{"data":{"fields":"term_definition","term_definition":"
A helical gear is essentially a spur gear with teeth winding around a cylinder. They are high-efficiency gears (Efficiencies of 98 to 99%) which are typically used when high speeds and high horsepower are involved. Helical gears are preferred over spur gears in many applications due to their smoother and quieter operation. The involute tooth profile is normally used for helical gears as small variances in center distance will not affect tooth action. However, helical gears require perfect tooth profiles and exactly equal helix angles for proper performance. These requirements can be difficult to fulfill, particularly when mating gears are of dissimilar materials. Its teeth are helical or of screw thread form in relation to the axes. The tooth profiles are similar in all normal plane sections. Line contact occurs with the line of contact being straight and oblique to the axes.\r\n\r\nA helical rack has straight teeth which are set at an angle to the direction of motion. The teeth of a LEFT HAND helical gear lean to the left when the gear is placed flat on a horizontal surface, observing the top area. The teeth of a RIGHT HAND helical gear lean to the right when the gear is placed flat on a horizontal surface, observing the top area. Helical gears of the same hand operate at right angles. Helical Gears of opposite hands run on parallel shafts. The helical tooth form is involute in the plane of rotation and can be developed in a manner similar to that of the spur gear. However, unlike the spur gear, which may be viewed as two-dimensional, the helical gear must be viewed as three-dimensional to show change in axial features.\r\n\r\nHelical gears offer additional benefits relative to spur gears, those being:\r\n-Improved tooth strength due to the elongated helical wrap-around.-Increased contact ratio due to the axial tooth overlap.\r\n-Helical gears thus tend to have greater load carrying capacity than spur gears of similar size.\r\n-Due to the above, smoother operating characteristics are apparent.\r\n-When helical gears are operated on other than parallel shafts, the tooth load is concentrated at a point, with the result that very small loads produce very high pressures. The sliding velocity is usually quite high and, combined with the concentrated pressure, may cause galling or excessive wear, especially if the teeth are not well lubricated. For these reasons, the tooth load which may be applied to such drives is very limited and of uncertain value, and is perhaps best determined by trial under actual operating conditions. If one of the gears is made of bronze, the contact area and thereby the load carrying capacity, may be increased, by allowing the gears to \u201crun-in\u201d in their operating position, under loads which gradually increase to the maximum expected.<\/div>
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