In most cases, an electric motor is not suitable for driving a load directly, as it rotates too fast and is not capable of delivering sufficient torque on its own. Speed reducers (or gearboxes) effectively convert some of that speed to torque, enabling the power produced by the motor to be used efficiently. Speed reducers represent a significant portion of the cost of putting together a drive. It’s essential to know how to select a speed reducer for your electric motor application that maximizes performance and efficiency while reducing the total cost of ownership.

Speed and Torque

The first thing you need to do to select the right speed reducer is to calculate the ratio required, which is the difference between the motor speed and the speed needed for the load. Once you have done this, check that the gearbox can withstand the torque produced by the motor (at the input) and the resistance of the load (at the output).

A speed reducer's thermal horsepower rating is the horsepower that it can transmit during three or more hours of constant usage without overheating. Overheating quickly deteriorates the lubrication inside, leading to damaged gears and expensive parts replacement. To ensure long and trouble-free service life, it’s essential to consider the torque capability of the speed reducer and its thermal horsepower rating and service factor.

Different types of speed reducers have different ratios and torque capabilities. Generally, planetary and helical gear reducers offer the best performance for high-torque, high-efficiency power transmission. In contrast, bevel gear and spur gear reducers are cheaper and suitable for lower ratio, lower torque applications.

Size and Configuration

Every electric motor drive application is different, and it is important to take into account the amount of space available for the speed reducer and the physical configuration of the motor and drive elements.

Speed reducers come in three basic configurations: coaxial, parallel shaft, and right-angled. Coaxial reducers, which are often spur gear and planetary gear types, have the output shaft aligned on the same axis as the input (motor) shaft. Parallel shaft reducers, usually spur gear and helical gear reducers, have the input and output shafts in the same direction but are offset by some distance. Right-angled reducers, such as bevel gear and worm gear reducers, transmit the power of the motor at right angles, changing the direction of the shaft through 90 degrees. The choice you make between these configurations depends on the layout and mounting position of the electric motor and the load.

It is also essential to take into account the size. Some applications may have very little space to work with, and in this case, a highly compact gearbox such as a planetary or worm gear reducer may be required. Compact gearboxes tend to overheat more easily, so check the thermal horsepower rating first.

In some cases, the reducer may be required to operate at an unusual angle, which can impact the capability of the lubrication to do its job and potentially make in-place maintenance difficult. Consult with your motor supplier regarding the suitability of a product for these cases.


As with electric motors, the maintenance requirements of a speed reducer will greatly impact its total cost of ownership.

Some types of reducers require more difficult maintenance. For example, planetary gear reducers are usually more complex than bevel gear reducers and will require more time and skill to disassemble and service. This potential downtime must be factored in at the planning stage.

Most reducers are designed for clean, non-abrasive, moderate temperature environments. Applications operating in more extreme conditions will require extra care and consideration to prevent excessive maintenance and parts replacement costs. Hygiene-sensitive applications such as food processing will require washdown capability, and humid or dusty environments will require reducers with sufficient ingress protection. Consult with your supplier if you are unsure of the suitability of a speed reducer for your application.


Calculating the total cost of ownership of a speed reducer means considering the initial cost and the cost of maintenance and parts replacement. A good speed reducer will often give 20+ years of service, but one that does not match the torque requirements, duty cycle, and operating conditions and characteristics of the application may last only a short period of time before it needs to be replaced. If you are frequently replacing your speed reducer, consult with your motor supplier to find better long term solutions.

The different types of reducers vary in capability and cost. Planetary reducers, with their high torque capability, high power transmission efficiency, compactness and low backlash, are typically expensive. In contrast, worm gear reducers, with a more straightforward design and lower transmission efficiency, tend to be relatively economical. Each type of reducer has its unique strengths and weaknesses that do not necessarily show up in price, so first make sure a speed reducer matches your requirements, and then factor in your budget.


Speed reducers have a significant impact on both the performance and the total cost of ownership of your drive application. Knowing how to select a speed reducer for your electric motor will save you a lot of money and hassle in the long run. At eMotors Direct, we understand how tough the decision can be, which is why we have created the Custom Gearbox Builder tool to help you put together a drive that meets all your needs from our extensive range of electric motors and speed reducers.