There are various types of couplings and backstops used in mechanical power transmission systems, each with their own features and benefits. Here are some common types:

1.​Rigid Couplings: Rigid couplings are used when shaft alignment is precise andthere is no need for compensation. They are typically made of metal and provide a solid, inflexible connection between two shafts.

2.​Flexible Couplings: Flexible couplings are used when there is slightmisalignment between the shafts. They are designed to compensate for misalignment, absorb shock and vibration, and protect machinery from overload. Common types of flexible couplings include jaw couplings, gear couplings, and disc couplings.

3.​Fluid Couplings: Fluid couplings are used to smoothly transmit power betweentwo shafts. They use hydraulic fluid to transmit torque from one shaft to the other, andcan also provide some degree of misalignment compensation.

4.​Sprag Clutches: Sprag clutches are a type of backstop used to prevent thereverse rotation of a machine or equipment. They consist of a cylindrical outer race andan inner race with sprags (small angled teeth) that engage in one direction but allow freerotation in the opposite direction.

5.​Roller Clutches: Roller clutches are another type of backstop that use rollersinstead of sprags to prevent reverse rotation. They are typically used in high-speedapplications and can handle high torque loads.

6.​Ratchet and Pawl Mechanisms: Ratchet and pawl mechanisms are used toprevent reverse rotation in low-speed applications. They consist of a ratchet (a toothedwheel) and a pawl (a spring-loaded lever) that engage in one direction but allow freerotation in the opposite direction.

Overall, the choice of coupling or backstop will depend on the specific requirements of the application, such as the amount of misalignment that needs to be compensated for, the amount of torque that needs to be transmitted, and the speed and direction of rotation.