General information and classification of gears

Virtually any mechanism in modern technology is partly or entirely composed of different types of gears. In most cases, the gear elements of the kinematic scheme are used as motion transmitting devices . In this article, the classification of gears will be considered in detail. We will talk about their varieties and peculiarities.

Definition

So, from a technical point of view, the gear transmission is a mechanism that serves to transfer rotation from one shaft to another and to change the rotational speed by means of racks and wheels.

Classification of gears says that the gear wheel located on the shaft transmitting the rotation is usually called the master, and the receiving rotation is called the slave. Also, the element that has a smaller size in the pair is called a gear, and the one that is larger is a wheel.

Scope of application

Gear transmissions, classification, basic parameters and features of the operation of which will be described below, are justifiably considered to be the most common parts in machine building and other branches of the national economy. Such a high demand is explained by the possibility of transmission with their help of capacities in the range from several parts to several tens of thousands of kilowatts. In this case, the circumferential speeds of rotation can be up to 150 m / s, and the gearing numbers range from hundreds to thousands. The diameter of the wheels themselves ranges from a few millimeters (sometimes even their fractions) to six or more meters.

Differentiation

The purpose and classification of gears provides for their separation according to the following features:

1. The location of the axles of the wheels in space:

• With parallel axes (cylindrical gears);
• With intersecting axes (bevel gears);
• With crossed axes (worm and screw gears).

2. By the type of relative rotation of the wheels and the location of the teeth:

• With external gearing (the wheels rotate in opposite directions relative to each other);
• With internal gearing (rotation of both wheels goes in one direction).

3. In the shape of the profile:

• Involute teeth;
• Cycloidal;
• With Novikov's gear.

4. The location of the theoretical line of the tooth:

• Straight wheels;
• Helical teeth;
• Chevron;
• Screw (with a circular tooth).

It should be noted that indirect gears have a great smoothness of their work, they have much less wear and noise compared to spur gear.

5. By the rate of peripheral speed:

• Low-speed transmissions (less than 3 m / s);
• Medium-speed (from 3 m / s to 15 m / s);
• High-speed (over 15 m / s).

Gradation by area of application

Classification of gears for functional purposes provides for their division into:

• Kinematic (reference) transmissions. They are used in a variety of instruments, counting-solving mechanisms. The main requirement for such transmissions is the observance of the highest kinematic accuracy, that is, there must be a clear coordination of the angles of rotation of both the driving and the driven wheels.
• Speed transmissions are used in gearboxes of turbomachines, gearboxes of cars. Requirements: maximum possible smoothness of work.
• Power transmissions are operated in crane and rolling mechanisms. They work at low speeds, but at the same time transmit impressive torques. The main requirement put forward for transmissions of this type is the tight contact of the teeth that are in mating with each other.

Additional criteria

Classification of gears for structural design takes into account that they can be of an open and closed type. Open transmissions can be operated either without lubrication (extremely rare) or treated with special grease.

Closed gears, in turn, are lubricated by immersing the teeth in a special oil, which is filled with a crankcase (immersion lubrication). In some cases, a centralized supply of the composition to the crankcase is provided. In this case, the flow of the lubricating fluid is regulated by means of special chokes.

Depending on how the speed changes, the gears are divided into:

• Lowering (they are called reducers). In such transmissions, the gear ratio is greater than or equal to one.
• Multipliers - gear ratio is less than one.

By the way, the gear ratios are both constant and step-adjustable due to the movement of the wheels directly over the shaft (for example, the gearbox).

Positive traits

Classification of gears will be incomplete if you do not consider their merits. In comparison with other types of transmission gears are characterized by:

• Technological.
• Constancy of the transmission ratio.
• High load capacity (up to 50,000 kW).
• An impressive coefficient of efficiency (up to 0.99).
• Small overall dimensions in comparison with other transmissions under the same conditions.
• Great reliability when working.
• Ease of maintenance.
• Relatively small loads on the supports and shafts.

Negative qualities

As for the disadvantages of gears, they include:

• No possibility to change the gear ratio steplessly.
• The accuracy of manufacturing and installation should be at a high level.
• The occurrence of noise at high operating speeds.
• Unsatisfactory cushioning properties.
• Large dimensions in cases where between the axes of the driven and drive shaft an impressive distance.
• Cutting teeth requires special equipment and tools.
• Inability to compensate for dynamic loads due to high cruelty.
• No safety function. The gear is not capable of protecting the machine or the mechanism from overload.

Also, gear drives (advantages and disadvantages, classification and types of which are indicated above) are irrationally using their teeth, which is manifested in the simultaneous operation of not more than two teeth of each of the wheels in conjugation.

Deformation of the teeth of the wheels

Correct design and operation of the gear transmission is manifested in the absence of strong noise and overheating during operation. If these two specified criteria still have inconsistencies, then this could well lead to the destruction of the teeth of the wheels. Classification gears for operational use also makes their own adjustments to the transmission, but in general the types of tooth fractures are as follows:

• Plastic deformation of working surfaces.
• Breaking.
• Seizing.
• Wear.
• Chipping.

In cases where the teeth break, often not only the transmission failure occurs, but also the damage to various adjacent parts, parts (for example, bearings, shafts are destroyed). This is due to the wedging action of the broken pieces.

Quite often, the teeth break down due to their "fatigue", which appears as a consequence of the appearance and progressive development of the crack. This kind of breakdown is most typical for closed transmissions.

Rubbing of the teeth is most often observed in open gears, which is explained by the penetration into the meshing zone of various metal particles, dirt, dust (abrasive wear). Also, the reason may be a poor lubrication, therefore, this type of wear is not covered by closed transmissions.

Manufacture of wheels

It is important to know that gear drives, advantages and disadvantages, the classification of which depends on their technological and physical properties, are made of different materials.

Most often in practice, they are:

• Carbon steel of ordinary quality (Ст6, Ст5).
• High-quality steel grades.
• Alloyed steel grades.
• Gray and high quality cast iron.
• Some nonmetallic materials (bakelite, textolite).

The most widespread gears with gear wheels made of steel, which is explained by the optimal combination of strength, reliability and weight. Such material is ideal for high-loaded gears.

In turn, gray cast iron is used for wheels operating infrequently, as well as slow-moving open gears. Cast iron is good in that the teeth of the wheels on its basis are not too demanding for lubrication and are well rubbed against each other.

Plastic cogwheels produce for mechanisms where maximum silent operation of high-speed transmission is required, without the need for high manufacturing accuracy.

Hardness and heat treatment

Gears, classification, the application of which is also dependent on the bearing capacity, necessarily pass heat treatment.

Cogwheels made of steel are conventionally divided into two groups:

• Wheels with tooth hardness of less than 350 HB. This indicator is formed due to the normalization or improvement of steel. The teeth are cut directly after heat treatment.
• Wheels whose hardness exceeds 350 NV. Such hardness is provided by chemical-thermal hardening: carburizing, nitridation, cyanidation, surface quenching by means of high-frequency currents.

Lubrication of gears

Classification of gears by the location of the teeth will be incomplete if you do not consider lubricating the gearing. In itself, the lubrication process is focused on reducing the wear rate of the teeth, removing heat and fine abrasive particles, increasing the efficiency of the entire transmission. Thanks to the use of high-quality lubricants, the resistance of the wheels to seizing increases. The role of lubricant can be plastic, liquid and solid materials.

Plastic lubrication is most often used in open gears, which operate at a temperature of not more than +120 degrees. Hard lubrication is also used in open gears, but in those whose operating temperature exceeds 100 degrees Celsius. The most popular lubricant is liquid. The greatest popularity was obtained by oil oils. As for synthetic lubricant materials, they are used only in special cases, since their price is quite high.

The designation of liquid oils is as follows:

• Industrial oil - the letter I.
• For use in hydraulic systems - G.
• For heavily loaded transmissions - T.
• Oil, which has anticorrosive, antioxidant, anti-wear additives, - S.
• Oil that does not have any additives - A.

Bevel gears

Classification of bevel gears in a simplified version is as follows:

• Wheels are conical toothed with straight teeth.
• With tangential teeth.
• With curved teeth.
• With circular teeth.
• With a line of teeth in the form of an involute.

Straight-toothed conical wheels are most often used in open gears, but elements with circular teeth are used in gearboxes.

Characteristics and symbols

The main parameters on which the classification of gears relies are as follows:

• Number of teeth - Z.
• Center distance - a.
• Width of wheel rim - b.
• The radial clearance is c.
• The height of the tooth is ha.
• The height of the tooth is h.
• The dividing diameter is d.
• The initial diameter is dw.
• The diameter of the troughs of the teeth is dr.
• The diameter of the vertices of the teeth is da.

Production of gears

Gear wheels are produced on automatic lines. These highly specialized lines are divided into short and complex. The first group is associated only with cutting and finishing the gears. The second is a set of machines of the most various purposes, which ensure the full production of gears. Such lines use semi-automatic machines for gear processing, additionally equipped with loading and unloading machines and other automation devices.

In technological lines of production of wheels between production machines, flexible transport links are used in the form of belt and chain conveyors, as well as mobile transfer trolleys, which exclude the occurrence of nicks and other defects.