subject: Something about Spur Gears and Helical Gears [print this page] Something about Spur Gears and Helical Gears
Spur gear is the simplest type of gear which consists of a cylinder or disk. Its form is not straight-sided, thus, the edge of each tooth is straight and aligned parallel to the axis of rotation. It is the most basic and available type.
As the most common type, spur gears are often used because they are the simplest to design and manufacture. Besides, they are the most efficient. Gear teeth could be manufactured with a wide variety of shapes and profiles.
Nowadays, the involute profile is the most commonly used system for gearing. An involute is a curve that is traced by a point on a taut cord unwinding from a circle, which is called a base circle. The involute is a form of spiral, the curvature of which becomes straighter as it is drawn from a base circle and eventually would become a straight line if drawn far enough.
An involute drawn from a larger base circle will be less curved (straighter) than one drawn from a smaller base circle. Similarly, the involute tooth profile of smaller gears is considerably curved, on larger gears is less curved and is straight on a rack, which is essentially an infinitely large gear. Additionally, gear teeth also have a close relationship with the whole mechanical part. Gear failure can occur due to tooth breakage (tooth stress) or surface failure as a result of fatigue and wear.
Helical gear is also an important type that can be widely applied. It offers additional benefits relative to spur gears. For instance, they have improved tooth strength due to the elongated helical wraparound and increased contact ratio due to the axial tooth overlap. Moreover, they also tend to have greater load carrying capacity than spur gears of similar size.
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 under loads which gradually increase to the maximum expected.
