Mechanical Engineering: Mechanisms, Joints, and Power Transmission Systems
A machine is a system composed of one or more mechanical parts with moving components and, at times, other assemblies. A mechanism is a set of mechanical elements that perform guidance and transmission functions related to the movements and forces within a machine. Joints can be classified as fixed or detachable.
Fixed Joints: Used when the union must withstand significant mechanical forces, and dismantling is not necessary.
Detachable Joints: These elements are linked but can be dismantled. Some examples include screws, cotter pins, and bolts.
Different Rivet Joints
- Lap Joints: When two pieces overlap.
- Butt Joints with Single Cover Plate: Both pieces are in the same plane and joined by a plate.
- Butt Joints with Double Cover Plates: Pieces are joined in the same plane with two plates.
Materials: Mild steel, copper, or aluminum.
Purpose of Riveting: Joining thin pieces that cannot support welding or where welding could cause internal stress and deformation.
Nut and Screw Characteristics
Nut Features: Screw pitch, screw angle, fillet geometry.
Screw Types: Square thread, sawtooth, trapezoidal, triangular, round.
Cotter Pins: Steel pieces generally located within notches or recesses.
Longitudinal Keys: Transmit torque while ensuring axial immobility of connected pieces.
Springs
Definition: Used to absorb energy or shock loads, produce power, or exert force/pressure.
Spring Classifications: Tension/Traction springs, Compression springs, Torsion springs, Leaf springs, Spiral springs, Rubber springs.
Rolling Bearings
Definition: An intermediate element mounted between two bodies in a mechanism, allowing rotation.
Bearing Parts: Outer ring, inner ring, rolling elements, separators.
Lubrication
Definition: Introducing substances (lubricants) between moving surfaces in contact to reduce friction.
Importance: Essential for reducing friction and wear.
Good Lubricating Characteristics:
- Reduces friction between moving parts.
- Acts as a coolant.
- Protects against contaminants.
Commonly Used Lubricants: Hydrodynamic (maintains a lubricant film between parts) and synthetic lubricants.
Shafts and Axles
Shaft: A generally cylindrical piece capable of transmitting circular motion and supporting one or two power sources.
Axle: A usually cylindrical piece around which other mechanical parts revolve.
Couplings
Definition: Joining two aligned shafts or axles without changing speed or timing.
Coupling Types: Rigid couplings, Flexible couplings, Mobile couplings (cardan or universal joints), Torque limiters (friction disc, shear pin).
Articulated Mechanisms
- Four-Bar Linkage: Four rods connected by joints. If all rods rotate, it’s a crank-crank mechanism. If one oscillates, it’s a rocker-crank mechanism.
- Crank-Rocker Mechanism: Converts circular motion to reciprocating motion and vice-versa.
Belt and Chain Transmission
Belt Transmission: Two pulleys linked by a flexible belt. Advantageous for long distances.
Chain Transmission: Two sprockets linked by a chain, eliminating slippage.
Transmission Ratio: The ratio of output shaft rotations to input shaft rotations.
Gears
Definition: Mechanisms for transmitting circular motion using toothed wheels.
Gear Types: Spur gears, Helical gears, Bevel gears, Internal gears, Rack and pinion, Worm gears.
Cams and Eccentrics
Various types exist, but the most common is the plate cam, consisting of a cam and a follower.
Centrifugal Governor (Watt Governor)
Used in steam engines to regulate steam flow to the cylinder.
Intermittent Mechanisms
- Geneva Drive (Maltese Cross): A rotating drive wheel and a driven wheel with radial slots.
Trains and Reducers
Trains: Combinations of mechanisms where one element drives the next.
Reducers: Train mechanisms that significantly reduce angular velocity.
Clutches
Devices that allow connection and disconnection of a device at will, sometimes automatically.