Mechanical Components and Power Transmission Systems
Fundamental Mechanical Definitions
Basic Rotating Components
Shaft
A shaft is a single piece, generally cylindrical, capable of transmitting circular motion and torque.
Axle
An axle is simply a piece, usually cylindrical, around which other parts of a mechanical assembly revolve.
Shaft Couplings
Couplings are devices used to connect two shafts together at their ends for the purpose of transmitting power.
Rigid Coupling
Rigid couplings join shafts using rigid metal pieces that are fastened by clamps or non-involute keys/splines. The shafts must not be misaligned.
Flexible Coupling
The coupling is achieved using a resilient rubber gasket, which allows a certain tolerance in shaft alignment and simultaneously absorbs vibrations.
Sliding Coupling (Cell Coupling)
Used when the separation between shafts is variable. The shafts are typically drawn from a docked sleeve (or yoke) which allows lateral displacement of the shaft while transmitting torque.
Cardan Joint (Universal Joint)
This union is performed using an element called the cross (or spider), onto which the yokes at the ends of the two shafts are coupled. This joint is used in cars and trucks to transmit movement between the gearbox and the differential.
Torque Limiting Devices
Torque Limiter
A device that limits the transmitted torque during direct transmission between shafts. (Examples: friction disc and shear pin).
Friction Disc Limiter
Coupled between the shaft and the pulley or gear wheel. It transmits torque via friction. The pressure between the disc, the shaft, and the wheel can be adjusted using a screw.
Shear Pin
The pin is placed between the shaft and the wheel. It is calibrated to withstand a determined shear force, beyond which it cuts off, stopping the transmission.
Linkage Mechanisms
Four-Bar Linkage Mechanism
A simple device consisting of four bars joined together by articulations (joints). If the fully connected bars spin, the structure is called a double-crank. If only the input link spins, the output link is called a rocker.
Crank-Slider Mechanism
Used to transform circular motion into reciprocating (alternate rectilinear) motion, or vice versa. Note that the rectilinear movement is limited and alternating.
Infinite Connecting Rod Mechanism
In this mechanism, the role of the crank causes a mobile part to move, achieving the desired linkage effects. This mechanism, similar to the crank-slider, can be found in old steam engines.
Transmission Fundamentals
Fixed Distance Transmission Elements
This type of joint is used when transmitting movement between two parallel axes separated by a fixed distance. These are basically represented by belt and chain transmissions.
Transmission Ratio (RT)
The transmission ratio ($R_T$) between two pulleys indicates the number of output shaft revolutions per input shaft revolution. ($W_1/W_2 = T_1/T_2$)
Gear Mechanisms
Gears are mechanisms used for the transmission of circular motion. Types include: straight (spur), helical, bevel, internal, rack-and-pinion, and worm screw.
Spur Gears (Straight Gears)
These transmit rotary motion between parallel axes located a short distance apart. The teeth are parallel to the axes. They are used when working at low speeds and when relatively small forces are transmitted.
Helical Gears
Unlike spur gears, helical gears maintain contact across the tooth face, allowing them to transmit larger forces.
Bevel Gears (Conical Gears)
These transmit motion between two axes that intersect. The most common types have straight teeth.
Internal Gears
These gears have teeth on the inside, meshing with an external gear. Therefore, the two axes are located on the same side and usually rotate in the same direction.
Rack-and-Pinion
This consists of a prismatic bar (the rack) with teeth that meshes with a wheel or gear (the pinion). It converts rotary motion into linear motion, or vice versa.
Worm Gear (Worm Screw)
This is the coupling between a screw (worm) and a gear (worm wheel), typically used to work with axes that cross at 90 degrees.