Practical Knowledge Points in Mechanical Design-Part 2

• 1. Belt drive is divided into: friction type and meshing type
• 2. The maximum instantaneous stress of the belt occurs when the tight edge of the belt begins to wrap around the small pulley; the stress changes four times in one circle of the belt
• 3. Tensioning of V-belt drive: regular tensioning device, automatic tensioning device, tensioning device using tensioning wheel
• 4. The number of links in a roller chain is usually an even number (the number of teeth on the sprocket is an odd number). When the number of links in a roller chain is an odd number, an intervening link is used.
• 5. The purpose of chain drive tensioning is to avoid poor meshing and chain vibration when the loose side of the chain is too sag, and to increase the meshing angle between the chain and the sprocket.
• 6. Gear failure forms: broken teeth, tooth surface wear (open gears), tooth surface pitting (closed gears), tooth surface bonding, plastic deformation (ridges appear on the driven wheel and grooves appear on the driving wheel)
• 7. Gears with a hardness greater than 350HBS or 38HRS are called hard-faced gears; otherwise, they are called soft-faced gears.
• 8. Improving manufacturing accuracy and reducing gear diameter to reduce circumferential speed can reduce dynamic loads. In order to reduce dynamic loads, the gear can be trimmed on the top of the tooth. The purpose of making the gear teeth into a drum shape is to improve the distribution of tooth loads.
• 9. Tanr=z1:q (diameter coefficient) The larger the lead angle, the higher the efficiency and the worse the self-locking performance.
• 10. Displace the worm wheel. After the displacement, the pitch circle and pitch circle of the worm wheel still coincide, but the pitch line of the worm has changed and no longer coincides with its pitch circle.
• 11. Failure forms of worm gear transmission: pitting, tooth root fracture, tooth surface bonding and excessive wear; failure often occurs on the worm wheel
• 12. Power loss of closed worm gear transmission: meshing wear loss, bearing wear loss, oil splash loss when parts entering the oil pool stir the oil
• 13. The worm gear transmission must be calculated for thermal balance based on the condition that the heat generated per unit time is equal to the heat dissipated per unit time. Measures: add heat sinks and increase the heat dissipation area, add fans to the worm shaft end to accelerate air flow, and install circulating cooling pipes in the transmission box.
• 14. Conditions for forming hydrodynamic lubrication: The two surfaces sliding relative to each other must form a converging wedge-shaped gap; the two surfaces separated by the oil film must have sufficient relative sliding speed, and their movement must make the lubricating oil flow from the large opening into the small opening and out; the lubricating oil must have a certain viscosity, and the oil supply must be sufficient.
• 15. Basic structure of rolling bearing: inner ring, outer ring, hydraulic body, cage
• 16. 3 tapered roller bearings, 5 thrust ball bearings, 6 deep groove ball bearings, 7 angular contact bearings, N cylindrical roller bearings 00, 01, 02, 03 are d = 10mm, 12mm, 15mm, 17mm respectively 04 means d = 20mm, 12 means d = 60mm
• 17. Basic rated life: The speed or operating hours at which 10% of a group of bearings suffer pitting damage and 90% do not suffer pitting damage is the life of the bearing.
• 18. Basic rated dynamic load: The load that the bearing can withstand when the basic rated life of the bearing is exactly 106 revolutions.
• 19. Bearing configuration method: two fulcrums are fixed in one direction, one fulcrum is fixed in two directions and the other fulcrum is floating, and both ends are floatingly supported

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