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Depending on their properties and design characteristics, there are different types of gear. Aside from understanding how gears work, it is essential for machining engineers to understand the different types and their peculiarities for the best possible results. These gears are outlined below.
Sporting a simple design, spur gears are the commonest types of gears in machines today. Spur gears belong to the class of gears where the tooth is parallel to the shaft. Considering that these gears have a parallel design, the efficiency of transmission of both motion and power is usually high.
One distinct feature of spur gears is that they do not transmit load in the axial direction. Also, the way spur gears are set up makes them the optimal gear for processes that do not require a high speed.
Helical gears are so named because of their helix shape, similar to helical springs. Like spur gears, helical gears are commonly used across manufacturing industries and machines.
However, unlike spur gears with teeth positioned parallel to the shaft, helical gears have their teeth set at an angle to the shaft. Also, more teeth are involved when transmitting motion and speed, which translates to helical gears having the ability to carry more load.
Again, helical gears transmit load in the axial direction, creating a thrust force and needing bearings. Helical gears are perfect for operations that require high speed. There are different variations of helical gears, and they are as follows:
Depending on whether the gears have teeth on one or both helixes, helical gears can be subdivided into two. If teeth are only on one helix, the gears are single helical. The hand (left or right) the teeth are on does not matter as long as only one side contains the teeth.
However, the gears are known as double helical if there are teeth on both hands. There is a gap between the two faces in a double helical gear. Of the two subtypes, the double helical gears are the more efficient and smother in transferring motion because there is more overlap of the teeth.
Screw gears are a pair of helical gears with the same hand and operate at a twist angle of 45 degrees. They have a similar design to many fasteners. These gears usually operate on gears that are neither parallel nor intersecting. Unlike conventional helical gears, screw gears only have one tooth as the point of contact, which affect their ability to carry high power and loads.
This type of helical gear is similar to double helical gears, with the difference being that the Herringbone gear is shorter, and there is no space between the two faces. While Herringbone gears are suitable for operations that involve a lot of vibration and shock, they are expensive and difficult to manufacture, implying that they are not widely used in industries.
This gear type has similarly-sized teeth equidistance to one another along a linear rod called the rack. Also, the gear rack has a pair of circular gears known as the pinion that engages the rack. The mesh between the pinion and the rack turns rotational into linear motion. This type of gear is mostly used in the automobile industry for the manufacture of steering.
A bevel gear is a cone-shaped gear with teeth around the cone that is used for the transmission of force between intersecting shafts. The shafts can intersect perpendicularly or at a specific angle. This gear type is not widely used in the industry because it is relatively expensive. Still, the gear is used in the manufacture of mixers and crushers.
There are several types of bevel gears, and they are highlighted below:
This subtype of bevel gears is the most typical in many industries, primarily due to their simple setup. While the gear’s pitch is conical, the teeth are upright.
This subtype of bevel gears has a curvature in their tooth line, which corresponds to excellent contact points between the teeth of the gear. The result is stronger gear with a smoother transmission. However, they are very expensive.
If a bevel gear has a gear ratio of one, it is referred to as a miter gear. This is an important feature to note, considering other gears of this type can have up to a 400:1 ratio. Miter gears are the best bevel gears for operations requiring efficient transmission at high speeds.
There is a striking similarity between spiral bevel gears and hypoid bevel gears, with the primary difference being whether or not the shaft intersects. If they do, the bevel gear is known as spiral, but if they don’t, the gear is hypoid.
Other types of bevel gears include:
This type of gear comprises two important components: the worm and the worm wheel, which are simply the shaft and mating gear. The shafts in this gear have a screw cut, like a drill, and do not intersect.
Transmission generally occurs very smoothly and quietly because there is limited friction between the hard worm and the soft worm wheel. However, this type of gear is not the most efficient. Worm gears are mostly used in agricultural machines.
For this gear type, there are teeth on the inside cones. For transmission of motion, there is an external gear paired with the internal gear, with both gears rotating in the same direction. Because the number of teeth on the internal and external gears often differs, several problems may arise when using this type of gear. Nonetheless, the gear is useful in shaft couplings.
A herringbone gear, a specific type of double helical gear,[1] is a special type of gear that is a side-to-side (not face-to-face) combination of two helical gears of opposite hands.[2] From the top, each helical groove of this gear looks like the letter V, and many together form a herringbone pattern (resembling the bones of a fish such as a herring). Unlike helical gears, herringbone gears do not produce an additional axial load.
Like helical gears, they have the advantage of transferring power smoothly, because more than two teeth will be enmeshed at any moment in time. Their advantage over the helical gears is that the side-thrust of one half is balanced by that of the other half. This means that herringbone gears can be used in torque gearboxes without requiring a substantial thrust bearing. Because of this, herringbone gears were an important step in the introduction of the steam turbine to marine propulsion.[citation needed]
Manufacture
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Precision herringbone gears are more difficult to manufacture than equivalent spur or helical gears and consequently are more expensive, so typically they are used in heavy machinery.
Where the oppositely angled teeth meet in the middle of a herringbone gear, the alignment may be such that tooth tip meets tooth tip, or the alignment may be staggered, so that tooth tip meets tooth trough. The latter alignment is the unique defining characteristic of a Wuest type herringbone gear, named after its inventor Caspar Wüst-Kunz.
This herringbone bevel gear was made by Citroën and installed around 1927 in the small Miřejovice hydropower plant on the River Vltava in the Czech Republic, connecting a Francis turbine to the generator. It worked flawlessly until 2011.A disadvantage of the herringbone gear is that it cannot be cut by simple gear hobbing machines, as the cutter would run into the other half of the gear. Solutions to this have included assembling small gears by stacking two helical gears together, cutting the gears with a central groove to provide clearance as per Wüst patent, and (particularly in the early days) by casting the gears to an accurate pattern and without further machining. With the first two methods of fabrication, herringbone gears had a central channel separating the two oppositely-angled courses of teeth. This was necessary to permit the shaving tool to run out of the groove.
The development of the Sykes gear shaper in the 1910s[3] made it possible to have continuous teeth with no central gap. Sunderland, also in England, also produced a herringbone cutting machine. The Sykes uses cylindrical guides and round cutters; the Sunderland uses straight guides and rack-type cutters. The W. E. Sykes Co. dissolved in 1983–1984, since then it has been common practice to obtain an older machine and rebuild it if necessary to create this unique type of gear.[citation needed] Recently, the Bourn and Koch company has developed a CNC-controlled derivation of the W. E. Sykes design called the HDS1600-300.[promotion?] This machine, like the Sykes gear shaper, has the ability to generate a true apex without the need for a clearance groove cut around the gear. This allows the gears to be used in positive displacement pumping applications, as well as power transmission. Herringbone gears with low weight, accuracy and strength may be 3D printed.
During both World Wars marine gearboxes for naval ships were a major production bottleneck for surging warship demand, and other propulsion options like triple-expansion steam engines and diesel-electric had to be implemented for less important and slower ships like destroyer escorts.
Citroën
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Citroën Type A final drive herringbone pinion and crownwheelThe logo of the car maker Citroën is a graphic representation of a herringbone gear, reflecting André Citroën's earlier involvement in the manufacture of these gears. Early Mors and Citroën cars used a herringbone bevel gear final drive in the rear axle.[4]
Panhard Dyna X and successor cars (1948–1967) used double helical gears in the transaxle and for the camshaft timing gears in the engine.
References
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