This article offers a detailed guide to pneumatic solenoid valves
Read further to learn more about:
- What are gear drives?
- Functions of gear drives
- Types of gears
- Common gear drives
- And much more…
Chapter 1: What are Gear Drives?
Gear drives, sometimes referred to as gear trains and gearboxes, are mechanisms consisting of an assembly of gears, shafts, and other machine elements for mounting the rotating parts. They form a mechanical system used for transmitting shaft power from a driver such as an engine, turbine, or motor to a driven piece of machinery. Gear drives can alter the transmitted power by using different configurations of gears.
Gear drives can increase or decrease the rotational speed of the output shaft. A common use of gear drives is for reducing speeds of motors and engines that typically run at thousands of revolutions per minute (rpm). These are known as speed reducers. By reducing the speed, torque is increased. This force amplification characteristic is one of the main functions of speed reducers.
Gears are the main components of gear drives. Gears are toothed rolling elements which mesh with one another by engaging their teeth. Because of the large dynamic forces involved, gears are made with alloyed steel. The properties of these metals are also modified by heat treatment to reach the right toughness and rigidity required for its application.
Other components of gear drives are the shafts, keys, couplings, bearings, housing, and flanges. The shafts are the components that connect the gear drive with input and output systems. Keys and couplings are used to lock the shafts of the driver and driven equipment onto the gear drive. Bearings are the machine elements that support the shafts while reducing friction. The housing and flanges are usually made monolithic. The housing encloses and supports the whole assembly while the flanges are used for mounting.
Chapter 2: Functions of Gear Drives
Gear drives are used for many different applications. Some of the popular uses of gear drives are automotive transmission systems, wheel differentials, marine equipment, turbines, and gear motors. Gear drives are preferred over other mechanical power transmission systems because of their high efficiency, high load capacity, and durability. The four main functions of gear drives are explained in detail below.
Changing the Speed of Rotation:
Gear drives can increase or decrease the speed of the driven shaft relative to the driver. This is achieved by using gears with different pitch diameters or numbers of teeth. A large driver coupled to a small driven gear increases the output speed. Conversely, using a small driver and a large driven gear produces the opposite result.
This relationship comes from the fact that the linear speed at a point of contact along the pitch circles of both gears must be constant. This is true for an ideal scenario. This is given by, where v is the linear speed (or velocity), ra and rb are the gear radii, and ωa and ωb are the angular speeds of the driver and driven gears, respectively.
The ratio between the number of teeth of the driven to the driver gear is known as the gear ratio. Other references define the gear ratio by dividing the number of teeth of the larger gear by the number of teeth of the smaller gear, regardless of the direction of power transmission. The relationship between the angular speed, pitch diameters, and number of gear teeth is expressed by the expressions, Where da and db are the pitch diameters, and Na and Nb are the numbers of teeth of the driver and driven gears, respectively.
Speed of rotation can also be altered using combinations of different gear types. Examples are worm drives and planetary gear drives. A worm drive consists of a gear with a screw-like profile called a worm, and an external gear similar to a spur gear called a worm gear or worm wheel. This arrangement produces output speeds with reduction ratios far higher than ordinary gear trains. However, they cannot be driven in reverse, unlike other gear trains.
Planetary gear drives, or planetary gearboxes, are an assembly of external and internal spur gears. The assembly is composed of three components. One component is a central gear called sun gear. Another component is a set of multiple gears revolving around the sun gear called planet gears. The last component is a single internal ring gear called the annular gear. Planetary gear drives can output three different speed ratios by holding one component stationary while the other two are used as input and output.
Increasing or Decreasing the Output Torque.
Changing the speed of rotation produces the opposite effect to the torque. Increasing the output speed decreases the torque and vice versa. This effect is known as mechanical advantage. It trades a magnitude of the angular speed to produce a larger force or torque.
This relationship is derived from the law of energy conservation. In an ideal gear drive system, the power transmitted must be constant. This is shown by the formula, where P is power, and τa and τb are the torques of the driver and driven gears, respectively.
Mechanical advantage is defined as the ratio of the output to the input force or torque. This is related to the angular speed shown by the expression below.
Changing the output torque is achieved the same way as altering the angular speed. This is done by using gears with a different number of teeth, using different gear types, or both.
Modifying the Axis of Rotation.
Gear drives are also used to modify the axis of rotation of the driven relative to the axis of the driver gear by
- Offsetting or translating the output shaft while being parallel with the input shaft.
- Changing the axis of rotation by turning it at an angle relative to the input shaft while still being at the same plane.
- Changing the axis of rotation while at the same time producing an offset. This creates a non-intersecting and non-parallel driver and driven shafts.
Spur and helical gears only transmit power to parallel shafts. For non-parallel shafts, there are two common gear systems used: a worm gear system and a bevel gear system. Worm gears transmit power between two non-intersecting and non-parallel shafts. Bevel gears are more versatile because of the existence of several different types. They can transfer power from intersecting and non-intersecting, non-parallel shafts. Straight, spiral, and Zerol bevel gears are used for intersecting shafts while hypoid bevel gears are used for non-intersecting, non-parallel shafts.
Reversing the Direction of Rotation.
A simple gear system composed of two meshing parallel gears always rotates in opposing directions. For transmission systems composed of several gears, the output shaft can either rotate clockwise or counterclockwise. This is further modified by placing idler gears between the power transmitting gears. Idler gears do not change the gear ratio nor produce a mechanical advantage, unlike the power transmitting gears.
This characteristic is particularly useful for manual automotive transmissions. The reverse gear is engaged by using an idler gear which reverses the rotation of the output shaft. Another application is a reversing gearbox. It is composed of three or four bevel gears. It transmits power without changing the speed and torque of the system, but the direction of the output shaft rotation is in reverse.
Chapter 3: Types of Gears
Knowing the basic types of gears helps you to understand gear drives better. There are different types of gears each designed for a specific purpose. Some types are modifications of the others intended to perform better at a specific aspect. Still, they have their caveats, particularly in terms of manufacture and cost. Enumerated below are the different types of gears and their brief descriptions.
External gear is a general classification of gears in which the teeth are cut at the outer surface of the stock. They mesh externally with other gears. These are the most common type of gears and are seen in every gear drive.
Internal gears have teeth formed on the inner surface. They are always mated with an external gear with a smaller number of teeth. Internal gears are used for more specific applications such as a planetary gear set. The short center-to-center distance of using an internal gear set makes it desirable for compact applications.
Spur gears are the most extensively used gear type. They are cylindrical with straight teeth formed along the lateral surface of the cylinder. Spur gears can be external and internal. An external spur gear is considered the usual type that has teeth on its outside surface. An internal spur gear, on the other hand, is a hollow cylinder with teeth cut on its inner lateral surface.
This type of gears has a cylindrical form similar to spur gears. The difference is their teeth which are cut in a spiral wrapping around the cylinder. Helical gears are used because of their smooth and quieter operation. They are also stronger and have a longer lifespan than spur gears with similar ratings. However, the downside of using helical gears is the larger thrust load imposed on the supporting bearings.
A herringbone gear is a combination of two opposite-handed helical gears placed side-to-side. As the name suggests, its two sets of helical teeth form a V-shape or herringbone pattern. Since they are made from two helical gears, they are sometimes referred to as double-helical gears. The design of a herringbone gear eliminates the effects of excessive thrust load evident in ordinary helical gears.
Straight Bevel Gear:
This is the simplest form of a bevel gear. The teeth are cut in a straight line that intersects at the axis of the gear when extended. Straight bevel gears have an instantaneous line of contact which makes them prone to vibration and noise.
Spiral Bevel Gear:
Spiral bevel gears have curved and oblique teeth. This results in more overlap between teeth which promotes smooth and gradual engagement upon contact. The improved smoothness results in minimal vibration and noise produced during operation. The disadvantage of spiral bevel gears, however, is their larger thrust load.
Zerol Bevel Gear:
Zerol bevel gears have teeth curved in the lengthwise direction. These gears are also somewhat similar to spiral bevel gears in terms of their profile. Due to its curvature, Zerol bevel gear teeth have a slight overlapping action. This makes the gears run smoother than straight bevel gears. Its advantage over spiral bevel gears is the minimal thrust load produced.
Also known as crown gear or contrate gear, face gears have teeth cut on the plane perpendicular to its shaft axis. They are sometimes regarded as bevel gear that has a pitch cone angle of 90°. Face gears mesh with spur gears and bevel gears.
Crossed Helical Gear:
Crossed helical, or cross-axis helical gears, are used in non-intersecting and non-parallel shafts. Unlike ordinary helical gears, crossed helical gears operate by sliding action similar to a screw.
This type of gear must not be confused with worm gears which are defined as the driven gear in a worm gear drive. A worm operates similarly to crossed helical gears. Both of these gear types have drivers that operate through screw action. However, a worm is modified such that its teeth mesh with a larger contact area.
A hypoid gear is a type of bevel gear designed to have an offset between the two shaft axes. Its teeth are similar to that of spiral bevel gears. Hypoid gears are used to match larger pinions to a particular size of a driven gear, making the pinion stronger and have a higher contact ratio to the larger gear.
Chapter 4: Common Gear Drives
Mating several gears by making their teeth engage creates a gear drive. A simple gear drive is composed of at least two gears. These two gears can be aligned in parallel or intersecting. Also, their shafts can be coplanar or non-coplanar. More complicated gear drives use more than two gears such as those found in multi-speed transmission systems and multi-stage gearboxes.
Gears can be matched, arranged, and oriented differently to serve any application. Custom gear drives feature different characteristics such as extremely high gear ratios, compactness, multi-speed transmission, and so on. For common applications, typical gear drive designs are widely available. Common gear drives are discussed below.
Parallel Gear Drives:
Parallel gear drives use gear sets that transmit power through shafts with parallel axes. Spur, helical, and herringbone gears are used for this type of gear drive. Parallel gear drives are the most common and extensively used in industries involving mechanical equipment.
Parallel gear drives have higher power transmission efficiency than other configurations. They are also easier to manufacture. On the other hand, parallel gear drives use large output gears for attaining high-speed ratios. Thus, they are not suitable for compact applications. To reduce the size of the driven gear, multiple gear stages are employed.
Multiple stages are used to create higher speed reduction. These are attained using compound gears. A compound gear is composed of two or more concentric gears placed next to each other. These adjacent gears have a different number of teeth. Since these gears are coupled by a single shaft and have different pitch diameters, they have the same angular speed but different linear speed.
Right-angle Gear Drives:
Right-angle gear drives, or simply right-angle drives, are power transmission systems that use gears that transfer shaft power at a 90° angle. Their input and output shaft axes are coplanar and intersecting. Right angle gear drives feature output shafts that extend in one direction or both. In terms of its orientation, it can be horizontal or vertical.
Most gears that fall into this category are bevel gears, namely straight, spiral, and Zerol bevel gears. Their nominal profile resembles a cone, in contrast to parallel axis gears that take the form of a cylinder. Gear geometries of bevel gears are much more complicated than spur and helical gears. Consequently, their manufacturing processes are also more difficult.
A simple right-angle gear drive uses two mating bevel gears. They are typically used as speed reducers where the pinion is the driver gear. For applications that only require changing the axis of the output shaft, miter gears are used. Miter gears compose a set of two bevel gears with the same number of teeth. This results in a constant speed and torque across the system.
Inline Gear Drives:
Inline gear drives or inline gear reducers have input and output shafts that align into the same axis. Since the shafts have the same centerline, these drives are also called concentric gear drives. Applications of these mechanisms have different designs and use multiple stages of speed reduction. They are primarily used in systems that require speed reduction or torque amplification without changing the orientation and position of the shaft.
Like parallel gear drives, inline gear drives use spur, helical, and herringbone gears. Simply mating two gears will create an offset between the shaft axes. Thus, an intermediate gear is used. Inline gear drives use compound gears to couple the driver and the output gear. This also becomes the second stage of speed reduction making the assembly much more compact.
Inline gear drives are usually used for reducing the speed of the motor. They are directly coupled into the motor shaft and mounted next to the motor. Motor manufacturers also supply products that have a built-in speed reduction gearbox that uses inline gear drives. These are known as gear motors.
Worm Gear Drives:
Worm gears are devices that transmit power through non-intersecting, non-parallel shafts. They are primarily used for high-speed reduction. Various mechanical equipment uses worm gears such as gates, conveyors, and elevators. Then again, the applications that make the most out of their unique characteristic are tuning and indexing mechanisms which require high precision adjustments.
Worm gear drives have two main components: the screw and the wheel. The screw, or worm, is the driver gear while the wheel, or worm gear, is the driven gear. Rotating the screw by 360° turns the wheel by the number of worm threads. Worms with more than one thread are called multi-start worms. The speed reduction ratio is equal to the number of teeth of the driven gear divided by the number of worm threads.
Another important characteristic of a worm gear drive is its self-locking ability. Other gear drives can reverse the direction of power transmission by rotating the driven gear instead of the driver gear. The case is different with worm gear drives since high friction from the sliding surfaces restricts the wheel from turning the worm. However, using a multi-start worm can reduce the high friction from reverse operation; thus, eliminating the self-locking effect.
Planetary Gear Drives:
As mentioned earlier, planetary gear drives or planetary gearboxes are an assembly of internal and external gears that can achieve multiple speed reduction ratios. By using internal gear, they are much more compact than gearboxes using external gear such as parallel and inline gear drives. This is done by locking one component, either the sun, planet, or annular gear.
When not locked, the planet gears rotate and revolve around the sun. One or more planet gears can be used which are connected by a mechanical link called a carrier. A shaft is connected to the carrier which is either locked or used as an output shaft. Planet gears also mesh with the annular gear. The annular gear circumscribes the sun and planet gears.
Planetary gear drives are much more versatile compared to other gear drives. The best application to explain its operation is in a car’s automatic transmission. In a typical automatic transmission, the input can either be the sun gear or the annular gear, while the output is the planetary gears or carrier shaft.
Aside from locking either of the input gears, their speeds are manipulated such that one may rotate faster than the other. Simply locking the annular gear will do the speed reduction required for the first gear of the transmission system. The transmission’s second gear is attained by also rotating the annular gear at a slower speed compared to the sun gear.
Rotating the annular gear and sun gear at the same speed causes the carrier shaft to rotate at the same speed as the input. This is known as direct drive. When the annular gear is locked and the sun gear is rotated in the opposite direction, the carrier shaft rotates in reverse.
Manipulation of both the annular and sun gear speeds is made possible by connecting multiple planetary gear sets. This is done by connecting the carrier shaft of a planetary gearset to either the annular gear or the sun gear of the next gear set. The more planetary gear sets are connected, the more speed reduction ratios are achieved.
Cyclo Gear Drives:
Cyclo gear drives or cyclo reducers are mechanisms that reduce the speed of a mechanical power transmission system. Like inline gear drives, they also have concentric input and output shafts. Other shaft axes configurations are achieved by using it in tandem with bevel and parallel gear sets.
Cyclo gear drives are different from the others since these assemblies rely on other machine elements such as cams, discs, and pins for power transmission. Their gears are also different which have a cycloidal profile rather than an involute as seen in other gear types.
The input component of a cyclo gear drive is an eccentric cam or bearing. As the eccentric cam rotates, it pushes the cycloidal disc against a cycloidal ring gear. As the cycloidal ring gear remains static, the disc eccentrically rotates in the opposite direction as the eccentric cam.
The output shaft has pins that go into holes on the disc. The holes have a slightly larger diameter to allow for eccentric movement. The eccentric rotation of the disc rotates the output shaft in the same direction as the input shaft.
- Gear drives are mechanisms used for transmitting shaft power from a driver such as an engine, turbine, or motor to a driven piece of machinery.
- They have four main functions: 1) to alter the output shaft speed, 2) to change the torque delivered, 3) to change the shaft axis alignment, and 4) to reverse the direction of rotation.
- Gears are the main components of gear drives. Different types are available such as spur, helical, herringbone, bevel, worm, and face gear.
- Gear drives are designed according to their application. Common designs are parallel, right angle, inline, worm, planetary, and cyclo gear drives.