Scoring, gear

Gear elements with adequate hardness and surface finish have a greater resistance to initial scoring, a destructive phenomena found in meshing teeth. 

The exact events that make up the Candidate Physical Ability Test vary from place to place, but the tasks you have to perform are almost always job-related—they are a lot like the physical tasks you will actually have to perform as a firefighter. Some tests are set up as obstacle courses others consist of a group of stations. In some, you are timed from start to finish with no breaks others allow a break period between stations. The tests are timed. Your performance on the test is scored depending on that time. Often you have to wear full (heavy) protective gear, including an air pack, throughout these events. Here is an example of the events in a test that you would typically have ten minutes, 20 seconds to complete ... 

FINKIN,E.F., GU,A. and YONG,L. A Critical Examination of The Elastohydrodynamic Criterion for the Scoring of Gears, Trans. ASME, J.Lub.Tech., July 1974, 418-425. 

Wear, scoring, material flow, pitting, fracture, creep, and fatigue cause plastic and metal gears to fail. Continuous lubrication can increase the allowable bending stress by a factor of at least 1.5. However there are plastics (acetals, nylons, fluoropolymers, and others) that operate efficiently with no lubrication. There are plastics with wear resistance and durability of plastic gears makes them exceptionally useful. 

Abrasion creates unique wear patterns on the teeth. The pattern varies, depending on the type of abrasion and its specific forcing function. Figure 14.36 illustrates severe abrasive wear caused by particulates in the lubricating oil. Note the score marks that run jfrom the root to the tip of the gear teeth. 

An investigation of the scoring resistance of the gearset, which is a function of the gear-tooth geometiy, the surface finish of the teeth, and the properties of the lubricant... 

Regardless of the application, the scoring or scuffing resistance of the gear teeth should be investigated. In many cases, it will be desirable to use an oil with appropriate extreme-pressure additives that greatly increase the antiweld or antiscoring characteristics of the lubricant. 

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