What should I select first when I am designing a mechanical system, the motor or drive?

It is recommended that a motor be selected before the drive so that you can ensure the motor performance as stated on the datasheet. If you select a drive first, you may not have enough current as specified on the motor datasheet to meet the continuous torque requirement. As an example, say you selected a drive that supplies 6A of current. Then you select the Omni+ motor OPN060013A , with a continuous torque of 0.52Nm, which meets your system requirements. However, you later notice that that motor has a continuous current rating of 17A. This means that the continuous torque value of 0.52Nm was rated at 17A. If you only have 6A to supply to the motor, the continuous torque output will drop significantly. See the continuous torque calculation with limited current below.

Continuous Torque = Kt*Continuous Current
0.52 = 0.03 * 17
X = 0.03 * 6
X = 0.18 Nm

Click for information on Omni+ motor and Ingenia servo drive compatibility chart.