# Unit 1 - Design of spur Gear Student copy.pdf

30 de May de 2023
1 de 43

### Unit 1 - Design of spur Gear Student copy.pdf

• 1. Subject : Machine Design-II T.Y. Mechanical Introduction to Machine Design-II Ms. S M Gujrathi Assistant Professor E-mail : gujrathisonammech@sanjivani.org.in by S M Gujrathi
• 2. Unit 1: Design of Spur Gears A)Introduction to gears Gear Selection, material selection, Basic modes of tooth failure, Gear Lubrication Methods. by S M Gujrathi
• 3. B)Spur Gears Number of teeth and face width Force analysis, Beam strength (Lewis) equation, Velocity factor, Service factor, Load concentration factor, Effective load on gear,  Wear strength(Buckingham’s)equation, Estimation of module based on beam and wear strength, Estimation of dynamic tooth load by velocity factor and Buckingham’s equation. by S M Gujrathi
• 4. C)Types of helical and Bevel gears, Terminology, Virtual number of teeth, force analysis of Helical and Straight Bevel Gear. Design of Helical and Straight Bevel Gear based on Beam Strength, Wear strength and estimation of effective load based on Velocity factor (Barth factor) and Buckingham’s equation. Mountings of Bevel Gear. (No numerical on force analysis of helical & Bevel Gear) by S M Gujrathi
• 5. 1.Introduction • A mechanical drive is defined as a mechanism, which is intended to transmit mechanical power over a certain distance, usually involving a change in speed and torque. 1.Generally prime mover rotates at high speed and machines requires low speed but high torque. Eg. Crane drum. the motor runs at 1440 rpm while the speed of the rope drum is as low as 20 rpm. 2. In certain machines, variable speeds are required for the operation, whereas the prime mover runs at constant speed. Eg. Lathe machine 3. Standard electric motors are designed for uniform rotary motion. However, in some machines like shaper or planer, linear motions with varying velocities are required. by S M Gujrathi
• 6. 1.1Classification of Drives Mechanical Drives Power transmission by Friction Ex.? Power transmission by Engagement Ex.? by S M Gujrathi
• 7. 1.2 The selection of a proper mechanical drive Depends upon: 1. centre distance, 2. velocity ratio, 3. shifting arrangement, 4. maintenance considerations 5. cost. General guidelines: by S M Gujrathi
• 8. Sr. Factor Type Conditions 1. Center Distance 1.Flat belts and roller chains are suitable for long centre distances. 2. V-belts have comparatively short centre distances 2.Gear drives have the smallest centre distance between two shafts 2. Speed Ratio 1.flat belt drives not recommended where constant speed is desirable.(Due to slipping condition) 2. Chain drives not recommended where constant speed is desirable.(Due to polygonal effect.) 3.Gear drives are preferred in applications which require constant speed. 3. Shifting mechanism 1.Flat belts with relatively long centre distances can be shifted from tight to loose pulleys. 2.V-belts/chain drives it is not possible to use the shifting mechanism. 3. Spur gears can be shifted on splined shaft 4. Maintenance 1. belt drives Maintenance of is relatively simple. It usually consists of periodic adjustment of centre distance in order to compensate the stretch of the belt 2.Chain and gear drives lubrication is an important consideration in maintenance. by S M Gujrathi
• 9. 2. GEAR DRIVES • Gears are defined as toothed wheels, which transmit power and motion from one shaft to another by means of successive engagement of teeth. • Advantages over chain and belt drives: (i)Velocity ratio remains constant because of a positive drive . (ii) Compact in construction due to less CD. (iii) It can transmit very large power, which is beyond the range of belt or chain drives. (iv) It can transmit motion at very low velocity, which is not possible with the belt drives. (v) The efficiency of gear drives is very high, even up to 99 percent in case of spur gears. (vi) A provision can be made in the gearbox for gear shifting, thus changing the velocity ratio over a wide range. by S M Gujrathi
• 10. Disadvantages • Gear drives are, however, costly and their maintenance cost is also higher. • The manufacturing processes for gears are complicated and highly specialized. • Gear drives require careful attention for lubrication and cleanliness. • They also require precise alignment of the shafts. by S M Gujrathi
• 11. 2.1 CLASSIFICATION OF GEARS Gear by S M Gujrathi
• 12. Gears Parallel shaft axes gear 1.Spur gear 2.Helical gears 3.Herringbone 4.Rack and pinion 5.Internal Gears Intersecting shaft axes gears Bevel Gears Non Intersecting and perpendicular shafts axes Worm Gears Non Parallel and non perpendicular shaft axes gears Spiral Gears by S M Gujrathi
• 13. by S M Gujrathi
• 14. by S M Gujrathi
• 15. Herringbone gear box by S M Gujrathi
• 16. Rack and Pinion by S M Gujrathi
• 17. Internal Gears by S M Gujrathi
• 18. Intersecting Shaft Axes Gear i)Bevel Gear: by S M Gujrathi
• 19. Non-Intersecting and perpendicular axis gears by S M Gujrathi
• 20. 2.2SELECTION OF TYPE OF GEARS by S M Gujrathi
• 21. Sr. Factor Type Conditions 1. General layout of shafts Spur and helical gears shafts are parallel bevel gears When the shafts intersect at right angles, Worm gears axes of shafts are perpendicular and non-intersecting crossed helical The axes of two shafts are neither perpendicular not intersecting 2. speed reduction or velocity ratio spur or helical 6 : 1 and rarely 10 : 1(because When the velocity ratio increases, the size of the gear wheel increases which increase the size of the gearbox and the material cost.) bevel gears The normal velocity ratio for a pair is 1 : 1, which can be increased to 3: 1 under certain circumstances. worm gears 60:1 or 100 : 1.( They are widely used in material handling equipment due to this advantage) 3. Power to be transmitted Helical gears high speed power transmission due to less noise. Spur Gears Low speed power transmission due to more noise. 4. Cost Spur Gears Not only Cheapest but many methods are available for manufacturing All other gears All other require specialized manufacturing process due complex shape. by S M Gujrathi
• 22. 3.Gear Terminology by S M Gujrathi
• 23. (1) Pinion A pinion is the smaller of the two mating gears. (2) Gear A gear is the larger of the two mating gears. (3) Velocity Ratio (i) Velocity ratio is the ratio of angular velocity of the driving gear(pinion) to the angular velocity of the driven gear(gear). It is also called the speed ratio. i = wp /wg (4) Transmission Ratio (i) The transmission ratio (i’) is the ratio of the angular speed of the first driving gear to the angular speed of the last driven gear in a gear train. (5) Pitch Surface The pitch surfaces of the gears are imaginary planes, cylinders or cones that roll together without slipping. (6) Pitch Circle The pitch circle is the curve of intersection of the pitch surface of revolution and the plane of rotation. (7) Pitch Circle Diameter The pitch circle diameter is the diameter of the pitch circle.(pitch diameter) by S M Gujrathi
• 24. (7) Pitch Point: The pitch point is a point on the line of centres of two gears at which two pitch circles of mating gears are tangent to each other. (8) Top land The top land is the surface of the top of the gear tooth. (9) Bottom land The bottom land is the surface of the gear between the flanks of adjacent teeth. (10) Addendum Circle The addendum circle is an imaginary circle that borders the tops of gear teeth in the cross section. (11) Addendum (ha) The addendum (ha) is the radial distance between the pitch and the addendum circles. Addendum indicates the height of the tooth above the pitch circle. (12) Dedendum Circle The dedendum circle is an imaginary circle that borders the bottom of spaces between teeth in the cross section. It is also called root circle. (13) Dedendum (hf) The dedendum (hf) is the radial distance between pitch and the dedendum circles. The dedendum indicates the depth of the tooth below the pitch circle. by S M Gujrathi
• 25. (14)Clearance (c) The clearance is the amount by which the dedendum of a given gear exceeds the addendum of its mating tooth. (15) Face of Tooth The surface of the gear tooth between the pitch cylinder and the addendum cylinder is called the face of tooth. (16) Flank of Tooth The surface of the gear tooth between the pitch cylinder and the root cylinder is called fl ank of the tooth. (17) Face Width (b) Face width is the width of the tooth measured parallel to the axis. by S M Gujrathi
• 26. (18)Centre Distance The centre distance is the distance between centres of pitch circles of mating gears. It is also the distance between centres of base circles of mating gears. (19) Pressure Angle The pressure angle is the angle which the line of action makes with the common tangent to the pitch circles. The pressure angle is also called the angle of obliquity. It is denoted by alpha. (20) Contact Ratio (mp) The number of pairs of teeth that are simultaneously engaged is called contact ratio. by S M Gujrathi
• 27. Circular Pitch The circular pitch (p) is the distance measured along the pitch circle between two similar points on adjacent teeth. Therefore, Diametral Pitch The diametral pitch (P) is the ratio of the number of teeth to the pitch circle diameter. Therefore, Module The module (m) is defined as the inverse of the diametral pitch. Therefore, Variation of no. of teeth wrt module by S M Gujrathi
• 28. by S M Gujrathi
• 29. Check your understanding: 1.If 2 pair of teeth is in contact with each other then contact ratio is……… 2. …………..is the amount by which the dedendum of a given gear exceeds the addendum of its mating tooth. 3. A pair of spur gears consists of a 20 teeth pinion meshing with a 120 teeth gear. The module is 4 mm. Calculate: (i) the centre distance=……… (ii) the pitch circle diameters of the pinion and the gear=……………. by S M Gujrathi
• 30. Modes of failures Due to static and dynamic loads Can be avoided by adjusting parameter such as the 1.module 2.he face width, so that the beam strength of the gear increases. Due to the surface destruction (i) Abrasive Wear (ii) Corrosive Wear (iii) Initial Pitting (iv) Destructive Pitting (v) Scoring by S M Gujrathi
• 31. SELECTION OF MATERIAL Desirable properties of gear material 1.The load carrying capacity: Material should have sufficient strength against fluctuating and/or dynamic loads 2. Material should have good ‘wear rating’which take care of grain size, percentage of carbon, and surface hardness. Material should have surface endurance strength to avoid failure due to destructive pitting. 3. Material should have low coefficient of friction to avoid failure due to scoring. 4. Material should be resistant to thermal distortion or warping. by S M Gujrathi
• 32. Material Applications Remark grey cast iron of Grades FG 200, FG 260 or FG 350 Large size gears 1.cheap and generate less noise compared 2.good wear resistance 3.drawback is poor strength. Case-hardened steel gears Best combination of a wear resisting hard surface together with a ductile and shock- absorbing core. The plain carbon steels are 50C8, 45C8, 50C4 and 55C8 medium duty applications alloy steels 4OCr1, 30Ni4Cr1 and 4ONi3Cr65Mo55 heavy duty applications, Bronze Worm wheels Low Coefficient of friction by S M Gujrathi
• 33. Material Applications Remark Non-metallic gears(molded nylon, laminated phenolics like Bakelite or Celeron) 1.Low load with low pitch velocity quiet operation 2.Water and oil. The nonmetallic pinions generally run with cast iron gears phenolic resins With marginal lubrication low modulus of elasticity by S M Gujrathi
• 34. by S M Gujrathi
• 35. by S M Gujrathi
• 36. by S M Gujrathi
• 37. ESTIMATION OF MODULE BASED ON BEAM STRENGTH by S M Gujrathi
• 38. by S M Gujrathi
• 39. by S M Gujrathi
• 40. by S M Gujrathi
• 41. ESTIMATION OF MODULE BASED ON WEAR STRENGTH The wear strength is the maximum value of the tangential force that the tooth can transmit without pitting failure by S M Gujrathi
• 42. by S M Gujrathi
• 43. by S M Gujrathi