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Ingenieurwesen

Gear and Gear trains

MANJUNATH NFolgen

- 1. Gears
- 2. Gears! Gears are most often used in transmissions to convert an electric motor’s high speed and low torque to a shaft’s requirements for low speed high torque: Speed is easy to generate, because voltage is easy to generate Torque is difficult to generate because it requires large amounts of current Gears essentially allow positive engagement between teeth so high forces can be transmitted while still undergoing essentially rolling contact Gears do not depend on friction and do best when friction is minimized
- 3. Gears A gear is a wheel with teeth on its outer edge. The teeth of one gear mesh (or engage) with the teeth of another. Above Gears meshing or engaged
- 4. Gears Driver and Driven Two meshed gears always rotate in opposite directions. Driver gear Driven gear Spur Gears
- 7. Spur Gears Teeth are parallel to the axis of the gear Advantages Cost Ease of manufacture Availability Disadvantages Only works with mating gear Axis of each gear must be parallel
- 9. Helical Gears Teeth are at an angle to the gear axis (usually 10° to 45°) – called helix angle Advantages Smooth and quiet due to gradual tooth engagements (spur gears whine at high speed due to impact). Helical gears good up to speeds in excess of 5,000 ft/min More tooth engagement allows for greater power transmission for given gear size. Disadvantage More expensive Resulting axial thrust component
- 10. Helical Gears Mating gear axis can be parallel or crossed Can withstand the largest capacity at 30,000 hp
- 11. Bevel Gears Gear axis at 90°, based on rolling cones Advantages Right angle drives Disadvantages Get axial loading which complicates bearings and housings
- 12. Spiral Bevel Gears Same advantage over bevel gears as helical gears have over spur gears!! Teeth at helix angle Very Strong Used in rear end applications (see differentials)
- 15. Worm Gears Gears that are 90° to each other Advantages Quiet / smooth drive Can transmit torque at right angles No back driving Good for positioning systems Disadvantage Most inefficient due to excessive friction (sliding) Needs maintenance Slower speed applications worm worm gear
- 16. Gears • Multiple gears can be connected together to form a gear train. Simple Gear Train Each shaft carries only one gear wheel. Intermediate gears are known as Idler Gears.
- 17. Gears Compound Gear Train Driver Compound Gear Driven If two gear wheels are mounted on a common shaft then it’s a Compound Gear train.
- 18. Gears Generally, the Gear Ratio is calculated by counting the teeth of the two gears, and applying the following formula: Gear ratio = Number of teeth on driven gear Number of teeth on driver gear Gear Ratio
- 19. Gears Gear Ratio - Calculation A 100 tooth gear drives a 25 tooth gear. Calculate the gear ratio for the meshing teeth. Gear ratio = Number of teeth on driven gear Number of teeth on driver gear Gear ratio = driven 25 = 1 driver 100 4 This is written as 1:4
- 20. Gears Gear Speed :- Calculation A motor gear has 28 teeth and revolves at 100 rev/min. The driven gear has 10 teeth. What is its rotational speed? Speed of driven gear = Number of teeth on driver gear x 100 Number of teeth on driven gear Speed of driven gear = driver = 28 x 100 = 280 rev/min driven 10 28 teeth, driver 10 teeth, driven
- 21. Gears The worm gear is always the drive gear Worm and wheel Worm gear and wheel
- 22. Gears The rack and pinion gear is used to convert between rotary and linear motion. Rack and Pinion Heavy Duty Car Jack
- 23. Gears Bevel gears are used to transfer drive through an angle of 90o . Bevel Gears Bevel gears
- 32. Gears used for Speed Reducer Recall the main purpose of mating/meshing gears is to provide speed reduction or torque increase. driver driven P G G P N N N N n n VRRatioVelocity ==== Pinion nP NP Gear nG NG ωω )2/(DRvspeedlinePitch t === )12/(min)/( Dnftvt π=
- 33. Example: Want a 3:1 reduction NP=22 teeth What is NG? Solution: VR = 3 = NG/NP NG = 3*22 = 66 teeth Figure 8-15, pg. 322
- 34. Engine Pump n1, N1 n2, N2 n3, N3 n4, N4 Given: n1 = 500 rpm, N1 = 20t N2 = 70t, N3 = 18t, N4 = 54t Find: n4 Example: Double Speed Reducer Solution: 1. n2 = 500 rpm*(20/70) = 142.8 rpm 2. n3 = n2 3. n4 = 142.8 rpm*(18/54) = 47.6 rpm 4. Total reduction = 500/47.6 = 10.5 (0r 10.5:1) Torque?? Increases by 10.5!! Power?? Stays the same throughout!
- 35. Gear Nomenclature N = Number of teeth Use subscript for specific gear NP=Number of teeth on pinion (driver) NG=Number of teeth on gear (driven) NP < NG (for speed reducer) NA=Number of teeth on gear A Circular Pitch, P is the radial distance from a point on a tooth at the pitch circle to corresponding point on the next adjacent tooth P=(π∗D)/N
- 36. Gear Nomenclature Gear Train Rule – Pitch of two gears in mesh must be identical πDG NG =P πDP NP GEAR PINION
- 37. Gear Nomenclature Diametral Pitch, (Pd) – Number of teeth per inch of pitch diameter *Two gears in mesh must have equal Pd: *Standard diametral pitches can be found in Table 8-1 and 8-2 D N =Pd DG NG ==Pd DP NP

- Helical gears offer a refinement over spur gears. The leading edges of the teeth are not parallel to the axis of rotation, but are set at an angle. Since the gear is curved, this angling causes the tooth shape to be a segment of a helix. The angled teeth engage more gradually than do spur gear teeth. This causes helical gears to run more smoothly and quietly than spur gears. Helical gears also offer the possibility of using non-parallel shafts. A pair of helical gears can be meshed in two ways: with shafts oriented at either the sum or the difference of the helix angles of the gears. These configurations are referred to as parallel or crossed, respectively. The parallel configuration is the more mechanically sound. In it, the helices of a pair of meshing teeth meet at a common tangent, and the contact between the tooth surfaces will, generally, be a curve extending some distance across their face widths. In the crossed configuration, the helices do not meet tangentially, and only point contact is achieved between tooth surfaces. Because of the small area of contact, crossed helical gears can only be used with light loads. Quite commonly, helical gears come in pairs where the helix angle of one is the negative of the helix angle of the other; such a pair might also be referred to as having a right-handed helix and a left-handed helix of equal angles. If such a pair is meshed in the &apos;parallel&apos; mode, the two equal but opposite angles add to zero: the angle between shafts is zero -- that is, the shafts are parallel. If the pair is meshed in the &apos;crossed&apos; mode, the angle between shafts will be twice the absolute value of either helix angle. Note that &apos;parallel&apos; helical gears need not have parallel shafts -- this only occurs if their helix angles are equal but opposite. The &apos;parallel&apos; in &apos;parallel helical gears&apos; must refer, if anything, to the (quasi) parallelism of the teeth, not to the shaft orientation. As mentioned at the start of this section, helical gears operate more smoothly than do spur gears. With parallel helical gears, each pair of teeth first make contact at a single point at one side of the gear wheel; a moving curve of contact then grows gradually across the tooth face. It may span the entire width of the tooth for a time. Finally, it recedes until the teeth break contact at a single point on the opposite side of the wheel. Thus force is taken up and released gradually. With spur gears, the situation is quite different. When a pair of teeth meet, they immediately make line contact across their entire width. This causes impact stress and noise. Spur gears make a characteristic whine at high speeds and can not take as much torque as helical gears because their teeth are receiving impact blows. Whereas spur gears are used for low speed applications and those situations where noise control is not a problem, the use of helical gears is indicated when the application involves high speeds, large power transmission, or where noise abatement is important. The speed is considered to be high when the pitch line velocity (that is, the circumferential velocity) exceeds 5000 ft/min.[3] A disadvantage of helical gears is a resultant thrust along the axis of the gear, which needs to be accommodated by appropriate thrust bearings, and a greater degree of sliding friction between the meshing teeth, often addressed with specific additives in the lubricant