When should I add a gearbox to a motor?

Gearboxes reduce speed and increase torque

Whilst there are occasionally gearboxes that will increase speed and reduce torque, in the vast majority of cases, a gearbox will be used to reduce the shaft speed of a motor and increase the torque.

This can be done for several reasons.

  1. You need to reach a certain torque amount and the most cost-effective method is to use a smaller motor with a gearbox, rather than using a larger motor.
  2. You need a certain amount of torque but are limited by space and are therefore unable to use a large motor that could otherwise meet the torque requirement.
  3. You need to run at such a slow speed range that a motor is simply not practical as a method by which you could operate it at. In other words a gearbox is the only real way of delivering the mechanical performance you need.

What are the benefits of adding a gearbox?

  1. A gearbox can add significantly more torque for a very small package and can therefore be hugely effective as a method for driving low speed, high torque applications from a small package.
  2. They can be tailored to deliver the exact torque and speed range you require. Whatever you need it is possible to get a motor and gearbox combination that will be able to output the torque and speed requirements you have. Gearboxes enable you to get much closer to the specific requirement by providing the flexibility and range that you will be unable to get from just a motor.
  3. They are compact and can therefore reduce weight when compared with a larger motor.
  4. In some cases gearboxes can mean that you are able to use a lower voltage for a particular application. For example a typical 10Nm motor application would normally function with a 12V or 24V supply, but with a gearbox added it is easily possible to reach these torque levels.

What are the downsides of using gearboxes?

There are a few downsides to using gearboxes. Some of these can be dealt with by using a high quality gearbox. Others are simply unavoidable parts of using a gearbox. The main issues that you might find with a gearbox are;

  1. Closed loop applications become more difficult. It is possible to mount encoders or pots on gearboxes but this usually a bit more difficult to achieve than without a gearbox due to typical gearbox design. Of course it is possible to monitor position by calculating where a shaft should be based on an encoder on the motor shaft and a programme which accommodates for the gear ratio but this will never be 100% accurate due to backlash and other issues with gearboxes.
  2. Inefficiency increases. Typical planetary gearboxes are often around 75% efficient, although they can be made much more efficient if made by very high quality manufacturers. This means that you are trading 25% of your potential mechanical rotary power for heat and noise energy. For some applications this may be less of an issue, but for others this can mean that significantly more input power is required and this can make the gearbox undesirable.
  3. It’s another part that can potentially go wrong. It sounds obvious and simple but there is a basic argument for this idea. The more complexity you add to an application, the higher the risk of failure. By adding an extra component an increasing the complexity of the drive system it is likely that the chances of a failure will increase.
  4. You can lose the direct control and responsiveness of a motor such as a stepper which can reduce its effectiveness in certain applications.

Ok, so what are the typical applications that are best suited to gearboxes…

Slow moving, high torque applications are by far the most common types of applications that these motors should be applied to. Whether it’s slow moving peristaltic pumps, large rotating turntables or robotics, a geared motor will move your application into the torque speed range that you need it to be in. This broadly applies to most industries depending on the specific type of project or application in question. For example, lots of pumps would not use gearboxes but then for those, such as large peristaltics, that require slow speeds and high torque, gearboxes would be the obvious choice.

Similarly in many direct drive applications (for example small robots), gearboxes are normally essential to get the drive down to the speeds and torque requirement without sacrificing efficiency by going for something like a stepper motor. In these applications, gearboxes enable the designers to benefit from a simple assembly process in which (for example) four wheels can all have independent drives that are able to deliver the required torque and speed. Without gearboxes this type of work would be impossible.

Conclusion – a hugely useful addition in the right application

Gearboxes can make a huge difference and are basically essential for certain types of application as can be seen from the examples outlined above. However, in other cases it is important to think about exactly what it is you are wanting to do and to explore every potential option first. Adding gearboxes will reduce efficiency and in certain types of application this can be avoided. However, in other applications there is simply nothing which can deliver the performance that a gearbox can and they are therefore to be considered as essential parts of the process.

If you have a project where you are looking at gearboxes as a potential option but are unsure on the best possible route to go down then please feel free to get in touch with us to discuss your requirements. Our UK based team will be more than happy to advise on the options you have available and work with you to find the most appropriate solution for your project.