10.2 HELICAL GEARS

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Helical gears are essentially spur gears, but with a bit of a twist in the normally straight profile of the teeth.

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The helical arrangement means that the teeth engage at one point (as opposed to a line of contact), and then slowly mesh along the face of the tooth. This also means that the number of engaged teeth (contact ratio) can be higher.

If we contrast spur and helical gears. The spur gear teeth contact fully all at once.

These gears mesh very smoothly, so they find application for,

- high speeds

- heavy loads

- gear noise must be reduced

- center lines of shafts do not intersect

These gears,

- require more effort during fabrication

- need additional bearings to resist axial thrust.

helical gears can be used to transmit torques between parallel, and non-parallel (often perpendicular) shafts.

To eliminate the need for an axial thrust bearing we can use a herringbone (double helical) gear. This is effectively two helical gears with opposite twists on the helix, and joined down a center line.

10.2.1 Design of Helical Gears

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We can describe the helix with a single angle,

This helix allows three different ways to measure the pitch and pressure angles.

We can now develop an equivalent pitch radius,

Typical design parameters include,

- produced for specialized applications, and custom designed

- typical pressure angle is 20°

- for helix angles from 0 to 30° use the normal diametral pitch to calculate tooth proportions

- helix angles greater than 45° are not recommended

- in mating parallel gears - one must have a left hand helix, and the other must have a right hand helix. Use the sign of the helix angle to indicate left or right handed.

The line of contact in spur gears is straight across the teeth. In helical gears, the line of contact is diagonal.

To measure the contact ratio we need to use three values to be effective,

10.2.2 Perpendicular Helical Gears

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If two shafts intersect at an angle (typically 90°) we can link them using helical gears. The angle between the shafts can be,

The example below shows one configuration for helical gears, including the location of thrust bearings.

When the shaft intersection angle is large (90°) you may use same handed gears to intersect. In the example above the gears are both right handed.

The pitch diameter for these gears can be found using,

A minimum contact ratio of 2 is recommended for these gears.

Typical parameters for cross-axis helical gears are given below,

10.2.3 Practice Problems

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