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Why do you use a gearbox with a motor?

A gearbox is a device that transfers mechanical power from a motor to another part of the machine. While a motor is designed to provide mechanical energy in the form of speed and torque, a gearbox is used in conjunction with a motor to reduce the output speed of the motor while increasing its output torque. Basically, when selecting a motor and gearbox combination, the speed and torque requirements are the first thing to consider. A gearbox helps to optimize these requirements by matching speed and torque requirements for a specific application. A gearbox also helps to match the load inertia to the motor inertia, which provides much better control and stability in the system.

 

Are there different types of gearboxes used for different applications?

Yes, there are many types of gearboxes, but the most common ones designed for drive systems are spur, helical, planetary, and worm/wheel gearing.

  • A spur gear is a gear wheel with teeth projecting parallel to the wheel’s axis, which produces inline motion from the motor shaft. They can be mounted in series, called spur trains, to achieve high gear reductions. Spur gears are used in a wide range of applications, from slow to moderate speeds. You typically can back drive, or manually spin the gearbox in reverse, but this is highly dependent on ratio and materials of the gearing. Typically, they provide up to a 10:1 reduction per gear stage and can be combined with a worm/wheel to provide unique geometries.
  • Helical gears provide similar characteristics to a spur gear, but the teeth have a helical cut, providing a more gradual and smooth gear interaction. This often results in a more efficient, higher torque and a quieter gearbox.
  • Planetary gears consist of one or more outer gears (or planet gears) revolving around a center gear (called a sun gear) and an outer ring gear. The relative size of each gear determines the speed reduction between the motor and the output of the gearbox. Planetary gears provide inline, high-performance motor operation and exhibit a highly efficient and power dense transmission, often up to 97% or so per stage. Planetary gears are used in a wide range of applications from slow to high speed, are often back drivable, and can be constructed with multiple stages to provide very high reduction ratios. Planetary gearboxes are often the choice for high performance, high duty cycle applications.
  • Worm/wheel gearing provides 90-degree motion from the motor shaft. These gears are typically used when high torque is required at low speeds. They provide very high gear reductions in a small space. For example, a 4:1 reduction is the same physical size as a 75:1 – solely based on the pitch of the worm. Compare this to a bevel gear, which can only provide up to about a 10:1 reduction in the same package size. Typically, at or above 25:1, a worm gear is going to be self-locking, meaning it will be difficult or impossible to back drive. Although this isn’t considered a safety feature, it could be very useful in certain applications. Worm/wheel gearing is often quieter than a spur or planetary gear, but there are exceptions.

 

What else do I need to think about when selecting a gearbox for my motor?

When selecting a gearbox, the primary goals are to select the correct speed and torque requirements while matching the motor to load inertias for optimal performance. Users should consider designing around the system dynamics, load types, duty cycle (run time versus non-run time), and other parameters, including physical size, ingress protection, external forces, backlash, and more. These requirements will determine the physical size of the gearbox, as well as the material selection, which dictates mechanical limitations, thermal limitations, audible noise level considerations and life expectancy of the gearbox.

 

Can you elaborate on importance of gear material selection?

Material selection of gearing determines sliding frictions between gears and impacts efficiencies and overall life expectancy of the gearing. For example, the first helical gear in the gear train is often a reinforced thermoplastic with hardened steel spur gears on subsequent stages. This configuration helps to reduce the audible noise of the gearbox and supports a higher output torque and longer life. Another example would be to use a bronze gear over a composite gear to support higher continuous or impact loads. The downside is the bronze gearing will have higher frictional loads and run hotter over a composite gear, which can accelerate the wear of the bronze gear. Ultimately, you should select the gearing material that supports the most and least important parameters for the application.

 

How about noise levels?

The dB ratings are arguably similar between bronze and composite gearing at low to medium speeds and marginally higher for bronze at 100+ rpm. However, the composite material reduces the higher frequency and increases the very low frequency levels so the perceived audible level is often worse for a bronze worm wheel.

 

So, which is better – a composite gear or a bronze gear?

It depends. A bronze gear can be pushed harder, offers a better continuous/peak load ratio, and handles peak loads better than a composite gear in a similar package size. Yet, if your loads are within the specifications of the gear, then the composite gear is fine and provides additional benefits, such as higher efficiencies, lower noise levels, and longer life. A composite gear will have almost no wear if it is used within its design parameters, and you can often see them last in excess of 5,000+ hours. Compare that to a bronze gear, where you have metal on metal that will wear over time. In addition, the particles that shed from the bronze gear can encroach on the lubricant and create a sandpaper-like effect, further reducing gear life.

 

What is your takeaway advice for someone selecting a gearbox?

When selecting a gearbox, there is a lot to consider. There is often more than one answer since different gearing types have different advantages and disadvantages. In addition to the performance of the gearbox, consider how the material selection of the gears can help achieve your application goals. Lastly, there are many parameters to consider, and calculations are critical to optimizing your application. That said, it is always good to test your solution to see how it reacts in the specific application.

John Uzzolino

John Uzzolino Business Development Manager – Parvalux, maxon USA

John Uzzolino heads up business development for the Parvalux product line at maxon, USA. As the Business Development Manager, John is responsible for managing and promoting the Parvalux products to  the North American market. John joined maxon in 2019, bringing 20 years’ experience in industrial motion control and mechatronics applications. John holds a BS in mechanical engineering from Rutgers University and started his career as a Sales Engineer working for a high-tech distribution company, focusing on solving motion control and mechatronic applications for machine builders in the North East U.S. John then continued his career, focusing on helping companies
bring new mechatronic products to the US market.

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