacts the cam at a single point. It is used where precise, point-to-point motion is required.
• Roller Follower: A follower with a roller that runs on the cam's surface. It reduces friction and wear, making it ideal for high-speed applications.

Gear Trains

A gear train is a system of gears that transmits motion and force from one part of a machine to another. Gears come in various types, including spur gears, bevel gears, and worm gears. Gear trains can be used to change the direction of motion, increase speed, or provide mechanical advantage.

Spur Gears: These are the most common type of gear, with teeth that are straight and parallel to the axis of rotation. They are used in systems where the input and output shafts are parallel.

• Example: A hand-cranked winch.

Bevel Gears: These gears are used when the direction of the shaft’s rotation needs to be changed by 90°. The teeth are cut at an angle.

• Example: Gear systems in power tools.

Worm Gears: These are used to transmit motion at right angles, like bevel gears, but with the added benefit of reducing speed while increasing torque. They also have the unique characteristic of allowing movement in one direction but preventing movement in the other.

• Example: In a screw jack or a conveyor belt system.

Gear Ratios: Gear ratios determine how much the output speed or torque is changed compared to the input. A larger gear driving a smaller gear will increase speed but decrease torque, while a smaller gear driving a larger gear will decrease speed but increase torque.

Pulleys and Belts

Pulleys and belts are used to transmit rotary motion and force from one part of a system to another, usually over a distance.

Pulleys:

A pulley consists of a wheel with a groove around its circumference, through which a belt runs. By using different sizes of pulleys, a system can increase or decrease speed and torque.

• Example: A pulley system in a crane used to lift heavy loads.

Types of Pulleys:

Fixed Pulley: The pulley is attached to a fixed point and does not move. It changes the direction of the applied force.

• Example: A flagpole pulley.

Movable Pulley: The pulley moves along the rope or belt, providing mechanical advantage by reducing the force needed to lift a load.

• Example: A pulley in a construction crane.

Compound Pulley: A combination of fixed and movable pulleys, which increases mechanical advantage, making it easier to lift heavy objects with less effort.

• Example: Pulley systems used in lifting mechanisms like elevators.

Belts:

• V-Belts: These are commonly used in applications where high torque and power transmission are needed, such as in car engines.
• Flat Belts: These are used for transferring motion over long distances and are often found in conveyor belts and machines.
• Timing Belts: Used in systems where precise synchronisation of motion is required, like in a car's engine.

Applications: Pulleys and belts are found in numerous applications, such as in conveyor systems, industrial machines, or vehicle engines.

Mechanical devices are fundamental to the design and operation of many everyday products and systems. By understanding different types of motion, mechanical linkages, cams, gears, and pulleys, designers can create efficient and functional systems that meet the specific requirements of a task. Whether you are designing a simple lever or a complex gear train, understanding how these devices work together is key to developing successful mechanical solutions.