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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 left-hand gear; the other a right-hand gear).

Helical Gears Connecting Non-Parallel Shafts Helical gears used to connect non-parallel 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 non-parallel 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 well-lubricated. Extreme pressure (E.P.) lubricants are recommended.

Extreme pressure (E.P.) lubricants are not recommended on bronze gears.

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