Gear Damage and Its Causes From Caterpillar - Gear Reuse Guidelines_SEBF8193

SEBF8193-03 April 2004 SMCS Code: 4050

GUIDELINE FOR REUSABLE PARTS AND SALVAGE OPERATIONS Reusability of Drive Train Gears

Abnormal Wear.....................................................27

Table of Contents Introduction .............................................................3

Frosting .............................................................27

References .............................................................3

Pitting ................................................................30 Corrosion / Corrosive Pitting .........................33 Destructive Pitting .........................................39

Tooling and Equipment ...........................................3

Spalling .............................................................43

Gear Nomenclature ................................................4

Pitting/Spalling ..................................................49

Glossary of Terms...................................................4

Scoring ..............................................................53

Gear Types..........................................................5

Case Crushing ..................................................61

Case hardened surface ..........................................7

Abrasive Wear...................................................65

Machine Application, Operation and Maintenance ...........................................................7

Nicks .................................................................69

Use of this Guideline...............................................3

Methods for Minimizing Gear Failures ....................8 Operational Characteristics of Gears......................9 Caterpillar Genuine Gears...................................9 Competitive Gears...............................................9 You should install only Caterpillar genuine gears into Caterpillar equipment.Metallurgy........9

Cracks ...............................................................71 Chipping ............................................................74 Uneven Contact ................................................77 Foreign Object Damage ....................................79 Rippling .............................................................87 Lipping...............................................................88

Case Hardened Surface......................................9

Gear Bores ...........................................................89

Rolling/Sliding Action...........................................9

Planet Gear Bores.............................................89

LPSTC/HPSTC..................................................10

Gears with Replaceable Bearing Races ...........97

Load/Contact Velocity .......................................10

Spider Gear Bores ............................................99

Lubrication Thickness........................................11

Splines................................................................100

Cleaning................................................................11

Thrust Faces.......................................................101

Inspection Procedure ............................................12

Notes ..................................................................103

Gear Inspection Process ...................................12 Crack Inspection Methods.................................14 Liquid Fluorescent Dye Penetrant Method ....14 Liquid Non-Fluorescent Penetrant Method....14 Dry Magnetic Particle Method .......................15 SOS Testing .........................................................15 Reconditioning ......................................................15 Marking Code Procedure ......................................16 Machining .............................................................17 Rough Machining ..............................................19 Finish Machining ...............................................20 Normal Wear.........................................................23 White Layer Flaking...........................................24 High Hour Wear.................................................26 SEBF8193-03

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Table 1 Summary of Revisions Page

When replacement parts are required for this product, Caterpillar recommends using Caterpillar replacement parts or parts with equivalent specifications including, but not limited to, physical dimensions, type, strength and material.

Description

All

Revised Format

All

Included drive train gears for all machines

100

Added Splines

101

Added Thrust Faces

Failure to heed this warning can lead to premature failures, product damage, personal injury or death.

Introduction This Guideline enables dealers and their customers to benefit from cost reductions made possible through an established parts reusability and salvage program. Every effort has been made to provide the most current and relevant information known to Caterpillar Inc. Since the Company makes ongoing changes and improvements to its products, this Guideline must be used with the latest technical information available from Caterpillar to ensure such changes and improvements are incorporated where applicable. You may also refer to “Analyzing Gear Failures”, Form SEBF8045.

References Table 2 References Form No.

Use of this Guideline This Guideline contains information, which can help to visually determine the reusability of drive train gears. This Guideline also contains the operational characteristics of drive train gears. The information that is presented in this guideline will help to explain the cause and progression of a failure. Never install a part that is shown in this Guideline as a part that cannot be used again. This Guideline can help identify the gears that can or cannot be used again, by illustrating the typical modes of failure. This Guideline cannot guarantee that any individual gear can be reused. There is always some risk of failure. Judgment based on experience and careful, complete inspection is the best guide.

SEBF8193-03

Title

SEBF8185

Salvage Procedure for Final Drive Planet Gears Used in Off-Highway Trucks

SEBF8187

Standardized Parts Marking Procedures

SEBF8148

General Salvage and Reconditioning Techniques

NENG2500

Caterpillar Tool and Shop Products Guide

SEBF8029

Index for all Reuse and Salvage guidelines

Tooling and Equipment Table 3 Tooling and Equipment Part No.

3

Description

⎯

Bright Incandescent Light

⎯

Small Flashlight

170-5903

Gear Inspection Stand

4C-4735

Magnetic Particle Test Kit (magnetic yoke, magnetic particle powders, and cleaner / remover fluid)

1U-5566 1U-6444

Black Light, 110 Volt or Black Light, 220 Volt

146-1738

Borescope

8S-2257

Loupe (magnifying glass)

154-1296

Service Technicians Aerosol Kit

9U-7377

Metal Etching Pen

8T-7765

Scotch Brite Pad

6V-2010

Polishing Stone

Gear Nomenclature

Glossary of Terms Profile: A gear tooth profile is known as the shape or outline of the gear tooth as it is seen from the side of the gear. Addendum: The addendum is the section of tooth that is above the pitch line. Pitch Line: The pitch line is located between the HPSTC and the LPSTC where there is no sliding action between the two mating gear teeth surfaces. Crown: A gear tooth crown is the outline that can be seen from the top of the tooth. You will be able to notice the slight convex or round outline (see item 23, Illustration 1). The slightly rounded shape of the tooth crown compensates for minor gear tooth misalignment. The crown can also help distribute the load from the center of the tooth outward for more efficiency. Dedendum: The Dedendum is the section of the gear tooth that is below the pitch line. HPSTC: Highest Point of Single Tooth Contact. LPSTC: Lowest Point of Single Tooth Contact.

Illustration 1 This illustration shows the key elements of a gear. The elements in this illustration are identified in the Chart A (Gear Tooth Nomenclature). Table 4 Chart A Gear Tooth Nomenclature Item

Description

12

End

13

Tip

14

Tip Land

15

Edge

16

Tooth Face

17

HPSTC (Highest Point of Single Tooth Contact)

18

Pitch Line

19

LPSTC (Lowest Point of Single Tooth Contact)

20

Fillet

21

Root

22

Start of Active Profile

23

Crown

24

Profile

25

Pitch Diameter of Gear

26

Thrust Face

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Gear Types

Illustration 3 Helical Gears Helical gears have parallel or perpendicular shafts. Helical gears are quiet. Multiple teeth carry the load.

Illustration 2 Spur Gears Spur gears have parallel shafts. The load is applied to one single tooth and one third of the adjacent tooth.

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Illustration 4

Illustration 5

Straight Bevel Gears Straight Bevel Gears have angled or perpendicular shafts. One and one third teeth carry the load. Some end thrust is produced.

Other Bevel Gear Designs (Hypoid, Spiral, Etc.) These gear shafts are perpendicular to each other. The gear teeth are curved in order to provide more contact area between mating teeth. This design creates quiet operation; more sliding tooth contact, and end thrust. Two or more teeth carry the load.

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Case hardened surface

Machine Application, Operation and Maintenance

All Caterpillar gears are heat-treated. A heat treat process can extend the life and durability of a gear. A case hardened surface is the result of this heat treat process. Caterpillar determines the application of the part before it goes to heat treatment. The heat treatment depth is based on that application. Final drive planet gears on Off Highway Trucks are highly loaded and have a heavy application. Therefore, a heat treatment process for this type of gear is critical. Small gears are heat treated to a depth of at least 0.2 mm (0.008 inch). Large gears are heat treated to a depth, which can range up to 12.7 mm (0.5 inch). A gear is not heat treated to the core. The soft inner core material is essential. It can actually provide flexibility during high load instances. The average case hardened surface thickness on smaller gears is about 0.5 mm (0.020 inch). Larger gears can have a case hardened surface thickness that is about 12.7 mm (0.5 inch). If the heat treat depth of the case is less than 0.10 mm (0.004 inch), do not reuse the gear.

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The information about the machine application, operation, and maintenance should always be used as an aid in any reuse decision. If the reuse decision for a gear is "Borderline", always consider the machine application, operation, and maintenance. If the history of the machine is not known, you should assume that the application was heavy, the hours were high, and the maintenance was insufficient. Never base a reuse decision on any one characteristic. Gears are designed to withstand the sliding action between the surfaces of mating teeth. Gears are not designed to withstand conditions of insufficient lubrication. Insufficient lubrication can cause metal-to-metal contact, which will increase resistance and therefore increase the operating temperatures (especially under load). If regular maintenance is ignored, inadequate lubrication will eventually occur and cause serious damage. These conditions can and will eventually contribute to abnormal wear or accelerated wear during operation.

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8. Handle gears carefully and store them properly during a rebuild procedure. Gears are easily chipped. A chipped tooth is shown in Illustration 7.

Methods for Minimizing Gear Failures 1. Gear systems are easily contaminated during field assembly. It is very important for you to use good housekeeping procedures at all times. Even if the service is being done in a Dealer Service Center, clean procedures will minimize the possibility for contamination. 2. Use only the recommended build procedures, maintain proper oil fill levels and use only Caterpillar recommended lubricants in order to minimize internally generated wear debris. 3. Replace any breathers that are dirty in order to minimize airborne contamination.

a.

Do not stack gears or set them on the floor as shown in Illustration 6.

b.

Stack the gears in the shape of a pyramid (from large to small). When you stack gears, separate them with cardboard.

c.

You should always use a nylon sling in order to lift a gear. Use of a nylon sling will help to prevent damage to the bore of the gear. Follow all Caterpillar safety standards when you operate a hoist.

4. Schedule a Caterpillar certified SOS (Scheduled Oil Service) oil analysis in order to target possible causes of oil contamination. An SOS oil analysis can help determine the cause of a failure and can assist in the prevention of possible future failures. Keep an accurate record of the machine hours and application. This information will be used during the SOS oil analysis in order to generate the most accurate diagnosis. 5. If your gear oil passes through a filtering system, always use the specified filters and change them at the recommended intervals. 6. Change the drive train oils at an interval that is recommended in the machine's Service Manual.

Illustration 7 If you mishandle a gear, the teeth may chip. The gear in the above illustration had been mishandled.

7. During a rebuild, be certain that all gears (including new ones) are free of rust and debris. Also examine the gears in order to make sure that there is no handling damage present.

9. Maintain a complete and accurate history file for the machine and the machine component hours. 10. Record the number of builds and hours on all gears. For more information about the standard parts marking procedure, refer to the "Marking Code Procedure" section of this guideline.

E71149

Illustration 6 Do not set gears on the floor or stack them on top of each other.

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Metallurgy

Operational Characteristics of Gears The information in this section will explain the different operational characteristics of gears. The basic gear terminology will also be explained in this section

Cat gears all have unique metallurgical compositions. The metallurgical content of each gear is tailored to its specific application. By utilizing special steel formulas, Caterpillar improves the impact resistance of its gears and ensures uniformity and proper load transfer.

Caterpillar Genuine Gears

Case Hardened Surface

Caterpillar gears deliver maximum performance and productivity while they transmit extreme loads. In order to convert engine horsepower into torque, drive train gear teeth move against each other and are under high stress. Caterpillar drive train gears are built to transmit extreme loads and work as a system with unique features like: • Smooth tooth surfaces. • Resistance to pitting and scoring. • Prevention of excess stress contact on meshing teeth. • Break-resistant tooth geometry. • Application-specific gear metallurgy. • Proprietary heat-treat processes.

Highly loaded drive train gears require a case hardened surface. All drive train gears are heat treated to a specified level of hardness. The depth of the case hardened surface depends on the size of the gear, the application of the gear, or the metallurgy of the gear. This Caterpillar patented heat treat process can offer extended gear life to highly loaded drive train gears. Generally, the smallest Caterpillar gears will be heat treated to a depth of at least 0.20 mm (0.008 inch). The larger Caterpillar gears can be heat treated to a depth of 12.70 mm (0.500 inch).

These properties differentiate Caterpillar gears from other replacement or remanufactured gears. By installing genuine Caterpillar gears in your Cat equipment, you can ensure maximum machine performance and productivity. Cat gear teeth are uniquely shaped in order to ensure that stress is distributed over a large area, preventing tooth breakage.

When gears rotate in and out of mesh, the teeth go through a combined rolling/sliding action. This action occurs during contact above and below the pitch line. Only at the operating pitch line is there rolling action alone.

Rolling/Sliding Action

A

B

Competitive Gears

R-S R

Some competitive gear manufacturers claim that their gears can be used in many different machines because they are designed for multiple pieces of equipment. Every Caterpillar gear has a specific tooth design for each application. These subtle yet important design differences are critical to the life of any drive train system. The use of Caterpillar gears will ensure correct gear tooth contact and efficient power flow under varying load conditions. You should install only Caterpillar genuine gears into Caterpillar equipment.

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PL

R-S

E71044

Illustration 8 The rolling/sliding action that occurs as a gear rotates is shown in this illustration. (A) - Loaded side of tooth (B) - Unloaded side of tooth (R) - Rolling action (R-S) - Rolling-sliding action (PL) - Pitch line

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LPSTC/HPSTC

Load/Contact Velocity

The lowest point of single tooth contact (LPSTC) is low on the gear tooth. Pitting normally occurs at the LPSTC. See callout 26, in Illustration 9. The highest point of single tooth contact (HPSTC) is high on the gear tooth. Scoring normally occurs at and above the HPSTC. See callout 28, in Illustration 9.

There is a relationship between gear tooth load and the gear tooth contact velocity. As the load increases, wear will increase. As the contact velocity increases, scoring increases.

Illustration 10 When you operate a gear in the “No Wear” zone (2) it will have maximum operating life, but the oil must be clean. Maximum gear life will occur under medium gear speeds, and low/medium gear loads. If the oil is contaminated, the wear line (1) will increase at a steeper angle, shifting the complete graph toward the origin. Illustration 9 LPSTC/HPSTC. (24) - Driven gear (25) - Driving gear (26) - Beginning of tooth contact with rolling and sliding action (LPSTC) (27) - Full tooth contact with rolling action (28) - End of tooth contact with rolling and sliding action (HPSTC)

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Lubrication Thickness

Cleaning

The thickness of the film of lubrication is important. The temperature of the lubrication, the amount of the lubrication, and speed of the gear can affect the thickness of the lubrication. Lubrication thickness can also affect the type of wear. Refer to the callouts in Illustration 10 for the following combinations of gear use. Slow gear speed: A slow gear speed can give the oil too much time to run off the gear surface. Slow gear speeds can cause "metal to metal" contact. At a slow gear speed, the gear teeth will have insufficient lubrication. Fast gear speed: A fast gear speed can cause the teeth to penetrate the thin film of lubrication during operation. Gear speeds that are too fast will usually result in “metal-to-metal” tooth contact. High gear load: A high load at a slow gear contact velocity will have accelerated wear characteristics (1). High loads can penetrate a thin film of lubrication. At both Slow and Fast gear speeds, a high load can break the film of lubrication. Accelerated wear (1) can happen at slow speeds and breakage (5) and pitting (4) can happen at high gear speeds. High gear contact velocity: A higher gear contact velocity will usually cause the gear to score (3). High contact velocities with elevated loads will usually cause pitting (4).

Gears must be cleaned prior to inspection. 1. Remove the gears from the machine or component, and clean the gears in a solvent that has a petroleum base. The dark brown or black appearance in a gear bore may be caused from gear oil and can be removed with an 8T-7765 Scotch Brite Pad and solvent. Lightly hand polish the stained area with the 8T-7765 Scotch Brite Pad.

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2. Make sure that the every gear tooth is clean. Wipe off the excess solvent with a clean, dry paper towel in order to remove any deposits or oil. 3. If the gears are to be stored after inspection put a thin layer of oil on the gears and then store them in a clean, dry place.

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Inspection Procedure

Gear Inspection Process

Accurate inspection is an important necessity for the life of any gear. The maximum life of any drive train can be obtained through proper inspection and repair procedures. For a list of inspection tooling, refer to the "Tooling and Equipment" section in this Guideline. When you examine a gear, always use good lighting. If an unacceptable gear is reused, it is likely that the gear will fail and destroy other components in the drive train system. The technician who is performing the gear inspection should be familiar with all types of gear wear and damage. The most common reasons for gear failure are described in this Guideline. Close inspection will help aid in the reuse decision.

1. Clean all gears thoroughly before you inspect them. Refer to the "Cleaning" section of this guideline. 2. Carefully inspect the entire surface area of the gear for pitting, spalling, cracking, or any other damage. Inspect every gear tooth because it is possible for only one tooth to be damaged. 3. Carefully inspect the gear bore with a bright light. Planet gear bearing bores can be sensitive to high loads and speeds; therefore careful inspection for damage is critical.

Illustration 11 During the inspection process, use a clean, white, nonglossy piece of paper in order to reflect light onto the face of each gear tooth.

Illustration 12 Notice the white plastic bench top. It is easy to clean, light reflective, and will not damage the gears if they are dropped onto it.

To help identify damaged areas during the inspection process, use a magnifying glass, a strong light source (sunlight is best), and a reflector made from a clean, white, non-glossy sheet of paper, as shown in Illustration 11. White paper can reflect the light onto dark surfaces. A magnifying glass or lighted magnifying lamp may also be helpful during the inspection process. For detailed inspection information, please refer to SEBF8148 “General Salvage and Reconditioning Techniques”.

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a. Gear rim. b. Splines (if applicable). c. Bearing surfaces (if applicable). 4. Bearing bores. Bearing bores may have minimal wear or damage only. 5. Follow and fill out the Gear Inspection Checklist (Chart B) in order to ensure that the gear is inspected for all types of possible damage.

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Table 6

Note: Please refer to SEBF8148 for general salvage and reconditioning techniques.

Chart C Tooling and Equipment

Table 5

Item Chart B Gear Inspection Checklist

Part Number

Polishing Stone

6V-2010

Black Light

4C-4715

Loupe

8S-2257

Borescope

8T-9290

Microscope (Optional)

6V-2196

Abnormal Wear

Crack Inspection Kit (Includes Cleaner, Dye, and Developer)

4C-4736

Frosting

Magnetic Particle Test Kit

4C-4735

Pitting

Penetrant

9U-6337

Initial

Scotch Brite Pad

8T-7765

Corrosive

Metal Etching Pen

9U-7377

Destructive

Developer

9U-6338

Spalling

Reflective Inspection Surface

-

Pitting \ Spalling

Lint Free Towels

-

Scoring

Bright Incandescent Light

-

Seal Pick

-

Item

Yes

No

Normal Wear White Layer Flaking High Hour Wear

Case Crushing Corrosion/Corrosive Pitting Abrasive Wear Nicks Cracks Chipping Uneven Contact Foreign Object Damage Rippling/Lipping Gear Bores Planet Gear Bores Gears with Replaceable Bearing Races

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4. Pat the surface dry with a clean lint free paper towel in order to eliminate the water from the surface of the gear.

Crack Inspection Methods If you suspect that a gear is cracked, use one of the following crack inspection methods. Caterpillar recommends the following methods.

5. Use a black light to locate the cracks. 6. Spray the 9U-6338 Developer on the cracked areas. After the developer has been applied, allow it to dry. During this time, the developer will draw the penetrant from the cracks up to the surface of the gear. The cracks should become visible during the first ten minutes of the application.

Follow these general precautions whenever using the listed methods of crack detection to prevent personal injury. Never shine the black light directly into anyone's eyes. Do not smoke or eat while using inspection chemicals. Wash hands thoroughly after using chemicals. Avoid getting chemicals on clothing. Avoid inhaling spray mist, airborne powder dust, and solvent vapors. Provide adequate ventilation. Store chemicals away from open flames or sources of heat.

Liquid Non-Fluorescent Penetrant Method Tooling for this method requires a 4C-4736 or a 9U-6335 Liquid Penetrant Kit. Note: This method will not always find shallow cracks of 0.05 mm (0.002 inch) or smaller.

Liquid Fluorescent Dye Penetrant Method

The gear must be clean and dry in order to use the following Liquid Non-Fluorescent Penetrant Method. 1. Refer to the cleaning section of this guideline in order to thoroughly clean the inspection surface. Make sure that the surface is dry before you continue with this procedure.

Tooling for this method requires a 1U-5566 or a 1U-6444 Black Light and a 9U-6335 liquid florescent dye penetrant kit. The gear must be clean and dry to use this method. This is the most desirable method for crack detection. Note: While doing the crack detection test, wear 1U-5571 Ultraviolet Light Absorbing Spectacles (glasses) in order to reduce eye fatigue and protect your eyes from the chemical. The 1U-5571 Ultraviolet Light Absorbing Spectacles will increase the contrast between the fluorescent area and the background of the gear.

2. Spray penetrant on the surface that is to be inspected. Allow the penetrant to remain on the surface for at least five minutes but no longer than thirty minutes. This will allow the penetrant to enter the smallest cracks. 3. Remove the excess penetrant from the surface of the gear with a clean moist paper towel. Do not wash too much of the penetrant off. If you over wash the gear, simply repeat the process from Step 1.

1. Refer to the cleaning section of this guideline in order to thoroughly clean the gear.

4. Pat the surface dry with a clean lint free paper towel in order to eliminate the water from the surface of the gear.

2. Spray the 9U-6337 Penetrant on the gear surface that is to be inspected. Allow the penetrant to remain on the surface for at least five minutes but no longer than thirty minutes. This will allow the penetrant to enter the smallest cracks.

5. Spray a light coat of developer solution on the surface that is to be inspected. A crack will appear as a red colored line in the white developer.

3. Remove the excess penetrant from the surface of the gear with a clean moist paper towel. Do not wash too much of the penetrant off. If you over wash the gear, simply repeat the process from Step 1.

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Dry Magnetic Particle Method

SOS Testing

Tooling for this method requires a 4C-4735 Magnetic Particle Test Kit. The kit includes a magnetic yoke, cleaner, and magnetic particle powders. The gear must be clean and dry to use this method. Follow these steps to use the dry magnetic particle method in order to check for cracks. 1. Refer to the cleaning section of this guideline in order to clean the inspection surface. Make sure that the surface is dry before you continue with this procedure.

Caterpillar recommends that you schedule a certified SOS oil analysis based on the machine application and an hourly schedule. Change your oil on a regular basis. Refer to the owning and operating manual in order to acquire the recommended oil change intervals, oil capacity, and oil type. Regular oil changes in combination with Scheduled Oil Samples are the best way to discover and or prevent oil contamination before it destroys components in the drive train system. An SOS oil analysis may also help determine the cause of a failure.

2. Place the magnetic yoke perpendicular (90°) to the surface that is to be checked.

Reconditioning

3. Turn on the magnetic yoke.

Some gears that have limited surface damage may be used again after the damaged area has been polished with a stone and oil. Caterpillar recommends that you use a stone and oil. Never use a die grinder in order to recondition a gear. A die grinder can easily remove the case hardened surface of a gear. (See illustration 14)

4. Apply the powder over the suspected cracked area. 5. The magnetic powder will fill the crack that is between the two prongs of the yoke. The particles that have just aligned to the crack will become visible to the unaided eye. Some of the powder will be attracted to the magnet and some will fall into the crack. The powder that falls into the crack will display the precise location and the precise dimensions of the crack.

NOTICE If the tooth has a raised metal area, remove only the raised portion on the tooth surface. Do not try to remove pitting or any other recessed surface damage. Do not use a grinder or power polisher; use only a handheld stone. Using power tools to remove damage can also remove the case hardened surface of the gear, causing eventual failure.

6. If you discover only a portion of a crack, it may be necessary for you to move the fork and apply more powder. Repeat the process from Step 2. Please refer to SEBF8148 “General Salvage and Reconditioning Techniques”, for further reconditioning information.

Illustration 13 A stone and oil should be used in order to polish a tooth. Be aware of the adjacent tooth. The corner of the stone can easily damage adjacent teeth.

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Illustration 14 A grinder had been used on this gear in an attempt to recondition the teeth. If you need to recondition the teeth on any gear, use the stone and the oil. Do not use a grinder. A grinding process can remove the case hardened surface of a gear.

Do not use again

Illustration 15 The marking code is "1-15". This gear has been through one rebuild. This gear had 15,000 hours on it when it was rebuilt.

Marking Code Procedure

EXAMPLE 2: In this example (Illustration 16), the marking code for the first rebuild is "1-12". The marking code for the second rebuild is "210". The "2" indicates that the gear had been rebuilt two times previously. The "10" indicates that there were 10,000 hours on the gear at the time of the rebuild.

The marking code is a Caterpillar standard that is used in order to track the life of an individual gear. The code will identify the number of rebuilds, and hours at the time of each rebuild. This information is critical for any reuse decision. You should always consider this information when you are making a reuse decision or when you are locating the root cause of a failure. When you mark a planet gear or a sun gear, make sure that the mark is on the side of the gear and is not covered by a mating part. Use the 9U-7377 Metal Etching Pen that is listed in the tooling and equipment section of this guideline in order to mark the code onto the gear. The gear parts marking procedure is a standard throughout Caterpillar Inc. This code is extremely helpful when the machine is sold into a different territory after the first rebuild. During an overhaul, a part's previous marking code(s) should never be removed. For additional information about the marking code procedure, refer to SEBF8187 "Standardized Parts Marking Procedures".

Illustration 16

EXAMPLE 1: In this example (illustration 15), the marking code is "1-15". The ‘1’ indicates that the component had been rebuilt one time previously. The '15' indicates that there was 15,000 hours on the gear at the time of rebuild.

SEBF8193-03

The marking codes are "1-12" and "2-10". This gear has been through two rebuilds. The gear had 12,000 hours on it when it was rebuilt for the first time. The gear had 22,000 hours on it when it was rebuilt for the second time.

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Machining This section covers the basic stages that are required in order to machine a gear.

Illustration 17 The horizontal marks (arrows) were created during the manufacturing process. These marks are rough machining marks. The marks may look like cracks but are not. Use one of the crack inspection methods in order to make sure that the marks are not cracks.

Use again

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Illustration 18 The burrs that are on this gear tooth may look like lipping, but they are not. The machining process can leave burrs on the edges of gear teeth. Dull cutting tools can cause these burrs.

Use again

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Rough Machining Gear teeth are often machined by using a hobbing process. If a gear has been through a hob process, you will easily notice the rough machining marks on the surfaces of the teeth and the burrs on the edges of the teeth.

Illustration 19 This gear has been through the rough machining process. A hob tool was used in this process.

Illustration 20 You can visually see the hob machining marks (1) on gear tooth face. The rough machining marks and burrs that are left from the hobbing process will be removed during the finish machining process.

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Finish Machining After the gear teeth have been cut with the hob process, a finish machining process is used. This process is called shaving. Illustration 21 shows the typical appearance of a new gear that has just passed through the shaving process. This gear is ready for heat treatment.

Illustration 21 This is a new gear that has not been through the heat treat process yet. The shaving marks (2) that are shown in this illustration will be visible after thousands of hours of use. These marks are positioned on the tooth face at a slight angle. Since the root fillet area is not a contact surface, it does not need to undergo the finish machining process. Therefore, the hob marks (1) will be left in the root fillet. This gear is ready for heat treatment.

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Illustration 22 This is a magnification of the gear that was shown in Illustration 21. You can see the shaver marks (2) more clearly. The hob marks (1) are also visible in the fillet.

Illustration 23 This gear tooth shows an acceptable, even contact wear pattern after thousands of hours of use. This gear's wear pattern is an ideal wear pattern. The pattern of wear is centered and even on the tooth. During operation of the gear train, the shaving marks will slowly wear away. Notice how the contact ends before it gets to the edges or the tip of the gear tooth. The gear teeth are designed this way in order to reduce the occurrence of chipping at the edges and scoring at the tips.

Use again

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Illustration 24 These marks (A) may look like cracks but they are actually shaver marks from the shaving process. Inspect this gear for cracks with any one of the crack inspection methods.

Use again

Illustration 25 Sometimes, a machining mark will look like a crack. If the gear appears to have a crack, inspect it with one of the crack inspection methods. Because machining marks are not usually deep, they should not show up during a crack detection process. Normal wear will smooth the surfaces and should be evenly distributed across the teeth.

Use again

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Normal Wear This section will describe the types of normal wear patterns that a gear can develop. The two types of normal gear wear are white layer flaking, and high hour wear. If a gear has only normal wear, it can generally be used again.

Illustration 26 Normal wear is shown in this example. The machining marks (arrows) on this gear are still visible.

Use again

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White Layer Flaking Nitride hardened gears develop a thin brittle skin (1), it may flake off during normal operation. White layer flaking is a normal type of gear wear. Most gears that show signs of white layer flaking can be used again.

Illustration 27 Although the flaking of the skin appears to be destructive, it is not and this gear may be reused.

Use again

Illustration 28 This is a magnified view of the thin brittle skin (1) that can develop on nitride-hardened gears.

Use again

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Illustration 29 The rough machining marks have caused the load to become concentrated in certain areas. Notice how the pattern of the white layer flaking follows the machining marks.

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High Hour Wear As gears are loaded, the gear surfaces begin to wear smooth. This smooth surface should occur over approximately two thirds or more of the length of the gear tooth. See Illustrations 30 and 31.

Illustration 30 This gear illustrates normal high hour wear. After you check for other types of abnormal wear, this gear may be reused. Adjust the alignment during the build so that the wear will be centered on the teeth of this gear.

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Illustration 31 Normal wear is shown in this example. The machining marks (arrows) are still visible on this gear.

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Abnormal Wear This section describes the different types of abnormal wear.

Frosting Frosting occurs when mating gear teeth undergo metal-to-metal contact. The metal-to-metal contact usually results from an inadequate film of oil between the surfaces of mating gear teeth. Frosting is not harmful to the gear as long as pitting has not started and as long as the profile has not changed. Frosting appears like a "cloudy gray" area on the gear contact surface. A gear that shows signs of frosting can be used again as long as it is free from other types of damage. The following may cause frosting: a. High Operating temperatures which cause the lubrication viscosity to become thin b. High loads c. Incorrect lubrication d. Incorrect part e. (Etc.) Frosting itself is not harmful to the gear. However, if the conditions that cause frosting are not corrected, pitting can start and lead to destructive pitting and spalling.

E71150 Illustration 32 Frosting appears on the gear contact surface and it usually occurs in an even wear pattern. Frosting is displayed in this illustration as small surface pits that are along the pitch line. Always check for pitting damage and profile damage before you reuse a frosted gear.

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Illustration 33 Correct the cause of frosting in order to avoid pitting. A classic example of frosting appears as a "cloudy gray" area on the gear contact surface. This condition is caused by inadequate oil film between two mating gear teeth surfaces. There is no pitting present in this example.

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Illustration 34 This is an example of frosting. Because there is no pitting present, this gear may be reused. Correct the load alignment during the installation process.

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Illustration 35 Pitting (1) and frosting (2) are both shown on the gear that is in this illustration. The frosting (2) is acceptable. However, because destructive pitting will usually occur with frosting (2), you must reference the Pitting section of this guideline in order to determine that the pitting (1) is not destructive. After you determine that the pitting (1) is not destructive, the gear may be reused.

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Illustration 36 Reuse of this gear depends on application, operation, and maintenance.

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Pitting Pitting is the development of small holes in the surface of a tooth. Pitting is a sliding contact stress fatigue failure on the surface of a gear tooth. The holes usually occur low on the gear tooth near the lowest possible single tooth contact (LPSTC). There are two types of pitting: a. Initial pitting will usually occur during the machine break-in period (up to 500 hours). Initial pitting is normal. As long as the application, operation, and maintenance are correct, instances of initial pitting will rarely progress into destructive pitting. When the break-in period is over, the initial pitting will usually stop. b. Destructive pitting will usually occur after the machine break-in period (over 500 hours). Destructive pitting will usually be found on most or all of the gear teeth. If the causes of destructive pitting are not corrected, the deep pits will progress into spalling, and then into tooth fracture.

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Illustration 37 Notice the small pits that are located near the LPSTC on this gear tooth.

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Illustration 38 The initial pitting (1), shown in this illustration should not progress into destructive pitting. Check for other types of abnormal wear before you reuse this gear.

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Illustration 39 The arrows show an instance of initial pitting. Lightly polish the area with an 8T-7765 Scotch Brite Pad and oil.

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Illustration 40 This gear has some initial pitting (arrow) just at the SAP (Start of Active Profile). Check for other types of abnormal wear before you reuse this gear.

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Illustration 41 This illustration is a magnification of Illustration 40.

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Corrosion / Corrosive Pitting Corrosion can be cause by an electrolyte that has entered the lubrication system. The most common electrolyte is water. Corrosion can lead to corrosive pitting. Correct the cause of corrosion before corrosive pitting occurs. If an electrolyte enters the drive train system, it can neutralize the additive package in the oil. Early stages of corrosion are not harmful. Most corroded gears can be used again as long as pitting has not occurred. During the rebuild, repair the entry point for the electrolyte. Rust is a common type of corrosion. Check for corrosive pitting after you remove the corrosion.

Illustration 42 Corrosion has caused corrosive pitting. The pitting covers a good percentage of this gear tooth.

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Illustration 43 Corrosive pitting, in an advanced stage, will take over the entire face of a gear tooth. This stage of corrosive pitting is usually followed closely by spalling.

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Illustration 44 The corrosive pitting on this gear has developed into a spall.

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Illustration 45 Because the corrosive pitting shown here is only located on the ends of the gear teeth, this gear may be used again.

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Illustration 46 This image shows a gear that has undergone corrosion. Corrosion may create shallow surface deposits. These deposits visually appear as discoloration. Use a microscope in order to examine the deposits. If the gear shows signs of pitting, do not reuse the gear. The gear shown above has been examined and does not show any signs of pitting.

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Illustration 47 Remove the rust that is on this gear. If the source of the problem (usually a source of Water) is not discovered and corrected immediately, corrosion will cause corrosive pitting.

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Illustration 48 The rust pits (1), shown in this illustration, are visible. Because rust has caused pitting on the teeth of this gear, it should not be used again.

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Illustration 49 The appearance of the teeth on this gear is the only thing affected by corrosion. The texture of this gear remains smooth and the gear does not show any signs of pitting. Corrosive pitting has not yet occurred on this gear. Correct the cause of the corrosion before you reuse this gear.

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Illustration 50 This gear may not be used again because it is heavily rusted. The cause of the rust is usually water mixing with the oil. Check other gears for rust and replace them if necessary. Before you rebuild the drive train, make sure the source of moisture has been eliminated.

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Illustration 51 The corrosion in this illustration has not caused pitting.

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Illustration 52 The corrosion that is shown in this illustration has progressed past the first stage in corrosion. The gear that is in this illustration may appear to be reusable, however, a good light source and a good magnification will reveal that the gear tooth is actually discolored and rough.

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Destructive Pitting Destructive pitting usually occurs only after a long period of operation (over 500 hours). Destructive pitting can be found on most or all of the gear teeth. Destructive pitting can start as micro pitting and progress into a spall. The spalled tooth will eventually fracture.

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Illustration 53 Destructive Pitting is shown in this illustration. The large pits across the LPSTC are destructive pits. This type of pitting can penetrate the case hardened surface of a gear.

Illustration 54 If the lubrication film is inadequate, micro pitting (1) may occur. Micro pitting is a type of destructive pitting that can be caused by “metal to metal” contact. The micro pitting (1) that is shown in this illustration is in its early stages. Therefore, this gear can be used again. Determine the cause of insufficient lubrication and correct it before you reinstall the gear.

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Illustration 55 Notice how the destructive pitting (2) is much deeper than initial pitting (1). The destructive pitting (2) is near the LPSTC and has penetrated the case hardened surface.

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Illustration 56 This is a magnified view of the pitting (1) that is shown in Illustration 55.

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Illustration 57 Although the pitting shown here is widespread, it has penetrated the case hardened surface.

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Illustration 58 Reuse of this gear depends on application, operation, and maintenance.

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Illustration 59 The pitting on this gear is borderline. Check for other types of abnormal wear. Base your decision on the application, operation, and maintenance of the machine.

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Illustration 60 Micro pitting (1) and macro pitting (2) are shown in this illustration. Because the macro pitting (2) is developing into a spall, this gear cannot be reused.

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Spalling Spalling is a type of subsurface fatigue failure. Spalling is a result from a small fatigue crack that has worked it's way up the face of the gear tooth while remaining under the case hardened surface. Either destructive pitting or case crushing can cause spalling. A destructive pit is usually the starting point for a spall. As a spall starts to progress, it will travel from the destructive pit, up the face of the tooth. Throughout this process, the crack will remain beneath the case hardened surface. When a crack travels beneath the case hardened surface of a gear tooth, it is traveling the path of least resistance. As the subsurface crack progresses towards the tip of the tooth, the case hardened surface will break away in small pieces. These pieces will continue to break away until the spall has reached the tip of the tooth. A spall will form in the shape of a "V". This process happens over a length of time. Once the protective case hardened surface has deteriorated, the weak inner core will be left unprotected. A tooth that has spalling damage will eventually fracture. In order to detect spalling damage in the drive train, schedule a Caterpillar certified SOS oil analysis. If you do not make the necessary repairs, spalled teeth can fracture and cause significant damage to the drive train system. Do not reuse any gear that has spalling damage. Note: Spalling is often confused with case crushing. It is important to know the differences. 1. Destructive pitting is the leading cause of spalling. Look for instances of destructive pitting. If destructive pitting is not present, the damage may be due to case crushing. 2. Examine the damaged area. Spalling will not leave pieces of crushed metal. Case crushing will leave pieces of the case. 3. When a gear tooth has spalling damage, the shape of the damage will be like a "V". Damage from a crushed case is usually in a rectangular shape. 4. Refer to the Case Crushing section in this guideline for more information. Spalling damage is shown in the following illustrations. Before a spall occurs, you will be able to see the outline in the face of the tooth. An outline of spalling damage will look like a tornado. Do not reuse any gear with a "tornado" shaped outline on the tooth face. Once the spall progresses, the case hardened surface will slowly break away. Examples of spalling damage are shown in the next several photos. Do not reuse any gear with spalling damage.

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E71188

Illustration 61 Notice how the spall starts at destructive pitting and climbs up the tooth face in a ‘V’ shaped pattern. Illustrations 62 thru 66 show the growth of a spall. For orientation purposes, the tooth tip (1) and the tooth fillet (2) will be called out in the following illustrations.

Illustration 62 Stage 1: The starting point of a spall (arrow) is pointed out below the pitch line.

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Illustration 63 Stage 2: A small fatigue crack at the surface of the gear is shown here. Metal starts to flake off the tooth face at this point.

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Illustration 64 Stage 3: The small subsurface cracks will join and the tooth face will flake away.

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Illustration 65 Stage 4 shows the small subsurface cracks. These cracks continue to join, as the spalling damage area grows larger.

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Illustration 66 Stage 5 shows where the spalling has progressed across the tooth face. Stage 6 will be a tooth fracture.

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Illustration 67 The arrow shows an area on the gear tooth where spalling is in an early stage. These pits are becoming connected by small subsurface cracks.

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Illustration 68 As the subsurface cracks grow, a piece of the tooth face will flake off.

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Illustration 69 Most spalls develop in a “V” shape. Sometimes, the "V" shape is not always distinctive. Even if the shape is not a complete "V", a spall will grow from narrow too wide as it progresses up the face of the tooth.

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Illustration 70 The spalling shown here has originated from pitting damage (arrow).

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Pitting/Spalling Pitting/spalling is the transition process between pitting and spalling. A gear with evidence of pitting/spalling should not be used again. Spalling can progress rapidly into a tooth fracture. Illustration 71 shows a cross-section view of spalling and it's present state just prior to a tooth fracture.

Illustration 71 This is a cross-section of the progression from pitting to spalling. Notice how the destructive pitting has caused subsurface cracks in hardened tooth surface.

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Illustration 72 This cross section shows a spalled gear tooth. Notice how the destructive pitting (arrow) can cause subsurface cracks in the case hardened surface of the tooth.

Illustration 73 Spalling is a type of subsurface fatigue fracture that removes flakes of metal and can progress rapidly into a tooth fracture. Notice how the subsurface cracking can cause a “tornado” outline on the face of the gear tooth. This area will eventually fracture and look similar to the damage that is shown in the next illustration.

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Illustration 74 Notice how this spall progresses up the tooth face from the destructive pitting at the bottom of the tooth. This spall is in the common “V” shape.

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Illustration 75 Destructive pitting (1) is shown along the LPSTC and destructive pitting (2) is shown near the pitch line. Subsurface cracking exists just beneath the destructive pitting (2). The destructive pitting (2) is in the process of progressing into a spall. The spall on the front tooth will look like the spall on the back tooth. The spall (3) is shown progressing from the destructive pitting to the HPSTC in a ‘V’ shaped pattern up the tooth.

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Illustration 76 This illustration shows a cross section view of a spall fatigue that’s in its early stages. A spall that’s in this stage is difficult to detect because it starts in the core material of the gear. Once the cracks reach the case hardened surface, a spall fatigue can rapidly progress into a tooth fracture.

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Scoring Scoring, also known as adhesive wear, can result from metal-to-metal tooth contact and can cause vertical scratches, which are perpendicular to the pitch line. Scoring begins above and/or below the pitch line where sliding action occurs between two mating teeth. Scoring damage should not occur at pitch line because only rolling action is present at this location. Scoring can occur when the oil film is not adequate, during periods of excessive load, and misalignment. Severe scoring will produce an irregular furrowed surface on the tooth face. Reuse a gear with scoring damage only if the tooth face is relatively smooth and only if the scoring has not altered the shape of the tooth profile. Before you reuse a gear with scoring damage, refer to the crack detection section in order to check for cracks.

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Illustration 77 Scoring is usually located above or below the pitch line. Scoring is the result of metal-to-metal tooth contact.

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Illustration 78 The scoring (1) that is shown in this illustration is in an early stage. Because of this, the gear may be used again.

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Illustration 79 This is a magnification of Illustration 78. The scoring has not penetrated the case hardened surface.

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Illustration 80 In this illustration, the scored areas (vertical scratches) are above and below the pitch line (PL). At the pitch line, there is only rolling action. Because of this, scoring will not occur at the pitch line (PL).

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Illustration 81 The shaver marks (arrows) in this illustration were made during the final machining operation in the manufacturing process. Shaver marks can be confused with scoring marks. Scoring marks are vertical. Shaving marks will be positioned at a slight angle and will cover the entire width of the tooth face. The shaving marks that are shown in the above illustration are not harmful (refer to the "Machining" section at the front of this Guideline for additional information).

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Illustration 82 The arrows in this illustration show scoring marks that are excessive enough to make this gear unusable. Notice that damage is below the pitch line.

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Illustration 83 Check the profile of each gear tooth. If the profile has changed do not use this gear again.

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Illustration 84 If scoring damage above the pitch line is smooth and the profile has not changed, this gear may be reused.

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Illustration 85 This illustration is a magnified view of Illustration 84.

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Illustration 86 The scoring (vertical scratches) is above the pitch line. Because the tooth face is smooth, the gear may be used again.

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Illustration 87 The lipping damage (1) has changed the tooth profile. A gear with lipping damage should not be used again.

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Illustration 88 Severe scoring has damaged the profile of the teeth on this gear.

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Illustration 89 The tooth profile (1) is damaged and irregular. Notice how the scoring has changed the tooth profile (1).

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Illustration 90 Scoring has damaged the tooth profile. A corrected load alignment will help to prevent the same type of damage to the replacement gear. Locate and correct the cause of load misalignment.

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Illustration 91 This gear has medium sized abrasive wear. Stone and lightly polish the rough edges before you reuse this gear. Because the abrasive wear is towards the tip of the teeth, the gear may be reused. Always check the profile in order to ensure that it is free from damage.

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Case Crushing Case crushing is a failure of the case hardened surface of a gear. The case (surface) of a tooth is much harder than the core (center) of a tooth. Case crushing has an appearance much like something that has shattered. When a case crushing failure occurs, the softer core material becomes exposed. Because the softer core material is unable to withstand the normal loads placed on the gear, a gear tooth with a crushed case will quickly fracture. Gears with evidence of case crushing cannot be used again.

E71201 Illustration 92 The case hardened wear surface has failed and the core is exposed. Gears with this type of wear should not be reused because the soft core is unable to withstand the normal loads placed on the teeth.

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1

2

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Illustration 93 This illustration shows a cross section of a gear. The components consist of a hardened case (1) and a softer inner core (2).

Illustration 94 This gear has been saw cut, polished, and acid etched. In this cross-section, you can see the case hardened surface. Every Caterpillar gear goes through a proprietary heat treat process.

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Illustration 95 A misaligned load or a shock load can cause case crushing. Case crushing may also occur if the case hardened surface is thin. A gear with evidence of case crushing should not be reused.

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Illustration 96 Case crushing (arrow) is shown in the illustration above. The cracks in the tooth face are hard to see in this early stage. You must use one of the crack inspection methods in order to detect the damage.

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Illustration 97 As a gear progresses through the stages of case crushing, horizontal cracks across the tooth face will become visible. The "shattered" appearance is unique to case crushing.

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Illustration 98 In the final stage of case crushing, material from the tooth face will “flake off”. The edges to the cavity are sharp and perpendicular to each other. Distinguishing the difference between case crushing damage and spalling damage can be difficult. The cavity that results from a crushed case is usually larger and deeper than the cavity that results from a spall. Experience is the best guide when you need to distinguish the difference between the two types of damage.

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Abrasive Wear Abrasive wear is caused by small contaminants such as soil, sand, or metal in the lubrication system. The small particles will eventually wear down the surface of the teeth and can even alter the gear profile. Abrasive material can travel throughout the drive train gear system and damage other gears as well. Therefore, if you find that one gear has abrasive wear, check the other gears in the drive train system as well. Light abrasive material will cause the gear to appear gray matte in color. The smallest contaminants will lightly polish a gear's teeth. If light abrasive wear is not corrected, the teeth will become shiny enough to resemble a mirror. Medium to large sized contaminants will usually produce scratches, cuts, and bruises on the gear teeth. Large particles can create deep grooves on the tooth face. If you find evidence of abrasive wear, correct the source of the contamination immediately. A gear with limited abrasive wear may be reused as long as the profile has not changed and as long as the gear does not have any other types of damage.

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Illustration 99 This illustration shows a diagram of advanced abrasive wear around the pitch line area. Advanced abrasive wear will alter the tooth profile. (1) Heavy wear (2) Pitch line

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Illustration 100 The gear that is shown in this illustration is in an early stage of the abrasive wear process. The teeth on this gear have a satiny appearance with no flat spots on the profile. The gear can be used again after you correct the source of the contamination.

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Illustration 101 The teeth on this gear appear to have a "satiny" finish. On this gear, the abrasive wear has advanced from the early stages of abrasive wear displayed in the previous illustration. Notice the machining marks and polishing marks on the right side of the teeth. The machining marks are still visible toward the tips of the gear teeth.

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Illustration 102 This is the next stage in the abrasive wear process. When abrasive wear is allowed to continue, the gear teeth will develop a mirror-like finish. The finish will be so shiny; each tooth will reflect an image of the adjacent tooth.

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Illustration 103 Notice the areas above and below the pitch line. The scratching on the surface of these teeth hints that large foreign particles were present in the lubrication system. The normal sliding action is responsible for this massive amount of wear above the pitch line. Bruising has also occurred on the teeth of the gear.

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Illustration 104 This gear has been exposed to a significant amount of abrasive wear. The arrow in this illustration indicates a heavy wear groove. A significant amount of abrasive wear is capable of causing a depression like this.

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Illustration 105 The case hardened surface has been worn away and has exposed the softer core. This damage will quickly progress into a tooth fracture.

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Nicks Nicks usually occur when a gear is mishandled before heat treatment. Because most nicks will occur before heat treatment, a gear with a nick can be reused.

Illustration 106 This gear has a nick on the tip of the tooth.

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Illustration 107 This nick (1) occurred before heat treatment. Before you reuse this gear, verify that the damage is not a crack.

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Illustration 108 This tooth has been nicked on the tooth face and in the fillet. Before you reuse this gear, verify that the damage is not a crack.

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Illustration 109 The nick that is shown on this gear is on the tooth tip land (arrow). Nicks on any tooth tip land or edge are acceptable as long as they do not extend onto the tooth contact surface. Use a stone and oil in order to polish the rough edges around the nick.

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Cracks It is difficult to visually distinguish a difference between a scratch and a crack. To determine if cracks are present, a crack inspection method must be used in order to inspect the gear. See the "Crack Inspection Methods" section of this guideline for more information about the necessary equipment and procedures that are needed in order to determine if cracks are present.

Illustration 110 These cracks are located in the fillet area of the tooth. A dye penetrant crack inspection method has been used in order to detect the cracks that are shown in this illustration.

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Illustration 111 This gear has a crack (arrow) towards the end of the tooth. The liquid florescent dye penetrant crack inspection method has been used in order to find the crack that is shown in this illustration.

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Illustration 112 Cracks have developed in the root areas of this gear. It was necessary to use the dry magnetic particle inspection method in order to locate these cracks.

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Illustration 113 The crack in the root area of the tooth has progressed toward the gear bore from the tooth the fillet. Sometimes it is very difficult to see a crack like this unless you use one of the crack detection methods.

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Illustration 114 This crack has developed in the root area in between two teeth and the crack has progressed all the way through the gear.

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Chipping

Illustration 115 The chip (arrow) on this gear is from mishandling. Unlike a nick, a chip will always occur after heat treatment.

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Illustration 116 This is a magnification of Illustration 115

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Illustration 117 The ends of the teeth are chipped because foreign objects have damaged them. Polish the sharp edges with a stone and oil. Inspect the gear for cracks using a crack inspection method.

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Illustration 118 The tooth end has been chipped because the gear had been mishandled. Since this chip extends into the fillet area (A), you should not reuse this gear. The tooth may break off during operation and cause significant damage to other components in the drive train system.

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Illustration 119 The chipping damage extends onto the contact surface of the gear tooth.

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Illustration 120 This chip (1) does not extend onto the contact surface. Polish the rough edges that are around the chip (1) with a stone and oil.

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Uneven Contact

2

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Illustration 121 The five most common types of uneven wear patterns are shown in this illustration. (1) Normal contact pattern. Maximum gear life (Application, operation, and maintenance procedures must be adequate.) (2) Tooth end contact pattern. Tooth breakage can occur. (3) Tooth tip contact. Scoring, pitting, and tooth breakage can occur. (4) Concentrated root contact. Scoring and pitting can occur. (5) Concentrated tip contact. Scoring, pitting, and tooth breakage can occur.

Illustration 122 In this illustration the contact pattern (1) is highly concentrated toward the end of the tooth. The contact pattern is a "Type 2" uneven contact pattern. Because the contact pattern (1) is not centered on the tooth, this gear should not be used again.

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Illustration 123 The contact pattern (arrow) is a "Type 2" uneven contact pattern. This is a typical example of a severe wear step. The contact pattern is not centered on the tooth.

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Illustration 124 The arrows in this illustration point out severe wear steps. This type of wear is also caused by a "Type 2" uneven contact pattern.

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Foreign Object Damage A foreign object is any object that is not supposed to be in the drive train system (gear tooth, roller bearing, shrapnel, etc.). Usually, a foreign object will come from the drive train itself. Under harsh conditions, gear teeth can break and roller bearings can become dislocated. These foreign objects can damage other drive train components. Broken teeth from other gears and pieces of failed bearings are the two most common types of foreign objects. Foreign object damage must not penetrate the case hardened surface of a gear. The contact surface of a gear may have only a small amount of foreign object damage. Foreign object damage must not exist in the fillet areas on the gear. Do not reuse a gear if you find that more than one of the following is true: • If you find a significant amount of foreign object damage on a gear. • If the foreign object damage will affect the distribution of the load. • If the foreign object damage affects the lubrication film. • If the foreign object damage exists in the root fillet area. • If the foreign object damage is questionable. If necessary, you may need to refer to the Reconditioning section of this guideline in order to recondition the foreign object damage. As long as the contact surface nearest the damage will not become highly stressed under a normal load, the gear may be reused.

Illustration 125 The foreign object damage to this ring gear has not penetrated the case hardened surface. The root fillet area is not damaged.

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Illustration 126 The foreign object damage (1) is large and deep. The damage is also near the fillet.

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Illustration 127 The foreign object damage on this gear is just above the pitch line.

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Illustration 128 The foreign object damage (arrows) is at the pitch line.

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Illustration 129 The foreign object damage (arrows) is at the pitch line and below the pitch line. The foreign object damage that is below the pitch line does not extend into the root fillet.

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Illustration 130 The foreign object damage (1), near the pitch line, is acceptable. However, because the foreign object damage (2) extends into the fillet area of the tooth, this gear cannot be reused.

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Illustration 131 This illustration is a magnification of Illustration 130. The fillet has foreign object damage (2).

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Illustration 132 The foreign object damage (arrow) has penetrated the case hardened surface.

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Illustration 133 The foreign object damage on this tooth exists in the fillet (arrow). Examine the tip of the meshing gear tooth for similar damage.

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Illustration 134 The tip of the mating gear tooth (arrow) is embedded into the fillet. Do not reuse this gear or the mating gear with the broken tooth. Inspect other gears in the drive train for damage.

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Illustration 135 The face of this tooth is severely cracked. Foreign object damage can cause a tooth to crack.

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Illustration 136 The face of this tooth is severely cracked, which is the result of foreign object damage. The wide gap that is between each stress line and the deep ridges indicates that this crack happened quickly.

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Illustration 137 The tips of these teeth are severely cracked, which is the result of foreign object damage.

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Illustration 138 The deep indentation that is shown on the gear above is from a roller bearing.

Do not use again

Illustration 139 This type of foreign object damage was caused by soft debris. Usually, soft debris will not cause a significant amount of damage.

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Rippling

Illustration 140 This gear has rippling damage (shown between the lines). This type of damage would be most common on highly loaded gear teeth.

Do not use again

Illustration 141 Rippling has advanced from scoring, to pitting, and then to a change in profile.

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Lipping

Illustration 142 Lipping is the permanent displacement of surface metal. Lipping will happen if a gear is too soft or if a gear is overloaded during adequate lubrication. If the lubrication were not adequate, the damage would be a combination of scoring, metal pullout, and smearing.

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Gear Bores Drive train gear bores are very sensitive to damage. If the gear is to be reused, the bore must not have any wear or damage. Like all other inspection processes, a good light source is required in order to fully inspect the bore of a gear. Refer to the cleaning section of this guideline in order to clean the bore of the gear. If gear bore has stains that cannot be removed with 8T-7765 Scotch Brite Pad and solvent, do not reuse the gear. A gear with minimal bore damage can be reused. Because gear bores are very sensitive to damage, it is important that you make a careful reuse decision for any gear bore. If a gear bore has two or more different types of damage, it should not be reused. Gears with lightly damaged gear bores may be reused as long as each of the following conditions are true: • • • • • •

The case hardened surface has not been penetrated The damage is light The damage is not located in the antifriction bearing contact areas The damage does not extend across more than 25% of the width of the thrust face The damaged areas are few in number The damage can be smoothed with a stone

Planet Gear Bores Planet gear bores are unique to other drive train gear bores. Planet gears are highly loaded. Therefore, it is important to inspect all planet gear bores with care.

Illustration 143 This bore is in excellent condition and there is no visible damage.

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Illustration 144 If a gear bore is used as a race for a bearing, it should not be worn or damaged. There is no wear or damage in this gear bore.

Use again

Illustration 145 Foreign objects have damaged the bore of this planet gear.

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Illustration 146 Use a stone and oil in order to remove the sharp edges around the nick (1) before reusing this gear.

Use again

Illustration 147 If a bore has stains like these, you should clean the bore of the gear. Refer to the Cleaning section of this guideline for more information. If you clean the gear bore and you still find instances of pitting, do not reuse the gear. The standard cleaning procedures were applied to the bore of the gear in this illustration. The discolored lines were still there. The discolored lines inside this gear bore are instances of pitting.

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Illustration 148 Like the bore in the previous illustration, the stains could not be removed from the bore of this gear. The stains are instances of pitting.

Do not use again

Illustration 149 This gear bore has been damaged in both the area of the bearing roller track (1) and the area of the retaining washers (2).

Do not use again

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Illustration 150 Use good lighting when you inspecting any gear bore. If the surface of the bore has an irregular contour, the beams of light will not be straight.

Do not use again

Illustration 151 The bearing retaining washers have spun inside the gear bore.

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Illustration 152 Brinelling will create a washboard appearance and texture on the surface of the bearing bore. Do not reuse a gear with a bore that has brinelling damage.

Do not use again

Illustration 153 This gear bore has corroded. Notice how the corrosion makes a pattern of parallel bands in the bore.

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Illustration 154 The bore on this planet gear has spalled (arrows).

Do not use again

Illustration 155 The gear bore that is shown in the above illustration has corroded.

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Illustration 156 Examine both edges of the gear bore. This nick is along the edge of the gear bore and is the only instance of damage to the bore. Therefore, the gear may be reused. The nick has not penetrated the case hardened surface. Polish any protruding edges that are around the nick with a stone and oil.

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Gears with Replaceable Bearing Races Damage to a bore with a replaceable bearing race occurs when the bearing cup rotates inside the gear bore. Planet gears with replaceable bearing races can often be used again if the race has only crept in the bore. Before you reuse the gear check the size of the gear bore in order to determine that it is still within reusability specifications. Refer to SEBF8185 “Salvage Procedure for Final Drive Planet Gears Used in Off-Highway Trucks”, for more information about gears that have replaceable bearing races.

Illustration 157 The bearing race has crept inside this bore and has caused only a small amount of damage. If the bore is still within specification, it may be used again. The dark brown or black appearance may be caused from gear oil and can be removed with an 8T-7765 Scotch Brite Pad and solvent. If the bore is not within tolerance, follow the Reuse and Salvage procedure that is outlined in SEBF8185 “Salvage Procedure for Final Drive Planet Gears Used in Off-Highway Trucks”, before you install the new bearing.

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Illustration 158 The bearing race that was in this gear has spun inside the gear bore. If a gear bore is blue in color, do not reuse it. The blue temper color in the bore was caused by extreme temperatures (overheating). If a gear bore becomes overheated, it cannot be used again.

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Spider Gear Bores

Illustration 159 The bore of this gear has spun around a shaft. The bore is beyond the limit for reuse. The teeth are also heavily damaged.

Do not use again

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Splines Spline wear can be determined by the use of a seal pick. With a seal pick held perpendicular to the spline tooth, check for wear steps in the spline teeth that stop the seal pick. If the pick is stopped by any of the wear steps, do not reuse the gear. Visually check for other types damage on the spline before you reuse it.

Illustration 160 Use a seal pick (1) in order to determine if a spline can or cannot be reused. Hold the pick in a position that is perpendicular to the spline tooth that you are checking. Drag the seal pick across the wear step (3). If the pick is stopped by the wear step, do not reuse the spline. The fillet radius (2) is also shown above.

Do not use again

Illustration 161 This gear has spline wear that does not stop a seal pick. Check the wear at the ends of the spline engagement as well. As long as there is no other damage, this spline may be used again.

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Thrust Faces The thrust face on a planet gear, sun gear, or side gear should not have smears or heat checks. Heat checks are shallow cracks that interrupt the lube film and can shave off material from the thrust bearing. Gears with light thrust face damage can be used again. Light scratches are acceptable only when they do not stop a seal pick. If you notice a sharp edge on the thrust face, remove it with a stone and oil.

Illustration 162 This planet gear thrust face is in good condition. The gear may be used again as long as there are no cracks in the surface of the thrust face. Use one of the crack inspection methods that are described in the "Crack Inspection Methods" section of this guideline.

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Illustration 163 This differential side gear is approaching the wear limit, however, because this gear does not have any other damage, it may be used again. The roughness and damage (arrows) are at the maximum allowable limit for reuse.

Use again

Illustration 164 The roughness and damage (arrow) level of the complete thrust surface does not meet the standards for reuse. Also note the discoloration.

Do not use again

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Notes

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