Inventory 67 key knowledge points in mechanical fundamentals
1. Simple machine composition: consisting of three parts: the prime mover part, the execution part, and the transmission part.
2. Motion pair: The connection that allows components to directly contact and maintain a certain form of relative motion is called a motion pair.
High pair: Any motion pair that is in point contact or line contact is called a high pair.
Low pair: Any motion pair that is in contact with a surface is called a low pair.
3. Local degrees of freedom: The 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 a degree of freedom.
A simple diagram illustrating the relative motion relationship between the components of a planar mechanism is called a mechanism motion diagram.
4. The regular thread profile angle is α= 60 °, trapezoidal thread profile angle is α= The tooth shape of a 30 ° rectangular thread is square. The thread profile with the highest transmission efficiency is rectangular thread (square). The best self-locking feature is the triangular thread profile.
5. What are the commonly used anti loosening methods?
(1) Friction anti loosening
(2) Mechanical anti loosening
(3) Non removable anti loosening
6. How do flat keys transmit torque?
Flat keys transmit torque through compression on the side of the key and keyway.
7. Single round headed keys are used in thin-walled structures, hollow shafts, and some situations where radial dimensions are limited.
8. The axial movement of the parts adopts guide flat keys or sliding keys.
9. What are the similarities and differences between couplings and clutches?
The commonality between couplings and clutches: Couplings and clutches are commonly used components in mechanical transmission.
They are mainly used to connect shafts to shafts, or shafts to other rotating parts to transmit motion and torque.
Differences: During machine operation, the coupling always connects the two shafts together. Only when the machine is stopped can the two shafts be separated by disassembly; The clutch can connect and separate the two shafts at any time during machine operation.
10. Couplings with compensation function belong to the flexible coupling type.
11. What are the forms of flexible couplings?
Flexible couplings are divided into flexible couplings without elastic components and couplings with elastic components.
Flexible couplings without elastic elements include (1+slider coupling, toothed coupling, universal coupling, chain coupling). Flexible couplings with elastic elements are further divided into elastic sleeve pin coupling, elastic pin coupling, and tire type coupling.
12. Clutches are divided into two categories: tooth embedded clutches and friction clutches.
13. The spring inside the steel tape measure is a spiral spring. The car shock absorber uses plate springs.
14. What are the basic forms of hinge four bar mechanisms? What are their characteristics?
There are three basic forms of hinge four bar mechanisms:
(1) Crank rocker mechanism
(2) Double rocker mechanism
(3) Double crank mechanism
Features:
(1) One connecting rod can rotate in a full circle, while the other connecting rod can only swing back and forth.
(2) Both connecting rods are rocker rods.
(3) Both connecting rods can rotate in a full circle.
15. Crank: a connecting rod that can rotate around the frame in a full circle.
Connecting rod: a component that is not directly connected to the frame
Connecting rod: a component connected to the frame by a rotating pair
Rack: a fixed component of a mechanism
16. What are the forms that hinge four bar mechanisms can evolve into?
(1) The rotating pair is converted into a moving pair, the crank rocker is converted into a crank slider or crank block, the double crank is converted into a rotating guide rod, and the double rocker is converted into a moving guide rod.
(2) Expand the rotating pair
17. What are the conditions for having a crank?
The crank is the shortest component, and the sum of the lengths of the shortest component and the longest component is less than or equal to the sum of the lengths of the other two components.
18. What is dead center position? Where did it happen? How to navigate through dead center positions?
Dead center position: Without considering the gravity, inertial force, and frictional resistance in the motion pair of the component:
When the rocker is an active component and the connecting rod and crank are collinear, the force passing through the center of the hinge does not produce a torque to the center point and cannot rotate the crank. This position of the mechanism is called the dead center position.
The inertia of the mechanism components is used to pass the system through the dead center at the position where the connecting rod and the follower are collinear.
19. What is the quick return feature? What is the significance?
When the crank rotates at a constant speed, the average speed of the reciprocating swing of the rocker is fast and slow, and this motion property of the mechanism is called the quick return characteristic.
Meaning: Shorten non working hours and improve production efficiency.
20. What is a cam mechanism? What are the components?
A cam is a component with curved contours or grooves, and the cam mechanism belongs to the high pair mechanism. It is composed of three basic components: a cam, a follower, and a frame.
21. What are the types of cams divided by shape?
Disc cam, moving cam, cylindrical cam.
22. What are the types of cam mechanisms in the form of followers?
Pointed follower, roller follower, and flat bottom follower.
23. The intermittent mechanism refers to a periodic motion in which the active component rotates uniformly while the driven component rotates and stops. There are ratchet mechanism, groove wheel mechanism, and incomplete gear mechanism.
24. The film feeding device of the movie projector belongs to the "groove wheel mechanism", and the internal structure of the bicycle flywheel belongs to the "ratchet mechanism".
25. The failure forms of belt transmission include slipping and fatigue damage (such as pulling, delamination, tearing, etc.).
The V-belt structure has a wedge angle of 40 °.
25. Neutral layer: The layer where the length and width of the V-belt remain unchanged when it is bent around the pulley is called the V-belt neutral layer.
26. What are the types of V available? Which section size is the smallest and which is the largest? Which type of transmission has the minimum power and which type has the maximum power?
There are 7 models, YZABCDE, known as cross-sectional. The cross-sectional size is the smallest in Y-shape and the largest in E-shape. The Y-type has the smallest transmission power and the E-type has the largest.
27. The wrap angle of the small pulley shall not be less than 120 °.
28. Suitable for staggered shaft transmission with staggered shaft helical gears; Cross axis hyperbolic bevel gear; There are three types of worm gear and worm gear.
29. What are tooth torque, tooth height, tooth thickness, tooth tip height, and tooth root height?
The arc length between two adjacent tooth surfaces on the same side of the indexing circle is called the tooth moment. The radial distance between the tooth tip circle and the tooth root circle is called the tooth height.
The arc length between the tooth profiles of a gear is called tooth thickness. The radial distance between the tooth tip circle and the indexing circle is called the tooth tip height. The radial distance between the indexing circle and the tooth root circle is called the tooth root height.
30. What is modulus? How to choose standard modulus?
The quotient obtained by dividing the pitch p by the circumference π is called the modulus. When selecting modules, the first series should be given priority, followed by the second series, and the modules in parentheses should be avoided as much as possible.
31. What are the basic parameters of gears?
Number of teeth, modulus, indexing circle diameter, tooth tip circle diameter, tooth root circle diameter, top clearance, and tooth shape.
32. What is pressure angle? What is the standard pressure angle in degrees?
The angle between the direction of normal pressure at any point on the involute and the direction of velocity at that point is called the pressure angle at that point. The standard pressure angle is 20 °.
33. What are the properties of involutes?
(1) The length of the line segment that rolls along the base circle is equal to the corresponding arc length that is rolled over the base circle.
(2) The normal at any point on the involute must be tangent to the base circle. In other words, the tangent of the base circle must be the normal of a point on the involute.
(3) The acute angle between the normal of a point on an involute tooth profile and the velocity direction of that point is called the pressure angle of that point.
(4) The shape of an involute depends only on the size of the base circle. The smaller the base circle, the more curved the involute; The larger the base circle, the flatter the involute. When the radius of the base circle is infinite, its involute will become a straight line.
(5) There is no involute inside the base circle.
34. What are the meshing conditions of the gear pair? What are the conditions for continuous operation?
Due to the standardization of modulus and pressure angle, the correct meshing condition for gears is that the modulus and pressure angle of the two gears are equal. And the base teeth of the two gears are equal.
The continuous transmission condition is that the previous pair of wheels has not yet finished meshing, and the subsequent pair of gears have entered a meshing state. That is, the ratio of the length of the actual meshing line segment to the normal tooth pitch Ea>1.
35. What is the minimum number of teeth for a gear?
The minimum number of teeth for involute standard spur gears is 17.
36. What are the forms of gear failure?
Gear breakage: pitting on the tooth surface; Tooth surface bonding; Tooth surface wear; There are 5 types of tooth surface deformation.
37. What is the self-locking of worm gear transmission?
The helix angle of the worm is very small, and the worm can only drive the turbine transmission, while the turbine cannot drive the worm to rotate.
The correct meshing conditions for worm gear transmission are:
(1) In the middle plane, the axial modulus mx1 of the worm gear is equal to the cross-sectional number mt2 of the turbine.
(2) In the middle plane, the tooth profile angle of the worm shaft surface α End face tooth shape angle of x1 and turbine α T2 is equal.
(3) Worm indexing circle lead angle γ 1. Spiral angle of turbine indexing cylindrical surface β 2 is equal and the rotation direction is consistent.
What are the conditions for meshing helical cylindrical gears?
(1) The normal surface modulus of two gears is equal.
(2) The normal tooth profile of two gears is equal.
(3) The helix angles of the two gears are equal and the rotation direction is opposite.
40. What is a fixed axle wheel system? What is a turnover gear system?
When the gear system is in operation, the axis of all gears is fixed relative to the position of the frame, which is called a fixed axis gear system. A gear system in which at least one gear axis can rotate around another axis during operation is called a planetary gear system (or epicyclic gear system).
41. What are the basic components of a planetary gear system?
The planetary gear system consists of planetary gears, sun gears, planetary carrier, and frame.
42. What is the function of the idler gear?
When the steering of the active shaft remains unchanged, the direction of the driven shaft can be changed by using the idler gear in the wheel system.
43. There is only one planetary degree of freedom and two differential degrees of freedom.
44. What are the types of axes? What kind of load are each bearing?
According to the different loads they bear, shafts can be divided into three categories: shaft, spindle, and transmission shaft.
The shaft that bears both bending moment and torsion during operation is called a shaft. This type of shaft is most common in various machines.
A shaft that only bears bending moments without torque is called a spindle. Mandrels are divided into two types: rotating mandrels and fixed mandrels.
The pulley is connected to the shaft by a key, and the pulley rotates together with the shaft. The two ends of the shaft are supported by a pair of sliding bearings, which are called the rotating spindle.
The pulley rotates on the shaft, and the two ends of the shaft are fixed on the frame, which is called a fixed spindle. A shaft that only bears torque and does not bear bending moment (or bending moment is very small) is called a transmission shaft.
45. The front and rear axles of a bicycle belong to the spindle, while the center axle belongs to the pivot.
46. How to classify rolling bearings? What kind of load are each bearing?
Axial bearing capacity of self-aligning ball bearings: a small amount;
Axial bearing capacity of self-aligning roller ball bearings: a small amount;
Axial bearing capacity of tapered roller bearings: relatively large;
Axial bearing capacity of thrust ball bearings: can only withstand unidirectional axial loads;
Bidirectional thrust ball bearing's load-bearing capacity: able to withstand axial loads in both directions;
Axial bearing capacity of deep groove ball bearings: a small amount;
Axial bearing capacity of angular contact ball bearings: relatively large;
Axial bearing capacity of cylindrical roller bearings without retaining edges on the outer ring: none;
Axial bearing capacity of cylindrical roller bearings without retaining edges on the inner ring: none;
Axial bearing capacity of needle roller bearings: none.
47. How to calculate the inner diameter of rolling bearings?
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 transmission method that utilizes pressurized liquid in a closed system to transmit motion and power.
The hydraulic system consists of five parts: power components, actuator components, control components, auxiliary components, and working medium.
54. The two basic parameters of hydraulic transmission are pressure and flow rate.
What are the main indicators for selecting hydraulic oil? What does it mean?
Viscosity is the main indicator for selecting hydraulic oil. The commonly used hydraulic fluids for hydraulic transmission are No. 32 (with an average kinematic viscosity of 32mm2/s), No. 46, or No. 68 universal hydraulic oils.
56. What is the difference between hydraulic pumps and hydraulic motors?
(1) The pump is driven by an electric motor to rotate and output pressure oil.
(2) When inputting pressure oil to it, the output mechanical energy (torque) is the motor. Under certain conditions, the two can be converted.
(3) Due to different uses, there are slight differences in structure between the two (such as comparison between blade type hydraulic motors and pumps);
a. The rotor of a blade type hydraulic motor has slots with springs inside to ensure that the blades are tightly attached to the rotor and there is sufficient torque during startup.
b. The blade type hydraulic motor requires forward and reverse rotation, so the inclination angle of the blades is equal to zero.
57. What is the function of a hydraulic cylinder?
There are three types of hydraulic cylinders, namely piston type hydraulic cylinders (which come in two forms: single rod and double rod), plunger type hydraulic cylinders, and swing type hydraulic cylinders.
The piston cylinder and plunger cylinder achieve reciprocating linear motion, output speed and thrust; The swinging cylinder realizes reciprocating rotation or swing, and outputs angular velocity (speed) and torque.
58. What is the function of differential connection?
Differential connection generally involves connecting the inlet and outlet oil of a hydraulic cylinder together, and connecting the rod chamber oil of the cylinder.
59. What are the types of control valves? What are their respective functions?
Control valves are divided into directional valves, pressure valves, and flow valves; The directional valve controls the direction of oil flow in the hydraulic system.
The flow valve controls the flow rate of oil in the hydraulic system. The pressure valve controls the pressure of the oil in the hydraulic system.
60. The connecting structures of directional valves are divided into ball valve type, slide valve type, and rotary valve type.
62. What are the functional meanings of the three position four-way directional valves O, M, P, and Y?
The central function of the three-way four-way directional valve: O-type meaning: all four oil ports P, A, B, and T are closed, the hydraulic cylinder is locked, and the hydraulic pump is not unloaded.
M-type meaning: P and T are connected, A and B are closed, hydraulic cylinder is locked, and hydraulic pump is unloaded.
63. Structure of speed control valve?
The speed control valve is a combination valve composed of a pressure reducing valve and a throttle valve in series.
What is the difference between an overflow valve and a pressure reducing valve?
The relief valve only operates when overloaded, while the pressure reducing valve operates normally.
65. The function of an accumulator is to supply a large amount of pressure oil in a short period of time to achieve rapid movement of the actuator, compensate for leakage to maintain system pressure, eliminate pressure pulsation, and alleviate hydraulic impact.
What is the function of a filter? What are the various forms?
Filters can filter out impurities in oil, including various types such as mesh, wire, sintered, and paper core.
67. What are the basic control circuits in hydraulic systems?
The basic circuit can be divided into pressure control circuit, speed control circuit, and direction control circuit.