What is a spiral bevel gear?

What is a spiral bevel gear? A spiral bevel gear is a type of conically shaped gear that has octoidal teeth.  The gear teeth of a spiral bevel pinion mesh with the teeth of a mating spiral bevel gear form a spiral bevel gear pair. They are a type of machine element commonly found in applications which require a change in direction and speed.

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Spiral bevel gears are used with spiral bevel pinions to create mechanical systems that change speed and torque primarily in perpendicular shaft applications. When the spiral bevel gear and the spiral bevel pinion have the same number of teeth, they are called spiral miter gears. For any spiral tooth spiral bevel gear combination, the mating pinion and the spiral bevel gear must be the same pitch, the same pressure angle, the same spiral angle, but opposite spiral direction. In addition, the pinion gear must be produced with a pitch angle that, when added to the pitch angle of the bevel gear, is equal to the reference cone angle. The reference cone angle is commonly known as the shaft angle. Figure 1 details this relationship.

A bevel gear can have straight teeth or spiral teeth. This article only covers spiral tooth bevel gearing. The teeth of spiral bevel gears are readily identifiable, as they are curved in nature and taper toward the intersection of the shaft axes. Spiral bevel gears can be grouped into two styles. One style is the Gleason type, and the other is the standard type.

Gleason type spiral bevel gears are designed specifically as profile shifted gears. The spiral bevel pinion is positively shifted, and the spiral bevel gear is negatively shifted. This is done in order to better distribute the gear strength within the pair. As miter gears are equal, there isn&#;t any shifting of Gleason miter gears. The tooth profile of a Gleason spiral bevel gear has the tooth depth h = 1.888m; tip and root clearance c = 0.188m; and working depth hw = 1.700m.             

Standard type spiral bevel gears do not have any profile shift and do exhibit some weakness when the number of teeth on the pinion is small relative to the number of teeth on the mating spiral bevel gear.

The teeth of a spiral bevel gear are generated using a specialized cutter on a spiral bevel generating machine. The cutting tool machines a section of the spiral bevel gear and then indexes. The number of teeth produced by each cutter is limited, as the cutter radius needs to account for the number of teeth on the mating spiral bevel gear and the required shaft angle. Spiral bevel gears can be produced from various materials; however, carbon or alloy steels are typically used. Softer materials including brass, bronze, or plastic are not suitable for the production of spiral bevel gearing. 

The geometry of a spiral bevel gear is defined by several parameters. The primary considerations of the spiral bevel gear are the outer diameter, the mounting distance, the cone distance, and the length through the bore. 

Table 1 details the calculations for a Gleason type spiral bevel gear pair.

The first value needed to produce a spiral bevel gear is the pitch. In the metric system, this is known as the module. As the value of the module increases, the size of the gear tooth increases. In the English standard system, the pitch of a helical gear is known as the diametral pitch (DP). It represents the number of teeth that are found on a gear with a one-inch reference diameter.

The pressure angle is the angle between the line of action of the gears and the tangent to the pitch circle. It determines the contact between the teeth of the gears and affects the load-carrying capacity and efficiency of the gears. In the English system, gears typically have values for pressure angle of 20 degrees or 14 degrees 30 minutes. For metric gears, the pressure angle is typically 20 degrees.

The number of teeth for the pinion is chosen by the end-user based on the speed ratio that is desired for the application and an understanding that values of less than 12 teeth is not practical for power transmission. The speed ratio of a singular pinion engaged with a spiral bevel gear is simply the number of teeth on the spiral bevel gear divided by the number of the number of teeth on the pinion. Speed ratios for straight spiral bevel pairs are limited by the size of the spiral bevel gear and the pitch angles. As such, they are limited in practice to ratios of 6:1 or less.

The addendum of a spiral bevel gear tooth is the linear distance between the pitch radius and the tooth tip measured at the heel of the spiral bevel gear tooth. Correspondingly, the dedendum is the linear distance between the pitch radius and the tooth root. The sum of the addendum and the dedendum determines the total tooth height.

Although not shown in Table 1, the value for backlash is very important for spiral bevel gear pairs. This value measures the distance between the spiral pinion gear teeth and the spiral bevel gear teeth when they are not in contact. It is necessary to have a minimum amount of backlash for the gear teeth to mesh properly and for lubricant to engage with the spiral bevel gear and spiral bevel pinion at their point of contact.

The design of a spiral bevel gear involves determining the pitch, pressure angle, shaft angle, mounting distance, and backlash. These factors are dependent on the desired speed ratio, power transmission requirements, and the design of the mechanical system. Spiral bevel gears will only transmit power between non-parallel axes. When the pinion is used as the driver, the pinion rotates; the teeth engage, and torque is transmitted from the pinion to the spiral bevel gear resulting in a reduction in speed, but an increase in torque. When the spiral bevel gear is used as the driver, the spiral bevel gear rotates, the teeth engage, and transmits torque from the spiral bevel gear to the pinion, resulting in an increase in speed but a decrease in output torque. This is a significant drawback if you are using spiral bevel gearing in a speed increaser.

Spiral bevel gears are a commonly used element in mechanical systems where a change in direction and speed is required, because they are simple in design, efficient in operation, and cost-effective. Due to the continuous tooth engagement, they are ideal for high-speed and high-torque applications.  Understanding the technical definitions and design principles of spiral bevel gearing is essential for anyone working with mechanical systems.&#;

Mechanical part

A spiral bevel gear is a bevel gear with helical teeth. The main application of this is in a vehicle differential, where the direction of drive from the drive shaft must be turned 90 degrees to drive the wheels. The helical design produces less vibration and noise than conventional straight-cut or spur-cut gear with straight teeth.

A spiral bevel gear set should always be replaced in pairs i.e. both the left hand and right hand gears should be replaced together since the gears are manufactured and lapped in pairs.

Handedness

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A right hand spiral bevel gear is one in which the outer half of a tooth is inclined in the clockwise direction from the axial plane through the midpoint of the tooth as viewed by an observer looking at the face of the gear.

A left hand spiral bevel gear is one in which the outer half of a tooth is inclined in the counterclockwise direction from the axial plane through the midpoint of the tooth as viewed by an observer looking at the face of the gear.

Note that a spiral bevel gear and pinion are always of opposite hand, including the case when the gear is internal.

Also note that the designations right hand and left hand are applied similarly to other types of bevel gear, hypoid gears, and oblique tooth face gears.[1]

Hypoid gears

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A hypoid is a type of spiral bevel gear whose axis does not intersect with the axis of the meshing gear. The shape of a hypoid gear is a revolved hyperboloid (that is, the pitch surface of the hypoid gear is a hyperbolic surface), whereas the shape of a spiral bevel gear is normally conical. The hypoid gear places the pinion off-axis to the crown wheel (ring gear) which allows the pinion to be larger in diameter and have more contact area. In hypoid gear design, the pinion and gear are practically always of opposite hand, and the spiral angle of the pinion is usually larger than that of the gear. The hypoid pinion is then larger in diameter than an equivalent bevel pinion.

A hypoid gear incorporates some sliding and can be considered halfway between a straight-cut gear and a worm gear. Special gear oils are required for hypoid gears because the sliding action requires effective lubrication under extreme pressure between the teeth.

Hypoid gearings are used in power transmission products that are more efficient than conventional worm gearing.[citation needed] They are considerably stronger in that any load is conveyed through multiple teeth simultaneously. By contrast, bevel gears are loaded through one tooth at a time. The multiple contacts of hypoid gearing, with proper lubrication, can be nearly silent, as well.

Spiral angle

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The spiral angle in a spiral bevel gear is the angle between the tooth trace and an element of the pitch cone, and corresponds to the helix angle in helical teeth. Unless otherwise specified, the term spiral angle is understood to be the mean spiral angle.

• Mean spiral angle is the specific designation for the spiral angle at the mean cone distance in a bevel gear.
• Outer spiral angle is the spiral angle of a bevel gear at the outer cone distance.
• Inner spiral angle is the spiral angle of a bevel gear at the inner cone distance.

Comparison of spiral bevel gears to hypoid gears

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Hypoid gears are stronger, operate more quietly and can be used for higher reduction ratios, however they also have some sliding action along the teeth, which reduces mechanical efficiency, the energy losses being in the form of heat produced in the gear surfaces and the lubricating fluid.

Hypoid gears are typically used in rear-drive automobile drivetrains.

A higher hypoid offset allows the gear to transmit higher torque. However increasing the hypoid offset results in reduction of mechanical efficiency and a consequent reduction in fuel economy. For practical purposes, it is often impossible to replace low efficiency hypoid gears with more efficient spiral bevel gears in automotive use because the spiral bevel gear would need a much larger diameter to transmit the same torque. Increasing the size of the drive axle gear would require an increase of the size of the gear housing and a reduction in the ground clearance, interior space, and an increase in weight.

The hypoid gear is also commonly used in some railcar transmissions with diesel power units - where the engine and gearbox are similar to those used in traditional trucks and busses (not diesel/electric hybrid type drive). The transmission, to allow the input shaft to always rotate in one specific direction (either clockwise or anti-clockwise) while allowing the output shafts to change their rotational direction; thus allowing a vehicle to drive either direction.[clarification needed]

Another advantage of hypoid gear is that the ring gear of the differential and the input pinion gear are both hypoid. In most passenger cars this allows the pinion to be offset to the bottom of the crown wheel. This provides for longer tooth contact and allows the shaft that drives the pinion to be lowered, reducing the "hump" intrusion in the passenger compartment floor. However, the greater the displacement of the input shaft axis from the crown wheel axis, the lower the mechanical efficiency.

See also

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References

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