Mechanical Engineering Basics: Mechanisms and Power Transmission
1. Introduction: A Brief History of Machines
Originally, our ancestors used simple tools or instruments to survive against predators. The need for more complex machines led to the invention of new mechanisms during the Renaissance. Some notable figures include:
- Leonardo da Vinci: Invented and gathered a multitude of mechanisms and machinery.
- Christopher Polhem: Invented a total of 80 simple machines, which he called the “mechanical alphabet.” He believed that any complex machine could be formed from these simple mechanisms, just as sentences are formed from letters.
- Constedt: Extended the mechanical alphabet to 103 mechanisms.
- Hachette: Created a functional classification of all known mechanisms based on their function. This method of analyzing mechanisms is still used today and includes:
* Receivers: Elements that receive movement from a prime mover.
* Regulators: Control the flow of energy (e.g., clutches, switches).
* Communicators: Transmit motion.
* Modifiers: Transform one type of movement into another.
* Operators: Mechanisms that produce a final effect.
2. Technical Systems or Machines
A machine or technical system is a combination of properly grouped mechanisms or devices that use a power source.
A. Driving Elements
These elements provide the necessary energy for movement.
- Prime Movers: These engines rarely provide energy directly to the machine. The energy is usually processed, often into electrical energy, to be used by a motor.
- Secondary Engines: These engines directly power machines. Examples include:
- Muscle power from animals or humans
- Thermal power: Produced by burning fuel. Depending on whether combustion occurs inside or outside a cylinder, we have:
- External combustion engines (e.g., steam locomotives)
- Internal combustion engines (e.g., gasoline, diesel engines)
- Electric power: The most popular secondary engines are electric motors and electromagnets.
B. Elements of Machines
Machines are composed of various elements, and the term “mechanism” is often used for systems with some form of mobility. These elements can be classified as:
- Mechanical: Transmitters of movement, movement transformers, auxiliary power.
- Electrical and Electronic Equipment: Generators, wires (cables), receivers (bulbs, resistors, motors, solenoids), accumulators, control elements (switches).
- Pneumatic and Oil-Hydraulic: Compressors, pipes, control valves, actuators.
3. Mechanical Elements – Transmitters of Motion
- Direct Transmission:
- Couplings
- Joints
- Trees
- Wheels
- Rope or cable
- Indirect Transmission:
- Chain
- Strap
4. Links Between Trees (Shafts)
A transmission shaft is a rotating element that transmits power or energy. An axis is a generally cylindrical machine element that supports different parts. Two types of couplings are used:
- Rigid Coupling: Shafts are placed along the same geometric axis and their position remains unchanged during rotation. Two common solutions are flanges and cymbals.
- Mobile Link: Allows for an inclination between transmission shafts. This means the geometric axes of the two shafts may not be aligned at all times. Examples include:
- Elastic joints
- Cardan or universal joints
- CV joints
- Spline shaft seals
- Oldham couplings
5. Friction Wheel Drive
- A. External friction wheels
- B. Internal friction wheels
- C. Conical friction wheels: Used to transmit motion between shafts whose extensions intersect. They have the shape of a truncated cone.
- D. Transmission of pulleys and belts: A pulley is a wheel used in transmissions with a strap or belt. The belt is a flexible cord that transmits motion. A belt drive consists of at least two pulleys and a belt. Transmission ratio is an important concept in pulley systems.
- Types of pulleys and belts:
- Keystone: Most commonly used for industrial purposes.
- Plan/rectangular: Often used to transmit small amounts of power.
- Round: Often used on machines that rotate at very low speeds.
6. Gear Drive
Gears are used to transmit large forces or when a constant gear ratio is required. They can transmit motion between parallel, perpendicular, and intersecting shafts.
A. Transmission between parallel shafts:
The set of wheels is called “interior.” Gear teeth types include:
- Spur gears: Characterized by straight teeth.
- Helical gears: Teeth are inclined transversely. The transverse angle is similar to spur gears.
- Herringbone gears (double helical gears): To compensate for axial forces, two helical gears with opposite helix angles are used.
- Epicyclic gears (planetary gears): Consist of an inner ring gear (annulus), a sun gear, and three smaller gears called planet gears. The planet gears revolve on their axes, which are attached to a planet carrier.
B. Transmission between perpendicular intersecting axes:
Two types of gears are used: straight bevel gears and helical bevel gears. Helical bevel gears are more complex to manufacture.
C. Transmission between perpendicular non-intersecting axes:
Three solutions are used: worm gears, hypoid gears, and crossed helical gears.
7. Gear Train
Each pair of meshing gears in a gear train is called a gear pair. A gear train is a set of two or more gear pairs that mesh together to vary the rotational speed of the final shaft.
8. Relationship Between Power and Torque
In addition to rotating, a motor also transmits power and torque to the final shaft. Torque is the product of force and distance.
9. Joints
Joints allow movement and transmit forces. They function as first-class levers. Different types of joints allow movement in opposite directions, the same direction, or different planes.
10. Wire Rope or Cable Elements
Used since ancient times, the most important are the simple pulley and the block and tackle (hoist).
11. Combination of Ropes, Wires, and Joints
Combining cables and joints can achieve various effects, such as the braking system in bicycles.
12. Chain and Belt Drives
These elements are ideal for transmitting motion between distant shafts.
- Chain drive: Ideal for dusty environments where durability is required.
- Timing belt: Very quiet and requires no lubrication.
13. Safety Standards and Use of Mechanical Elements
A basic safety rule is to avoid touching moving parts until they have completely stopped. International laws, especially European ones, have strict safety requirements for manufactured products. Generally, the following must be adhered to:
- “All moving parts of products that transmit movement must be protected.”
- If a machine is potentially dangerous during operation and operators must be away from it, safety systems must be incorporated to prevent operation while it’s being handled.
14. Machine Efficiency
Not all power or energy transmitted from the engine reaches the final shaft. Some is lost due to friction, slippage, and the design of the elements.