Almost any mechanism in modern technology partially or completely consists of various types of gears. In most cases, it is the gear elements of the kinematic scheme that are used as transmission devices for the movement . This article will examine in detail the classification of gears. We will talk about their varieties and features.
Definition
So, from a technical point of view, a gear is a mechanism that serves to transfer rotation from one shaft to another and to change the speed using racks and wheels.
The classification of gears says that a gear located on a shaft transmitting rotation is called the leading one, and the receiving rotation is the driven one. Also, that element that has a pair of smaller sizes is called a gear, and that which is large is called a wheel.
Scope of application
Gears, classification, the basic parameters and features of which will be described below, are justifiably considered the most common parts in mechanical engineering and other sectors of the economy. Such a high demand is explained by the possibility of transferring power with their help in the range from several fractions to several tens of thousands of kilowatts. In this case, peripheral rotation speeds can be up to 150 m / s, and gear ratios range from hundreds to thousands. The diameter of the wheels themselves ranges from a few millimeters (sometimes even their shares) to six or more meters.
Differentiation
The purpose and classification of gears provides for their separation according to the following features:
1. According to the location of the axles of the wheels in space:
- with parallel axes (cylindrical gears);
- with intersecting axes (bevel gears);
- with crossed axles (worm and screw drives).
2. According to the type of relative rotation of the wheels and the location of the teeth:
- with external gearing (wheels rotate in opposite directions relative to each other);
- with internal gearing (rotation of both wheels goes in one direction).
3. According to the profile form:
- involute teeth;
- cycloidal;
- with Novikov gearing.
4. According to the location of the theoretical line of the tooth:
- spur gears;
- helical;
- chevron
- screw (with a circular tooth).
It is worth noting that indirect gears have a great smoothness of their work, they have much less wear and noise compared to spur gears.
5. In terms of peripheral speed:
- low-speed gears (less than 3 m / s);
- medium speed (from 3 m / s to 15 m / s);
- high-speed (over 15 m / s).
Application Graduation
The classification of gears according to their functional purpose provides for their division into:
- Kinematic (reference) transmission. They are used in a variety of devices, counting and 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 rotation angles of both the driving and driven wheels.
- High-speed transmissions are used in turbomachine gearboxes, car gearboxes. Requirements: the maximum possible smoothness of work.
- Power transmission are operated in crane and rolling mechanisms. They work at low speeds, but at the same time they transmit impressive torques. The main requirement put forward to gears of this type is tight contact between the teeth in conjunction.
Additional criteria
The classification of gears by design takes into account that they can be open and closed. Open transmissions can work either without lubrication (extremely rarely), or they can be treated with special greases.
Closed gears, in turn, are lubricated by immersing the teeth in a special oil, which fill the crankcase (immersion lubrication). In some cases, a centralized supply of the composition to the crankcase is provided. At the same time, the flow of lubricating fluid is regulated using special chokes.
Depending on how the speed changes, the gears are divided into:
- lowering (they are called reducers). In such gears, the gear ratio is greater than or equal to one.
- Multipliers - gear ratio less than one.
By the way, gear ratios are both constant and step-adjustable due to the movement of the wheels directly along the shaft (for example, gearbox).
Positive traits
The classification of gears will be incomplete if you do not consider their merits. In comparison with other types of gears, gears are characterized by:
- Manufacturability.
- The constancy of the gear ratio.
- High load capacity (up to 50,000 kW).
- Impressive efficiency (up to 0.99).
- Small overall dimensions in comparison with other transmissions under the same conditions.
- Great reliability during operation.
- Ease of maintenance.
- Relatively small loads on bearings and shafts.
Negative qualities
As for the shortcomings of gears, they include:
- The inability to change the gear ratio steplessly.
- The accuracy of manufacture and installation must be at a high level.
- Noise at high speeds.
- Poor shock absorbing properties.
- Large dimensions in cases where there is an impressive distance between the axes of the driven and driving shafts.
- Tooth cutting requires special equipment and tools.
- Inability to compensate for dynamic loads due to high severity.
- Lack of safety function. The gear train is not able to protect the machine or mechanism from overload.
Also, gears (advantages and disadvantages, the classification and types of which are indicated above) irrationally use their teeth, which is manifested in the simultaneous operation of no more than two teeth of each of the wheels in conjunction.
Wheel tooth deformation
Proper design and operation of the gear train is manifested in the absence of loud noise and overheating during operation. If these two specified criteria still have inconsistencies, then this may well lead to destruction of the teeth of the wheels. The classification of gears for operational purposes also makes adjustments to the work of the gear, however, in general, the types of tooth damage are as follows:
- Plastic deformation of work surfaces.
- Breakage
- Seizing up.
- Wear.
- Chipping.
In those cases when the teeth break, often there is not only a breakdown of the transmission, but also damage to various adjacent nodes, parts (for example, bearings, shafts are destroyed). This is due to the jamming effect of the broken pieces.
Quite often, the teeth break due to their “fatigue”, which appears as a result of the occurrence and progressive development of the crack. This type of failure is most characteristic of closed gears.
Abrasion of teeth is most often observed in open gears, which is explained by the penetration of various metal particles, dirt, dust into the engagement zone (abrasive wear). Poor lubrication can also be the reason, therefore closed gears are not insured against this type of wear .
Wheel production
It is important to know that gears, advantages and disadvantages, the classification of which depend on their technological and physical properties, are made of various materials.
Most commonly used in practice are:
- Carbon steel of ordinary quality (St6, St5).
- High quality steel grades.
- Alloy steel grades.
- Gray and high quality cast iron.
- Some non-metallic materials (bakelite, textolite).
The most widespread are gears with steel gears, which is explained by the optimal combination of strength, reliability and weight. This material is ideal for heavy duty gears.
In turn, gray cast iron is used for wheels that work infrequently, as well as low-speed open gears. Cast iron is good in that the teeth of the wheels based on it are not too demanding on lubrication and are well rubbed against each other.
Plastic gears are produced for mechanisms where the maximum noiselessness of high-speed transmission is required, while high precision manufacturing is not required.
Hardness and heat treatment
Gears, classification, the use of which are also dependent on the bearing capacity, undergo mandatory heat treatment.
Steel gears are conventionally divided into two groups:
- Wheels with a hardness of teeth less than 350 HB. Such an indicator is formed due to the normalization or improvement of steel. The teeth are cut directly after heat treatment.
- Wheels whose hardness exceeds 350 HB. This hardness is ensured by chemical-thermal hardening: cementation, nitriding, cyanidation, surface hardening using high-frequency currents.
Gear lubrication
The classification of gears by tooth arrangement will be incomplete if the issue of gear lubrication is not considered. The lubrication process itself is focused on reducing the wear rate of the teeth, removing heat and small abrasive particles, increasing the efficiency of the entire gear. Thanks to the use of high-quality lubricants, the resistance of the wheels to seizing increases. Plastic, liquid and solid materials can act as lubricants.
Grease is most often used in open gears that operate at a temperature of no more than +120 degrees. Solid lubricant is also used in open gears, but in those whose operating temperature exceeds 100 degrees Celsius. The most popular lubricant is liquid. The most popular oil oils. As for the 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 - letter I.
- For use in hydraulic systems - G.
- For heavily loaded gears - T.
- Oil having anticorrosive, antioxidant, antiwear additives - C.
- Oil without any additives - A.
Bevel gears
The classification of bevel gears in a simplified form has the following form:
- Bevel gear wheels with straight teeth.
- With tangential teeth.
- With curved teeth.
- With circular teeth.
- With a line of teeth in the form of an involute.
Spur bevel wheels are most often used in open gears, but elements with circular teeth are involved in gearboxes.
Characteristics and designations
The main parameters on which the classification of gears is based are as follows:
- The number of teeth is Z.
- Center distance - a.
- Wheel rim width - b.
- Radial clearance - p.
- The height of the tooth leg is ha.
- Tooth height - h.
- Dividing diameter - d.
- The initial diameter is dw.
- The diameter of the tooth cavities - dr.
- The diameter of the tops of the teeth is da.
Gear manufacturing
Cog wheels are made on automatic lines. These highly specialized lines are divided into short and complex. The first group is associated only with cutting and finishing gears. The second is a combination of machines for various purposes, which provide a complete manufacture of gears. In these lines, semi-automatic gear cutting machines are used, additionally equipped with loading and unloading and other automation devices.
In the technological lines of wheel production between production machines, flexible transport links in the form of belt and chain conveyors, as well as mobile transfer trolleys, which exclude the occurrence of nicks and other defects, are most often used.