How to Fix Gearbox Failures

1. Before disassembling the gearbox, inspect the exterior. a.) Note the condition of the seals and keyways and whether they are damaged. This could indicate if extra contaminants could get into the gear box or if there is leakage from the gear box. b.) Take the gear tooth contact patterns.

2. Disassemble the gearbox. a.) Examine the gear teeth and bearings and record their condition. b.) Obtain the following information to calculate the load capacity of the gear set: number of teeth, outside diameter, face width, gear housing center distances for each gear set, whole depth of teeth, tooth thickness. c.) Look for signs of corrosion, contamination, and overheating.

3. Wash the parts with solvent and re-examine them per step 2C.

4. Check the oil filter and magnetic plug for wear debris and contaminants.

Now that you have an understanding of the environment, you can begin determining the type of gear failure:

1. Bending failure - It is caused by repeated loading, starts as a crack that crows until the part fractures. The fatigue crack is called a "beach mark", resembling a shell. The crack origin will have several beach marks around it. Most fatigue failures occur in the tooth root fillet, where the gear tooth meets the gear wheel.

2. Contact failure - It is caused by repeated stresses that cause cracks and detachment of metal fragments from the tooth contact surface. The common forms are pitting. Micro-pitting will have a frosted matte, usually a gray-stained appearance. This most often occurs when a gear is under too much load.

3. Wear - Tooth wear is caused by mechanical, chemical, or electrical action. It can take 3 forms: adhesion, abrasion, or polishing. a.) Adhesion is the transfer of material from one tooth to another due to welding and tearing. Mild adhesion occurs during gear set run in. Adhesion will subside after wearing imperfections on the surface. Mild adhesion occurs in patches. Moderate adhesion will remove machining marks from the contact surface, and will not subside and will affect the entire tooth and probably multiple gear teeth. Surfaces will have a rough or matte texture. b.) Abrasion is caused by contaminants in the lubricant; scale, rust, chips, grinding dust, weld splatter, and wear debris. It will appear as smooth, parallel scratches or gouges. When severe, it will remove all machining marks, may wear steps at the end of the contact surface, and can reduce tooth thickness. This kind of wear can be confirmed by tests of the hydraulic fluid for contaminants. c.) Polishing is caused by a fine abrasion that gives a mirror-like finish to gear teeth. Severe polishing removes all machining marks and can leave the surface wavy. This kind of wear can also be confirmed by tests of the hydraulic fluid for contaminants.