Augury specifies two types of Looseness in their fault cards: Structural Mechanical Looseness and Rotating Mechanical Looseness.
The structure of a rotating asset plays a vital role in determining the reliability and life span of the asset. It is critical to identify and correct any type of faults that exist with the structure to prevent small faults from maturing into larger faults which ultimately lead to the failure of the equipment.
One of the most common faults found on a structure is looseness. Structural looseness refers to excessive movement or play between structural components, leading to abnormal motion and the potential to overstress the various components in the system. Structural looseness is a fault that is typically excited by a separate forcing fault, such as imbalance or resonance. Some examples of structural looseness are:
- Cracked or broken frame
- Loose mounting bolts
- Loose baseplate
- Damaged or inadequate grout
- Loose anchor bolts
- Soft foot
- Excessive flexibility of machinery support structure
Its vibration signature is high vibration at 1x the running speed, typically in either the vertical or horizontal direction. This vibration is created by the motion of the equipment rocking or traveling back and forth at the frequency of rotation. The direction of vibration will be based on the degree of freedom created by the looseness. Another distinguishing trait of the signature is that the axis with looseness will be much higher than the other 2 axis. This trait uniquely identifies looseness from an imbalance or resonance fault.
One important thing to consider is that the vibration signature changes when the structural looseness is isolated to the mounting of the bearing itself. Some examples of this scenario are:
- Loose bearing mount bolts
- Cracked pillow block housing
- Crack in bearing mount pedestal
The main identifying trait of this signature is elevated amplitudes at 2x the running speed which are often higher than both 1x and 3x. Below is an example of the vibration signature. This can be helpful when trying to identify the specific location of the structural looseness.
In addition to vibration analysis, one of the most effective tools for pinpointing and correcting structural looseness is motion amplification. This technique can remove the guesswork on remediation and greatly reduce the time and cost of fault correction.
Occasionally the points of interest are guarded and difficult to obtain visibility with the camera. In this scenario, the best method to correct this fault is to check bolt torques and inspect for excess clearances in the bearing housing. It may also be necessary to check for cracks on the various components with a dye penetrant test.
Structural looseness is less likely to exist as a fault on its own due to the forces required to create the fault. For this reason, the root cause of structural looseness is often tied to an unaddressed fault like imbalance or resonance and the best preventative action is to correct these faults.
There will always be some level of vibration faults in all rotating equipment. Modification to the structure is only recommended when the structure is not capable of handling the forces generated in an asset that is considered to have healthy vibration levels. The table below shows some common root causes of Structural looseness for an otherwise healthy machine.
Insufficient Tightening Torque
- Use calibrated torque wrenches
- Follow the manufacturer's recommended torque specifications
Loosening from Vibration
- Apply thread-lock (Loctite) to fasteners
- Use lock nuts where possible
- Consider using vibration-isolating pads
Thermal Expansion and Contraction
- Schedule preventive torque checks when equipment experiences large temperature swings
Improper Design or Fabrication
- Ensure proper alignment and fit of components during the fabrication and install / commissioning
- Take baseline readings during the commissioning
Fatigue and Stress Concentration
- Use fatigue-resistant materials in critical areas
- Perform stress analysis to identify high-stress points
- Perform NDT inspections for cracks for known failure points
Inadequate Support or Foundation
- Use epoxied anchor bolts
- Consider using vibration-isolating pads
Eliminating structural looseness is critical to ensure the reliability and maximize the lifecycle of all rotating assets. It is important to remember that structural looseness is often the result of additional underlying faults but can also exist alone due to poor design, installation, or repair.
The best method for eliminating structural looseness is to continuously monitor for vibration faults and address them when present. Motion amplification should also be used when possible to help pinpoint the problematic location.
Addressing these faults early as they appear is critical to eliminate any impacting stress that greatly accelerates the severity of the fault. Doing so will ensure asset uptime and guarantee the longevity of the asset's useful life.