Gearbox. a mechanism that changes the torque and engine power, is present in almost any machine and machine tool. It is part of the vehicle’s transmission and regulates movement with precision in precision instruments. What is a gearbox from a technical point of view? This is one or more gears interacting with each other and reducing the number of engine revolutions to an acceptable speed of rotation of the executing node. Instead of a pinion gear there may be a worm.
Device and principle of operation
A gearbox without gas or hydraulic add-ons means a mechanical device for changing angular velocity and torque. It works according to the Golden Rule principle, when the power transmitted by rotation remains practically unchanged, and decreases by efficiency.
The simplest gear unit is gearing and gearing. Torque is transmitted through direct contact of the teeth. the elements of the part. They move with the same linear speed, but different angular. The number of rotations of the gear and wheel per unit time is different, depending on the diameters of the parts and the number of teeth.
Gears and wheels are fixedly mounted on shafts or are made together with them. In the housing may be from one to several pairs of gears. On the assembly drawing of the gearbox, its structure and components are clearly visible:
- case cover;
- pairs in engagement;
The housing at the very bottom has a hole for draining oil and a level control device for lubricants, eye or dipstick. The connector with the cover coincides with the plane of the axis.
On the kinematic diagram of the gearbox, gear joints, shaft arrangements and the direction of rotation are schematically indicated. Also shown is the type of tooth, straight or inclined. According to the kinematic scheme, you can determine the number of stages, gear ratio and other characteristics of how this gearbox works.
The principle of operation of a mechanical gearbox is based on the transmission of torque from one shaft to another through the interaction of gear parts fixedly mounted on them. The linear speed of the teeth is the same. It cannot be different, because the contact is hard.
The principle of operation of the gearbox is the pressure of the tooth on a surface similar to that of an adjacent part and the transmission of the force moving the driven wheel. As a result, the rotation speed decreases. A force is created on the output shaft that can drive the actuator.
The main pair is always the first, high-speed gear or worm connected to the engine and its corresponding wheel. By its type, the entire node is also determined. The number of steps is equal to the number of gears having a gear ratio greater than 1.
In addition to working gears, parasites can be used. gears that do not change the torque, only the direction of rotation of the wheel and, accordingly, of the shaft on which it is located.
The gear unit symbol contains a series of numbers and letters indicating its parameters and type. The first is an indication of the number of steps and the type of gearing:
- cylindrical. C;
- worm. H;
- conical. K;
- globoid. G;
- wave. In;
- planetary. P.
Combined models are indicated by several letters, starting with the first pair:
- cylindrical-worm. CC;
- worm-cylindrical. CC;
- conical-cylindrical. CC.
The number of gears of this type is indicated by a number before the letter.
Horizontal location is considered normal and does not have its own designation. For a vertical unit, after designating the type of gears, the letter B is put. B. means a high-speed model. Behind it is the conventional numerical designation of the assembly option.
Next, the distance between the axes of the drive and the output shaft, the gear ratio with numbers and the shape of the output shaft with a letter designation, for example, C. cylindrical shank, K. conical, are indicated.
The marking may contain an indication of the climatic modification, for example, for the tropics, northern regions, according to which guest is executed.
For example: 1Ts2U-250-31.5-22-M-U2. Two-stage cylindrical with horizontal arrangement. The center distance of the shafts of the low-speed stage is 250 mm, the gear ratio is 31.5. Assembly option for assembly 22, shank by coupling type, climatic version complies with GOST 15150-69.
An electric drive. a motor and a transmission unit in one housing, has slightly different markings. First, there is the letter designation of the brand of the prefabricated drive, the output wheel rotation speed is indicated, since it is constant, connected to one electric motor.
Gearboxes differ externally in size and shape. The internal structure is diverse. All of them are united by a list of technical characteristics by which they are selected for various machines and machine tools. The main parameters of the gearbox include:
- gear ratio;
- gear ratio;
- gear torque value;
- number of steps;
The gear ratio is taken as the total of all gears, and at the same time a table of gear ratios is indicated if the unit has 2 or more stages. On it, a node is selected that converts the rotation of the electric motor or motor with the desired number of revolutions.
It is important to know the amount of torque on the output shaft of the gearbox to determine whether there will be enough power to drive the unit.
The main characteristic of gearing, by which all other parameters are determined. Shows how many revolutions less the wheel makes relative to the gear. Gear ratio formula:
where U is the gear ratio;
Z1 the number of gear teeth;
Z2 the number of gear teeth.
The gear module of the gear and wheel is the same. Their number depends on the diameter. Therefore, you can use the formula:
Where D2 and D1 the diameters of the wheels and gears, respectively.
The calculation of the total gear torque is defined as the product of the gear ratios of all pairs:
Where uR gear ratio;
U1, U2, Un gear ratios of gear pairs.
When calculating the gear ratio, the ratio of the number of teeth of the wheel and the visits of the worm is taken.
In chain transmissions, the gear ratio calculation is done similarly, by the number of teeth on the sprockets and by the diameters of the parts.
When determining the gear ratio of the belt pair, the number of teeth is replaced by the pulley diameters and everything is multiplied by the slip coefficient. Unlike a gear train, the linear speed of the extreme points on the pulleys is not equal to each other. The engagement is not rigid, the belt slips. The transmission efficiency is lower than that of a gear and chain transmission.
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When designing a new unit with predetermined characteristics, the power of the future gearbox is taken as a basis. It is determined by the magnitude of the torque:
where u12. gear ratio;
The “-” sign indicates the reverse direction of rotation of the wheel and shaft on which it is located. With an odd number of gears, the driven wheel spins in the opposite direction with respect to the drive, towards it. With an even number of bevel gears, both shafts rotate in the same direction. You can make it spin in the right direction by installing an intermediate part. parasites. She has the number of teeth like a gear. The parasitic changes only the direction of rotation. All other characteristics remain the same.
The determination of the torque on the shaft is necessary, it allows you to know the power at the output of the gearbox, the values are connected by a directly proportional ratio.
The torque of the input motor at the input is multiplied by the gear ratio. To obtain a more accurate actual value, multiply by the efficiency value. The coefficient depends on the number of steps and the type of gearing. For a spur conical pair, it is 98%.
The purpose of the mechanism
A reducer is a node that changes power. This may be the pressure of gas and liquid in gas cylinders, pipelines and distribution substations. Mechanical gearboxes change the speed and angular speed.
What is needed in the mechanism and the machine gear transmission mechanism. It reduces the angular speed of the engine, while increasing the same amount of torque. the force with which the output shaft can act on the actuator.
The functions of the unit are to reduce the rotational speed by tens of times and increase the torque equally. the force with which the machine will perform work.
Types of gearboxes
A gearbox is a mechanism that transmits torque. The simplest mechanical units that transmit torque are considered to be belt and chain transmissions. They transmit rotation from one part to another and at the same time change the angular velocity.
The largest group of gearboxes that are widely used in all mechanisms, from coffee grinders to blast furnaces, mechanical gear reducers. They are divided into groups according to several parameters:
- type of gearing;
- number of gears;
- installation method;
- spatial position of axes and gear joints.
Usually the drive shaft of the gearbox is high-speed. It is rigidly connected to the engine and rotates at the same speed, up to 1500 rpm. In the opposite relation, when the wheel is leading and the output rotation speed increases and the torque decreases, the assembly is called a reduction.
According to the type of gearing and the shape of the gear, they are divided:
- worm gears;
Combined models can have different types of gears.
The largest number of cylindrical gearboxes is produced. The working surface of the wheel and gear has the shape of a cylinder. Models are characterized by high efficiency, ease of execution and a wide variety of details. Single-stage units are called transmission gears. It is compact, lowers rotation speed and simultaneously transmits torque.
According to the shape of the tooth, cylindrical models are divided:
- spongy teeth;
According to the kinematic scheme, they are straightforward and branched.
A straight tooth has a rounded surface that contributes to the maximum possible contact area. When engaged, the teeth contact along the entire length. Friction is minimized. Efficiency of spur gearing is the highest, 99%.
The advantages of spur gears include minimal bearing load, low friction, the mechanism does not heat up.
Lack of high noise during operation and low power. To betray a lot of effort, the wheels must be made wide, oversized.
The oblique tooth is at an angle. The contact area is larger with the same wheel rim width. The teeth mesh gradually. The helical pair works quietly, smoothly and is able to withstand heavy loads.
The involute friction area is larger, the details are heated. Helical gear efficiency 98% and below. Making parts with an oblique tooth is more difficult, especially tooth milling. Greater accuracy is required when setting up the cutting tool. The inclined position of the tooth creates additional axial loads on the bearings and shortens their life.
To compensate for the negative axial forces of helical gears, chevron gears have been created. They represent two wheels on the same shaft with the teeth tilted in the opposite direction. Thus, the power is further increased.
Chevron gears work quietly. Lack of sophisticated and lengthy tooth cutting technology.
The number of gears can be any. Arrangement of shafts parallel, horizontal and vertical in one plane. With a large number of gears in one housing, a two-row arrangement of shafts is possible.
Cylindrical models are widely used in all fields. From household appliances, coffee grinders, drills, to the metallurgical and mining industries. Each machine has one or more gearboxes. In especially severe conditions, chevron gears are used.
The gear and wheel have a tapered surface. The shafts are angled. The tooth on the gear is straight and radial. Often bevel gears are used in combined or lowering units. The direction of rotation is possible in any direction. The wheel can act as a leader.
How many gears are in the bevel gear depends on its purpose. Usually one. The most famous example of helical gearing is the rear axle differential, which reduces the torque of the assembly. From one wheel, 2 gears rotate synchronously in one direction.
Instead of a pinion gear, there is a worm with a threaded thread in the gearing. There are 1, 2, 4 threads. They do not make another number of calls. The axis of the shafts are perpendicular in different planes.
The worm interacts with several teeth of the wheel during rotation. From strong friction at an angle, a braking moment occurs. He does not allow the wheel to turn and move the worm. Self-braking is used in hoisting mechanisms. A suspended load cannot go down. A worm gear can move the wheel and its associated mechanism with great precision. This is used in instruments and machines for fine-tuning the position of the tool.
Worm gearboxes create with one and two gears. Often make combined with tapered gears.
The worm gearbox has a quiet and smooth ride, the largest gear ratio of one pair is up to 80 units.
The disadvantage is low efficiency and strong heating during operation. it is necessary to make a cooling system.
Planetary models are structurally different from all others. Their wheel is fixedly fixed in the casing. In gearing with it 4 satellites. gears, which synchronously rotate from the central gear.
The carrier connected to the output shaft rotates around the sun gear. The shafts of the satellites are fixed in it through bearings.
The sophisticated design of the planetary gearbox is compensated by its high power, compact size and quiet running. Planetary models are used for work in mines, metallurgy, mining industry.
Gearboxes in which various types of gears are installed are called combined. Most often, cylindrical pairs with worm or conical pairs are connected in one housing.
Gear motor. engine and transmission unit assembled in one housing. The drive is usually made with tapered or worm pairs. The number of gears is one and two.
In wave models, oscillations of the gear located inside the wheel are used for rotation. The model has not yet become widespread.
How to choose a gearbox instead of a broken one, on existing equipment and when creating mechanisms yourself. The main one is the output shaft power. It is calculated based on the engine speed of the gear ratio.
You should pay attention to the location of the shafts; it can be in one direction in cylindrical models.
Mounting is carried out using a flange directly to the motor shaft and is installed on the platform with holes in the sole.
The marking indicates the center distance between the shafts. This size is of constructive importance when installing the node and connecting it to the engine and the shaft of the working mechanism.
You should see which pair in the gearbox is the first, its gear ratio, gearing. The choice of gear includes the location of the shafts in space. They can be located at right angles and be in different planes. The type of bearings is indicated in the technical documentation. There is also a table of the service life of different nodes.
When designing a machine, the selection of a worm gear is based on the power and location of the gearing. With lower engagement, the steam is well lubricated, does not require additional cooling and is able to work for a long time. Pay attention to the operating mode. The node is not always able to work for several hours continuously. The worm joint overheats quickly.
Damage to the gearbox can be avoided with proper operation and regular maintenance. You should carefully study the passport. It indicates the types of maintenance and their frequency. It is necessary to regularly change the oil, constantly add it. Compliance with the operating mode will save the unit intact.
The main malfunction of the gearbox is associated with its overheating. This occurs in the absence of lubrication and the use of oils of other brands. Otherwise, the unit will overheat, the gearing may jam.
Bearings have their own margin of safety. Their period of operation is indicated in the passport. If you do not change to new ones in time, the nodes begin to crumble. The balls will fall out, and the shaft will begin to rotate with great effort, jerking.
Between the housing and the covers: the upper and the side, along the plane of the connector, sealant is laid during assembly. It does not allow oil to flow out. If you do not change it in time, fluid will flow from all connectors.
Overload, a sharp inclusion leads to tooth decay. When the transmission mechanism does not match the engine, it will not last long.