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.
E71138
1
2
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
E71138
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.
Use again
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.
Use again
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.
Do not use again
Illustration 103
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.
Do not use again
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.
Do not use again
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.
Use again
Illustration 107
This nick (1) occurred before heat treatment. Before you reuse this gear, verify that the damage is not a crack.
Use again
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.
Use again
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.
Use again
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.
Do not use again
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.
Do not use again
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.
Do not use again
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.
Do not use again
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.
Chipping
Illustration 115
The chip (arrow) on this gear is from mishandling. Unlike a nick, a chip will always occur after heat treatment.
Use again
Illustration 116
This is a magnification of Illustration 115
Use again
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.
Use again
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
Illustration 119
The chipping damage extends onto the contact surface of the gear tooth.
Do not use again
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.