Introduction
Couplings are a component used to attach individual shafts together for the purpose of power transmission. They allow for multiple components to operate as a system and provide a means to simplify the disassembly process to isolate or replace single components. They are often problematic due to the complexity of aligning multiple components and also because they are typically covered by a guard and often ignored.
Coupling Types
Couplings come in a wide variety of types to fit the various applications. They are generally divided into 2 main categories which are rigid couplings and flexible couplings. Rigid couplings are used when precise alignment between the 2 shafts are required and there is little to no relative movement between the shafts. Flexible couplings are used when some relative motion between the shafts is required. Flexible couplings have a wider tolerance for alignment between shafts.
Coupling selection is a critical consideration to ensure the reliability of a rotating asset. This is to avoid premature coupling failure and also to ensure that the bearings of the components being joined are not over stressed. The main things to consider are:
- Maximum Torque Requirements
- Restoring Force - The force required to misalign the coupling
- Misalignment Tolerances
- If the coupling should be Torque limiting for protection
The table below shows some of the characteristics for the more common couplings:
As mentioned earlier, there is a wide range of couplings available in the market to fit the various applications to ensure the reliability of the system. If coupling degradation is a common failure mode for a properly aligned asset, it is likely the coupling selection is incorrect.
Detecting Coupling Degradation
One of the most effective ways of preventing and identifying coupling degradation is with vibration analysis. The vibration signature for coupling wear is similar to the signature for misalignment. It is also true that misalignment is the primary cause of coupling wear. This is due to the relative movement between the shafts as they rotate which creates a cyclic stress on the coupling.
Coupling wear is detected by an increase in vibration amplitudes at harmonics of the running frequency. When this increase starts to appear at 3x, 4x, and 5x the shaft speed, it’s usually an indication that the coupling is starting to fail. Below is an example of this signature:
Another effective method for identifying coupling wear is to use a stroboscope. A stroboscope emits a bright flash and has the functionality to adjust the timing of the flash to match the speed of the coupling rotation. This creates the appearance that the coupling is not rotating and allows for a visual inspection to be done. A link to a video showing this process is provided at the end of this article.
Root Causes of Coupling Wear
Misalignment is by far the most common root cause of coupling degradation. This is true for just about all couplings. In the case of a rigid coupling or a coupling with a high restoring force, the misalignment can also cause premature bearing failure. Additional root causes of coupling degradation are listed below:
- Overloading: Operating equipment beyond its design capacity can cause overloading of the coupling. This can lead to fatigue failure, shearing of fasteners, and deformation of coupling elements. Impact and Shock loads are a well known cause of this.
- Poor Lubrication: Inadequate lubrication of the coupling components can result in increased friction and wear. Lubrication is crucial for reducing frictional heat and preventing corrosion.
- Corrosion: Corrosion can occur due to exposure to moisture, chemicals, or environmental factors. It weakens the material and can lead to coupling degradation or even catastrophic failure.
- Material Fatigue: Repeated cyclic loading and unloading of the coupling can cause fatigue in the material, leading to cracks and failure over time.
- Excessive Vibrations: Vibrations in the equipment can accelerate coupling deterioration. High levels of vibration can cause fretting, fatigue, and stress concentration in coupling components.
- Thermal Cycling: Rapid temperature changes can result in thermal cycling of the coupling, leading to differential expansion and contraction of coupling elements. This can cause stress and distortion, ultimately leading to failure.
- Improper Installation: Incorrect installation practices, such as improper torqueing of fasteners or not aligning the coupling correctly, can introduce stress concentrations and lead to premature failure. One critical thing to consider here is a tight fit of the coupling on the shaft.
- Contamination: The presence of contaminants, such as dirt, debris, or foreign particles, within the coupling can cause abrasive wear and damage to coupling surfaces. This is specifically critical for gear type couplings.
- Aging: Over time, materials can degrade and lose their original properties. Aging can make coupling components more susceptible to failure. This is specifically true for couplings that use non metallic inserts.
Summary
Couplings are a relatively simple component that are often problematic. The process of aligning separate shafts and installing a coupling requires a strong understanding of geometric tolerances and an attention to detail which can be difficult to obtain in the field. These aspects along with guarding which often makes coupling inspections difficult are the primary reason couplings fail.
Condition monitoring virtually eliminates this by providing continuous data for most of the variables that cause a coupling to wear. By monitoring both the vibration and temperature of the bearings next to the coupling, the most common coupling degradation root causes can be identified and corrected resulting in more reliable assets and reduced maintenance costs.
Coupling Visual Inspection Video 👇
