Home  Guides Gearbox Science I: Predicting Rate of Fire for Gearsets
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Gearbox Science I: Predicting Rate of Fire for Gearsets |
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Written by Jay
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Monday, 07 February 2005 |
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Page 3 of 10 Gear Ratios:
When several gears are used in conjunction in a gear train or transmission, the relative sizes of the gears plays the determining role in describing the characteristics of the system. Using gears of different sizes or types, the input torque and speed can be modified to sacrifice one for the other, depending on the requirements of the system. In an AEG mechbox, the output speed of the AEG motor must be “stepped down” in order to achieve a torque gain enough to drive the main spring. The gear ratio between 2 circular gears in contact is calculated via the ratio of their circumferences. Gears which are designed to mesh with each other are typically designed with identical tooth profiles so that the gear ratio between them remains constant during operation. This design feature also provides a short-cut for the calculation of gear ratios, removing the need to actually measure the dimensions of the gears in question. Instead, the ratio of the number of teeth between the 2 gears can be used. When a smaller gear, such as the AEG motor pinion with 10 teeth, is connected to a larger gear, such as the AEG bevel gear with 30 teeth, a 3:1 ratio is achieved between the gears which magnifies the output torque of the motor by 3x while simultaneously reducing the output speed by 1/3. That is to say, if the motor spins at 1000rpm and puts out 200 in-lbs of torque, the bevel gear now spins at 1000/3 = 333rpm and puts out 200x3 = 600 in-lbs of torque. As more gears are added to the system to form a gear train, the ratios continue to multiply, greatly increasing the desired final output. The standard Tokyo Marui gear ratio can be determined easily by counting the relative number of teeth between each gear and multiplying the ratios as described above:
| Motor Pinion | Bevel to Motor | Bevel to Spur | Spur to Bevel | Spur to Sector | Sector to Spur | Final Ratio | | 10 | 30 | 10 | 39 | 20 | 32 | 18.72 |
*the colored cells delineate each gear intersection: blue = bevel, orange = spur, green = sector.
Bevel to Motor Pinion Ratio: 30:10 = 3:1
Spur to Bevel Ratio: 39:10 = 3.9:1
Sector to Spur Ratio: = 32:20 = 1.6:1
Final Ratio = 3 x 3.9 x 1.6 = 18.72
By altering the standard ratio of teeth between the various gears in the mechbox, aftermarket parts manufacturers can choose to enhance the final torque output as in the case of “torque-up” gearsets or the final output speed as in the case of “hi-speed” gearsets.
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