Considering the subject of machine parts, one can not but pay attention to such an important technical indicator as the gear ratio. This term exists already for a long time and its meaning 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 by having at least two gears (gears) that are in mesh with each other. Such a coupling is called a gear transmission.
The easiest way to calculate the gear ratio is to calculate the number of teeth on each of the available wheels, and then to divide the number of teeth of the driven gear by the number of wheels of the drive gear. This rational number will be the transfer 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.
To date, there are such types of mechanical transmission:
In general, mechanical transmissions are divided according to the following criteria:
- Depending on the transfer of motion from the leading link to the slave: transmission by friction and transmission by gearing.
- Depending on the ratio of the speeds of the slave and the driving link: slowing gears (they are also gearboxes), accelerating gears (multipliers).
- Depending on the location of the shaft axes: gears with intersecting, 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 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 drive shaft, and also by the ratio of the speed of the driven and the drive shafts.
The range of the gear ratio can be very large and reach large values. In this case, the transmission ratio of the gear train itself is characterized by constancy, since the gear is not susceptible to slipping. The efficiency of such a transmission is in the range 0,97-0,98.
Gear transmissions transmit rotation between shafts, which can have parallel, crossing or perpendicular axes. In addition, such transmissions are able to transform (translate) the translational motion into rotary and vice versa (an "screw-nut" front).
The engagement in the gearing can be either external or internal. Wheels are made with straight, oblique or chevron teeth.
Gear transmissions are capable of transmitting high rotational speeds with a constant gear ratio and have a high efficiency.
The gear ratio of the reducer is one of the most important criteria in the process of its selection. The second most important indicator is the distance between the axes.
Used to transfer rotational motion in cases where the shafts are crossed.
The worm can be cylindrical or globoid, depending on the surface of the thread, 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 dissipation.
- Frequent seizure and low efficiency.
Its 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 transfer is a kind of planetary transmission.
The wave gear includes a rigid gear wheel with internal teeth, and a rotating flexible wheel with external teeth. Both wheels are engaged with each other thanks to a wave generator connected directly to the transmission housing.
Due to the available design features, the wave transfer has the following advantages:
- Small size and weight.
- High kinematic accuracy.
- The gear ratio of transmission in one stage has a large indicator and can well reach 300.
- Ideal damping ability.
- Formation of a large ratio in one stage.
The disadvantages are:
- Very complex design.
- High loss of power for friction and deformation of a flexible wheel (the efficiency is about 0.7-0.85).
Most often used in the textile industry, machine-tool construction and other industries, except aircraft construction. The transmitted power can reach 10 kW. At high rates it is very difficult to guarantee the required pressing force of the rollers.
In the transmission there are three types of slip: geometric, elastic and slippage.
For a normally functioning transmission, elastic sliding is characteristic, while skidding indicates the presence of an overload.
Like the jagged it is very common. Depending on how the shafts and belt are arranged, the transmission can be:
The belt can be round, flat, trapezoidal.
The gear ratio in such gears is in the range 1: 4, 1: 5 and only in rare cases can be equal to 1: 8.
To the positive qualities of the belt drive is:
- Simplicity of design.
- The possibility of locating both pulleys at a great distance from each other (over 15 meters).
- Quiet and smooth operation.
- Protection of mechanisms from overloads due to the elastic properties of the belt and its ability to slip on the surfaces of pulleys at certain times.
- Work at high angular velocities.
Disadvantages of the transfer are:
- Elongation of belts (their stretching) in the process of work, that is, fragility.
- Variability of the gear ratio, which is explained by the inevitable slippage of the belt.
- Large enough.
Unlike the belt analog it is not susceptible to slippage. Calculation of the gear ratio is performed by analogy with the gear train, because the transmission sprockets, in fact, represent the same gear wheels.
A characteristic feature of the chain transmission is that the rotation is transmitted only in the presence of parallel shafts. Between the axes of the asterisks the distance must be taken not less than one and a half diameters of the large sprocket. In this case, the gear ratio can reach 1:15.
It is also important to note that the chain is worn on sprockets not with interference, like belts, but with a certain degree of sagging. The tension adjustment is carried out using a special screw.
Advantages of the transfer are as follows:
- Little sensitivity to the inaccuracy 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 deterioration of chains in case of poor-quality installation and with poor maintenance.
In many machines and units, not only continuous rotational motion is used, but also intermittent, which is carried out with the aid of a ratchet, pawl and lever.
In addition to rotation, the ratchet also performs a safety function. So, for example, in hoisting winches, a ratchet together with a pawl does not allow the drum to turn in the opposite direction, securely fixing it in the required spatial position.
The considered types of mechanical transmissions are used practically in any branch of the national economy and have received the widest distribution due to their technical capabilities.