Helical gears have been developed from spur gears, and their teeth lie at an angle to the axis of the shaft. Contact between the teeth in mesh acts along the diagonal face flanks in a progressive manner; at no time is the full length of any one tooth completely engaged. Before contact ceases between one pair of teeth, engagement commences between the following pair. Engagement is therefore continuous, and this fact results in a reduction of the shock which occurs when straight teeth operate under heavy loads. Helical teeth give a smooth, quiet action under heavy loads; backlash is considerably reduced; and, due to the increase in length of the tooth, for the same thickness of gear wheel, the tooth strength is improved.
Figure 24.20 illustrates the lead and helix angle applied to a helical gear. For single helical gears, the helix angle is generally 12°-20°.
Since the teeth lie at an angle, a side or end thrust occurs when two gears are engaged, and this tends to separate the gears. Figure 24.21 shows two gears on parallel shafts and the position of suitable thrust bearings. Note that the position of the thrust bearings varies with the direction of shaft rotation and the 'hand' of the helix.
End thrust Driver
End thrust Driver
In order to eliminate the serious effect of end thrust, pairs of gears may be arranged as shown in Fig. 24.22 where a double helical gear utilizes a left- and a right-hand helix. Instead of using two gears, the two helices may be cut on the same gear blank.
Where shafts lie parallel to each other, the helix angle is generally 15°-30°. Note that a right-hand helix engages with a left-hand helix, and the hand of the helix must be correctly stated on the drawing. On both gears the helix angle will be the same.
Fig. 24.22 Double helical gears. (a) On same wheel (b) On separate wheels
angle less than 90°, but the hand of helix should be checked with a specialist gear manufacturer. The hand of a helix depends on the helix angle used and the shaft angles required.
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