To choose the right gearbox for your electric motor drive application, it’s important to understand the fundamentals of gearboxes, the key differences between gear designs, and how to match a gearbox with the requirements of your application.
A gearbox is a gear train designed to modify the speed and torque characteristics of a motor. When gearboxes reduce speed, they simultaneously increase the torque (turning force) at the output. When they increase speed, they reduce torque.
Speed reducers are a popular term for gearboxes that reduce the speed of an electric motor and increase the torque. Because almost every application of gearboxes involves reducing speed and increasing torque, these terms are often used interchangeably. However, speed reducers are really a subcategory of gearboxes, and it is possible to use a gearbox to increase the speed of the output at the cost of torque.
Types of Gearboxes
The two main types of gearboxes are in-line gearboxes and right-angled gearboxes, each using specific types of gears.
In-line gearboxes transmit the RPM of the motor to an output shaft that is parallel to the motor shaft. Depending on the gear train, the output shaft can be coaxial (aligned) with the motor shaft or offset from it by a small distance. In-line gearboxes typically use the following types of gears:
- Spur gears
- Helical gears
- Herringbone gears
- Planetary gears
1. Spur Gears
Spur gears are the most common gear design and are relatively economical due to the standardization of their design. However, they don’t offer as much torque capability as some other gear designs.
2. Helical Gears
Helical gears have better meshing characteristics than spur gears, making them quieter in operation and able to handle more torque. However, due to their design, they produce axial forces that make them unsuitable for some very high torque applications.
3. Herringbone Gears
Herringbone gears are essentially composed of two opposite-hand helical gears mounted side-by-side. This retains the benefits of helical gears while removing the issue of axial forces, making them useful for very high torque applications such as heavy-duty power transmission.
4. Planetary Gears
Planetary gears typically provide the greatest speed reduction in the smallest volume and are very efficient. They also have a very low backlash, making them especially suited to applications with frequent stops and starts.
Right-angle gearboxes transmit the RPM of a motor to an output shaft that is at right-angles (90 degrees from) the motor shaft axis. Right angle gearboxes typically use the following types of gears:
- Bevel gears
- Worm gears
1. Bevel Gears
Bevel gears are essentially spur or helical gears that are tapered such that they can mesh with another gear oriented at a different angle, making them suitable for creating a right-angle turn in the drive system. They offer similar characteristics to standard spur and helical gears.
2. Worm gears
Worm gears are essentially a screw with a single tooth that spirals the length of the shaft, meshing with the teeth of a second gear mounted at right angles to the worm. Because the teeth interact in a sliding motion, they are usually quieter than other gear types. However, due to the friction produced, they have an overall lower efficiency, with a sharp drop in efficiency once the lead angle of the screw approaches 15 degrees. The friction also generates a significant amount of heat within the gearbox, and the correct maintenance of the lubrication system is important.
Selecting a Gearbox
Selecting a gearbox for your motor depends on the following gearbox characteristics:
- Torque output
- Speed output
- Service factor
- Mounting and connection characteristics
- Radial load
- Moment of inertia
The gearbox must output enough torque to run the machinery that the gearbox is connected to. This includes the torque required to start and accelerate the machinery – which may be substantially higher than the normal operational torque – as well as the torque that is created during jams and load shocks.
If the application requires a specific speed output, the gearbox must be able to maintain that speed while providing sufficient torque to the driven load.
If the motor output power closely matches the power requirements of the driven load, the gearbox must convert that power efficiently. Efficiency is also important when one of the goals is to reduce the operational costs of running the motor, such as power costs.
The service factor of the gearbox is the percentage of its rated torque that it can sustain for short periods of time. A substantial service factor is required if the gearbox will experience frequent fast accelerations or occasional load spikes on the output. As gear reducers do not have a cooling fan, the service factor is also based on how many hours a day it is used for. The longer the production shift, the larger the case needs to be to dissipate the produced heat.
Mounting and Connection Characteristics
If the gearbox mounting area has space and profile restrictions, the gearbox size, shape and configuration is an important factor in determining which option is suitable. Different types of gears have different size efficiencies, with planetary gear trains generally the most compact, and spur gears requiring a larger volume. Also, in smaller applications, it may be possible to utilize a gearbox that provides the mounting for the motor, whereas in larger applications it is usually necessary to mount them separately to a chassis.
Backlash is the amount of error or plays in the meshing of the gears in the gearbox, which results in the gearbox experiencing mechanical shock when started or stopped. For applications where the motor is run intermittently, it may be advisable to utilize a gearbox with a low amount of backlash, such as a planetary gearbox, to avoid premature mechanical failure.
Radial load refers to the force on the gearbox output shaft that occurs at right angles to it, such as when a belt pulley or sprocket is used to connect the gearbox to the driven load. Gearboxes have different radial load tolerances, so it’s important to make sure that a gearbox can withstand the operational conditions of your application.
Moment of Inertia
The moment of inertia of the gearbox is essentially the resistance of the gearbox to sudden acceleration. In applications that require responsive speed control such as robotic servosystems, it is important to choose a gearbox that features a low moment of inertia.
Gearbox Installation and Maintenance
The most common cause of premature gearbox failure is not the quality and workmanship of the gearbox, but the conditions in which it is run and the service factor it was designed for. The most important factor is to maintain the gearbox properly – ensuring adequate lubricant is always present in every part of the gearbox and replacing the lubricant when it wears down from mechanical pressure and heat. Also, it is essential to properly install the gearbox to reduce vibration and stress. Always ensure that the alignment error between the motor and the gearbox, and between the gearbox and the driven load, is within the acceptable limits of the gearbox.
Gearboxes have a variety of characteristics that affect their suitability for different applications. When choosing a gearbox, it is important to carefully analyze your specific application so that you can select an option that efficiently meets all the design criteria. When you have chosen a gearbox, it is essential to install and maintain it properly to ensure the longest possible service life.
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