1. Common materials for mechanical parts include: ordinary carbon structural steel (yield strength), high-quality carbon structural steel (with an average carbon content of 0.20%), alloy structural steel (with an average mass fraction of about 2% for Mn2), cast steel (with a yield point not less than 230 and a tensile strength not less than 450), and cast iron (with a tensile strength of HT200 gray cast iron).
2. Common heat treatment methods include: annealing (slow cooling in the furnace), normalizing (cooling in air), quenching (rapid cooling in water or oil), tempering (reheating the quenched part to a certain temperature below the critical temperature, holding for a period of time, and then cooling in air), quenching and tempering (quenching followed by high-temperature tempering), and chemical heat treatment (carburizing, nitriding, carburizing and nitriding).
3. Structural processability of mechanical parts involves: ease of manufacturing blanks for parts, ease of machining parts, ease of assembly and reliable positioning of parts.
4. Common failure modes of mechanical parts include: fracture due to insufficient strength; excessive elastic or plastic deformation; excessive wear, slipping or overheating of friction surfaces; loosening of connections; leakage of containers, pipelines, etc.; inability to achieve design accuracy requirements.
5. Stress can be classified into static stress and variable stress. The most basic variable stress is stable cyclic stress, which includes asymmetric cyclic stress, pulsating cyclic stress, and symmetric cyclic stress.
6. Fatigue failure and its characteristics: Fatigue failure under variable stress is called fatigue failure. Characteristics: sudden fracture after multiple applications of a certain type of variable stress; the maximum stress of the variable stress at the time of fracture is far less than the yield limit of the material; even for plastic materials, there is no obvious plastic deformation at the time of fracture. When determining the fatigue limit, the magnitude of the stress, the number of cycles, and the cycle characteristics should be considered.
7. Characteristics of contact fatigue failure: Under the repeated action of contact stress, cracks first appear on the surface or near-surface of the part, and then during the rolling contact process, due to the lubricating oil being trapped in the crack, high pressure is generated, causing the crack to propagate. Finally, the surface metal peels off in small pieces, forming small pits on the part surface, known as contact fatigue pitting. Hazards of contact fatigue pitting: Reduces the contact area, damages the smooth surface of the part, reduces its bearing capacity, and causes vibration and noise. Contact fatigue pitting is the main failure mode of gears, rolling bearings, and other parts.
8. Reasons for introducing virtual constraints: To improve the stress distribution of components (multiple planetary gears), enhance the stiffness of the mechanism (shaft and bearing), and ensure the performance of mechanical operation.
9. Types of threads: ordinary threads, pipe threads, square threads, trapezoidal threads, sawtooth threads.
10. Self-locking conditions: λ ≤ ψ, that is, the helix angle is less than or equal to the equivalent friction angle.
11. Transmission and connection of helical mechanisms: Ordinary threads have a large tooth angle β and good self-locking property, so they are often used for connections; rectangular threads, trapezoidal threads, and sawtooth threads have a small β and high transmission efficiency, so they are often used for transmission.
12. Efficiency of helical pairs: η = effective power / input power = tanλ / tan(λ + ψv). In general, the helix angle should not exceed 40°. Under the conditions of d2 and P, increasing the number of locking thread turns n will increase λ, and the transmission efficiency will also increase accordingly. Therefore, to improve the transmission efficiency, a multi-threaded helical transmission can be used.
13. Types and applications of helical mechanisms: (1) Converting reciprocating motion to linear motion: transmission screw (jack, press, tiger pliers), guiding screw (window lifter feed mechanism), adjusting screw (micrometer, indexing mechanism, adjustment mechanism, fine adjustment mechanism for feed amount of tools) (2) Converting linear motion to rotary motion.
14. Characteristics of helical mechanisms: large reduction ratio; large gain in travel; self-locking in reverse can be achieved; smooth transmission, low noise, reliable operation; significant differences in mechanical efficiency among various helical mechanisms (helical pairs with self-locking ability have an efficiency of less than 50%).
15. Reasons for the widespread application of link mechanisms: They can achieve the conversion of various forms of motion; all the motion pairs in the link mechanism are low pairs, with small pressure, light wear, easy lubrication, and long service life; the contact surface is cylindrical or flat, making manufacturing relatively simple and high manufacturing accuracy easy to achieve.