Copyright Goodheart-Willcox Co., Inc. 593 Chapter 17 ∣ Final Drives When adhesive wear occurs due to a lack of lubrication, the component is darkened and discolored. A colored oxide layer known as a heat stain will form on carbon steel due to overheating. See Figure 17-34. A color scale can be used as a reference to gauge the temperature reached by the component. Examples of color and tem- perature are as follows: • Temperatures above 700°F/371°C form a dark gray oxide layer and cannot be precisely determined by the color of the oxide layer. • Gray (700°F/371°C). • Light blue (650°F/343°C). • Dark blue (590°F/310°C). • Purple (540°F/282°C). • Brown (500°F/260°C). • Straw color (400°F/204°C). Note A different color temperature chart is used for metal that is being actively heated. However, that chart is not used for failure analysis because a technician is not able to observe the component in the process of being overheated. Foreign particles, or contaminants, in the lubricant cause abrasive wear to compo- nents. Abrasive wear normally takes longer to cause component failure than adhesive wear. To detect abrasive wear, follow proper oil sampling techniques at the recommended manu- facturer’s service interval. By comparing the sample results over a period of time, it can be determined whether the final drive fluid meets cleanliness specifications or whether it contains contaminants. Analysis of the sample can identify any contaminants present. The equipment personnel can then determine whether the fluid needs to be changed to extend the life of the component and take appropriate action to prevent further contamination. Corrosion wear is the damage to a component caused by wear that occurs in a cor- rosive environment due to the effects of chemical reaction and mechanical action. Water is the substance that most commonly causes corrosion wear. See Figure 17-35. Fractures A component fracture is a failure that occurs when a single part breaks into two or more pieces. Fractures propagate from an area of concentrated stress called a stress riser. Stress risers can be caused by part design, material imperfections, poor material handling pro- cesses, and poor manufacturing processes. Overloads can cause a fracture. Overloads are generally the result of a single stress load that exceeds the component’s tensile strength or yield strength. Tensile strength is the amount of stress that can be placed on a component (such as a bolt or shaft) before it stretches and breaks. Yield strength is the point at which a component deforms plastically (does not return to its original condition after deformation). A cyclical overload failure can occur when an operator repeatedly exceeds the machine’s operat- ing capacity. For example, a ring gear that has been Goodheart-Willcox Publisher Figure 17-34. These gears have been overheated due to a lack of lubrication. The gear on the left shows adhesive wear. Heat caused a softening of the metal and a darkening of the gear. Goodheart-Willcox Publisher Figure 17-35. When water entered this transmission case through a faulty seal, the oil turned to a thick sludge, resulting in gear noise caused by corrosive wear.