Gears. The mechanism and types of gears
Gears are widespread in industrial units and household appliances. They act as an intermediate link between the source of rotational-translational motion and the node acting as the final consumer of this energy. Moreover, the transmitted power can be calculated as negligibly small units (watch movements and measuring instruments), as well as by enormous efforts (turbines of power stations).
Modes of motion transmission
The engine that generates energy, and the final unit that consumes it, often differ in characteristics such as rotation speed, power, angle of application of force. In addition, one source of rotational energy can be used to actuate several different nodes or aggregates at once. To ensure the delivery of torque in such conditions, intermediate modules are needed that would transfer this force with minimal losses.
If as a result of such a distribution or transformation, the revolutions of the drive shaft become larger than those of the follower, then it is customary to speak of a reduction gear. In this case, the loss of speed is compensated by an increase in the load on the driven axle and an increase in the power of the consuming node. In the case when an increase in the number of revolutions is observed, such a transfer will be increasing. Accordingly, this will be accompanied by a decrease in the force on the driven shaft.
Features gear mechanism
The belt drive assumes the presence of an intermediate link between the pulleys on the connected shafts - a flexible belt. Gear mechanism from such a connection is characterized by the presence on the surface of the mating parts of the teeth of the gearing. They are identical in profile and size.
The head of the tooth of the wheel engages with the repetitive hollow profile on the gear. When the drive shaft rotates, the driven shaft turns in the opposite direction. Between them, the minimum possible gap is provided constructively, providing sliding, thermal expansion and lubricant to prevent jamming. In this case, the leading part of the pair mechanism is called the wheel, and the driven part - the gear.
In the belt drive, the plane of engagement of the belt with a pulley is at least one third of the circumference. In a gear mechanism between the drive wheel and the driven gear under load, one pair of teeth is in constant contact. Wheels and gears on shafts are usually mounted on a keyed joint.
Gears are widespread. They are durable and reliable in operation, while adhering to acceptable load levels and an adequate level of service. The compact mechanism provides high efficiency and can be used for a wide range of speed changes.
The presence of teeth of the gearing allows you to achieve the constancy of the gear ratios between the adjacent shafts due to the inability to slip. In this case, the load on the shafts do not exceed the permissible limits.
Gears have a number of features that can be attributed to their shortcomings. In terms of operation - such a mechanism makes noise at high speeds of rotation. It cannot respond flexibly to changing loads, as it is a rigid structure with fine adjustment.
Technologically, this is the difficulty of making pairs of gear wheels. For this type of gear, increased accuracy is required, as the teeth are meshed with constantly changing voltage. In such conditions, possible fatigue damage to the material.
This occurs when the permissible loads are exceeded. Teeth can crumble, partially or completely break. Breakaway fragments get into the mechanism, damage adjacent mating sites, which leads to jamming and failure of the entire node.
The most widespread cylindrical gear. It is used in knots and mechanisms with a parallel arrangement of shafts. By design features distinguish teeth with a straight, oblique and chevron profile.
For intersecting shafts use a worm, helical cylindrical transmission, and for intersecting - conical. The rack and pinion gear is different in that the gear in the overall twin mechanism is replaced by a working plane. At the same time teeth, identical in wheel profile, are cut on it. As a result, the rotational motion is converted to translational.
Also share the transfer speed rotation: low, medium and high-speed. By appointment, they are divided into power and kinematic (not transmitting significant power). In addition, gears can be classified according to the gear ratio, axle mobility (ordinary and planetary), the number of degrees, the accuracy of the gearing (12 classes), the method of manufacture. The shape of the tooth profile can be evolvent, cycloid, pinch, circular.
All types of gears are widely used in various branches of industrial production. The annual production of various wheelsets in the millions. The scope of their application is so extensive that a rare device, mechanism, or aggregate that uses rotational motion in its work does not incorporate any type of toothed movable joint.
Spur gear is used to convert rotational motion with a reduction or boost factor. Examples: internal combustion engines, gearboxes in rolling stock, machine tools, drilling, metallurgical, mining and all types of industries.
Bevel gearing is used to a lesser extent due to the complexity in the process of manufacturing wheelsets. It is used in complex and combined mechanisms, where there is rotational movement with variable angles and load changes. Special gearboxes typically use bevel gears. Examples: driving axles of cars, agricultural machinery, locomotives, wheel sets of conveyors, drives of various industrial equipment.
They are used most widely, since the technology for manufacturing wheelsets is relatively simple and well-developed. Spur gear is used to transfer torque between shafts located in parallel planes. They differ in the shape of the teeth: with a direct arrangement, oblique and chevron. In rare cases, when the shafts are intersected and under minor loads, a screw profile is used.
Straight teeth are used the most. They are used to transmit torque with little or average load, as well as in cases when there is a need to shift the wheels during operation along the axis of the shaft.Oblique teeth used for smoothness. They are used for responsible mechanisms and at elevated loads. Chevron profile (two rows of oblique teeth on the edges, arranged in the form of a Christmas tree) is characterized by a high balance of axial displacement forces, which are a lack of helical wheel pairs.
Spur gears can be open or closed. In the latter case, the teeth of one of the wheels are not located on the outer, but on the inner surface of the circle.
In conditions where the torque from the source to the consuming unit needs to be delivered with an angular displacement, use intersecting shafts. Their axes are most often at a 90 degree angle. In such cases, bevel gears are commonly used.
It is called so because of the design features of pairs of gears. They have the shape of a cut cone and mate with their side planes, on which teeth are cut. In profile, they are higher at the base and decrease towards the top.
The ring gear can be straight, tangential or curvilinear cutting. If along the profile it is made in the form of a helical spiral, and besides the intersection, the shafts also have an axial displacement, then this bevel gear is called hypoid.It has a smooth running and low noise, but has an increased tendency to seize, therefore, special lubricants are used for it.
In comparison with cylindrical gears, bevel gears can provide only 85% of their bearing capacity. According to the technology of manufacture and assembly, they are the most complex. However, the ability to transmit torque with angular displacement makes them indispensable in complex assemblies and mechanisms.
Rack and belt gear
When it is necessary to transform the rotational motion into translational or vice versa, one of the wheels is replaced by a plane with cut teeth. Rack and gear are distinguished by ease of manufacture and installation, reliability and good load characteristics. It is used in machine tools and for drives, which use the forward movement: slotting machines, conveyors with alternate feed.
The gear-belt drive is a hybrid model that incorporates the positive qualities of both types. It is characterized by the constancy of the gear ratio due to the lack of slippage. Silent operation at high revs and loads is achieved by using flexible belts with a core. Often used in electric motor drives.
On the paired pulleys of the unit assembly and on the elastic belt, their binding, there are identical teeth on the profile. Transmission does not work on the principle of friction, but uses a gearing mechanism. At the same time, on the one hand, there is no need for strong tension between the pulleys and precise adjustment, on the other hand, lubrication between the mating metal parts.
Gears must have reliability in the robot at different speeds and loads, the strength of the teeth, their wear resistance and the ability to resist sticking. Steel is used as a base material for wheel pairs. It can be heat treated or contain alloying additives and impurities. Cast iron can act as a material for low-speed mechanisms having large dimensions and an open type of construction.
To prevent sticking, twin wheels are made of a different strength material. If high carbon steel is used for the wheel and gear, then different degrees of heat treatment are used. Also used bronze, brass, caprolon, textolite, plastics and formaldehyde.
Blanks for wheel pairs of gears can be made by casting or stamping. In the future, they are subjected to additional processing, and the teeth are cut. Use for this disc and finger cutters, shaped grinding wheels.
The gear mechanism of a conical type cannot be made using the method of finishing cutting with a mill or grinding, since the profile of the projections and depressions is not constant. This can be done only at the initial stage of roughing. Further fine-tuning is performed on the machines in the process of running with gearing. To do this, use a pair of high-strength wheel material, repeating the main profile. It acts as a cutting tool.
Carbon steels are hardened, carburized, nitrided or cyanated. For non-critical components, heat treatment can be carried out after the teeth are cut. For wheel sets of high precision, additional finishing sanding or running in is required.
During normal operation, the gear mechanism works smoothly, and the process is accompanied by monotonous moderate noise.The presence of extraneous sounds and uneven rotation indicate wear of the meshing surfaces, or a violation of the adjustment.
During maintenance, during inspection, check for the absence of cracks, damage to the teeth or their chips. Particular attention is paid to the correct engagement of the wheelsets and the absence of gaps. During operation, the face runout is checked and friction surfaces are monitored.
The correctness of the gearing is determined by applying paint to the gear teeth. Before it has dried, the shafts are turned several times and examine the places of contact between the working surfaces. The shape of the print (it should be in the form of an ellipse) determine the overall transmission status.
Pay attention to the touch points. They should be approximately in the middle of the height of the tooth. A stain of paint should occupy 70 - 80% of its length. Adjustment basically comes down to increasing or decreasing the thickness of the gaskets under the bearings.
Depending on the type of assembly, the lubrication of the open mechanism can be carried out periodically by hand with plastic material. For enclosed structures, it should be carried out by spraying or dipping part of the crown of the impeller into the lubricated bath.
To characterize the mechanism of engagement, determine the diameters of the pitch and the main circle, the center distance and the possible displacement of the shafts. The relationship of the number of teeth of the driving and driven wheels determines the gear ratio. It is based on the source data allows to calculate the speed for a pair of gearing.
The gear wheel is initially characterized by the number of teeth and the module. It is standardized and displays the length of the pitch circle per tooth. Determine the diameters of the protrusions and depressions. Calculate the total length, height and thickness of the tooth, as well as its individual parts - the head and leg.
Calculated pitch diameter. The ratio of the width of the gear rim is used. In the case of oblique teeth, they are determined with their angle of inclination. It should be borne in mind that in bevel and cylindrical gears it is different.
In addition to the above, the angle of the profile, the ratio of the end overlap and displacement, the line of engagement is also used. For worm gears calculate the number of turns, diameter and type of worm.
Calculation of gears
Before designing, the initial data should be studied and the conditions of the planned operation of the mechanism should be determined.The original contour, type and type of transmission, its location in the unit, allowable loads, material for wheel pairs and their heat treatment are taken into account. At this stage, taken into account the frequency of rotation of the shafts and their diameters, torque, gear ratio.
To calculate the gear, you need to decide on the overall gearing module, the number of teeth for the gear and the wheel, their profile, the angle of inclination and the location. The center distance is determined, the width of the gear rims of the pair is selected.
Geometrical indicators of machine gearing for which a gear is designed are calculated. The drawing should display at least two projections: frontal and side view on the left with measurements made. Additionally, a table of basic geometric and structural parameters is compiled, and graphs are constructed.
Values are calculated using formulas, tables, coefficients and ratios are applied, using the original data of the wheels and gears. The calculation algorithm for individual programs may contain up to fifty or more steps and logical steps. The optimal solution to the question of detailed design is the use of a specialized computer program.
The dimensions of the slots for the keys or slots are selected by standards. In general terms, the drawing of mounting wheels on shafts is developed separately.
Is gearing normalized? GOST, currently in force, determines the tolerances for the finished wheelsets. The accuracy of blanks is set depending on the technological features and can be adjusted for each industry or manufacturer separately.
For each type of gear, there are rules of interchangeability. Some standards have lost their relevance in general, some are valid only in certain regions. However, the norms developed earlier are used for general terminology, notation, the procedure for developing documentation and constructing drawings.
The GOST standards regulate the parameters for calculating the geometry of gear-wheel pairs, their modules, initial contours, degrees of accuracy and types of interfaces. Other standards set standards for individual parts of the parts, and others - for ready-made components and assemblies.