{"data":{"fields":"term_definition","term_definition":"<div class=\"definition\">When one gear tooth drives another, the driving force F(n) acts along the line of action. This driving force has two components: - A tangential force F(t), and - A radial force F(r) The tangential component represents the work function transmitting the load from the driving shaft to the shaft being driven. The radial component represents the work expended in an attempt to separate the shafts, i.e., work dissipated in the form of heat generated in the bearings. Many factors determine the operating pressure of a pair of gears, but in applications calling for maximum gear efficiency, the operating pressure angle should be kept as low as other considerations will permit. Each tooth may be considered to be a cantilever beam supported at one end. Contact with its mating tooth exerts both bending and shearing forces, causing the requirement that the gear material have sufficient stiffness, flexural strength and shear strength to withstand the load. During gear travel, the mating teeth both roll on and slide over each other, causing a combination of compressive stress and stress due to friction on the tooth surface at point of contact. First, there is an initial compressive loading caused by contact. The rolling action of the two surfaces pushes this compressive stress just ahead of the contact point. At the same time, sliding occurs causing frictional forces to develop a region of tensile stress just behind the point of contact. The areas in which these two stresses interact are the stress centers. In some gear designs, rolling forces will predominate; in others, sliding forces may be more critical, but all will have a combination of both no matter how well-designed or expertly manufactured. When the contact point on the tooth and the pitch point coincide, the sliding forces on each tooth face change direction and nullify each other; only rolling forces will apply. However, this does not mean that the pitch point is less susceptible to surface failure. In fact, it is at the pitch point that serious failure is most likely to occur. Profile Bearing Length Bearing While the pitch point does not experience compound stresses, it does bear high unit loading. This is because at both start and end of tooth contact, the previous pair of teeth should still be bearing some of the load, if the gears have been properly designed. At mid-contact, however, one tooth pair is usually bearing the entire load at or slightly above the pitch line. This high unit loading can cause fatigue, severe heat build-up and or surface deterioration that may lead to failure. Contributing to this stress is the factor of non-uniform wear above and below the pitch line. Because the sliding forces are not at work in this area, the pitch point tends to wear less than the rest of the tooth face. If wear is particularly severe on the balance of the tooth surface, the region of the pitch line can become elevated. This concentrates and increases the loading pressure of contact which, in turn, causes proportionally higher fatigue a d heat build-up in the area. Analyzing for stress in any gear design must take all the effects of gear action into account in order to ensure optimum gear performance.<\/div>"}}