67 Key Points of Mechanical Fundamentals

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1 Simple machine composition: It consists of three parts – the prime mover, the actuator, and the transmission mechanism. 2 Kinematic pair: A connection that allows components to be in direct contact while still permitting a certain form of relative motion is called a kinematic pair. High pair: A kinematic pair that involves point contact or line contact is called a high pair. Low pair: Any kinematic pair that involves surface contact is called a low pair. 3 Local degrees of freedom: Degrees of freedom that are independent of the motion of the entire mechanism are called local degrees of freedom. Degree of freedom: The independent motion of a component is called the degree of freedom. Motion diagram of planar mechanisms: A simple diagram that illustrates the relative motion relationships between the various components of a mechanism is called a motion diagram of the mechanism. 4. For ordinary threads, the thread profile angle is α=60°; for trapezoidal threads, it is α=30°. The profile of square threads is square-shaped. The thread profile with the highest transmission efficiency is the rectangular thread (square thread). The triangular thread profile has the best self-locking property. 5 What are the common methods for preventing loosening? (1) Frictional anti-loosening (2) Mechanical anti-loosening (3) Non-removable anti-loosening. 6 How do flat keys transmit torque? A flat key transmits torque through the compression between the key and the sides of the keyway. 7 Single round-head keys are used in thin-walled structures, hollow shafts, and in situations where radial dimensions are limited. 8 The axial movement of the parts is achieved using guide splines or sliding splines. 9 What are the similarities and differences between couplings and clutches? Common points of couplings and clutches: Couplings and clutches are commonly used components in mechanical transmission. They are mainly used to connect shafts to each other, or a shaft to other rotating components, in order to transmit motion and torque. Difference: While the machine is in operation, the coupling keeps the two shafts connected together; only when the machine stops running can the two shafts be separated by removing the coupling ； The clutch can connect and disconnect the two shafts at any time while the machine is in operation. 10 Couplings with compensating functions belong to the flexible coupling type. 11 What are the types of flexible couplings? Flexible couplings are divided into those without elastic elements and those with elastic elements. Flexible couplings without elastic elements include (1) cross-slider couplings, gear couplings, universal couplings, and chain couplings. Flexible couplings with elastic elements are further divided into elastic sleeve pin couplings, elastic pin couplings, and tire-type couplings. 12 Clutches are divided into two main categories: splined clutches and friction clutches. 13 The spring in the steel tape measure is a helical spring. Car shock absorption uses leaf springs. 14 What are the basic forms of hinge four-bar mechanisms? What are their respective features? The hinge four-bar mechanism has three basic forms: (1) crank-rocker mechanism, (2) double-rocker mechanism, (3) double-crank mechanism. Features: (1) One set of linkages can rotate a full circle, while the other set can only swing back and forth. (2) Both connecting rods are rocker arms. (3) Both connecting rods can rotate a full circle. 15 Crank: A connecting rod that can rotate a full circle around the frame. Link: A component that is not directly connected to the frame. Connecting rod: A component that is connected to the frame through a pivot joint. Frame: The fixed component of a mechanism. %16 What forms can a four-bar mechanism with hinges be transformed into? (1) A rotating pair is converted into a translating pair; a crank-rocker mechanism is converted into a crank-slider or crank-rocker mechanism; a double crank mechanism is converted into a rotating guide rod; a double rocker mechanism is converted into a translating guide rod. (2) Expand the rotating pair. 17 What are the conditions for having a crank? The crank is the shortest member, and the sum of the lengths of the shortest and longest members is less than or equal to the sum of the lengths of the other two members. 18 What is the dead center position? Where did it happen? How to pass through the dead center position? Dead center position: under the assumption that the weight of the components, their inertial forces, and the frictional forces in the kinematic pairs are negligible ； When the rocker acts as the active element and the connecting rod and crank are collinear, the force passing through the center of the hinge exerts no torque on that center point, and thus cannot cause the crank to rotate; this position of the mechanism is known as the dead center position. When it occurs at a position where the connecting rod and the follower are collinear, the inertia of the mechanism components is used to enable the system to pass through the dead center. 19 What is the quick-return characteristic? What’s the point? When the crank rotates at a constant speed, the average speed of the rocker’s back-and-forth motion varies between fast and slow; this characteristic of the mechanism’s motion is known as quick-return behavior. Significance: Reduce non-working time and improve production efficiency. 20 What is a cam mechanism? What components does it consist of? A cam is a component with a curved profile or grooves. A cam mechanism belongs to the category of higher-order mechanisms, and it consists of three basic components: the cam, the follower, and the frame. 21. What are the different types of cams based on their shape? Disc cam, moving cam, cylindrical cam. 22 What are the different types of cam mechanisms based on the type of follower? Spire follower, roller follower, and flat-bottom follower. 23 A mechanism in which the driving member rotates uniformly while the driven member performs periodic motion of rotating and stopping is called an intermittent mechanism. There are ratchet mechanisms, cam mechanisms, and incomplete gear mechanisms. 24 The film feed mechanism of a movie projector belongs to the \"sliding pulley mechanism\", while the internal structure of a bicycle flywheel belongs to the \"ratchet mechanism\". The failure modes of belt drives include slippage and fatigue failure (such as breaking, delamination, tearing, etc.). The 24 V belt structure has a wedge angle of 40°. 25 Neutral layer: The layer of the V-belt whose length and width remain unchanged when the belt bends around the pulley is called the neutral layer of the V-belt. What types are available for 26 V? Which cross-sectional dimension is the smallest, and which is the largest? Which one transmits the least power, and which one the most? 7 models, YZABCDE, are known as truncated forms. The cross-sectional dimension is smallest for type Y and largest for type E. The transmitted power is minimum for type Y and maximum for type E. 27 The wrap angle of the small pulley must be at least 120°. 28 Suitable for interaxial drives; it refers to helical gears with intersecting axes ； Interaxial hyperbolic bevel gear ； There are 3 types of worm gears. 29 What are tooth pitch, tooth height, tooth thickness, addendum height, and root height? On the pitch circle, the arc length between two adjacent tooth flanks on the same side is called the tooth pitch. The radial distance between the addendum circle and the dedendum circle is called the tooth height. The arc length between the tooth profiles of a tooth is called the tooth thickness. The radial distance between the addendum circle and the pitch circle is called the addendum height. The radial distance between the pitch circle and the root circle is called the root height. 30 What is a modulus? How to choose the standard module? The quotient obtained by dividing the tooth pitch p by pi is called the module. When selecting modules, the first series should be given priority, followed by the second series; modules in parentheses should be used as little as possible. 31 What are the basic parameters of gears? Number of teeth, module, pitch circle diameter, addendum circle diameter, dedendum circle diameter, clearance, tooth profile. 32 What is the pressure angle? What is the standard pressure angle in degrees? The angle between the direction of the normal pressure at any point on the involute and the direction of the velocity at that point is called the pressure angle at that point. What are the properties of an involute with a standard pressure angle of 20° 33'? (1) The length of the line segment along which the generating line rolls on the base circle is equal to the corresponding arc length on the base circle that is rolled over. (2) The normal at any point on the involute must be tangent to the base circle. In other words, the tangent to the base circle must be the normal at a certain point on the involute. (3) The acute angle between the normal to a point on the involute tooth profile and the direction of velocity at that point is called the pressure angle at that point. (4) The shape of the involute depends only on the size of the base circle. The smaller the base circle, the more curved the involute is ； The larger the base circle, the straighter the involute. When the base circle radius is infinite, its involute becomes a straight line. (5) There are no involutes inside the base circle. 34 What are the meshing conditions for gear pairs? What are the conditions for continuous operation? Since the module and pressure angle are already standardized, the correct condition for gear meshing is that the module and pressure angle of the two gears be equal respectively. Moreover, the base circle tooth pitches of the two gears are equal. The condition for continuous transmission is that the previous pair of gears has not yet finished engaging, while the subsequent pair of gear teeth has already entered the engaged state. That is: the ratio Ea of the actual meshing line length to the normal pitch is greater than 1. What is the minimum number of teeth for a gear with 35 teeth? For standard involute spur gears, the minimum number of teeth is 17. 36 What are the forms of gear failure? Tooth fracture ； tooth surface pitting ； tooth surface adhesion ； Tooth surface wear ； Five types, including tooth surface deformation. 37 What is the self-locking property of worm drive? When the helix angle of the worm is very small, the worm can only drive the worm gear, whereas the worm gear cannot drive the worm to rotate. 38 The correct meshing condition for worm drive is: (1) In the intermediate plane, the axial module mx1 of the worm is equal to the end-face module mt2 of the worm gear. (2) In the intermediate plane, the axial tooth profile angle αx1 of the worm is equal to the end-face tooth profile angle αt2 of the worm gear. (3) The lead angle γ1 of the worm’s pitch circle and the helix angle β2 of the worm gear’s pitch cylinder are equal, and their rotation directions are the same. 39 What are the conditions for the meshing of helical cylindrical gears? (1) The normal module of the two gears is equal. (2) The tooth profile angles on the generating surfaces of the two gears are equal. (3) The helix angles of the two gears are equal, but their rotation directions are opposite. 40 What is a fixed-axis gear train? What is a planetary gear train? When a gear train is in operation, the positions of the axes of all gears relative to the frame remain fixed; such a gear train is called a fixed-axis gear train. A gear train in which, during operation, the axis of at least one gear can rotate around another axis is called a planetary gear train (or epicyclic gear train). 41 What are the basic components of a planetary gear set? A planetary gear set consists of planetary gears, a sun gear, a planet carrier, and a housing. 42 What is the function of a flywheel? When the direction of rotation of the driving shaft remains unchanged, idler gears in the gear train can be used to change the direction of rotation of the driven shaft. A 43-degree-of-freedom planet has only one differential degree of freedom, while it has two. What are the categories of 44 axes? What types of loads do they each bear? Based on the loads they bear, shafts can be divided into three categories: rotating shafts, arbor shafts, and drive shafts. An axis that is subjected to both bending moment and torsional shortening during operation is called a rotating shaft. This type of shaft is most common in various machines. A shaft that bears only bending moment and no torque is called a mandrel. Spindles are further divided into rotating spindles and fixed spindles. The pulley is connected to the shaft by a key; the pulley and shaft rotate together. The two ends of the shaft are supported by a pair of sliding bearings, which is known as a rotating mandrel. The pulley rotates on an axis, with both ends of the axis fixed to the frame; this is known as the fixed mandrel. A shaft that only experiences torque and no bending moment (or only a very small bending moment) is called a drive shaft. 45 The front and rear axles of a bicycle are spindle types, while the middle axle is a rotating shaft type. 46 How are rolling bearings classified? What types of loads do they each bear? Self-aligning ball bearing Axial load capacity: small ； Self-aligning roller ball bearing Axial load capacity: small ； Tapered roller bearing axial load capacity: high ； Thrust ball bearing Axial load capacity: Can only withstand unidirectional axial loads ； Axial load-carrying capacity of bidirectional thrust ball bearings: capable of withstanding axial loads in both directions ； Deep groove ball bearing axial load capacity: small ； Angular contact ball bearing: High axial load capacity ； Cylindrical roller bearing without outer ring flanges: Axial load capacity: None ； Cylindrical roller bearing without inner ring flange: Axial load capacity: None ； Roller needle bearing Axial load capacity: None. 47 How is the inner diameter of a rolling bearing calculated? Inner diameter code/bearing inner diameter (mm) 00/10 ； 01/12 ； 02/15 ； 03/17 ； 04~99/Number X5. 48 What is hydraulic transmission? What are the components of a hydraulic system? Hydraulic transmission is a method of transmitting motion and power by using pressurized fluid in a closed system. A hydraulic system consists of five components: power elements, actuating elements, control elements, auxiliary elements, and working fluid. 54 The two basic parameters of hydraulic transmission are pressure and flow rate. 55 What are the main criteria for selecting hydraulic oil? What does it represent? Viscosity is the main parameter for selecting hydraulic oil. The commonly used hydraulic fluids are Grade 32 (with an average dynamic viscosity of 32 mm2/s), Grade 46, or Grade 68 general-purpose hydraulic oils. 56 What is the difference between hydraulic pumps and hydraulic motors? (1) A pump is one that is driven by a motor to rotate and generate pressure oil. (2) When pressure oil is supplied to it, the mechanical energy (torque) outputted is: a motor. Under certain conditions, the two can be converted into each other. (3) Due to their different applications, the two have slight structural differences (for example, when comparing vane hydraulic motors and pumps). a. The rotor of a vane hydraulic motor has slots, and springs are located within these slots; these springs keep the vanes in close contact with the rotor, ensuring sufficient torque at startup. b. Vane hydraulic motors need to be capable of rotating in both directions, so the inclination angle of the vanes is zero. 57 What is the function of a hydraulic cylinder? There are three types of hydraulic cylinders: piston hydraulic cylinders (which come in single-rod and double-rod versions), plunger hydraulic cylinders, and swing hydraulic cylinders. Piston cylinders and plunger cylinders achieve reciprocating linear motion, with output speed and thrust ； A swing cylinder enables reciprocating rotation or swinging, delivering angular velocity (rotation speed) and torque. 58 What is the purpose of a differential connection? In a differential connection, the inlet and outlet ports of the hydraulic cylinder are usually connected together, thereby allowing the fluid in the rod side chamber of the cylinder to flow. 59 What are the different types of control valves? What is each one’s function? Control valves are divided into direction valves, pressure valves, and flow valves ； Direction valves control the flow direction of the oil in a hydraulic system. The flow valve controls the amount of fluid flow in the hydraulic system. The pressure valve controls the pressure of the oil in the hydraulic system. The connection structures of 60 directional control valves include ball valve type, spool valve type, and rotary valve type. 62 What do the functions of O, M, P, and Y in a three-position four-way directional control valve mean? Neutral function of a three-way four-position directional control valve: The O position means that all four ports – P, A, B, and T – are closed, the hydraulic cylinder is locked, and the hydraulic pump is not unloaded. Meaning of M type: P and T are connected, A and B are closed, the hydraulic cylinder is locked, and the hydraulic pump is unloaded. 63 Structure of the speed control valve? The speed control valve is a combined valve consisting of a pressure reducing valve and a throttle valve connected in series. 64 What is the difference between a relief valve and a pressure reducing valve? The relief valve operates only under overload conditions, while the pressure reducing valve works throughout normal operation. 65 The function of the accumulator is to supply a large amount of pressurized oil in a short time to enable rapid movement of the actuator, to compensate for leaks and maintain system pressure, to eliminate pressure fluctuations, and to mitigate hydraulic shocks. 66 What is the function of a filter? What are the different forms? Filters can remove impurities from oil, and there are various types such as mesh type, wire type, sintered type, and paper core type. 67 What are the basic control loops in a hydraulic system? The basic control loops can be divided into pressure control loops, speed control loops, and direction control loops.