When to Use Spiral Bevel Gear for Trains？

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.&#;

Cone- or frustum-shaped gears for shafts whose axes intersect

Bevel gears are gears where the axes of the two shafts intersect and the tooth-bearing faces of the gears themselves are conically shaped. Bevel gears are most often mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well.[1] The pitch surface of bevel gears is a cone, known as a pitch cone. Bevel gears change the axis of rotation of rotational power delivery and are widely used in mechanical settings.

Bevel gear on roller shutter door. Regardless of the operating angle, the gear axes must intersect (at a point O) Bevel gear lifts floodgate by means of central screw. Bevel ring gear on the rear wheel of a shaft-driven bicycle Spiral bevel gear

Introduction

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Two important concepts in gearing are pitch surface and pitch angle. The pitch surface of a gear is the imaginary toothless surface that you would have by averaging out the peaks and valleys of the individual teeth. The pitch surface of an ordinary gear is the shape of a cylinder. The pitch angle of a gear is the angle between the face of the pitch surface and the axis.

The most familiar kinds of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This type of bevel gear is called external because the gear teeth point outward. The pitch surfaces of meshed external bevel gears are coaxial with the gear shafts; the apexes of the two surfaces are at the point of intersection of the shaft axes.

The use of a genuine bevel gear has even greater importance for the reliability of the axle than any other spare part. Bevel gears that have pitch angles of greater than ninety degrees have teeth that point inward and are called internal bevel gears.

Bevel gears that have pitch angles of exactly 90 degrees have teeth that point outward parallel with the axis and resemble the points on a crown, whence the name crown gear.

Hypoid bevel gear

Mitre gears

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Miter gears

Mitre gears are a special case of bevel gears that have equal numbers of teeth. The shafts are positioned at right angles from each other, and the gears have matching pitch surfaces and angles, with a conically-shaped pitch surface.[2]

Mitre gears are useful for transmitting rotational motion at a 90-degree angle with a 1:1 ratio.

Geometry of a bevel gear

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A double-helical bevel gear made by Citroën in for the Miřejovice water power plant

The cylindrical gear tooth profile corresponds to an involute (i.e. a triangle wave projected on the circumference of a circle), whereas the bevel gear tooth profile is an octoid[definition needed] (i.e. a triangle wave projected on the normal path of a circle of a sphere). All traditional bevel gear generators (such as Gleason, Klingelnberg, Heidenreich & Harbeck, WMW Modul) manufacture bevel gears with an octoidal tooth profile. IMPORTANT: For 5-axis milled bevel gear sets it is important to choose the same calculation / layout like the conventional manufacturing method. Simplified calculated bevel gears on the basis of an equivalent cylindrical gear in normal section with an involute tooth form show a deviant tooth form with reduced tooth strength by 10-28% without offset and 45% with offset [Diss. Hünecke, TU Dresden]. Furthermore, those "involute bevel gear sets" cause more noise.

Teeth

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There are two issues regarding tooth shape. One is the cross-sectional profile of the individual tooth. The other is the line or curve on which the tooth is set on the face of the gear: in other words the line or curve along which the cross-sectional profile is projected to form the actual three-dimensional shape of the tooth. The primary effect of both the cross-sectional profile and the tooth line or curve is on the smoothness of operation of the gears. Some result in a smoother gear action than others.

Tooth line

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The teeth on bevel gears can be straight, spiral or "zerol".

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Straight tooth lines

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In straight bevel gears, the teeth are straight and parallel to the generators of the cone. This is the simplest form of bevel gear. It resembles a spur gear, only conical rather than cylindrical. The gears in the floodgate picture are straight bevel gears. In straight bevel gear sets, when each tooth engages, it impacts the corresponding tooth and simply curving the gear teeth can solve the problem.

Spiral tooth lines

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Spiral bevel gears have their teeth formed along spiral lines. They are somewhat analogous to cylindrical type helical gears in that the teeth are angled; however, with spiral gears, the teeth are also curved.

The advantage of the spiral tooth over the straight tooth is that they engage more gradually. The contact between the teeth starts at one end of the gear and then spreads across the whole tooth. This results in a less abrupt transfer of force when a new pair of teeth come into play. With straight bevel gears, the abrupt tooth engagement causes more noise, especially at high speeds, and impact stress on the teeth which makes them unable to take heavy loads at high speeds without breaking. For these reasons, straight bevel gears are generally limited to use at linear speeds less than feet/min; or, for small gears, under rpm.[3]

Zerol tooth lines

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Zerol bevel gears are an intermediate type between straight and spiral bevel gears. Their teeth are curved, but not angled. Zerol bevel gears are designed with the intent of duplicating the characteristics of a straight bevel gear, but they are produced using a spiral bevel cutting process.

Manufacturing bevel gears

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Materials used in gear manufacturing process

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Bevel gearing

Applications

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The bevel gear has many diverse applications such as locomotives, marine applications, automobiles, printing presses, cooling towers, power plants, steel plants, railway track inspection machines, etc.

For examples, see the following articles on:

• Bevel gears are used in differential drives, which can transmit power to two axles spinning at different speeds, such as those on a cornering automobile.
• Bevel gears are used as the main mechanism for a hand drill. As the handle of the drill is turned in a vertical direction, the bevel gears change the rotation of the chuck to a horizontal rotation. The bevel gears in a hand drill have the added advantage of increasing the speed of rotation of the chuck and this makes it possible to drill a range of materials.
• Spiral bevel gears are important components on rotorcraft drive systems. These components are required to operate at high speeds, high loads, and for a large number of load cycles. In this application, spiral bevel gears are used to redirect the shaft from the horizontal gas turbine engine to the vertical rotor. Bevel gears are also used as speed reducers

Bevel gears on grain mill at Dordrecht. Note wooden teeth inserts on one of the gears.

Advantages

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• This gear makes it possible to change the operating angle.
• Differing of the number of teeth (effectively diameter) on each wheel allows mechanical advantage to be changed. By increasing or decreasing the ratio of teeth between the drive and driven wheels one may change the ratio of rotations between the two, meaning that the rotational drive and torque of the second wheel can be changed in relation to the first, with speed increasing and torque decreasing, or speed decreasing and torque increasing.

Disadvantages

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• One wheel of such gear is designed to work with its complementary wheel and no other.
• Must be precisely mounted.
• The shafts' bearings must be capable of supporting significant forces.

See also

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References

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