60 Basics of Mechanical Design: Reviewing the Old to Learn the New

       I. Fundamentals of Machines and Design

1. A machine consists of a prime mover section, a transmission section, an executing section, and a control section.

2. The main failure modes of belt drives are fatigue damage of the belt and slipping.

3. The significance of implementing standardization, serialization, and generalization (the “Three Standardizations”) in mechanical design:
   ① Reducing the design workload;
   ② Standard parts and components are mass-produced by specialized factories, resulting in high efficiency, low cost, and reliable quality;
   ③ Facilitating maintenance and use as well as replacement and repair;
   ④ The “Three Standardizations” are a principle that should be implemented in design and also a national technical policy.

4. Connections can be divided into detachable connections and non-detachable connections.

   II. Connecting Parts

   5. Threaded connections can be classified into: bolt connections, stud connections, and screw connections.
   6. Anti-loosening measures for threaded connections: friction locking, mechanical locking, and permanent locking.
   7. Pin connections are classified into: locating pins, connecting pins, and safety pins.
   8. Key connections are divided into: parallel key connections, Woodruff key connections, and spline connections.

   III. Shafts and Couplings

   9. Shafts are classified by function into: transmission shafts, axles, and rotating shafts.
   10. Couplings are divided into two major categories: rigid couplings and flexible couplings.

   IV. Bearings

   11. Bearings are divided into sliding bearings and rolling bearings; sliding bearings are divided by the load they bear into radial bearings and thrust bearings.
   12. Oil-impregnated bearing:

   12. Oil-impregnated bearing:
   Metal powders such as bronze, iron, or aluminum are uniformly mixed with graphite and pressed into a bearing bush. After high-temperature sintering, a porous bearing bush is obtained. After being fully impregnated in lubricating oil, the micropores are filled with lubricating oil; hence it is called an oil-impregnated bearing. It is made of powder metallurgy materials.
   Characteristics: relatively low strength, not resistant to impact, simple structure, and low price.

   13. Advantages of rolling bearings:
   ① Low friction resistance, sensitive starting, high efficiency, little heat generation, and low temperature rise;
   ② Axial dimensions are conducive to making the whole machine compact and simplified;
   ③ Small radial clearance; the clearance can be adjusted by preloading, giving high rotational accuracy;
   ④ Simple lubrication, low oil consumption, and convenient maintenance;
   ⑤ Standard parts, mass-produced, high cost-effectiveness, and easy to replace.
   Disadvantages: relatively large radial dimensions, low capacity to withstand impact loads, relatively loud noise at high speeds, and short service life.

   14. Composition of rolling bearings: outer ring, inner ring, rolling elements, and cage.
   15. The code of a rolling bearing consists of a prefix code, a basic code, and a suffix code; the basic code consists of the bearing type code, dimension series code, and bore diameter code.
   16. Structural forms of rolling bearings: two-support one-way fixed support, single-support two-way fixed support, and two-support floating support.
   17. Lubricants are divided into: lubricating oil and lubricating grease.
   18. Seals for rolling bearings are divided into: contact type and non-contact type.

   V. Springs

   19. Functions of springs:
   ① Cushion impacts and absorb vibrations;
   ② Control motion;
   ③ Store and release energy;
   ④ Measure and indicate;
   ⑤ Maintain elastic contact.
   20. Springs are classified by shape into: helical springs, leaf springs, spiral springs, disc springs, and ring springs.
   21. Winding methods for springs: cold winding and hot winding. Manufacturing process of cylindrical helical springs: winding and forming, end processing or hook making, and heat treatment.

   VI. Kinematic Pairs and Mechanisms

   22. A movable connection between components in machinery that both maintains contact and can produce determinate relative motion is called a kinematic pair.
   23. If the relative motion between the components of a kinematic pair occurs in the same plane or in mutually parallel planes, it is called a planar kinematic pair; otherwise, it is a spatial kinematic pair.
   24. Planar kinematic pairs are divided, according to different contact forms, into planar lower pairs and planar higher pairs.
   25. Common spatial kinematic pairs include: screw pairs and spherical pairs.
   26. Lower pairs can be divided, according to the form of relative motion between two components, into revolute pairs and prismatic pairs.
   27. Examples of lower pairs: door and window hinges, ceiling fans; the contact is surface contact, which can bear relatively large loads, wears slowly, has a long life, and has low contact pressure.
   Examples of higher pairs: gear meshing, rolling bearing balls and rings, train wheels and rails, cams and followers; the contact is line or point contact, with high contact pressure, can achieve relatively precise motion, and has high manufacturing requirements.

   VII. Gear Transmission (Supplement)

   28. Gear transmission should meet two basic requirements: accurate and smooth transmission and strong load-bearing capacity.
   29. Involute gear transmission has a constant transmission ratio.
   30. The main failure mode of closed gear transmission with soft tooth surfaces is tooth surface pitting. It is designed according to tooth surface contact strength and checked for tooth root bending strength.
   31. The main failure mode of closed gear transmission with hard tooth surfaces is tooth root breakage. It is designed according to tooth root bending strength and checked for tooth surface contact fatigue strength.
   32. The main failure mode of open gear transmission is abrasive wear.

   VIII. Part Failure and Design Criteria (Supplement)

   33. Failure modes of mechanical parts:
   overall fracture, excessive residual deformation, surface damage of parts (corrosion, wear, and contact fatigue), and failure caused by disruption of normal working conditions.
   34. Requirements that part design should meet: requirements to avoid failure within the expected service life (strength, stiffness, and life), structural processability requirements, economic requirements, low mass requirements, and reliability requirements.
   35. Design criteria for parts: strength criterion, stiffness criterion, life criterion, vibration stability criterion, and reliability criterion.
   36. Design methods for parts: theoretical design, empirical design, and model test design.

   IX. Commonly Used Materials and Heat Treatment (Supplement)

   37. Commonly used materials for mechanical parts: ordinary carbon structural steel (such as Q235), high-quality carbon structural steel (such as No. 20 steel and No. 45 steel), alloy structural steel (such as 20Mn2), cast steel (such as ZG230-450), and cast iron (such as HT200).
   38. Commonly used heat treatment methods: annealing (slow cooling with the furnace), normalizing (cooling in air), quenching (rapid cooling in water or oil), and tempering (reheating after quenching to below the critical temperature).

   X. Other Commonly Used Mechanisms (Supplement)

   39. In a hinged four-bar mechanism, if one of the two links connected to the frame is a crank and the other is a rocker, it is called a crank-rocker mechanism.
   40. Advantages of cam mechanisms: simple and compact structure, and the ability to realize complex motion laws.
   41. Clutches are divided into: jaw clutches, friction clutches, and overrunning clutches.
   42. Brakes, clutches, and couplings are important components in shaft systems, used to control the transmission and interruption of power.
   43. Gear trains can achieve speed change, direction reversal, large transmission ratio transmission, and transmission over a relatively long distance between two shafts.
   44. Principles for selecting commonly used materials for mechanical parts: meet usage requirements, have good processability, and be economically reasonable.
   45. The general procedure of mechanical design includes: clarifying the design task, scheme design, kinematic analysis and force analysis, part design, checking calculations, and review of structural processability.
   46. Main requirements for machines: functionality, reliability, economy, safety, and environmental friendliness.
   47. The strength of mechanical parts is divided into static strength, fatigue strength, and contact strength.
   48. Stress can be divided into static stress and variable stress; variable stress can further be divided into stable cyclic variable stress, unstable cyclic variable stress, and random variable stress.
   49. The calculation of contact stress is based on the Hertz formula and is applicable to high-pair contact parts such as gears and rolling bearings.
   50. Stiffness refers to the ability of a part to resist elastic deformation under load.
   51. Wear resistance refers to the ability of a part to resist wear and is an important indicator of surface integrity.
   52. Reliability refers to the ability of a part to complete a specified function under specified conditions and within a specified time.
   53. Formula for calculating the degrees of freedom of a kinematic pair: F = 3n - 2P_L - P_H (for planar mechanisms).
   54. Local degrees of freedom refer to the degrees of freedom possessed by certain components in a mechanism that do not affect the motion of other components.
   55. The frame is the fixed component in a mechanism.
   56. The driving link is the component in a mechanism that receives external input motion and power.
   57. The driven link is the remaining component in a mechanism that moves along with the driving link.
   58. A component is an independent unit of motion; a part is an independent unit of manufacturing.
   59. A mechanism is a system of components used to transmit motion and force, in which one component is the frame and the components have determinate relative motion.
   60. Standardization refers to establishing unified standards and specifications for the types, dimensions, structures, performance, etc., of parts.