32. Belt drive advantages and disadvantages: ① Advantages: It has good elasticity and can buffer and absorb vibration. Especially since the V-belt has no joints, the transmission is smooth and the noise is low; when overloaded, the belt slips on the pulley, which can prevent damage to other components; simple structure , easy to manufacture and maintain, low cost; suitable for transmission with large center distance; ② Disadvantages: elastic sliding during operation, which reduces the transmission efficiency and cannot accurately maintain the speed ratio relationship between the driving shaft and the driven shaft; the outer surface of the transmission The profile size is large; due to the need for tensioning, the force on the shaft is greater; the belt drive may be electrified by friction and produce sparks, so it cannot be used in flammable and explosive situations.
33. Factors that affect the carrying capacity of belt transmission: initial tension Fo, wrap angle a, friction coefficient f, belt mass per unit length q, speed v
34. The main failure modes of belt transmission: slipping and fatigue damage; design criteria: on the premise of no slipping, it must have a certain fatigue strength and lifespan.
35. Elastic sliding and slipping: Sliding: the comprehensive sliding of the belt on the pulley caused by overloading can be avoided; elastic sliding: the sliding of the belt on the pulley caused by the elastic deformation of the belt is inevitable
36. The basic types of threaded connections: bolted connections (ordinary bolted connections, reamed hole bolted connections), stud connections, screw connections, and tight screw connections.
37. Anti-loosening of threaded connections: friction anti-loosening (spring washers, double nuts, oval self-locking nuts, transverse cut nuts), mechanical anti-loosening (split pins and slotted nuts, stop washers, round nut stop washers, Series steel wire), permanent anti-loosening (punch point method, end welding method, bonding method)
38. Methods to improve the strength of bolted connections: avoid additional bending stress; reduce stress concentration
39. Key connection types: flat key connection (side), semicircular key connection (side), wedge key connection (upper and lower), spline connection (side)
40. Determine the cross-sectional size of the flat key: cross-sectional size of the key b × h (key width × key height) and key length L
41. The difference between a coupling and a clutch: Even these are devices used to connect two shafts (or rotating parts on the shaft and the shaft) to make them rotate together and transmit torque. Two shafts connected by a coupling only They can be separated by disassembly after stopping operation; the clutch can engage or separate the two axles at any time according to work needs without stopping.
42. Coupling classification: rigid coupling (without compensation capability) and flexible coupling (with compensation capability)
43. Selection of coupling type: For low-speed, high-rigidity short shafts, rigid couplings can be used; for low-speed, low-rigidity long shafts, flexible couplings without elastic elements can be used; for large transmission torques For heavy-duty machinery, gear couplings can be used; for machinery with high speed, vibration and impact, flexible couplings with elastic elements can be used; for two axes with large changes in axis position, a cross-axis universal should be used Coupling
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. Check the bearing pressure p:
Control the pressure per unit area to prevent excessive wear of the bearing; Calculation pv: Control the friction power consumption per unit area fpv per unit time to prevent the bearing from generating excessive heat during operation and causing damage to the friction surface; Calculation v: When the pressure For relatively small bearings, bearings that are qualified in both p and pv calculations may be scrapped due to excessive wear due to too high sliding speeds, so it is necessary to ensure that v ≤ [v]
46. The main failure modes of non-liquid friction sliding bearings are wear and gluing.
47. Classification of shafts: spindle (rotating spindle, fixed spindle; only bears bending moment but not torque), rotating shaft (bears both bending moment and torque), transmission shaft (mainly bears torque, does not bear or bears very little bending moment)
48. Notes on axis calculation:
① When there is a keyway on the shaft, enlarge the shaft diameter: one keyway 3°--5°; two keyway 7°--10°
② In the formula, the bending stress is a symmetric cyclic variable stress. 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, α=0.6 is taken; if the torsional shear stress is a symmetric cyclic variable stress. When , take α=1 (α is the conversion coefficient)
49. General principles for shaft structure design: The force on the shaft is reasonable and conducive to meeting the strength conditions of the shaft; the shaft and the parts on the shaft must be reliably fixed at the accurate working position; the shaft should be easy to process; the parts on the shaft should be easy to process. Disassembly and adjustment; minimize stress concentration, etc.
50. Factors affecting the selection of rolling bearing type: speed, axial force or radial force, load size, installation size requirements, etc.
51. Mechanical speed fluctuation:
①Reason: The driving force of the prime mover and the resistance of the working machine both change. If the two cannot adapt to each other from time to time, it will cause fluctuations in the mechanical speed. When the driving work is greater than the impedance work, the machine has surplus work, the kinetic energy of the machine increases, and the angular velocity increases, and vice versa.
②Hazards: Speed fluctuations will cause additional dynamic pressure in the kinematic pair, cause mechanical vibration, reduce the life of the machine, and affect mechanical efficiency and work quality;
③Adjustment method: Periodic: add a rotating flywheel with a large moment of inertia to the machine; non-periodic: use a speed regulator to adjust.