Using only the motor size and speed analysis can be problematic because most applications require a specific speed and or torque to meet the overall power requirements of the application. To keep the ideal package size, a gearbox with a motor can be used to reduce the speed and increase the torque to meet design parameters. It is a gear motor kit. The aforementioned “sweet spot” illustrates the point where the ratio of size versus motor speed is optimized. Using the correct gearbox allows the designer to optimize the motor while still meeting the required output speed and torque.
Though there are a variety of gearbox options, four specific types will be highlighted: the right angle worm, right angle bevel, planetary, and parallel shaft. Each gearbox type possesses unique design attributes that will dictate the performance of the reducer within the application. It is also important to consider gearing components when making a gearbox choice. The worm, bevel, spur and helical gearing options complete the reducing process according to their unique design attributes, contributing to the overall gearbox performance.
Similar to the motor selection process, it is important to build a reducer matrix to match the proper gearbox with the specific application. The key reducer parameters to note are duty cycle, maximum input speed, maximum torque, efficiency, noise, limitations, and back driveability (table 1). While these are common parameters and work well in the initial data-gathering phase, there are many additional items that should be considered before a final gearbox choice is made. It should be noted that the parameters listed in the selection matrix are common guidelines for standard, off-the-shelf products. Custom designs can be tweaked to run outside the normal parameters.
Table 1: Gearbox Quick Reference Guide – compares some of the typical parameters used during the gearbox selection process.
|Planetary||Parallel Shaft||Right Angle||Right Angle|
|Gears||Spur or Helical||Spur or Helical||Worm||Bevel|
|Max Input Speed||3500||4000||2800||3500|
|Back Driveability||Good||Good||Ratio Dependent||Good|
Duty cycle, maximum input speed and maximum torque are three of the most common parameters to consider in the gearbox selection process (Table 2). These values can be easily gathered from the application design inputs and motor outputs. Efficiency is a parameter that becomes critical to meeting performance specifications while optimizing package size. The efficiency of the gearbox is affected by gear quality, ratio, gear type, lubrication, bearing types, and side loading. Generally, higher efficiency is better but high efficiency can also drive cost. Referring back to the criteria listed in Table 2, noise is a very subjective term that is greatly affected by the environment and performance criteria required in the final product. A design that is considered very quiet in a loud industrial setting might sound noisy in a household or subdued setting.
One of the final parameters in Table 1 is limitations. Understanding the limitations of each gearbox is very important in making a gearbox selection. Looking at the parallel shaft and bevel right-angle gearboxes, these designs are limited in output torque by the mechanical wear on components (e.g.shafts, bearings, gears, etc.) known as a mechanical limitation(Table 2).On the other hand, the output torque of a worm gearbox is limited by the heat that is generated within the gearbox, known as a thermal limitation. This thermal limitation is directly related to the efficiency and life of the gearbox. Heat generation causes premature life issues with lubrication which, in turn, affects gear life. High heat is also a sign that the gearbox is not running as efficiently as it could. Looking at the worm gearbox, its general performance shows the lowest efficiency rating of all the gearbox ratings. Surprisingly, a planetary reducer can have both thermal and mechanical limitations, even though it has a high-efficiency rating.
Questions that often come up are “what size gearbox should be used for a given torque” and “how much continuous duty torque can come from a cubic inch of gearbox volume?” The measure of torque per gearbox volume (in-lb/in³) is called torque density.
Table 2: Torque Density Comparison – an example of torque density values for a right angle worm, right angle bevel, parallel shaft and planetary gearboxes.
|Right Angle Worm||6 In-Lb/In3|
|Right Angle Bevel||10 In-Lb/In3|
|Parallel Shaft Gearbox||12 In-Lb/In3|
For example, the worm gear will get about 6 in-lbs of torque per cubic inch of gearbox volume, while the planetary gets about 21 in-lbs of torque per cubic inch – almost 3 times as much for a given output (Table 2). The high torque density of the planetary gearbox indicates that it can be loaded to a relatively high torque in a small package. Higher output torques will generate higher temperatures in a reducer since there is less gearbox volume to serve as a heat sink, dissipating the heat that builds up as a result of the higher output torque. High torque density and a small package size mean a planetary gearbox can sometimes be limited by temperature.‡ Evaluating torque density, package size and resulting gearbox volume in the application will ensure an effective gearbox choice that will correctly fit the required form factor of the application.
In addition to the criteria in the Gearbox Quick Reference Guide (Table 1) there are other design criteria that need to be considered in the gear motor selection processes, such as overhung load, envelope size, cost, lubrication, and mounting options. Evaluating these additional specifications will help the designer identify application-specific limitations that can quickly eliminate one or two gearbox types. For example, if the gear motor will be placed in a tube within the application a right angle worm gearbox will be eliminated as an option because its form factor does not work well in a cylindrical space.