A hypoid gear is a style of spiral bevel gear whose main variance is that the mating gears' axes do not intersect. The hypoid gear is offset from the gear center, allowing unique configurations and a large diameter shaft. The teeth on a hypoid gear are helical, and the pitch surface is best described as a hyperboloid. A hypoid gear can be considered a cross between a bevel gear and a worm drive.

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Operation

Hypoid gears have a large pitch surface with multiple points of contact. They can transfer energy at nearly any angle. Hypoid gears have large pinion diameters and are useful in torque-demanding applications. The heavy work load expressed through multiple sliding gear teeth means hypoid gears need to be well lubricated, but this also provides quiet operation and additional durability.

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Specifications

Hypoid gears are common in truck drive differentials, where high torque and an offset pinion are valued. However, an offset pinion does expend some mechanical efficiency. Hypoid gears are very strong and can offer a large gear reduction. Due to their exclusive arrangement, hypoid gears are typically produced in opposite-hand pairs (left and right handedness). 

Dimension Specifications 

Gears mate via teeth with very specific geometry. Pressure angle is the angle of tooth drive action, or the angle between the line of force between meshing teeth and the tangent to the pitch circle at the point of mesh. Typical pressure angles are 14.5° or 20°, but hypoids sometimes operate at 25°. Helix angle is the angle at which the gear teeth are aligned compared to the axis.

Helix angle identified - Image credit: Wikimedia

Selection tip: Gears must have the same pitch and pressure angle in order to mesh. Hypoid gear arrangements are typically of opposite hands, and the hypoid gear tends to have a larger helical angle.

Mounting Specifications

The offset nature of hypoid gears may limit the distance from which the hypoid gear's axis may deviate from the corresponding gear's axis. Offset drives should be limited to 25% of the of the mating gear's diameter, and on heavily loaded alignments should not exceed 12.5% of the mating gear's diameter. 

Hypoid Gear Accessories 

To cope with the sliding action and heavy work loads for hypoid gears, high-pressure gear oil is necessary to reduce the friction, heat and wear on hypoid gears. This is particularly true when used in vehicle gearboxes. Care should be taken if the gearing contains copper, as some high-pressure lubricant additives erode copper.

Image credit: Jaytec

Applications

Application requirements should be considered with the workload and environment of the gear set in mind.

Power, velocity and torque consistency and output peaks of the gear drive so the gear meets mechanical requirements.

Inertia of the gear through acceleration and deceleration. Heavier gears can be harder to stop or reverse.

Precision requirement of gear, including gear pitch, shaft diameter, pressure angle and tooth layout. Hypoid gears' are usually produced in pairs to ensure mating.

Handedness (left or right teeth angles) depending the drive angle. Hypoid gears are usually produced in left-right pairs.

Gear lubrication requirements. Some gears require lubrication for smooth, temperate operation and this is especially true for hypoid gears, which have their own types of lubricant.

Mounting requirements. Application may limit the gear's shaft positioning.

Noise limitation. Commercial applications may value a smooth, quietly meshing gear. Hypoid gears offer quiet operation.

Corrosive environments. Gears exposed to weather or chemicals should be especially hardened or protected.

Temperature exposure. Some gears may warp or become brittle in the face of extreme temperatures.

Vibration and shock resistance. Heavy machine loads or backlash, the deliberate surplus space in the circular pitch, may jostle gearing.

Operation disruption resistance. It may be necessary for some gear sets to function despite missing teeth or misalignment, especially in helical gears where axial thrust can reposition gears during use. 

Materials

Gear composition is determined by application, including the gear's service, rotation speed, accuracy and more.

• Cast iron provides durability and ease of manufacture.
• Alloy steel provides superior durability and corrosion resistance. Minerals may be added to the alloy to further harden the gear.
• Cast steel provides easier fabrication, strong working loads and vibration resistance.
• Carbon steels are inexpensive and strong, but are susceptible to corrosion.
• Aluminum is used when low gear inertia with some resiliency is required.
• Brass is inexpensive, easy to mold and corrosion resistant.
• Copper is easily shaped, conductive and corrosion resistant. The gear's strength would increase if bronzed.
• Plastic is inexpensive, corrosion resistant, quiet operationally and can overcome missing teeth or misalignment. Plastic is less robust than metal and is vulnerable to temperature changes and chemical corrosion. Acetal, delrin, nylon, and polycarbonate plastics are common.
• Other material types like wood may be suitable for individual applications.

Resources

Gear Info - Hypoid Gears

Access Engineering Library - Standard Handbook of Machine Design, Ch. 11 Bevel and Hypoid Gears

Wikipedia - Spiral Bevel Gears

Agro Engineers - Bevel Gears

Image credit:

Weichai Power Co., Ltd.