Helical Gear: Types, Nomenclature, Advantage & Disadvantage

Helical gear:

It is Most expensive method of power transmission with heavy load to one shaft to another shaft than spur gear.A simple configuration modified in the helical gear as compared to ordinary spur gear. Helical gear have helix form around the teeth or gear. That is Most common gear used in modern industry for automobiles,turbine and highest speed application. It configure the teeth of two wheel are of opposite direction.The helixes may right handed on one wheel and left handed on other wheel.

single helical gear set
single helical gear set

Advantages of helical gear in modern industries:

That is three main reason for better as compared to spur gear. 1) Noise 2) load carrying capacity 3) manufacturing method


The less noise produced during the operation as compared to spur gear of equivalent in quality because the total contact ratio will be increased. When the engagement of teeth at any time more than one pair of teeth ( up to 10 pair of teeth). The load is transmitted for one shaft to another shaft for gradually and uniformly as successive teeth come into engagement. And also this helical gear operate in smoothly.


The more load carrying capacity capable of helical gear as compared to spur gear for very large capacity. The total length of line of contact will be increased. when the spur gear having the line of contact parallel to axis of rotation and the total length of line contact is equal to face width. For helical gear, the total length of line contact is diagonal across face of teeth. So in this case, the line of contact is more than face width in helical gear, which is lower unit loading and increase load carrying capacity.

helical geal line of contact
helical geal line of contact


The Limited number of standard cutter are used to cut the variety of helical gear. That is simply by varying helix angle.

Disadvantages of helical gear teeth

Teeth are inclined to axis of the rotation, so gear are subjected to axial thrust load. Which thrust load eliminated by using herringbone gear( double helical gear).

Different types of helical gears

Parallel axis helical gear

The gear are operate in two parallel shaft
The magnitude of helical angle same for pinion and gear
They have opposite of helix angle

Crossed helical gear(spiral gear)

That operate on two non parallel shaft
Girls having same or opposite hand of helix.

Kinematics of helical gear and Nomenclature

They have view at three ways of teeth on rack.

Transverse direction

The pair of helical gear view in direction of motion of the rack (section A-A as shown in fig), this orientation is called transverse direction. For this transverse plane( the plane perpendicular to action of the gear), all helical gear geometry is identical to that of spur gear.

Helical gear Geometry of Nomenclature
Helical gear Geometry of Nomenclature
Helical Gear Transverse Plane
Helical Gear Transverse Plane
Helical Gear Transverse Plane
Helical Gear Transverse Plane

Normal direction

If the teeth are consider for direction that aligned with the slant of teeth (section B-B as shown in fig), which orientation is called normal direction.

Axial direction

The teeth are viewed in direction on the axis of the rack ,which organisation is called actual direction.

β = Helix angle
Pt= Transverse circular pitch
Pn= Normal circular pitch
Pa= Axial pitch
Pd= Diametral pitch
∝1 and ∝2 = Transverse and normal pressure angle
Mt and Mn= Transverse and normal module
Z1 and Z2 = Number of teeth on pinion and gear
d1 and d2= Pitch circle diameter for pinion and gear respectively
N1 and N2= Speed of pinion and gear
a = Standard distance between gear and pinion

Helix angle or spiral angle

It is angle between tooth axis and plane containing the wheel axis. It is known as helix angle. The Helix angle varies from 15° to 25°.


The distance measured for each teeth per revolution along the axis of parallel to the axis.

Transverse circular pitch

The distance measured by corresponding point on the adjacent teeth in plane perpendicular to the axis of the shaft is known as transverse circular pitch.

Pt = πd1/z1 = Pn/ cos ß

Normal circular pitch

The distance measured by corresponding point on the adjacent teeth in plane perpendicular to helix is known as normal circular pitch.

Pn = Pt x cos ß = π.Mn

Axial pitch

The distance measured by corresponding point on the adjacent teeth in parallel plane to the axis shaft is known as axial pitch.

Pa = Pt x tanß = Pn/sinß = π.Mn/sinß

Normal diametral pitch

That is reciprocal of normal module is known as normal diametral pitch.

Pd = 1/Mn = π/Pn

Transverse pressure angle(∝t)

The pressure angle measured in transverse plane along the plane (A-A) is known as transverse pressure angle.

Normal pressure angle(∝n)

The pressure angle measured in normal plane along with the plane (B-B) is known as normal pressure angle.

cosß = tan ∝n / tan ∝t

Pitch circle diameter of pinion and gear

Pitch circle diameter of pinion d1 = (Mn/ cosß) Z1
Pitch circle diameter of gear d2 = (Mn/ cosß) Z2

Centre distance(a)

The centre to Centre distance between gear is called Centre distance.

a = (Mn/ cosß) x (Z1+Z2)/2

Speed ratio(i)

The ratio between speed of driving gear to driven gear or the ratio between number of teeth of driven gear to driving gear.

Herringbone gear or double helical gear

herringbone gear or Double gear Teeth
herringbone gear or Double gear Teeth

The herringbone gear or helical gear having consist of teeth,right and left helix cut on the same blank as shown in figure. The main drawback of the herringbone gear is single helical gear for existence of axial thrust load. They will be eliminated by herringbone configuration, because of the thrust force of right hand is balanced by the that of left hand helix. In this configuration the absence of thrust reaction, when the helix angle usually greater for herringbone gear than single helical gear.

Crossed helical gear or spiral gear or screw gear or skew gear

The pair of crossed helical gear is known as spiral gear. The spiral gear is used to connect and transmit motion in between two non parallel shaft and non intersecting shaft. In spiral gear the contact between mating teeth is always consider as a point, This spiral gear is suitable only for transmit small amount of power. In order for two helical gear are operate as crossed helical gear, they having same normal diameteral pitch and normal pressure. But the gear do not need the same helix angle or being opposite of hand. The most of crossed gear application should be having same hand.

Most advantages of spiral gears

That provide noiseless operation due to smooth engagement.
The spiral gear permit wide range of speed for without change of centre distance or gear size.
The spiral gear can be used at any angle other than 90 degree

Some limitation of spiral gear drive:

That transmit only small amount of power because point contact between pair of teeth.
During the operation the sliding action provided, So the possible of low efficiency than that of toothed gear.


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