There are all kinds of mechanical devices. Some of them have been familiar to us since childhood. This, for example, watches, bicycle, yule. We learn about others when we get older. These are car engines, hoisting cranes and others. Each moving mechanism uses some kind of system that makes the wheels spin, and the car works. One of the most interesting and sought after is the planetary gear. Its essence lies in the fact that the machine is driven by wheels or gears interacting in a special way. Let's consider it in more detail.
General information
The planetary gear and planetary gear are so named by analogy with our solar system, which can be conventionally represented as follows: in the center there is a "sun" (the central wheel in the mechanism). Around him are moving "planets" (small wheels or satellites). All these parts in the planetary gear have external teeth. The conditional solar system by its diameter has a boundary. Its role in the planetary mechanism is performed by a large wheel or epicycle. It also has teeth, only internal. Great work in this design performs the carrier, which is a lever mechanism. The movement can be carried out in different ways: either the sun will rotate, or the epicycle, but always together with the satellites.
During the operation of the planetary mechanism, another design can be used, for example, two suns, satellites and a carrier, but without an epicycle. Another option is two epicycles, but without the sun. A carrier and satellites should always be present. Depending on the number of wheels and the location of the axes of their rotation in space, the design can be simple or complex, flat or spatial.
To fully understand how such a system works, you need to understand the details.
Arrangement of elements
The simplest form of planetary gear includes three sets of gears with varying degrees of freedom. The above satellites rotate around their axes and at the same time around the sun remaining in place. The epicyclic connects the planetary mechanism from the outside and also rotates by alternately engaging the teeth (it and the satellites). This design is able to change the torque (angular velocity) in one plane.
In a simple planetary mechanism, the sun and satellites can rotate, and the epicenter remains fixed. In any case, the angular velocities of all components are not chaotic, but have a linear dependence on each other. As the media rotates, a low-speed output with high torque is provided.
That is, the essence of the planetary gear is that such a design is able to change, lay out and add up the torque and the angular velocity being carried out. Rotational movements in this case occur in one geometric axis. The necessary transmission element of various vehicles and mechanisms is installed.
Features of structural materials and schemes
However, a fixed component is not always necessary. In differential systems, each element rotates. Planetary mechanisms like this include one output controlled by two inputs. For example, the differential that drives the axle in a car is a similar gear.
Such systems operate on the same principle as structures with a parallel shaft. Even a simple planetary gear has two inputs, a fixed ring gear is a constant input of zero angular velocity.
Detailed device description
Mixed planetary designs can have a different number of wheels, as well as different gears through which they are connected. The presence of such parts greatly expands the capabilities of the mechanism. Composite planetary structures can be assembled so that the shaft of the carrier platform moves at high speed. As a result, some problems with reduction, sun gear and others can be eliminated in the process of improving the device.
Thus, as can be seen from the above information, the planetary mechanism works on the principle of transferring rotation between links that are central and mobile. Moreover, complex systems are more in demand than simple ones.
Configuration options
In the planetary mechanism, you can use wheels (gears) of various configurations. Suitable standard with straight teeth, helical, worm, chevron. The type of engagement will not affect the general principle of operation of the planetary mechanism. The main thing is that the axis of rotation of the carrier and the central wheels coincide. But the axis of the satellites can be located in other planes (intersecting, parallel, intersecting). An example of crossover is the cross- wheel differential, in which the gears are conical in shape. An example of crossing ones is a limited-slip differential with a worm gear (Torsen).
Simple and complex devices
As already noted above, the scheme of the planetary mechanism always includes a carrier and two central wheels. Satellites can be any number. This is the so-called simple or elementary device. In such mechanisms, structures can be: "SHS", "SVE", "EVE", where:
- C is the sun.
- In - drove.
- E - the epicenter.
Each such set of wheels + satellites is called a planetary series. In this case, all wheels must rotate in the same plane. Simple mechanisms are single and double row. In various technical devices and machines they are rarely used. An example is the planetary gear of a bicycle. The sleeve works by this principle, due to which the movement is carried out. Its design was created according to the "SVE" scheme. Satellites in not 4 pieces. In this case, the sun is rigidly attached to the axis of the rear wheel, and the epicenter is movable. A cyclist pushing the pedals forces him to rotate. In this case, the transmission speed, therefore, the rotation speed may vary.
More often, you can find complex gear planetary mechanisms. Their schemes can be very different, depending on what this or that design is intended for. As a rule, complex mechanisms consist of several simple ones created as a general rule for planetary transmission. Such complex systems are two-, three- or four-row. It is theoretically possible to create designs with a large number of rows, but in practice this does not occur.
Flat and spatial devices
Some people think that a simple planetary gear must be flat. This is only partially true. Complex devices can also be flat. This means that the planetary series, no matter how many are used in the device, are in one or in parallel planes. Spatial mechanisms have planetary rows in two or more planes. In this case, the wheels themselves may be smaller than in the first embodiment. Note that the planar planetary mechanism is the same as the spatial one. The difference is only in the area occupied by the device, that is, in compactness.
Degrees of freedom
This is the name of the set of rotation coordinates, which allows you to determine the position of the system in space at a given time. In fact, every planetary mechanism has at least two degrees of freedom. That is, the angular rotation speeds of any link in such devices are not linearly connected, as in other gears. This allows you to get the output angular velocity is not what it is at the input. This can be explained by the fact that in the differential connection in the planetary mechanism there are three elements in any row, and the rest will be connected with it linearly, through any one element of the series. Theoretically, you can create planetary systems with three or more degrees of freedom. But in practice they turn out to be inoperative.
Planetary gear ratio
This is the most important characteristic of rotational motion. It allows you to determine how many times increased the moment of force on the driven shaft in relation to the moment of the leading shaft. You can determine the gear ratio by the formulas:
i = d2 / d1 = Z2 / Z1 = M2 / M1 = W1 / W2 = n1 / n2, where:
- 1 - leading link.
- 2 - slave link.
- d1, d2 are the diameters of the first and second links.
- Z1, Z2 - the number of teeth.
- M1, M2 - torques.
- W1 W2 - angular velocity.
- n1 n2 - rotational speed.
Thus, when the gear ratio is higher than unity, the torque increases on the driven shaft, and the frequency and angular velocity decrease. This should always be taken into account when creating the structure, because the gear ratio in planetary gears depends on how many teeth have wheels, and which particular element of the row is the leading one.
Application area
In the modern world, there are many different machines. Many of them work with planetary gears.
They are used in automobile differentials, planetary gearboxes, in kinematic diagrams of complex machine tools, in aircraft engine gearboxes, in bicycles, in combines and tractors, in tanks and other military equipment. According to the principles of planetary gear, many gearboxes operate in electric generator drives. Consider another such system.
Planetary rotation gear
This design is used in some tractors, tracked vehicles and tanks. A simple diagram of the device is shown in the figure below.
The principle of operation of the planetary rotation mechanism is as follows: the carrier (position 1) is connected with the brake drum (2) and the drive wheel located in the caterpillar. The epicycle (6) is connected to the transmission shaft (position 5). The sun (8) is connected to the clutch disc (3) and the rotation brake drum (4). When the locking clutch is turned on and the belt brakes are turned off, the satellites will not rotate. They will become like levers, since they are connected with the sun (8) and epicycle (6) through teeth. Therefore, they force them and the carrier to simultaneously rotate around a common axis. In this case, the angular velocity is the same.
When the locking clutch is turned off and the rotation brake is turned on, the sun will stop and the satellites will begin to move around their axes. Thus, they create a moment on the carrier and rotate the drive wheel of the track.
Wear
As for the service life and depreciation, in the linear mechanisms of planetary systems, the load distribution is noticeable among the main components.
Thermal and cyclic fatigue can increase in them due to the limited load distribution and the fact that planetary gears can rotate quite quickly along their axes. Moreover, at high speeds and gear ratios of the planetary mechanism, centrifugal forces can significantly increase the magnitude of the movement. It should also be noted that as the accuracy of production decreases and the number of satellites increases, the tendency to imbalance increases.
These devices and their systems may even undergo wear or damping. Some designs will be sensitive even to small imbalances and may require high-quality and expensive assembly components. The exact location of the planetary pins around the axis of the sun gear may be the key.
Other planetary gear arrangements that help balance loads include the use of floating subunits or “soft” mounts to provide the most durable movement of the sun or epicenter.
Fundamentals of the synthesis of planetary devices
This knowledge is needed in the design and creation of machine components. The concept of "synthesis of planetary mechanisms" is to calculate the number of teeth in the sun, epicenter and satellites. In this case, it is necessary to observe a number of conditions:
- The gear ratio should equal the set value.
- The gearing of the teeth of the wheels should be correct.
- It is necessary to ensure the alignment of the input shaft and output.
- Neighborhood required (satellites should not interfere with each other).
Also, when designing, it is necessary to take into account the dimensions of the future design, its mass and efficiency.
If the gear ratio (n) is specified, then the number of teeth in the sun (S) and on planetary gears (P) must satisfy the equality:
n = S / P
If we assume that the number of teeth at the epicenter is early (A), then with a locked carrier, the following equality must be observed:
n = -S / A
If the epicenter is fixed, then the following equality will be true:
n = 1+ A / S
So the calculation of the planetary mechanism.
Advantages and disadvantages
There are several types of transmission that are safely used in various devices. The planetary among them is distinguished by the following advantages:
- A smaller load on each tooth of the wheels (both the sun, and the epicenter, and satellites) is provided due to the fact that the load on them is distributed more evenly. This positively affects the life of the structure.
- At the same power, the planetary mechanism has smaller dimensions and mass than when using other types of transmission.
- The ability to achieve a higher gear ratio with fewer wheels.
- Providing less noise.
Disadvantages of planetary gears:
- Need increased accuracy in their manufacture.
- Low efficiency with a relatively large gear ratio.