Considering the topic of machine parts, it is impossible not to pay attention to such an important technical indicator as the gear ratio. This term has been around for a long time and its value in mechanics should be discussed separately.
The gear ratio is one of the most important technical characteristics of any mechanical transmission of rotational motion. From a practical point of view, describe the indicator helps to understand how many times increases the moment of force resulting from the operation of the transmission. Determination of gear ratios in any mechanism is one of the most important tasks in mechanics and mechanical engineering.
The gear ratio is determined when there are at least two gears (gears) that are meshed with each other. Such mating is referred to as gear.
The easiest way to calculate the gear ratio is to count the number of teeth on each of the available wheels, and then divide the number of teeth of the driven gear by the number of wheels of the pinion gear. This rational number will be the gear ratio.
It is important to keep in mind that in the case of determining gear ratio in a gear transmission having multiple gears, it is necessary again to divide the number of teeth of the drive wheel the number of the slave. The parameters of the intermediate gears are not considered.
Today there are such types of mechanical gears:
In general, mechanical transmissions are divided according to the following criteria:
- Depending on the transfer of motion from the leading link to the slave: the transfer of friction and transmission gearing.
- Depending on the ratio of the speeds of the driven and driving links: slowing gears (they are also gearboxes), accelerating gears (multipliers).
- Depending on the location of the axles of the shafts: gears with overlapping, intersecting and parallel axes.
Worth pointing out that slowing the transfer is applied more often than accelerating. This fact is explained by the fact that the motor rotation speed is often much higher than desired speed shaft of the actuator or machine.
The gear ratio of the gear train can be calculated not only by the ratio of the teeth of its wheels, but also by dividing the angular velocity of the driven shaft by the angular velocity of the driving shaft, as well as by the ratio of the number of revolutions of the driven and driving shafts.
The gear ratio range can be very large and reach high values. In this case, the gear ratio of the gear train itself is characterized by constancy, since the gearing is not subject to slippage. The efficiency of such a transmission is in the range of 0.97-0.98.
Gears transmit rotation between shafts, which may have parallel, intersecting or perpendicular axes. In addition, such gears are able to transform (transform) the translational motion into rotational and vice versa (“screw-nut” gear).
Gearing in gears can be both external and internal. Wheels are made with straight, oblique or chevron teeth.
Gears are capable of transmitting high rotational speeds with a constant gear ratio and have high efficiency.
The gear ratio of the gearbox is one of the most important criteria in the process of its selection. The second most important indicator is the center distance.
Used to transmit rotational motion in cases where the axes of the shafts intersect.
The worm can be cylindrical or globoid depending on the surface of the thread location, and can also be involute or archimedean (in this case, the thread profile plays a decisive role).
The main disadvantages of the described transmission can be considered:
- High heat emission rate.
- Frequent sticking and low efficiency.
Her work is based on the principle of transformation of motion parameters due to wave deformation of the flexible element of the mechanism. In fact, such a transmission is a kind of planetary transmission.
The wave transmission includes a hard gear wheel with internal teeth and a rotating flexible wheel with external teeth. Both wheels interlock with each other thanks to a wave generator connected directly to the gear housing.
Due to the existing design features, wave transmission is endowed with the following advantages:
- Small dimensions and weight.
- High kinematic accuracy.
- The gear ratio of the transmission in one stage has a large figure and can easily reach 300.
- Perfect damping ability.
- Formation in one stage of a large gear ratio.
The disadvantages include:
- Very complex design.
- High power losses due to friction and deformation of the flexible wheel (efficiency is about 0.7-0.85).
Most often used in the textile industry, machine tools and other industries other than aircraft. Transmitted power can reach 10 kW. At high rates it is very difficult to guarantee the required pressing force of the rollers.
In the transfer, there are three types of slip: geometric, elastic and skidding.
For a normally functioning transmission, it is the elastic slip that is characteristic, while the skidding indicates the presence of an overload.
Like gear, it is very common. Depending on how the shafts and belt are arranged, the gear may be:
The belt can be round, flat, trapezoidal.
The gear ratio in such programs is in the range of 1: 4, 1: 5 and only in rare cases can it be equal to 1: 8.
The positive qualities of a belt drive include:
- The simplicity of the design.
- The possibility of the location of both pulleys at a great distance from each other (over 15 meters).
- Noiseless and smooth operation.
- Protection of mechanisms against overloads due to the elastic properties of the belt and its ability to slip at certain points on the surfaces of the pulleys.
- Work at high angular speeds.
The disadvantages of the transfer are:
- Lengthening of belts (their stretching) in the process of work, that is, fragility.
- The variability of the gear ratio, which is explained by the inevitable slippage of the belt.
- Large enough.
Unlike the belt analog, it is not subject to slippage. Calculation of the gear ratio is performed by analogy with a gear, because the sprockets, in fact, are the same gears.
A characteristic feature of the chain transmission is that rotation is transmitted exclusively in the presence of parallel shafts. Between the axes of the asterisks, the distance must be at least one and a half times the diameter of a large asterisk. In this case, the gear ratio can reach 1:15.
It is also important to note that the chain is put on the sprockets not with a tightness like belts, but with a certain degree of sagging. Tension adjustment is carried out using a special screw.
The advantages of the transfer are:
- Low sensitivity to inaccuracies of shaft installation.
- Transmission of rotation can be carried out by one chain at once to several asterisks.
- Rotation can be transmitted over long distances.
The disadvantage is the high noise and wear of the chains in case of poor installation and poor maintenance.
In many machines and units, not only continuous rotational movement is applied, but also intermittent too, which is carried out with the help of a ratchet, pawl and lever.
The ratchet mechanism besides rotation also carries out safety function. So, for example, in hoisting winches, the ratchet together with the dog does not allow the drum to turn in the opposite direction, fixing it reliably in the required spatial position.
The considered types of mechanical transmissions are used in virtually every sector of the national economy and are widely used due to their technical capabilities.