Case Study: Troubleshooting Vibration Issues at a Quarry Plant
The Challenge: Uncontrolled Vibration Disrupting Operations
A recycling and sieving plant was facing significant vibration issues during operation, specifically with structural vibration. The vibrations were so intense that workers struggled to hold onto railings for support, endangering their safety and disrupting operations.
Despite attempts by the plant’s manufacturer to stiffen the structure, the vibration persisted, leaving the plant’s management frustrated. The client even threatened to withhold payment unless the issue was resolved.
Initial Observations: Identifying the Root Cause
Upon visiting the site, we immediately noticed a few key issues:
Severe vibration: The screens' operation caused widespread vibration across the plant. As off-balance motors slowed down, they triggered the plant's natural frequencies, creating visible resonance (mode shapes), which was clearly observable to the naked eye.
Structural concerns: The plant's frame lacked diagonal stiffness beams, which could have helped reduce vibration. It was possible that the system was operating at one of its natural frequencies, leading to excessive oscillation.
Inadequate vibration isolation: The anti-vibration (AV) mounts, which were supposed to isolate the screens from the structure, were not functioning as expected.
Our Approach: Comprehensive Troubleshooting
We employed a multifaceted approach to pinpoint the exact causes of the vibration and develop an effective solution.
Finite Element Analysis (FEA):
We conducted a detailed FEA of the structure to identify the natural frequencies and mode shapes. This analysis provided insights into how the structure was responding to the operating frequencies of the screens, when the off-balance motors coasted to a halt, when switched off.
Experimental Modal Analysis (EMA):
On-site, we performed EMA to confirm the natural frequencies and to visually observe how the structure responded under various operational conditions.
Measurement of Anti-Vibration Mounts:
We measured the vibration transmissiblity across the AV mounts, which are designed to isolate the screens from the main structure. Our data revealed that the mounts were not performing as expected.
Key Findings: Identifying the Issues
The root cause of the excessive vibration was traced to two key factors:
Inappropriate AV mounts:
The AV mounts installed on the system were not as specified in the data sheet. These mounts were copies of a well-known brand's product (Firestone donut isolators), but they did not have the same material characteristics. This mismatch in stiffness led to poor isolation.
Insufficient isolation at high displacement:
The displacement or "throw" of the screens was too large for the mounts to absorb effectively. At high displacements, the mounts behaved more like bump stops (non-linear characteristics) rather than providing resilient isolation. This caused the mounts to couple directly with the structure, transmitting vibrations into the structure, rather than absorbing them.
The Solution: Custom Anti-Vibration Mounts
To resolve this issue, we recommended a two-pronged approach:
Redesigning the AV mounts:
Given the large displacement of the screens, and a possible requiremnt of increasing the "throw", we specified new AV mounts that were better suited to the operating conditions. These mounts were designed to handle the greater throw of the screens, ensuring effective vibration isolation, and decoupling the motion from the structure.
Testing and validation:
We conducted spring test bench testing to confirm the new AV mounts’ stiffness characteristics. Unlike the previous mounts, the new design provided the required resilience and was able to absorb the high displacement without transferring vibration to the structure.
The Outcome: Eliminating Vibration and Restoring Stability
By specifying the appropriate anti-vibration mounts and ensuring they were properly matched to the equipment’s operating conditions, we successfully eliminated the vibration issues. The plant was able to operate smoothly and as intended, the workers regained their ability to safely hold onto the railings, and the end customer was satisfied with the results.
The solution not only resolved the immediate vibration issue but also ensured the longevity and reliability of the plant’s equipment, restoring the manufacturer's confidence and securing payment from the customer.
Conclusion: The Importance of Proper Vibration Isolation
This vibration troubleshooting case highlights the critical role that vibration isolation plays in industrial equipment performance. Properly specified AV mounts, suited to the specific operational characteristics of the equipment, are essential for avoiding disruptive vibrations and ensuring the safety and efficiency of plant operations. By thoroughly analysing the issue and applying targeted engineering solutions, we were able to resolve a potentially costly problem for our client.
Engineering tools and methods used to resolve this vibration issue:
Root cause analysis - troubleshooting!
Finite element analysis - vibration analysis - stress analysis
Experimental modal analysis
Structural dynamics
Eigenvalues and eigenvectors