1. Common materials for mechanical parts: ordinary carbon structural steel (Q yield strength), high-quality carbon structural steel (with an average carbon mass fraction of 20/10000), alloy structural steel (with an average manganese mass fraction of about 2% in 20Mn2), cast steel (ZG230-450 yield point not less than 230, tensile strength not less than 450), cast iron (HT200 gray cast iron tensile strength)
2. Common heat treatment methods: 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 (the process of quenching+high-temperature tempering), chemical heat treatment (carburizing, nitriding, carbonitriding)
3. The structural processability of mechanical parts: facilitates the manufacturing of raw parts, facilitates the mechanical processing of parts, facilitates the loading and unloading of parts, and ensures reliable positioning
4. Common failure modes of mechanical parts: fracture due to insufficient strength; Excessive elastic or plastic deformation; Excessive wear, slippage, or overheating of the friction surface; Loose connection; Leakage of containers, pipelines, etc; The motion accuracy does not meet the design requirements
5. Classification of stress: divided into static stress and variable stress. The most basic type of stress is stable cyclic stress, which includes asymmetric cyclic stress, pulsating cyclic stress, and symmetric cyclic stress.
6. Fatigue failure and its characteristics: The failure under the action of variable stress is called fatigue failure. Characteristics: Sudden fracture after multiple applications of a certain type of stress; The maximum stress under stress during fracture is much lower than the yield limit of the material; Even for plastic materials, there is no significant plastic deformation when they break. When determining the fatigue limit, the magnitude of stress, number of cycles, and cycle characteristics should be considered
7. The characteristics of contact fatigue failure: Under the repeated action of contact stress, the parts first generate initial fatigue cracks on the surface or surface layer. Then, during the rolling contact process, the lubricating oil is trapped in the cracks, causing high pressure and crack propagation. Finally, the surface metal peels off in small pieces, forming small pits on the surface of the parts, known as fatigue pitting corrosion. Fatigue pitting hazard: reduces the contact area, damages the smooth surface of the parts, reduces their load-bearing capacity, and causes vibration and noise. Fatigue pitting causes gear damage. The main failure modes of rolling bearings and other components
8. The reason for introducing virtual constraints is to improve the stress situation of the components (multiple planetary gears), enhance the stiffness of the mechanism (shafts and bearings), and ensure the mechanical operating performance
9. Types of threads: ordinary thread, pipe thread, rectangular thread, trapezoidal thread, serrated thread
10. Self locking condition: λ≤ψ means that the helix angle is less than or equal to the equivalent friction angle
11. Spiral mechanism transmission and connection: Ordinary threads are often used for connection due to their large helix angle β and good self-locking properties; Rectangular thread, trapezoidal thread, serrated thread, due to its small β value and high transmission efficiency, is often used for transmission
12. The efficiency of a helical pair: η=effective work/input work=tan λ/tan (λ+ψ v). Generally, the helix angle should not exceed 40 °. When d2 and P are constant, as the number of locked threads n increases, λ will also increase, and the transmission efficiency will correspondingly increase. Therefore, to improve transmission efficiency, multi line spiral transmission can be used
13. Types and Applications of Spiral Mechanisms: ① Transforming Rotary Motion into Linear Motion, Transmitting Spiral (Jack, Press, vise), Conducting Spiral (Window Feed Spiral Mechanism), Adjusting Spiral (Micrometer, Dividing Mechanism, Adjusting Mechanism, Fine Adjustment Mechanism for Prop Feed) ② Transforming Linear Motion into Rotary Motion
14. The characteristics of spiral mechanism: it has a large reduction ratio; Having a large gain; Reverse travel can be self-locking; Smooth transmission, low noise, and reliable operation; The mechanical efficiency of various screw mechanisms varies greatly (screw pairs with self-locking ability have an efficiency of less than 50%)
15. The reason for the widespread application of linkage mechanisms is that they can achieve the conversion of various forms of motion; Each motion pair in the linkage mechanism is a low pair, with low pressure, light wear, easy lubrication, and long service life; Its contact surface is cylindrical or flat, making it relatively easy to manufacture and achieve high manufacturing accuracy.
16. Conditions for the existence of cranks:
① Shortest pole length+longest pole length ≤ sum of the other two poles
② The shortest pole is a connecting pole or frame.
17. Cam motion law and impact characteristics:
① Constant speed: rigid impact, low-speed light load
② Waiting for acceleration and deceleration: flexible impact, medium speed light load
③ Cosine acceleration: flexible impact, medium speed medium load
④ Sinusoidal acceleration: no impact, high-speed light load
18. The relationship between the pressure angle of the cam mechanism and the base circle radius: r0=v2/(ω tan α) - s, where r0 is the base circle radius and s is the displacement of the push rod
19. Roller radius selection: ρ a=ρ - r, when ρ=r, sharp points appear on the actual contour of the cam, that is, the phenomenon of becoming sharp, and the sharp points are easily worn; When ρ<r, the actual contour lines intersect, and the upper part of the intersection line is cut off during actual processing, making it impossible to achieve the motion law of the push rod in this part, that is, motion distortion; So it should be ensured that ρ>r, usually taking r ≤ 0.8 ρ. Generally, the base circle radius can be increased to increase the curvature
20. Advantages and disadvantages of gear transmission:
① Advantages: Wide range of applicable circumferential speeds and power; Accurate transmission ratio; High mechanical efficiency; Reliable work; Long lifespan; Can achieve transmission between parallel axes, intersecting axes, and intersecting axes; Compact structure;
② Disadvantages: High manufacturing and installation accuracy is required, and the cost is relatively high; Not suitable for long-distance transmission between two shafts
21. Characteristics of involute:
① The length of the line rolling over the base circle is equal to the arc length rolled over the base circle;
② The normal of any point on the involute must be tangent to the base circle, and the involute at point N is at the center of curvature at point K, with line segment NK being its radius of curvature; Official account "Mechanical Engineering Literature", engineer's gas station!
③ The pressure angles at each point on the involute curve of cos α k=ON/OK=rb/rk are unequal. The larger the radial diameter rk, the greater the pressure angle, and the pressure angle on the base circle is zero;
④ The shape of the involute depends on the size of the base circle. As the radius of the base circle increases, the curvature radius of the corresponding point on the involute line also increases. When the base circle is infinitely large, the involute becomes a straight line, so the tooth profile of the involute gear rack is a straight line;
⑤ There is no involute within the base circle.
22. Gear meshing conditions: It is necessary to ensure that each pair of gears on the meshing line can enter the meshing state correctly, m1=m2=m; α 1=α 2=α, which means that both the modulus and pressure angle are equal; The helical gear also requires that the helix angles of the two wheels must be equal in size and opposite in rotation direction; Cone gears also require equal taper distance between the two wheels; The worm gear requires that the lead angle of the worm gear be equal in size to the helix angle of the turbine and rotate in the same direction
23. Continuous transmission conditions for gear teeth: The coincidence degree ε=B1B2/ρ b>1 (the actual length of the meshing line segment B1B2 is greater than the normal pitch of the gear teeth)
24. Basic law of tooth profile meshing: For a pair of tooth profiles engaged in planar meshing, the common normal of their instantaneous contact points must intersect at the corresponding node C, which is inversely proportional to the angular velocity of the gear.
25. Root cutting:
① Reason for occurrence: When machining gears with rack type tools (or gear type tools). If the number of teeth of the processed gear is too small, the tooth crest line of the prop will exceed the meshing limit point of the wheel blank, and at this time, the blade will cut off a part of the involute tooth profile at the root of the gear, that is, root cutting;
② Consequence: Weakening the root of the gear, reducing its bending resistance, and decreasing the degree of coincidence;
③ Solution: Positive displacement gear
26. Advantages of positive displacement gears: It can process gears with fewer teeth than Zmin without root cutting, reducing the size of the gear transmission structure; Select appropriate displacement to meet the requirements of actual center distance; Improve the bending resistance of small gears, thereby enhancing the overall strength of a pair of gear transmissions
27. Failure modes of gears: gear breakage, tooth surface pitting, tooth surface bonding, tooth surface wear; The main failure modes of open gears are gear wear and tooth breakage; Closed gears mainly suffer from tooth surface pitting and tooth breakage; The failure modes of worm gear transmission are tooth bonding, pitting, and wear
28. Gear design criteria: For commonly used gear transmissions, calculations are usually only based on ensuring the contact fatigue strength of the tooth surface and the bending fatigue strength of the tooth root.
29. Parameter selection:
① Number of teeth: Keeping the diameter of the dividing circle constant, increasing the number of teeth can increase the degree of coincidence, improve the smoothness of transmission, and save manufacturing costs. Therefore, under the condition of meeting the fatigue strength of tooth root bending, having more teeth is better; Closed z=20~40 Open z=17~20;
② Tooth width coefficient: large gear tooth width b2=b; Small gear b1=b2+(2~10) mm;
③ Tooth ratio: straight tooth u ≤ 5; Diagonal teeth u ≤ 6-7; Open gear or manual gear u can be taken up to 8-12
30. The spur gear transmission has poor stability, high impact and noise; The helical gear transmission is smooth, with low impact and noise, suitable for high-speed transmission.
31. The function of the wheel system: to obtain a large transmission ratio (reducer); Realize variable speed and directional transmission (automotive gearbox); Realize the synthesis and decomposition of motion (differential, car rear axle); Realize compact high-power transmission (engine main reducer, planetary reducer)
32. Advantages and disadvantages of belt transmission: ① Advantages: It has good elasticity, can buffer and absorb vibrations, especially the V-belt has no joints, the transmission is relatively smooth, and the noise is low; When overloaded, slipping on the pulley can prevent damage to other components; Simple structure, easy manufacturing and maintenance, low cost; Suitable for transmissions with large center to center distances; ② Disadvantages: There is elastic sliding during work, which reduces transmission efficiency and cannot accurately maintain the speed ratio between the driving shaft and the driven shaft; The outer dimensions of the transmission are relatively large; Due to the need for tension, there is a significant force on the shaft; Belt drive may generate sparks due to friction, so it cannot be used in flammable and explosive environments.
33. Factors affecting the load-bearing capacity of belt transmission: initial tensile force Fo, wrap angle a, friction coefficient f, unit length mass q, speed v of the belt
34. The main failure modes of belt transmission are slipping and fatigue damage; Design principle: It has a certain fatigue strength and lifespan without slipping.
35. Elastic sliding and slipping: Slip: The comprehensive sliding of the belt on the pulley caused by overload can be avoided; Elastic sliding: The sliding of the belt on the pulley caused by the elastic deformation of the belt, which is inevitable
36. Basic types of threaded connections: bolted connections (ordinary bolted connections, bolted connections with hinged holes), double headed bolted connections, screw connections, and tight screw connections.
37. Anti loosening of threaded connections: friction anti loosening (spring washer, double nut, elliptical self-locking nut, transverse cut nut), mechanical anti loosening (open pin and groove nut, stop washer, round nut stop washer, serial steel wire), permanent anti loosening (punching method, end welding method, bonding method)
38. Methods to improve the strength of bolt connections: avoid generating additional bending stress; Reduce stress concentration
39. Key connection types: flat key connection (side), semi-circular key connection (side), wedge key connection (top and bottom), spline connection (side)
40. Determination of cross-sectional dimensions for flat keys: the cross-sectional dimensions of the key b × h (key width × key height) and the key length L
41. Difference between Couplings and Clutches: Couplings are devices used to connect two shafts (or rotating parts on shafts), allowing them to rotate together and transmit torque. The two shafts connected by couplings can only be separated by dismantling them after they have stopped running; The clutch can engage or disengage the two shafts at any time without stopping according to work needs during operation.
42. Coupling classification: rigid coupling (without compensation capability) and flexible coupling (with compensation capability)
43. Selection of coupling types: For low-speed and rigid short shafts, rigid couplings can be used; For long shafts with low speed and low rigidity, flexible couplings without elastic components can be selected; Gear couplings can be used for heavy machinery with high torque transmission; For high-speed, vibration and impact prone machinery, flexible couplings with elastic elements can be selected; For two shafts with significant changes in axis position, a cross axis universal joint should be selected
44. Bearing friction states: dry friction state, boundary friction state, liquid friction state, mixed friction state; Boundary and mixed friction are collectively referred to as non liquid friction
45. Verify the bearing pressure p:
Control the pressure per unit area to prevent excessive wear of the bearing shell; Calculate PV: Control the frictional power consumption FPV per unit area per unit time to prevent excessive heat generation during bearing operation, which may cause adhesive damage to the friction surface; Calculation v: When the pressure is relatively low, bearings with both p and pv calculations that are qualified may also be scrapped due to excessive wear and tear due to high sliding speed. Therefore, it is necessary to ensure that v ≤ [v]
46. The main failure modes of non liquid friction sliding bearings are wear and bonding
47. Classification of shafts: Spindle (rotating spindle, fixed spindle; only bears bending moment and not torque), shaft (i.e. bears both bending moment and torque), transmission shaft (mainly bears torque, does not bear or bears very small bending moment)
48. Attention to axis calculation:
① When there is a keyway on the shaft, enlarge the shaft diameter: one keyway is 3 ° -5 °; Two keyways 7 ° -10 °
② The bending stress in the equation is a symmetric cyclic variable stress, and when the torsional shear stress is a static stress, α=0.3 is taken; When the torsional shear stress is a pulsating cyclic variable stress, take α=0.6; If the torsional shear stress is a symmetric cyclic variable stress, take α=1 (α is the conversion factor)
49. General principles for shaft structure design: Reasonable force on the shaft is conducive to meeting the strength conditions of the shaft; The shaft and its components should be reliably fixed in the accurate working position; The shaft should be easy to process; The parts on the shaft should be easy to disassemble and adjust; Minimize stress concentration as much as possible
50. Factors affecting the selection of rolling bearing types: high or low speed, axial or radial force, load size, installation size requirements, etc
51. Mechanical speed fluctuation:
① Reason: The driving force of the prime mover and the impedance force of the working machine are both changing. If they cannot adapt to each other at all times, it will cause fluctuations in mechanical speed. When the driving power is greater than the impedance power, the machine produces surplus power, the kinetic energy of the machine increases, the angular velocity increases, and vice versa.
② Harm: Speed fluctuations can cause additional dynamic pressure in the motion pair, leading to mechanical vibration, reducing the lifespan of the machinery, and affecting mechanical efficiency and work quality;
③ Adjustment method: Periodic: Add a rotating component flywheel with a large moment of inertia to the machine; Non periodic: using a speed regulator to regulate.
52. Common software for mechanical design
1. AutoCAD: AutoCAD is an automated computer-aided design software that can be used for 2D drafting and basic 3D design. It is widely used in fields such as civil engineering, construction, decoration, industrial drawing, mechanical drawing, etc. Using AutoCAD makes it easy to draw drawings of various mechanical parts and perform operations such as dimension annotation and material calculation.
SolidWorks: SolidWorks is an original 3D design software based on the Windows system, and its easy-to-use and user-friendly interface makes it widely popular in mechanical design. SolidWorks is capable of fully automatically capturing design intent and guiding design modifications throughout the entire product design process. In addition, in SolidWoks' assembly design, new parts can be directly generated by referencing existing parts.
3. Pro/Engineer: Pro/Engineer software is known for its parameterization and was one of the earliest adopters of parameterization technology. It occupies an important position in the current field of 3D modeling software. Pro/Engineer is widely used in industries such as electronics, machinery, molds, industrial design, and automotive manufacturing. It can provide flexible modeling methods, allowing users to easily create complex 3D models.
4. UG: UG is a product engineering solution produced by Siemens PLM Software, which provides digital modeling and verification methods for users' product design and manufacturing processes. UG is widely used in various fields such as aviation, aerospace, automotive, and machinery, and its powerful functions can meet the design and manufacturing needs of complex models.
In addition to the aforementioned software, Catia, Inventor, and other software are also widely used in the field of mechanical design. These software have their own advantages and applicability, and mechanical designers can choose the software that suits their needs and habits for design.
