What device is used to connect two short shafts into a single long shaft?

The correct answer is that a rigid coupling is designed to connect two short shafts into a single long shaft. Rigid couplings are typically used in situations where alignment between the two shafts is precise and where torque transfer needs to be direct and strong. They effectively transmit power and rotational motion without any play or flexibility.

When using a rigid coupling, there are a few key advantages. First, it provides excellent torque transmission, making it suitable for applications where high speed and torque are involved. Furthermore, they ensure a solid connection that helps maintain the alignment necessary for efficient operation. This is particularly important in machinery applications where even slight misalignments can lead to increased wear or mechanical failure.

In contrast, other options serve different purposes. Universal joints, for instance, allow for angular movement between two shafts, accommodating misalignment but not suitable for rigid connections. Flexible couplings provide some degree of flexibility to compensate for misalignment and vibration but do not create a solid connection like a rigid coupling does. An adapter sleeve is typically used for mounting shafts or bearings rather than connecting two shafts directly.