1. Selecting the coupling too late in the design process
Couplings are often selected extremely late in the application design process, without meeting the complex requirements of the system. By considering couplings early on in the design process, each criteria can be considered individually, ensuring that the coupling chosen is suitable for the functions required.
2. Selecting the wrong type of coupling for the application
Several criteria must be considered when deciding on a type of coupling, including the type of application, torque, misalignment, stiffness, inertia, RPM, shaft mounting, environmental factors, space limitations, service factors, and cost. Each criteria must be individually considered to ensure that the coupling will be suitable for the application and not result in premature failure. This process of evaluation must also be repeated for any change in conditions throughout the application’s lifecycle.
3. Selecting the wrong type of coupling for the application misalignment conditions
An essential consideration when selecting a coupling is the misalignment conditions of the application. This may be angular, parallel or axial, or a combination of more than one misalignment (complex misalignment). Flexible couplings should be considered in these conditions, although the type will depend on the type of misalignment present. For example, an oldham coupling is suitable for large amounts of parallel misalignment, but cannot tolerate a high level of angular or axial misalignment, whilst a single beam coupling can withstand large amounts of angular and axial misalignment, but not parallel.
4. Failure to correct excessive misalignment
Even flexible couplings which are designed for use on misaligned shafts have their limits. A common point of failure is the under-estimation of the degree of misalignment, creating loads that surpass the coupling specifications. This causes the coupling to wear at an accelerated rate, and has the potential to cause other components, such as bearings, to also fail prematurely. Where misalignment exists beyond the manufacturer specifications for the coupling, this should first be rectified with shaft realignment before installing the coupling.
5. Selecting the wrong coupling for the torque in the application
The torque of an application is frequently under-estimated. The maximum instantaneous torque for the application needs to be considered, in addition to the steady state torque. Flexible couplings have different static torque ratings depending on the design type. For example, a double disc coupling will typically offer a 15-20% higher static torque rating than an identically sized Oldham coupling with an acetal disc.
6. Lack of consideration for windup
Windup is also known as torsional compliance or torsional rigidity, and is present in all couplings. It is the rotational deflection between the driver and the load, similar to winding up a spring. The most significant problem with this is maintaining accuracy of location due to a difference in angular displacement from one end of the coupling to the other.
7. Lack of consideration for backlash
Backlash is the loss of motion momentarily in a coupling. For example, when torsion is applied in one direction, the coupling bends and compresses under that stress. When the direction of torsion is changed, backlash is experience within the coupling. Any amount of backlash in a motion control application could be detrimental to the application, potentially causing lack of accuracy in positioning, and difficulty in tuning the system. Zero backlash couplings should be considered in these scenarios.
8. Selecting a coupling with the wrong amount of dampening
Dampening refers to the minimisation of shock and vibration and is particularly important in motion control and power transmission applications to reduce the waste of energy and the unnecessary stress on system components. Shock dampening helps to reduce the effects of impulse loads, minimising shock to the motor and other sensitive equipment. The potential for premature coupling failure can be accelerated when the selection of coupling type does not fully take into consideration the dampening levels required.
