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How to Select the Appropriate Gearbox

Author : Date : 2014-3-24 16:59:31
How to Select the Appropriate Gearbox(Speed Reducer)?
When considering a gearbox, many factors need to be considered to meet specific application requirements:
Output Torque
Output torque is dependent on the gear ratio used. To obtain a high output torque, a large gear ratio would be selected. Using a large gear ratio will lower the output shaft speed of the motor. Inversely, using a lower gear ratio, a smaller output torque value would be delivered into the system, with a greater motor speed at the output shaft. This statement illustrates the relationship that both torque and speed are inversely proportional to one another.
Gear Ratio
Gear ratios are defined as the correlation between the numbers of teeth of two different gears. Commonly, the number of teeth a gear has is proportional to its circumference. This means that the gear with a larger circumference will have more gear teeth; therefore the relationship between the circumferences of the two gears can also give an accurate gear ratio. For example, if one gear has 36 teeth while another gear has 12 teeth, the gear ratio would be 3:1.
Speed (RPM)
Speed is proportional to the gear ratio of the system. For example, if the input gear has more teeth than the output gear, the result will be an increase in speed at the output shaft. On the other hand, having the reverse scenario with more gear teeth at the output compared to the input will result in a decrease of speed at the output shaft. In general, the output speed can be determined by dividing the input speed by the gear ratio. The higher the ratio the lower the output speed will be and vice versa.
Gear Arrangement
Gear arrangement is an ingenious engineering design that offers various benefits over the traditional fixed axis gear system design. The unique combination of both power transmission efficiency and compact size allows for a lower loss in efficiency. The more efficient the gear arrangement, (i.e. spur, helical, planetary and worm) the more energy it will allow to be transmitted and converted into torque, rather than energy lost in heat. 
Another application factor to be taken into account is load distribution. Since the load being transmitted is shared among multiple planets, the torque capacity is increased. The higher number of planets in a gear system will increase the load ability and enhance torque density. Gear arrangements improve stability and rotational stiffness because of a balanced system, but it is a complex and more costly design.
Fixed-Axis vs Planetary Gear System
Figure 1: Fixed-Axis vs. Planetary Gear System
In Figure 1, the gear arrangement on the left is a traditional fixed axis gear system with a pinion driving a larger gear on an axis parallel to the shaft. On the right, is a planetary gear design system with a sun gear (pinion) surrounded by more than one gear (planet gears) and is encompassed in an outer ring gear. The two systems are similar in ratio and volume, but the planetary gear design has three times the higher torque density and three times the stiffness due to the increased number of gear contacts.
Fixed Axis Gear System:
Volume = 1, Torque = 1, Stiffness = 1
Planetary Gear System:
Volume =1, Torque = 3, Stiffness = 3
Other gear arrangements as mentioned in the Types of Gearboxes segment of this guide are bevel, helical, cycloid, spur and worm.
Backlash is the angle in which the output shaft of a gearbox can rotate without the input shaft moving, or the gap between the teeth of two adjacent gears. It is not necessary to consider backlash for applications which do not involve load reversals. However, in precision applications with load reversals like robotics, automation, CNC machines, etc., backlash is crucial for accuracy and positioning.

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