Shaft couplings connect one shaft to another shaft, such as the drive shaft of a motor to the shaft that turns the gears. Many different types of shaft couplings are used for making these types of connections. The shaft coupling makes the two shafts rotate without allowing one shaft to backlash. The use of couplings to connect shafts comes with its disadvantages.

Misalignment

One of the disadvantages of using a shaft coupling is the possibility and effects of misaligning the shafts, then attaching a coupling. The stiff or rigid coupling damages the components more than other types of couplings when misalignment occurs, but all couplings will cause some damage. Even a slight misalignment will create excessive force on the coupling, shaft and bearings, causing them to wear prematurely.

Vibration

Shafts can vibrate during rotation. The vibration can loosen the shaft coupling preventing the rotation between shafts from remaining constant. A loosened shaft coupling will cause one shaft to turn faster than the other creating other problems, such as worn components or slow gear speed. If you can determine that misalignment and excessive vibration will occur between shafts, it is best to use a flexible coupling. The use of a flexible coupling will come with other disadvantages.

Velocity

The use of flexible couplings, such as a leaf spring coupling, can cause a loss of velocity between shafts. The loss of velocity will influence the position of the shafts. Along with a loss of velocity, the shaft vibration will increase, causing the premature wearing of bearings and gears. You will also get more drive noise when the shafts are rotating.

Backlash

Many times mechanics will use a correcting coupling if they anticipate misalignment or excessive vibration. The correcting coupling works fine to decrease the damage of these shaft problems but over time will create more problems. New correcting couplings have gaps that will correct any misalignment and vibration between shafts, but over time, these gaps will increase. As the gaps increase, more backlash develops between shafts. The backlash places more pressure and torque on the couplings, as well as the motor, bearings and gears. You will begin to hear a loud slapping noise when this backlash problem develops.

Loosening

All types of couplings can loosen over their work life. The couplings are a wearable component and slowly loosen under normal operating conditions. Once the coupling begins to loosen, the velocity, drive speed, alignment and vibration between shafts will be harmed or damaged. All of these problems create major damage to the other components. A good periodic maintenance inspection can catch this disadvantage of using shaft couplings before the problem develops into an extreme defect.