16 44113 Double Helical Gears 1 3 Fig 11a Double Helical Gear 1 A double

16 44113 double helical gears 1 3 fig 11a double

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164.4.1.1.3 Double Helical Gears [1, 3] Fig. 11.a. Double Helical Gear [1] A double-helical gear is equivalent to a pair of helical gears secured together, one having a right-hand helix and the other a left-hand helix. The teeth of the two rows are separated by a groove used for tool run out. The purpose of this orientation is to cancel out the axial or thrust force, which occurs when using single-helical gears, as described previously. This is because the axial thrusts of the two rows of teeth cancel each other out. These gears can run at higher speeds with less noise and vibrations as compared to other helical gear applications. Fig. 11.b. Herringbone Gear [1] If the left and right inclination of a double-helical gear meets at a common apex and there is no groove in between, the gear is known as herringbonegear (fig .11.b). The continuous tooth form permits this type of gear to transmit heavy loads at very high pitch line velocities (up to 5000 fpm) which translates to high rotational speeds. These gears are particularly suited for shock and vibration applications, or where a high single speed reduction is required at very high horse power requirements. For example, these gears are used extensively in enclosed gear boxes which are used in cogeneration power plants due to the large horsepower requirements needed on order to transmit the power from steam into high rotational speeds for turning generators to produce electricity. This gear
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17type can withstand high speed and power requirements while delivering smooth operation coupled with long operational life and reliability. 4.4.1.1.4 Internal Gears [1] Most of the gears that we have studies have teeth located outside, or on the outer circumference of the gear. But an internal gear consists of a cylindrical ring with teeth located in the inner circumference of the gear meshing with another gear (pinion gear with external teeth) forming the internal gear set (see Figure 12). The tooth profile can be either a spur gear type or a helical gear type. Figure 12. Internal Spur Gear [1] The advantage of using internal gears is that they can be used for applications where space is a constraint. Also the pinion and driver gear have the same direction of rotation thus eliminating the need for an idler gear. This configuration has led to the development of systems known as planetary gearing systems which will be covered later in this chapter. 4.4.1.1.5 Spur Rack and Pinion [1,] Figure 13. Spur Rack and Pinion [1]
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18The spur rack is a special case of a spur gear where it has an infinite diameter but can have a similar pitch as other common spur gears. It is designed using a straight rectangular bar with straight sided teeth perpendicular to its length (Figure 13). The rack is then driven by a small spur gear known as the pinion, thus the term “Rack and Pinion.” The spur rack and pinion combination converts rotary motion into linear motion, or vice-versa. Spur racks are available in the same pressure angles as spur gears and are used generally for applications which involve oscillations or reversing of direction. Examples
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