Couplings are used to connect a drive shaft and a driven part.
For example, a motor shaft and a ballscrew for the purpose of transmitting the torque. Also couplings introduce mechanical flexibility providing misalignment for the shafts. As a result the coupling flexibility can prevent uneven wear, vibration, and other mechanical troubles due to misalignment. The main roles of a coupling include:

• Connecting drive shafts of motors, etc. with driven shafts of ballscrews etc. to transmit power
• Provide tolerance for errors (misalignment) between the cores of the drive shaft and the driven shaft
• Absorbing equipment impacts and vibration
• Improving equipment performance