Selecting and Sizing Your Motor (and gearboxes)

Choosing a motor is an essential task in developing robots. In this post, we will focus on how to measure your engine and choose the right gear.

The first step

The first step in choosing a machine is to determine the operating conditions and the maximum conditions that the device will see.

Robots / tables activated

We need to know the mass of the load/plow to determine the engine selection torque. Taking mass estimation (or even better, actual mass) is very important for machine selection. If you plan based on mass estimates, you should use a good range for mass increase. The torque constant can be found on the motor datasheet to find out how much torque you will make per ampere.


After determining your needed power/torque, what should follow is to estimate the required wheel speed. You must first determine the desired wheel speed (i.e. ultimate power) and then start choosing the motor and gear. With mini servos providing the best actuators, total efficiency can be obtained.

Form factor

Once you know the efficiency of the motor that you need, make sure the motor stack (encoder + breaker + motor + gearbox) matches your robot and can be neatly incorporated into your robot.


With which operating voltage can you operate the motor? Usually, the higher the voltage, the higher the engine speed. Voltage constants can be found on the motor datasheet to find out how fast you will move per volts.


This is often not a problem, but if your motor is in a closed space, you want to make sure overheating doesn’t occur. You should also think about the gearbox lubrication temperature range (see below for more information).


Sometimes it’s enticing to make a gearbox from scratch because it’s cheaper. However, giving the needed time to design, assemble, and test the new gear, it is often less expensive that buying the gearbox. You also get extra reliability when you buy a gearbox from a recognized company.

Precision and accuracy

What is the slush limit you take in your gearing? Most times, you can control a small degree of precision and accuracy in wheel motors. However, in a robotic arm or tool, you often need a precise and accurate low clearance system.

Wheel Diameter

The latter is essential to determine the torque needed for the driving motor. Remember, the bigger the wheel, the longer the distance covered per wheel rotation.

Determine The Torque Needed

Generally, determining the required output torque is simple, so is working reverse to find your gearing and motor. Ignoring inertia which is a torque limiting factor required in some cases. Conservation ( in most cases) can be achieved by ignoring inertia. In almost all cases, you need to add a factor of safety that exceeds your calculated value.

To determine the torque needed for your robot, there are several different approaches. The topmost is “Red’s rule”, which states that any drag on the rim (how many wheels can pull) must be enough to pull the whole robot. Although this allows the robot to work when several actuators are damaged (or skidded), it is also possible to tear the robot when each wheel is pulled in a different direction. Another problem with this approach is that even if your engine can apply so much torque, often there isn’t enough friction for the wheels to grasp and work.

The second approach is to build robots with sufficient torque to climb walls. This simulates the emergence of extreme tilt and level of obstacles with a smaller potential hazard for robots and without completely going overboard and excesive mobility system designing. Making an assumption that;


The general case for calculating the torque per wheel needed by a robot is:

Simplification to obtain holding torque per wheel on a flat surface is;

Following the Red’s rule, you won’t be dividing by the number of wheels.

Other aspects are not only slopes but also obstacles. To calculate this, you can simplify the problem by deriving the moment when constraints cross the robot’s wheel. If you want to simulate wall climbing, you can adjust the obstacle contact point to the wheel radius.

Based on your system, your choice can be either to use the slope method or the obstacle step method. A slope and holding torque on a flat surface will be the best fit for a field robot. For interior robots, you should consider the wheel intercept system for smaller obstacles.

Torque curve (and power curve)

To determine engine power (because some engine controllers have measurable output power), specify the point of the torque curve that you operate on, and that is your performance. So power = torque * speed.


Three types of gears are common in robots.


– 60-80% efficiency

– low reversal

– Suitable for low velocity and high-speed torque applications

– Gear reduction in height for its size

– Bad for shock loads


– Performance better than harmonics can exceed 90% in some cases.

– Cheaper than other equipment options

– The low reduction ratio of the gearbox

– Suitable for high-speed and low torque applications


– Greater form factor

– Suitable for surprises and vibrations

– Gear reduction in height for its size

– Efficiency can be up to 80%

In many cases, several types of gears are used in a system. For example, on a low-speed input harmonic drive, it is common to place a planetary drive stage between it and the motor to reduce the overall input speed.


There are two main types of gearbox lubricants which are:


– Simple standard options

– Can limit engine speed

– Set & Forget -no grease refilling

– Can keep dirt from the gearbox (must be sealed and not required)


– time to time inspection and refilling of oil level

– higher speed

– in comparison to grease, efficiency is reduced.

With all these options, you should ask the manufacturer for advice. Various types of grease are available to serve different purposes (such as heat).

Additional pointers

– Make sure the engine clutch to the gearbox is resistant to small leaks (eccentricities) in the motor shaft.

– Consider transferring torque from the stack to the wheels. For example, a motor may only have one rod, whereas harmonic drives have a round plate with the position of several pins

– Check fasteners that connect the engine stack to the wheels. Often people choose straps that are not strong enough for the torque transmitted.

– Make sure your motor has a rear axle when trying to install brakes or encoder/motor splitters.

Installing an encoder and brake requires you to talk with your manufacturer about your options. You shouldn’t make assumptions.