Gear is one of the components that have an overwhelming use almost in all kinds of machines. Here in this article, we are going to talk about gears and their different types. So, let’s proceed.
What is a Gear?
We can say that gear is a machine component with teeth cut around a cone-shaped or cylindrical surface with equal spacing. Usually, two gears are meshed and used to transmit forces and rotations to the driven shaft from the driving shaft. Gears can be segregated based on their shapes like cycloidal, involute, and trochoidal gears.
Besides, gears can also be classified based on their shaft’s position like intersecting shaft gears, parallel shaft gears, non-intersecting, and non-parallel shaft gears. According to Archimedes, the use of gears had come under the limelight in ancient Greece in B.C. However, with time their new types kept on emerging.
Types of Gears
Gears can be classified into different types like spur gears, helical gears, worm gears, gear rack, bevel gears, etc. Typically, their classification can be made considering their axes’ position, such as intersecting shafts, non-intersecting shafts, and parallel shafts.
For the mandatory transmission of force in mechanical designs, it is inevitable to understand different gear types. Even if you have chosen a general kind of gear, it is still recommended to consider factors like standard of precision grade, dimensions, need for heat-treating or teeth grinding, efficiency, and allowable torque.
Next, we will give a general overview of different gears. Meanwhile, you may consult their technical aspects to have deeper and technical information about these types of gears.
So, let us start with these different types of gears:
- Čelní ozubené kolo
Gears that have cylindrical pitch surfaces are referred to as cylindrical gears. Technically, Spur gears belong to the parallel shaft gear group. In these gears, there is a tooth line that is parallel and straight to the shaft.
For having greater accuracy and smooth transmission of power, spur gears are widely used in diversified industries. The second factor that makes them a suitable choice is their easy to manufacture process that includes lower costs. These gears don’t support loads in their axial direction. The transmission of power is made possible with the meshing of two gears: one is slightly bigger, which is called gear, and the second one is somewhat smaller, which is called the pinion.
Figure 1 Sketch of Spur Gear
- Helical Gear
Similar to spur gears, helical gears are also used with parallel shafts. These are cylindrical gears that possess winding tooth lines. Compared with the spur gears, helical gears have better meshing of teeth that work with more incredible quietness than spur gears. As helical gears can conveniently transmit greater loads, they are usually preferred for high-speed applications.
Unlike spur gears, helical gears have loads in an axial direction that brings the need for thrust bearing. Helical gears come with both left-hand and right-hand twisting options, and for the meshing pair, there needs to be opposite-hand gear.
Figure 2: Sketch of Helical Gear
- Gear Rack
Gear rack is referred to as same sized and same shaped teeth cut at equal distance along a straight rod or a flat surface. Again, a cylindrical gear has a radius equal to the pitch cylinder, and it transmits power by meshing with a cylindrical gear pinion. It converts the rotational motion into linear motion.
Meanwhile, a gear rack can also get developed for helical tooth racks and straight tooth racks, but with the same straight tooth line. When it comes to connecting gear racks end to end, it is done by machining the gear rack’s ends.
Figure 3: Sketch of Gear Rack
- Kuželové ozubené kolo
Bevel gears with their cone shape are used to transmit force between two shafts that intersect each other at one point, which is referred to as intersecting shaft. It has a cone shape because its teeth and pitch surface are cut along the cone shape.
In addition to it, bevel gear can be further divided into different types:
- Helical bevel gears
- Straight bevel gears
- Angular bevel gears
- Spiral bevel gears
- Hypoid gears
- Zerol bevel gears, and
- Miter Gears
Figure 4: Sketch of Bevel Gear
- Spiral Bevel Gear
As evident with the name, the spiral bevel gear is the type of Bevel gear, but with curved tooth lines. The tooth contact ratio for spiral bevel gear is greater than the straight bevel gears. That is why spiral bevel gears offer greater strength and better efficiency compared to straight bevel gears. But, due to increased tooth contact ratio, spiral bevel gears create more noise and vibration.
On the other hand, the manufacturing of spiral bevel gear is more complicated than straight bevel gears. As the teeth are curved, the thrust forces are in an axial direction.
Along with it, if the twisting angle is zero for the spiral bevel gear, it will be referred to as zero bevel gear.
Figure 5: Sketch of Spiral Bevel Gear
- Screw Gear
Two same hand helical gears form a screw gear, while the angle of twist between them is 45 degrees on the non-intersecting and non-parallel shaft. The carrying load capacity is low for screw gear, as the point of contact between two gears is also very small. So, screw gears are certainly not suitable for the transmission of greater power.
In screw gears, power is transmitted by the sliding of tooth surfaces, which necessitates lubrication for proper service out of these gears. Meanwhile, there is no limitation on the number of gears you want to attach, and you can form your desired combination of several teeth.
Figure 6: Sketch of Screw Gear
- Miter Gear
Bevel gears with a speed ratio of 1 are called miter gear. Miter gears are usually used to change the direction of power transmission without affecting the speed. Mainly, there are two types of miter gears: straight miter gear and spiral miter gear.
Spiral miter gears cause thrust force in the axial direction, and this is the reason behind the use of thrust bearing with spiral miter gears.
Furthermore, miter gears other than the shaft angle of 90 degrees are known as angular miter gears.
Figure 7: Sketch of Miter Gear
- Šneková převodovka
The worm gear is made up of two different components, the first one is the worm formed by the screw shape cut on the shaft, and the second component is a mating gear that is a worm wheel. Both of these components on a non-intersecting shaft are called worm gear. In the given sketch, both the worm and the worm wheel are cylindrical, but they might also be in some other shape.
The contact ratio between worm and worm wheel is relatively lower, which puts check on the transmission of greater loads. However, with the help of the hour-glass type, the contact ratio can get increased.
Besides, the contact between the worm and worm wheel is sliding, so lubrication is needed to reduce the friction. Secondly, the worm is made up of a rigid material, and a worm wheel is made up of soft material to reduce friction. Although this assembly is only suitable for more miniature load transmission, it is pretty smooth.
Furthermore, when the lead angle between the worm and worm wheel is slight, it can experience a self-locking feature.
Figure 8: Sketch of Worm Gear
- Vnitřní převod
Internal gears possess teeth in the cone or cylinders’ inner side, and each internal gear is paired with external gear. The primary purpose of using internal gears is gear type shaft coupling and planetary gear drives. When it comes to internal and external gear, there are certain limitations in the number of teeth, and these limitations are due to involute interference, trimming problems, and trochoid interference.
When internal and external gears are in mesh, both of the gears’ rotational direction is identical. But, when internal and external gears are in mesh, their rotation’s focus is the opposite.
Figure 9: Sketch of Internal Gear
Accordingly, these are some of the commonly used types of gears. Now, let us have a look at essential terminologies used in gears and their nomenclature:
Terminologies and Nomenclature of Gears
Knowing the terminologies used for gears becomes unavoidable to have a deeper insight into gears’ intricate concepts.
This visual representation will help you better understand the working mechanism of gears. Meanwhile, understating the terminologies for gears will also be easy to comprehend:
- Worm wheel
- Miter gear
- Spiral bevel gear
- Internal gear
- Gear coupling
- Screw gear
- Straight bevel gear
- Spur gear
- Involute spline shafts and bushings
- Helical gear
According to the orientation of the axes of gears, they can be classified into the following categories:
- For spur gear, internal gear, gear rack, and helical gear, the orientation axes are parallel.
- The intersecting axes support miter gear, straight bevel gear, and special bevel gear.
- Worm, worm wheel, worm gear, and screw gear have non-parallel and non-intersecting axes.
- Gear coupling, involute spline shaft and bushing, pawl, and ratchet possess other axes.
What is the Difference Between Sprocket and Gear?
We know that gear works in assembly and meshes with other gear, but sprocket meshes with a chain instead of gear. Very nest to the sprocket, there is an item that somehow looks like the gear, but it is ratchet, and it is allowed to move only in one direction.
Classification of Different Gears from the Point of Positional Relations to Attached Shaft
- Spur gears, helical gears, rack gears, and internal gears use parallel shafts. Usually, these gears are to transmit greater power.
- If the two shafts of gears are intersecting, the gear type will be beveled gear. Bevel gears also have high transmission efficiency.
- If the shafts of two gears are neither parallel nor intersecting, the gear type might be worm or screw gear. As there is a sliding contact between these, the lower transmission of power is only preferred using these gears.
Precision Class of Gears
The precision class is brought to use when different types of gears are grouped based on their accuracy. The precision class is usually set by various standards like JIS, AGMA, DIN, ISO, etc.
For example, JIS defines helix deviation, tooth profile error, runout error, and pitch error.
Existence of Teeth Grinding
The existence of teeth grinding has a significant effect on the performance of the gear. Therefore, when types of gears are considered, teeth grinding is given an important part. Grinding of the teeth gear enhances the gear quality so that its working becomes quieter and smoother, increases the force transmission capacity, and affects the precision glass. But grinding increases the gear’s cost, which is not preferable for all gears, so we use another cost-effective technique to increase the precision called shaving with shave clutters.
Kinds of Tooth Shape
Gears are classified by tooth shape into categories as
- Involute tooth shape
- Cycloid tooth shape
- Trochoid tooth shape
In the above-mentioned toothed gears, involuted gears are mainly used. Their quality of being effortlessly produced and correctly meshed, even if the center distance is slightly off, makes them desirable to be used widely. Cycloid tooth shapes are primarily consumed in clocks production, while trochoid tooth shapes are used in pumps.
Creation of Gears
It’s said about gears that
“Gears are the wheels with teeth and sometimes are called teethed wheels.”
Mechanical components used to transmit the rotation and power from one shaft to the other are called gears. If one shaft contains perfectly shaped teeth on the circumference of it in a way that when it rotates, these teeth perfectly fit in between the spaces of the teeth of another shaft. Therefore, it is a mechanical component that transmits power on the driver shaft principle, pushing the driven shaft into motion. It’s a rare case when one side is undergoing linear motion (also called rotational motion about an infinite point); it’s termed a rack.
Power and rotation can be transmitted from one shaft to another in many ways, e.g., rolling friction and wrapping transmission. Despite being small in size and very simple in structure, gears serve us in many advantageous ways like transmission of power, very accurate angular speed and ratio with minimal loss of power with long-lasting service.
Gears are widely used, from clocks, watches, and small precision measuring instruments to airplanes and ship transmission systems. They are considered one of the most important mechanical components with diverse applications and are listed with screws and bearings for their importance.
There are numerous gears, but the most common are those used to transmit speed ratio between two parallel shafts placed at a defined distance. The gears shown in the figure have their teeth parallel to the shaft and are called spur gears. These are the most popular type of gears.
Figure 10: Spur Gear
There are other types of gears called friction drives. These are the most straightforward and widely used components to transmit angular speed ratio between two parallel shafts. This process is carried out with two cylinders with diameters inversely related to their speed. One is driving the other smoothly and without any slippage. For transmission of speed in the opposite direction, the contact of cylinders is from the outer side. And for the same direction, the connection is from the inner side. Transmission occurs due to friction between the surfaces of two cylinders.
Yet, we can’t avoid slipping between these two due to the contact’s nature, so desirable transmission is not obtained. For the transmission on a large amount of power, heavy contact forces are required, leading to high bearing loads. This kind of system is not suitable for transmitting a significant amount of power due to the above reasons. So, to avoid such problems, the idea of creating teeth on the surface of cylinders works, from which a pair or more will always remain in contact with each other, providing more friction and a solid grip to drive.
The driving shaft teeth push the teeth of the driven shaft setting it into motion, assuring the power transmission. It is known as cylindrical gear, while the other one on which teeth are carved is called pitch cylinder. Spur gears are a further development of cylindrical gears.
Figure 11: Pitch Cylinders
When two shafts intersect, the reference for the carving teeth is the cones in contact. These gears are named bevel gears, as shown in the figure. The base where teeth are carved is called a pitch cone.
Figure 12: Bevel Gears
Figure 13: Pitch Cones
In the case of two non-parallel non-intersecting shafts, there are no rolling contact points on curved surfaces. Depending on what type of gear we are making, teeth are carved on the surfaces that are rotating and in contact. In all the gear systems, it’s important to consider tooth profiles to make the relative motion of rotating and contacting reference surfaces happen and make them in a match with each other.
During movement, gears are considered rigid bodies. The two gears’ typical speed components must be equal to maintain the angular speed ratio at the point of contact of gears’ teeth surfaces without crashing into each other or separating. We can also say this so that the relative motion in the expected direction and motion only happens at the teeth surfaces’ contact point.
For the tooth forms to comply with the requirements mentioned above, a general method of enveloping surfaces can give us desired tooth form.
Please choose the one side of gear A and consider it curved surface FA. And set both gears into relative motion. Then draw the successive positions of curved surface FA on the coordinate system attached to gear B. Conceive its surface FB of gear B by considering the envelope of this group of curves. It can be inferred from the envelop theory that the two gears are in relative motion being in line contact with each other.
Tooth forms can also be obtained by the following method. In addition to gears A and B, consider a gear C in the mesh with relative motion. This imaginary gear C in the mesh has a surface FC and appropriate relative movement. Utilizing the first method, we will envelop the successive positions on surface FC in relative motion with FA with line contact IAC. Repeat the process with surface FB with FC. Now tooth surfaces of FA and FB can be known by using imaginary surface FC.
Ways of Gear Utilization in Mechanical Systems
The primary purpose of gears is to transmit powers, but depending on ideas, they can also be used as machine elements in multiple ways. Following is a brief description of some of the methods:
- Grasping Mechanism:
Two spur gears can get used to making a grasping mechanism to hold the workpiece in different situations. It works on the principle that both of the gears are of the same diameter, and they move incoherence so that if one driver reverses, the driven also reverses. We can firmly grasp the workpieces of different sizes in claws connected with these gears by adjusting the opening angle. In this way, a versatile grasping machine can be made of them.
- Intermittent Motion Mechanism
Geneva Mechanism is also known as intermittent motion mechanism. Because of the highly specialized mechanical components used in it, it’s expensive. A low-cost, simple intermittent mechanism can also get obtained by utilizing missing teeth gears. Missing teeth here means that any number of teeth removed from the root of the gear’s surface. A gear coupled with missing tooth gear will rotate as long as it’s in contact with present teeth, and movement will stop as it confronts the blank space of the driving gear. At the same time, it has a dismal effect of shifting if pushed by any external force when gears are disengaged. It is imminent to maintain its position, which a friction brake can do.
- Special Power transmission Mechanism:
The one-way clutch is a mechanism that allows the rotational movement in one direction only. If mounted on a speed reducer gear stage, a mechanism can get created to transmit unidirectional rotational motion.
This mechanism can create a system that will work well with a motor when electric power is on, but it is driven by spring force when turned off.
Speed reducer is operated by internally mounting a spring, whether torsion coil spring or spiral spring, set so that the driven shaft moves in the opposite direction. After the spring’s complete winding, the motor stopped rotation, and the electromagnetic brake system comes into play. When the motor is turned off and the brake is applied, the spring force drives the output shaft in the opposite direction to what the motor is working. This type of machine is mainly used to close the valves in case of a power failure and is pronounced as spring return emergency shutoff.
Why Procurement of Gears is Difficult
No Gear Standard
Gears are widely used globally in almost every complex mechanical system from ancient times and are crucial, but there are no set standards to design the gear. Concerning class and precision of gears, different countries use different industry standards such as AGMA(US), JIS(Japan), DIN(Germany), etc. But there are no specified standards for the core factors that define gear like diameter, size, bore diameter, material strength, teeth formation. No unified approach is applied, but everyone designs the gear according to his specific requirements.
Diverse Gear Specifications
As discussed in the former paragraph, there are plenty of gear specifications. With simple gears as an exceptional case, it’s not an exaggeration to claim that “there are as many kinds as there are places where gears are used.” It’s common among gears that when specifications like tooth pitch, number of teeth, and pressure angle are matched, various other specifications determine a gear, like a face width, heat treatment, bore size, surface roughness after grinding, final hardness. That is why gear is almost impossible to replace with another one. The possibility of gear being compatible with others is very low.
No Obtainability of Desired Gears
Gear in the machine might be worn out or broken, and we searched the market for that gear but in vain. This problem can be easily solved if there is a drawing of the gear in the machine’s user manual. You can that gear manufactured again. Or the other possibility is that you can contact the machine manufacturer and he will agree to make a new gear of the kind for you. But what will happen if, unfortunately, both of these ways aren’t available; there isn’t any drawing on the user manual, and the manufacturer is also not available?
You can get a manufacturing drawing of the gear drawn, but it requires specialized gear knowledge and is not an easy task. Gear manufacturers can also face this problem due to a lack of gear specification knowledge. It requires a great deal of engineering work to rebuild worn-out or broken gear.
Production Cost Is High in The Case of One Gear
When a machine using gear is produced on a larger scale, gear is also produced in bulk quantity with the exact specifications, and the cost remains within a limit. More significant production utilizes the same amount of work with less cost per piece, which, when compounded to great quantity, drastically decreases gear costs. But what if we need to manufacture one or two gears for our machine. It’s quite an expensive task. Production of gear in one shot for 500 machines compared with the output of one or two pieces shows a considerable cost difference. Such a situation is also faced when someone is producing a new machine prototype and has to make a nominal quantity of gear.
Possibility of Using Gear Standards
If you are designing a new machine and its gear specifications match some of the manufacturer’s gears, the problems discussed above can get solved in these ways.
- During designing the machine, you can avoid creating new and specific gear for the machine.
- 2D and 3D CAD models, strength calculations, and printable part drawings provide by the gear manufactured can be utilized.
- If you need only one gear for the machine’s test trial, manufacturers produce standard gears that you can use.
When you are using gear in the machine and need to replace it, you can do so with some of the manufacturer’s standard gear or gear with the secondary operation. You can avoid the inconvenience of following tasks in the above-discussed way.
- Sketching a new model
- Look for the drawing
- Looking for a manufacturer for gear manufacturing
- High cost of production