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5 Common Classes of Girth Gear Potential Failure Modes

Tooth Fracture is generally considered an ultimate failure of the gear set as is often results in the inability to rotate the set.

Here are 5 classes of girth gear potential failure modes that can be detected and mitigated with routine inspections:

Wear-Abrasion

Abrasive wear is the removal or displacement of material along the tooth flank due to the presence of hard particles. These particles can come from external sources such as slurry, dust, contamination, or may be self-generated hard metal fragments from other failure modes such as macropitting or scuffing.

The tooth profile is most affected resulting in poor mesh action and localized overloading.

Note: Often severe wear will progress to macropitting as contact stresses increase from localized overloading

Scuffing

Scuffing is a severe form of adhesive wear caused by the transfer of material from one tooth surface to another due to microwelding and tearing.

Scuffing occurs when the lubricant film cannot completely separate the metal surfaces. Machining marks are removed and deeper craters are observed. In the severe form surface temperatures may become significantly elevated causing localized metallurgical changes to occur. Proper lubricant usage and application is the best prevention.

 

Note: Severe scuffing can initiate flank cracks

Plastic Deformation (Indentation)

Indention is caused by hard foreign material that becomes trapped between mating teeth, causing an indentation in the tooth surface. The area around the indentations is typically raised from plastic deformation of the metal.

Note: Severe scuffing can initiate flank cracks

Plastic Deformation (Cold Flow, Tip to Root Interference, Tight Mesh)

The topland of this gear is completely rounded from plastic deformation where the gear is in tight mesh with the pinion and experiencing tip to root interference.

Note: Significant edge burrs have developed form the tight mesh condition

Hertzian Fatigue (Spalling)

Spalling occurs when macropits form, then grow is size and coalesce into larger cavities on the gear tooth surface. Asperities and high pressures from variations in manufacturing tolerances. This form of macropitting quickly stops once the load redistributes. The image adjacent is a gearbox gear, showing spalling across the entire face.

Note: Spalling can be easily identified from stop action photographs of pinion teeth. This is a severe failure mode which can quickly lead to tooth cracking and fracture.

by William Quinn William Quinn No Comments

Girth Gear Pinion Infrared Imaging and Temperature Measurements

Monitoring pinion temperatures on mill gear pinions is an effective and efficient means of monitoring alignment and operating condition. Pinion temperature measurement when applied properly can adequately predict multiple failure modes, including misalignment, overheating from a loss of lubricant event, overheating from improper lubricant, and temperature anomalies from severe contamination.

Girth Gear Pinion Infrared Imaging and Temperature Measurements

Pinion Locations shown on a pinion with misalignment

Mill Pinion IR Temperature Profile
AlarmShutdown
°F°C°F°C
Temp A to E1583017
Temp A to C*30174525
Temp E to C*30174525
Any temp (MAX)*20596225107

*Some mills depending on application may operate at higher temperatures than recommended in this chart, consult with a specialist if temperatures operate normally above 200 F


Infrared sensors can be installed that continuously sample the radiation given off by an object in the infrared spectrum, logic in the device then translates this data into a temperature reading displayed to the user. Alarms and interlocks can be set up as an automated protective system. In order to obtain an accurate infrared reading the measured objects effective emissivity must be known. Effective emissivity will vary based on an objects material, color, surface (shinny or dull), geometry, and in certain cases temperature. When applying infrared technology to mill gear pinions the user must be aware of these factors which may affect readings, and understand the proper calibration of the temperature sensors.

Pinion Temperature Profiles

Example Pinion Temperature Profiles

Girth Gear Pinion Infrared basics

The infrared alignment technique should only be applied to mill gear pinions not kiln or dryer gears. Kiln and drier systems typically run slow and do not develop sufficient temperatures from the mesh forces and can be significantly affected by heat transfer from the kiln or drum.  However valuable information can be deduced from monitoring the mesh temperature on these systems.

Pinion misalignment is a common failure cause which can lead to a variety of gear tooth failure modes through overloading. Measuring temperature differentials from end to end across an operating pinion is an indirect measurement of the misalignment. When a gear is misaligned in operation the temperature profile will shift to the side correlating with the increase in load in that area.

Overall pinion temperatures can also indicate an issue related to lubrication including lack of lubricant. A typical alarm and shutdown chart is provided below. In rare cases the normal operating end to center temperature may be elevated above 20 °C in this event consultation with a mill gearing expert should be done to ensure damage is not done from running the set.

When an array of online infrared sensors are used the following chart should dictate the alarm and interlock settings. Please consult with a mill gear specialist before setting alarms and interlocks as certain operating conditions may require adjustments to these general alarm levels.

by Tom Shumka Tom Shumka

Benefits of ASTM E2905 vs Ultrasonics

Ultrasonic Inspection is very slow and has a “blind” spot between the probe and 3mm into the surface. Ultrasonics can miss cracks. However, it will provide information deep into the gear cast.

An argument can be made on the relevance of finding casting imperfections:

  • Ultrasonic inspection would detect porosity and inclusions in the cast but it is harder to detect surface cracks unless they propagate to the surface from the inclusion. It’s important to understand that porosity and inclusions found in the cast that have not propagated to the surface cannot be repaired.
  • ASTM E2905 detects 100% of all gear teeth surface indications larger than .40mm.
  • The reason why casting imperfections are not as important is because there is no way to repair them.
  • It’s only when the casting imperfection creates a crack that propagates to the surface that it becomes a problem. These are on cast gears only.
  • Any casting imperfections should be identified by the manufacturer’s Quality Control Program before the gear leaves the plant.

by Tom Shumka Tom Shumka

ASTM E2905 vs Eddy Current Method

The advantage of inspecting mill gear drives utilizing ASTM E2905 over Eddy Current, or for that matter, all other methods of NDT for gearing applications, is recorded data is utilized. ASTM E2905 is cleaner, faster, documentable, and covers a larger area in less examination time better than any other traditional Non-Destructive Testing methods today for gearing applications. Drastically reduces inspection time. Covers a large area in one single pass. Provides real-time mapping of the inspected region, facilitating data interpretation. Improves reliability and Probability of Detection (POD). Unlike some other inspection methods, this will size cracks accurately, and all of the data is electronically archived for future reference in C- Scan imagery.

The main advantage of E2905 are the 2 and 3D displays that are produced. With these types of displays, it is possible to characterize the defect.

Eddy Current (Disadvantages)

For example, Eddy-Current gives us only a phase angle and amplitude signal that we must interpret and thus make a judgment call on the type of defect. This judgment call relies solely on the inspector. See Figure 1.

  • Single Eddy Current Coil Must Be Moved across the addendum, dedendum and root.
  • Difficult to achieve full coverage of gear flank and root. Discontinuities can be missed. Figure 2
  • 15 to 20 Hour Time Frame.
  • Non-compliance with ASTM E2905.
  • Hand Written Reports

ASTM E2905 (Advantages):

  • With 2 and 3D Dimensional Isometric Displays, we can see the defects as it is in the material.
  • Knowledge of the defect type helps in determining the root causes and eliminates the potential for errors.
  • The system will display the actual characteristics of the defects, (see Figure 3), this is a Pit.

Figure 3: E2905 illustrating a Pit on a gear flanks

  • Using E2905, it is very easy to see a crack or a pit on a gear tooth compared to interpreting the Eddy Current signal in Figure 5.
  • In other words a pit looks like a pit (see Figure 3) and a crack looks like a crack (see Figure 4).
  • E2905 is used in this configuration and has consistently shown the ability to discriminate between dents, pits and cracks.

Figure 4: E2905 illustrating a crack on a gear flank

by Tom Shumka Tom Shumka

ASTM E2905 vs Magnetic Particle & Dye Penetrant Examinations

Benefits of ASTM E2905 over Magnetic Particle and Dye Penetrant for Cleaning and Inspections of Mill Gear Drives

  • ASTM E2905 – Standard Practice for Cleaning and Examination of Mill Girth Gear Teeth – Electromagnetic Methods.
  • AGMA 919-1-A14, Condition Monitoring and Diagnostics of Gear Units and Open Gears, recognizes E2905 as an acceptable inspection method.
  • Cleaning times on a 10m girth gear is under 1 hour. No need to wipe gear teeth after cleaning, saving numerous person hours.
  • Inspection time on a 10m, 406 tooth gear set is 7 hours.
  • E2905 is cleaner, faster, documentable, and covers a larger area in less examination time and greatly improves Probability of Detection better than any other traditional Non-Destructive Testing methods today for gearing applications.
  • Covers a large area in one single pass. Provides real-time mapping of the inspected region, facilitating data interpretation. Improves reliability and Probability of Detection.
  • Complies with Insurance companies requirements and could reduce insurance premiums.
Preparation / Inspection TimesMagnetic Particle / Dye PenetrantE2905
Cleaning10-12 hours1 hour
Inspection20-30 hours8 hours
ReportPaperFull Electronic Backup
Crack SizingNoYes
Total Time for Inspection25 – 35 hours8 hours
Compliance with ASTM E2905NoYes

Much faster than traditional methods.

Provides more production revenue than using traditional methods.

All data is recorded electronically; not on paper as in Magnetic Particle or Dye Penetrant.