



Helical Gears


The teeth on helical gears are cut at an angle to the face of the gear. When two
teeth on a helical gear system engage, the contact starts at one end of the
tooth and gradually spreads as the gears rotate. Two mating helical gears must
have equal helix angle but opposite hand. They run smoother and more quietly.
They have higher load capacity, are more expensive to manufacture. Helical
gears can be used to mesh two shafts that are not parallel and can also be used
in a crossed gear mesh connecting two perpendicular shafts. They have longer
and strong teeth. They can carry heavy load because of the greater surface
contact with the teeth. The efficiency is also reduced because of longer
surface contact. The gearing is quieter with less vibration.
One interesting thing about helical gears is that if the
angles of the gear teeth are correct, they can be mounted on perpendicular
shafts, adjusting the rotation angle by 90 degrees.


Right Hand Helical Gears

Left Hand Helical Gears

A helical gear is similar to a spur gear except that the teeth of a helical gear
are cut at an angle (known as the helix angle) to the axis (or hole). Helical
gears are made in both right and left hand configurations. There are two configurations of helical gear connections:
those connecting parallel shafts and those connecting nonparallel
shafts.
Helical Gears Connecting Parallel Shafts Helical gears connecting parallel shafts will run more
smoothly and quietly than spur gears, particularly when the
helix angle is great enough to ensure that there is continuous
contact from one tooth to the next. A pair of helical gears
used to connect parallel shafts must have the same pitch,
pressure angle and helix angle, but they will be opposite
hand gears (that is, one will be a lefthand gear; the other
a righthand gear).
Helical Gears Connecting NonParallel Shafts
Helical gears used to connect nonparallel shafts are
commonly called spiral gears or crossed axis helical gears.
If the shaft angle is 90 degrees, the gears will be of the same
hand and the sum of the helix angles will be equal to the
shaft angle (90 degrees).
Helical gears used on nonparallel shafts must have the same
normal pitch and normal pressure angles. They may, however, be
of the same or opposite hand depending on the shaft angle.


Fig. 1 Outside Diameter

Fig. 2 
A helical gear has two major circles:
1) the outside circle and 2) the pitch circle.
The outside circle is the distance around the outer edge
of the gear’s teeth. (1 and 2) The diameter of the
outside circle is called the outside diameter.
(Figure 1)
The pitch circle is the imaginary circle found at the
point where the teeth of two gears mesh (come in
contact, See 2 and 4).The diameter of the pitch circle
is called the pitch diameter. (Figure 2)



Terms used in helical gears:
1. Helix angle: It is a constant angle made by
the helices with the axis of rotation.
2. Axial pitch: It is the distance, parallel to the axis, between similar faces of adjacent teeth. It is same as circular
pitch and is therefore denoted by p_{c}. The axial pitch may also be defined as the circular pitch in the plane of rotation
or the diameteral plane.
3. Normal pitch: The normal pitch of a helical gear is the pitch of the tool
used to cut the teeth. It is measured in a plane perpendicular
to the direction of the teeth.
Normal diametral pitch (D.P.) = 3.146 ( Normal circular pitch (C.P.))
4. Transverse Pitch: The transverse pitch of a helical gear corresponds to the pitch
of a spur gear with the same number of teeth and the same
pitch diameter. It is measured in the plane rotation of the gear.
Transverse diametral pitch (D.P) = 3.1416 (Transverse circular
pitch (C.P.))
5. Normal Pressure Angle: Normal pressure angle is the pressure angle in the normal
plane of a helical gear tooth.



Helical Gear Formula


S. no. 
To Find 
Having 
Rule

1 
Transverse
Diametral Pitch 
Number of Teeth and Pitch Diameter 
Divide the Number of Teeth
by the Pitch Diameter

2 
Transverse Diametral Pitch 
Normal D.P. and Helix Angle 
Multiply the Normal D.P. by the cosine of the Helix Angle 
3 
Pitch Diameter 
Number of Teeth and Transverse D.P. 
Divide the Number of Teeth by the Transverse D.P. 
4 
Normal Diametral Pitch 
Transverse D.P. and Helix Angle 
Divide the Transverse D.P. by the cosine of the Helix Angle 
5 
Helix Angle 
Transverse D.P. and Normal D.P. 
Divide the Transverse D.P. by the Normal D.P. — Quotient is the
cosine of the Helix Angle

6 
Transverse Pressure Angle 
Normal P.A. and Helix Angle 
Divide the tangent of the Normal P.A. by the cosine of the Helix Angle.
Quotient is tangent of Transverse P.A.

7 
Normal Circular Tooth Thickness 
Normal Diametral Pitch 
Divide 1.5708 by the Normal Diametral Pitch

8 
Addendum 
Normal Diametral Pitch 
Divide 1 by the Normal Diametral Pitch

9 
Outside Diameter 
Addendum and Pitch Diameter 
Add 2 Addendums to the Pitch Diameter

10 
Whole Depth (Coarser than 20 D.P.) 
Normal Diametral Pitch 
Divide 2.250 by the Normal Diametral Pitch

11 
Whole Depth (20 D.P. and Finer) 
Normal Diametral Pitch 
Divide 2.200 by the Normal D.P. and add .002

12 
Clearance 
Addendum and Whole Depth 
Subtract 2 Addendums from the Whole Depth



Applications:
These are highly used in transmission because they are quieter even at higher speed and are durable. The other possible applications
of helical gears are in textile industry, blowers, feeders, rubber and plastic
industry, sugar industry, rolling mills, food industry, elevators, conveyors, cutters, clay working machinery, compressors and in oil industry.


Disadvantage:
A disadvantage of helical gear is the resultant thrust along the axis of the
gear, which needs to be accommodated by appropriate thrust bearings. This can be
overcome by the use of double helical gears by having teeth with a 'v'
shape.


Lubrication
Helical gears should be properly lubricated to: minimize
wear, prevent the generation of excessive heat, improve
efficiency through the reduction of friction between the
mating tooth surfaces, reduce noise and inhibit the
formation of rust.
Good lubrication depends on the formation of a film thick
enough to prevent contact between the mating surfaces.
The relative motion between gear teeth helps to produce
the necessary film from the small wedge formed adjacent
to the area of contact.
It is important that an adequate supply of the correct
lubricant is properly applied.
A straight mineral oil lubricant should be used for
most parallel shaft applications. Under heavy load
conditions, mild extreme pressure (E.P.) lubricants
are suggested.
Helical gears operating at right angles must always
be welllubricated. Extreme pressure (E.P.) lubricants
are recommended.
Extreme pressure (E.P.) lubricants are not
recommended on bronze gears.
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