Structures, Forces, Machines, and Motion Mechanisms
Structures, Forces, and Motion Mechanisms
A structure is a collection of elements arranged in an optimum way to support a body or an object. The structure of an object is the orderly distribution of its essential parts. This is what gives the object its shape.
Types of Structures
- Mass structures: These are solid, heavy, and very sturdy structures, e.g., walls, dams, etc.
- Frame structures: These structures are formed by columns and beams that act as supporting elements, e.g., the structure of a building.
- Truss structures: These structures are composed of bars that form triangles. They are lightweight and sturdy.
- Suspended structures: These are held in place by cables and suspenders attached to sturdy supports, e.g., tents, marquees.
Forces and Stresses
A force is an influence that can deform a body, change its movement, or produce motion in a body at rest. Stresses are the physical demands that a body or object must withstand when one or more external forces are applied to it.
The strength of a material is its ability to withstand external forces without breaking. The basic stresses are:
- Compression
- Tension
- Bending
- Torsion
- Shear
Equilibrium and Stability
A body is in equilibrium when its structure remains stationary. We say that a structure is stable when it remains in equilibrium even when a force is applied to it.
The center of gravity is the point where the weight of an object is considered to be concentrated.
Elements That Increase Rigidity
- Trussing: The triangle is the only figure that cannot be deformed.
- A brace is a part that is positioned diagonally between two bars to connect them by forming a right angle.
- Stay cables
- Sections: like beams and columns.
Machines and Mechanisms
We use the word machine to describe a set of elements or parts that work together and use energy to perform a task and obtain a result.
- Driver element: Feeds energy to the machine.
- Driven element: Receives the force produced by the machine.
- Converting and/or transmitting element: Transmits and/or converts the motion from the driver element to the driven element.
Devices that transmit and/or convert motion are called mechanisms.
Types of Motion
- Linear motion: The object moves in a straight line.
- Rotary motion: The object moves in circles.
- Reciprocating motion: The object moves forwards and backwards in a straight line.
- Oscillating motion: The object moves backwards and forwards, tracing a curve.
Motion Transmission Mechanisms
In this type, the mechanism is fed energy from its driver element and simply transfers it to the driven element. We can classify motion transmission mechanisms as:
- Linear transmission mechanisms: These are a type of simple machine, e.g., pulleys and levers.
- Rotary transmission mechanisms: These include friction wheels, belt and pulley systems, gears, chain and sprocket systems, and worm gears.
Motion Conversion Mechanisms
In this type, the mechanism converts the movement from the driver element into a movement that the driven element can use.
Specific Mechanisms
- Pulleys are used to lift heavy loads. They are wheels with a grooved rim.
- Friction wheels are a set of two wheels located one next to the other. Motion is transmitted between the wheels (driver and driven) by the friction between them.
- The transmission ratio (i) is the ratio between the size of the friction wheels and the speed of rotation.
- A gear is a mechanism consisting of two wheels with teeth.
- Chain and sprocket systems: These systems consist of two toothed wheels separated by a distance and connected by a chain that meshes with the teeth of the two wheels.
- Worm gear: This rotary transmission mechanism consists of a worm that is fitted to the driver shaft and a toothed wheel that is fitted to the driven shaft.