Helical Gears
The spur gear that has helix teeth (helicoids teeth) is called Helical Gear. Helical gears can transmits force between parallel shafts and non parallel shafts. They are widely used in different industries, such as the automotive, and in industrial machinery. Helical gears can bear load more than spur gears and work more quietly. Spur gears make noise due to sudden contact between gear teeth at the time of engagement, but helical gears make smooth contact from one edge of the tooth the other. This provides smooth and noiseless operation with higher efficiency than spur gears.
Classification of Helical gears
Parallel helical gears (used to transmit power between parallel shafts)
Double helical gears or herringbone gears.
Cross helical gears or skew gears (used to transmit power between non parallel shafts)
1. Parallel helical gears 2. Double helical gears 3. Cross helical gears
Parallel helical gears
Helical gears that are used to transmit power between two parallel shafts are called as parallel helical gears.
Double helical gears
Double Helical gears have two sections of teeth, right hand and the other left hand. They are designed to eliminate the thrust load induced in helical gears.
Cross helical gears
Cross helical gears are used in intersecting shaft with the shaft angle at 90 degrees.Crossed Helical Gear are only recommended for a narrow range of applications, with relatively light loads. What happens is that the for the two gears contact takes place only in a point, and not a line.
Transverse module and Normal module
The difference between transverse module and normal module is defined as the difference of basic tooth form. As shown in the figure below the module of tooth datum orthogonal to the center axis of gear is called transverse module. The module of tooth datum orthogonal to the thread helix is called normal module.
Difference between normal and transverse module
The characteristics of each module is Shown below
Force Analysis
Because of the helix in the teeth of helical gears, while gears mesh thrust forces is developed in the axial directions. The axial thrust forces will force the gear to move towards the direction of bearing. Hence thrust bearing have to be placed in the direction of axial force. The axial thrust bearings must be able to resist the developed thrust forces. The direction of the thrust forces depend on the helix hand and the direction of rotation as shown below. The position of thrust bearings is also shown in the figure below. Where, Ft is the tangential force, Fr is the radial force, Fx is the axial force. Larger the helix angle of teeth, larger the thrust.
