How you select correct stepper motor for your robot?

Choosing a stepper motor for sale that is suitable for your task is one of the most important parts of planning a robotics project. The good news is that there are many types of motors from which to choose and, as the joke goes, the bad news is that there are many types of motors from which to choose.
To choose the electric motors that can fit for your project you should consider some important motor specifications:

Torque
Speed
Precision and Accuracy
Voltage
Cost
Form Factor
Torque is a measure of a motor’s ability to provide a “turning force”. In a robot, the motor torque is conveyed to a wheel or a lever, which then causes the robot to move or the lever to lift, push, or pull something. Torque is measured in terms of force times the perpendicular distance between the force and the point of rotation, i.e. the shaft of motor. It is usually given in terms of ounce-inches (oz-inch), gram-centimeters (gm-cm) or foot-pounds (ft-lbs). Ounce-inches (oz-in) is the most common.

In order to drive the robot, the motor torque must at a minimum overcome the external torque of the friction force acting on the radius of the wheel. Use the following equation to find required torque:

T = 8 x C x W x D

where:

T is the torque in oz-in
C is the coefficient of friction
W is the weight in lbs
D is the wheel diameter in inches
Friction varies from 0.001 to 0.03. As an example, for a C= 0.03, the minimum torque to move a 5 lb robot with 4 inch diameter wheels would be:

T = 8 x 0.03 x 5lb x 4in = 4.8 oz-in

A motor can maintain a constant speed only if the torque is greater than the combined forces in opposite of the robot movement. In case that the motor torque is smaller than the opposition torque, the motor will stop and may be damaged since the electrical energy cannot be converted into torque.

After determining how much force/torque you need, the next step is to determine the speed that the wheel needs to turn. Speed requirement is easier to estimate and depends on how fast your robot should run. DC motors run at speeds of thousands of RPMs with low torque but most robots required less speed compare to this. The output torque is much too low to move the robot. So, this is not suitable for driving a robot. 
Output Speed = Motor Speed / Gear Ratio

Output Accuracy = Motor Accuracy / Gear Ratio

Although the reduction ratio plays a large part in determining the Gearbox Output Torque, there is also an inefficiency that is introduced through the use of a gearbox. Some of the torque of the motor is converted into heat and lost due to friction between the gears. 

Some applications require high speed and light weight such as multi copter and drone, in that case high efficient brushless dc motor is used.

Another important consideration is operating voltage. Before planning what battery packs will be used in the project, you have to find the nominal voltage where the motor runs Typically the higher the voltage the higher the speed of the motor. You can look at the Voltage Constant from the motor data sheet to figure out how fast you will go per volt.

Most common electric motors used in robotics projects are the DC motors. Common preferred voltages for brushless DC motors for sale are 3, 6, 12 and 24 Volts. If to a motor is applied a voltage lower than the voltage listen in the data sheet, the torque will not overcome the internal friction – mostly from the brushes. Also, if a higher voltage than that supported is applied to the motor, it may heat up and can be damaged.

Article source: https://article-realm.com/article/Finance/4354-How-you-select-correct-stepper-motor-for-your-robot.html