The simplest chain drive (Fig. 3) consists of two sprockets (1 and 2), each fixed on its own shaft, the smaller of which is most often the leading one, and a chain 3 enclosing them, made up of many rigid links that can rotate relative to each other friend.
Chain drives are widely used in general industrial machines.
Chain drives are widely used in various lifting (for example, multi-bucket elevators) and conveying devices. The use of chain drives in these cases simplifies the design of machine units, increases their reliability and productivity. These devices use a wide variety of circuit types.
Chain transmissions are used both to reduce (reduce speed during transmission) rotary motion and to multiply it (increase speed).
Advantages of chain drives: 1. Possibility of transmission of motion over sufficiently long distances (up to 8 m). 2. Possibility of transmission of motion by one chain to several shafts. 3. Absence of slippage and, consequently, stability of the gear ratio with a reduced lateral load on the shafts and on their bearings. 4. Relatively high efficiency (0.96 ... 0.98 with sufficient lubrication).
Disadvantages of chain drives: 1. Increased noise and vibration activity during operation due to the pulsation of the chain speed and the resulting dynamic loads. 2. Intensive wear of the chain joints due to impact interaction with the sprocket groove, sliding friction in the joint itself and the difficulty of lubrication. 3. Extension of the chain (increase in the pitch between the link joints) due to the wear of the joints and the elongation of the plates. 4. Relatively high cost.
Classification:
Chains for their intended purpose can be divided into:
1. traction chains designed to move loads on a horizontal or inclined surface;
2. load chains designed for lifting loads;
3. drive chains designed to transmit motion, most often rotational, in chain drives.
Roller, bush and toothed chains are most widely used as drive chains. These three types of circuits are standardized.
8. Gears, schemes, purpose, advantages, disadvantages, classification.
Gear- a three-link mechanism, which includes two movable links interacting with each other through a higher gear kinematic pair and forming lower (rotational or translational) kinematic pairs with the third fixed link
Rice. 1. Types of gears
The smaller gear involved in the engagement is commonly referred to as gear, more - cogwheel, a link of a gear transmission performing a rectilinear movement is called a gear rack (Fig. 1, j).
Rice. 2. Gear diagram and its parameters
The purpose of the gear transmission is the transmission of motion (most often rotational) with the transformation of parameters, and sometimes its type (rack and pinion). Rotary gears are the most common in technology (Fig. 5). They are characterized by the transmitted power from microwatts (quartz movement wrist watch) up to tens of thousands of kilowatts (large ball mills, crushers, kilns) at peripheral speeds up to 150 m / s.
The advantages of gears:
1. High reliability of work in a wide range of loads and speeds.
2. Great resource.
3. Small dimensions.
4 High efficiency.
5. Relatively low loads on shafts and bearings.
6. Constancy of the gear ratio.
7. Simplicity of service.
Disadvantages of gears:
1. The complexity of manufacturing and repair (high-precision specialized equipment is required).
2. Relatively high level noise, especially at high speeds.
3. Irrational use of teeth - usually no more than two teeth of each of the meshing wheels are involved in the transmission.
Gear classification:
1. By the size of the gear ratio:
1.1. with a gear ratio u> 1 - reducing (gearboxes are the majority of gear drives);
1.2. with gear ratio u<1 – мультиплицирующие (мультипликаторы).
2. By the relative position of the shafts:
2.1. with parallel shafts - spur gears
2.2. with intersecting shaft axes - bevel gears
(bevel gears with an angle of 90 degrees between the axes of the shafts are called orthogonal);
2.3. with crossed shaft axes - worm, screw (Fig. 5, i), hypoid;
2.4. with motion transformation - rack and pinion
3. By the location of the teeth relative to the generatrix of the wheel surface:
3.1. spur teeth - the longitudinal axis of the tooth is parallel to the generatrix of the wheel surface;
3.2. helical - the longitudinal axis of the tooth is directed at an angle to the generatrix of the wheel surface;
3.3. chevron - the tooth is made in the form of two helical gears with an opposite inclination of the axes of the teeth;
3.4. with a circular tooth - the axis of the tooth is made in a circle relative to the generatrix of the surface of the wheel.
4. By the shape of the engaging links:
4.1. with external gearing - the teeth are directed by their tops from the axis of rotation of the wheel;
4.2. with internal gearing - the teeth of one of the gearing wheels are directed by their tops to the axis of rotation of the wheel;
4.3. rack and pinion - one of the wheels is replaced by a straight toothed rack;
4.4. with non-circular wheels.
5. According to the shape of the working tooth profile:
5.1. involute - the working profile of the tooth is outlined along the involute of the circle (a line described by a point of a straight line rolling without sliding around a circle);
5.2. cycloidal - the working profile of the tooth is outlined along a circular cycloid (a line described by a point of a circle rolling without sliding along another circle);
5.3. tarsus (a type of cycloidal) - the teeth of one of the wheels engaging in engagement are replaced by cylindrical fingers - tarsus;
5.4. with a circular tooth profile (Novikov gearing) - the working tooth profiles are formed by circular arcs of almost identical radii.
6. According to the relative mobility of the geometrical axes of the gears:
6.1. with fixed wheel axles - ordinary gears (Fig. 5);
6.2. with movable axles of some wheels - planetary gears.
7. According to the rigidity of the toothed rim of the wheels entering the engagement:
7.1. with wheels of constant shape (with a rigid crown);
7.2. including wheels with a crown of varying shape (flexible).
8. According to the circumferential (tangential) speed of the teeth:
8.1. low-speed (Vz< 3 м/с);
8.2. medium-speed (3< Vз < 15 м/с);
8.3. high-speed (Vz> 15 m / s).
9. By design:
9.1. open (frameless);
9.2. closed (case).
The most widely used are reducing gear drives of rotary motion, including in multi-purpose tracked and wheeled vehicles (gearboxes, final drives, drives various devices). Therefore, the following description, if it is not specifically mentioned, only concerns the transmissions of rotary motion.
The chain transmission in the most common form consists of two wheels located at some distance from each other, called sprockets, and a chain that encompasses them (Fig. 1, a). Driving sprocket rotation is converted to driven sprocket rotation by the chain engaging the sprocket teeth. Sometimes chain drives with several driven sprockets are used. Chain drives operating at high loads and speeds are placed in special casings, called crankcases (Fig. 1, b), which provides constant abundant lubrication of the chain, safety and protection of the transmission from contamination and reducing the noise that occurs during its operation. Sometimes he uses chain variators, arranged according to the scheme of block-belt variators with sliding cones. Due to the stretching of the chains as they wear, the chain tensioner must adjust the chain tension. This regulation, by analogy with belt drives, is carried out either by moving the shaft of one of the sprockets, or by means of adjusting sprockets or rollers.
Rice. 1Advantages of chain drives compared to belt drives:
no slippage,
compactness (they take up significantly less space in width),
lower loads on shafts and bearings (no need for high initial chain tension).
The efficiency of the chain drive is quite high, reaching a value η = 0.98.
Disadvantages of chain drives:
- lengthening of the chain due to wear of its joints and stretching of the plates, as a result of which it has a restless course;
- the presence of variable accelerations in the elements of the chain, which cause dynamic loads, the higher the speed of the chain and the fewer teeth on a smaller sprocket;
- noise during work;
- the need for careful maintenance during its operation.
Chain drives are used at large center distances, when gear drives cannot be used due to their bulkiness, and belt drives - due to the requirements of compactness or constancy of the gear ratio. Depending on the design of the chains, transmissions with a power of up to 5000 kW are used at peripheral speeds up to 30 ... 35 m / s. The most common chain drives with power up to 100 kW at peripheral speeds up to 15 m / s. Chain drives are used in transport, agricultural, construction, mining and oil machines, as well as in machine tools.
Chains in chain drives are called drive chains. By design, drive chains are distinguished:
- sleeve, roller(GOST 13568-75),
- toothed(GOST 13552-81)
- shaped-link.
The main geometric characteristics of the chain are the pitch, that is, the distance between the axes of the two nearest joints of the chain, and the width, and the main power characteristic is the breaking load of the chain, established empirically.
Single row sleeve chain.
Single-row sleeve chain (Fig. 2, a) consists of inner plates 1 pressed onto bushings 2 freely rotating on rollers 5 on which external plates 4... Depending on the transmitted power, the drive bush chains are manufactured single row(PV) and double row(2PV). These chains are simple in design, have a low weight and are the cheapest, but less wear-resistant, so their use is limited to low speeds, usually up to 10 m / s.
Rice. 2
Drive roller chains according to GOST 13568-75 are distinguished:
- single row normal (PR),
- single-row long-link lightweight (PRD),
- single-row reinforced (PRU),
- two (2PR),
- three (ZPR),
- four-row (4PR),
- with curved plates (PRI).
A single-row roller chain (Fig. 2, b) differs from a bushing chain in that its bushings 2 install freely rotating rollers 5... The rollers replace the sliding friction between the bushings and the sprocket teeth in the bushing chain with rolling friction. Therefore, the wear resistance of roller chains is much higher compared to sleeve chains and, accordingly, they are used at peripheral gear speeds up to 20 m / s. Of roller single-row chains, the most common normal PR... Long link lightweight PRD chains manufactured with a reduced breaking load; the permissible speed for them is up to 3 m / s. Reinforced PRU chains are made of increased strength and accuracy; they are used at high and variable loads, as well as at high speeds.
Multi-row chains (Fig. 2, c) allow increasing the load in proportion to the number of rows, therefore they are used when transmitting large powers. Roller chains with bent plates (Fig. 2, d) of increased flexibility are used for dynamic loads (impacts, frequent reversals, etc.).
Toothed chain.
The toothed chain (Fig. 2, e) in each link has a set plates 1(their number is determined by the width of the chain) with two protrusions (teeth) and with a cavity between them for a sprocket tooth. This chain is manufactured with rolling friction joints. In the holes of the plates of each hinge, two prisms 2 and 3 with curved work surfaces. One of the prisms is connected to the plates of one link, and the other - to the plates of the neighboring link, as a result of which, in the process of movement of the chain, the prisms roll over one another.
Toothed chains with sliding friction joints are also used. The durability of toothed chains with rolling friction joints is approximately twice as long.
Toothed chains for protection against slipping off the sprockets and for operation are provided with guides plates 4 which are regular inserts, but without the sprocket teeth grooves. These plates require grooving of the corresponding grooves on the sprockets (see Fig. 4, b).
Toothed chains due to better conditions The sprocket teeth meshes operate with less noise, which is why they are sometimes referred to as silent. Compared to other gear chains, they are heavier, more difficult to manufacture and more expensive, therefore they are used of limitedly. Since the width of the toothed chains can be anything (there are chains up to 1.7 m wide), they are used to transmit high powers.
Shaped chains are of two types: hooked(Fig. 3, a) and pin(Fig. 3, b). The hook chain consists of links of the same shape, cast from ductile iron or stamped from ZOG strip steel without additional details... The assembly and disassembly of this chain is carried out by the mutual inclination of the links at an angle of 60 °. Cast in pin chain links 1 made of ductile iron are connected by pinned steel (from steel St3) pins 2... Shaped link chains are used when transmitting small powers, at low speeds (hook up to 3 m / s, pin up to 4 m / s), usually in conditions of imperfect lubrication and protection. The links of the shaped link chains are not processed. Due to their low cost and ease of repair, shaped link chains are widely used in agricultural machines.
Rice. 3
Drive chain lubrication.
Grease drive chains prevents them from rapid wear. For critical power chain drives, continuous crankcase lubrication is used, carried out at a speed of up to 8 m / s with a chain dipping in oil bath to a depth not exceeding the width of the plate and at a higher speed - by forced circulating supply of lubricant from the pump (see Fig. 1, b). In the absence of a sealed crankcase and a chain speed of up to 8 m / s, a consistent internal hinge lubrication is used, carried out periodically after 120..180 hours by immersing the chain in a lubricant heated to liquefaction. Sometimes drip lubrication is used instead of grease. When the transmission is operating intermittently with a peripheral speed of up to 4 m / s, periodic lubrication of the chain is also used, carried out by a manual oiler after 6 ... 8 hours.
Chain and sprocket material.
From material and heat treatment chains and sprockets depends on the durability of the chain drives.
Rice. 4
Bush, roller and toothed chain elements are made of following materials: plates - from medium-carbon or alloy steels 40, 45, 50, 30KhNZA with hardening to hardness HRC32 ... 44, and rollers, bushings, rollers and liners - from case-hardened steels 10,15, 20, 12KHNZA, 20KHNZA, 30KHNZA with heat treatment up to hardness HRC40 ... 65. Bush and roller chains are used, inside the steel bushings of which plastic bushings are placed, freely rotating both on the rollers and inside the steel bushings. Such chains are used when the joints are operated without or with poor lubrication.
Chain sprockets are designed in the same way as toothed wheels. Depending on the size, material and purpose, they are made whole (Fig. 4) or composite (Fig. 5).
Rice. 5
Sprockets for bush and roller chains are narrow. They are usually made of two parts - a disc with teeth and a hub, which, depending on the material and purpose, the sprockets are welded (Fig. 5, a) or connected with rivets (bolts) (Fig. 5, b). Sprockets for toothed chains (see Fig. 4, b) are wide, they are made intact. Whole sprockets and disks of compound sprockets are mainly made of medium-carbon or alloy steel 40, 45, 40X, 50G2, 35XGSA, 40XN with hardening to hardness HRC40 ... 50 or case-hardened steel 15, 20, 15X, 20X, 12XH2 with heat treatment to hardness HRC50 ... 60. Low speed sprockets at chain speed v≤3 m / s and in the absence of dynamic loads, they are also made of gray or modified cast iron СЧ15, СЧ18, СЧ20, СЧ30 with a surface hardness up to HB260 ... 300... Sprockets with a toothed ring made of plastic (duroplast or vulkolana) are used. Vulkolan is a type of polyurethane with special qualities. The design of such sprockets is shown in (Fig. 5, e). On the rim of the metal part of the sprocket, a groove is made in the form dovetail, interrupted by several transverse recesses, in which a gear ring made of plastic is placed. The advantage of plastic sprockets over metal sprockets is the reduction in chain wear and transmission noise.
§ 1. GENERAL INFORMATION
The chain transmission consists of a driving and driven sprocket and a chain that encompasses the sprockets and engages in their teeth. Chain drives with several driven sprockets are also used. In addition to the basic elements listed, chain drives include tensioners, lubricators and guards.
A chain is made up of pivot-linked links that provide mobility or "flexibility" to the chain.
Chain drives can be performed in a wide range of parameters.
Chain drives are widely used in agricultural and lifting and transport vehicles, oil drilling equipment, motorcycles, bicycles, cars.
In addition to chain drives, mechanical engineering uses chain devices, i.e., chain drives with working bodies (buckets, scrapers) in conveyors, elevators, excavators and other machines.
The advantages of chain transmissions include: 1) the possibility of using in a significant range of center-to-center distances; 2) smaller than that of belt drives, dimensions; 3) no slipping; 4) high efficiency; 5) small forces acting on the shafts, since there is no need for a large initial tension; 6) opportunity easy replacement chains; 7) the ability to transmit motion to multiple stars.
At the same time, chain drives are not without drawbacks: 1) they operate in the absence of fluid friction in the hinges and, therefore, with their inevitable wear, which is significant in case of poor lubrication and ingress of dust and dirt; wear of the joints leads to an increase in the pitch of the links and the length of the chain, which necessitates the use of tensioners; 2) they require more high precision installation of shafts than V-belt transmissions, and more complex maintenance - lubrication, adjustment; 3) transmissions require installation on crankcases; 4) the speed of the chain, especially with small numbers of teeth of the sprockets, is not constant, which causes fluctuations in the gear ratio, although these fluctuations are small (see § 7).
Chains used in mechanical engineering, according to the nature of the work they perform, are divided into two groups: drive and traction. The chains are standardized, they are produced in specialized factories. The production of drive chains alone in the USSR exceeds 80 million cubic meters per year. More than 8 million cars are equipped with them annually.
Roller, bush and gear chains are used as drive chains. They are characterized by small steps (to reduce dynamic loads) and wear-resistant hinges (to ensure durability).
The main geometric characteristics of the chains are the pitch and width, the main power characteristic is the breaking load, which is established empirically. In accordance with international standards use chains with a step multiple of 25.4 mm (i.e. ~ 1 inch)
In the USSR, the following drive roller and bushing chains are manufactured in accordance with GOST 13568-75 *:
PRL - single-row roller of normal accuracy;
PR - roller of increased accuracy;
PRD - roller long link;
PV - bushing;
PRI - roller with curved plates,
as well as roller chains in accordance with GOST 21834-76 * for drilling rigs (in high-speed gears).
Roller chains are chains with links, each of which is made of two plates pressed onto pins (outer links) or bushings (inner links). The bushings are put on the rollers of the mating links and form the hinges. The outer and inner links in the chain alternate.
The bushings, in turn, carry rollers that fit into the tooth spaces on the sprockets and engage with the sprockets. The rollers replace the sliding friction between the chain and the sprocket with rolling friction, which reduces wear on the sprocket teeth. The plates are outlined with a contour that resembles the number 8 and brings the plates closer to bodies of equal tensile strength.
The rollers (axes) of the chains are made stepped or smooth.
The ends of the rollers are riveted, so the chain links are one-piece. The ends of the chain are connected by connecting links with the fastening of the rollers with cotter pins or riveting. If it is necessary to use a chain with an odd number of links, special transition links are used, which, however, are weaker than the main ones;
therefore, they usually tend to use chains with an even number of links.
At high loads and speeds, in order to avoid the use of chains with large steps, unfavorable in terms of dynamic loads, multi-row chains are used. They are made up of the same elements as single-row ones, only their nodules have an increased length. The transmitted power and breaking loads of multi-row chains are almost proportional to the number of rows.
The characteristics of high-precision roller chains PR are given in table. 1. Roller chains of normal accuracy PRL are standardized in the range of steps 15.875 ... 50.8 and are designed for breaking load 10 ... 30% less than that of chains of high precision.
Long-link roller chains of the PRD are performed in a double step compared to conventional roller chains. Therefore, they are lighter and cheaper than conventional ones. It is advisable to use them at low speeds, in particular, in agricultural engineering.
PV bush chains have the same design as roller chains, but do not have rollers, which makes the chain cheaper and reduces the overall dimensions and weight with an increased projection area of the hinge. These chains are made with a pitch of only 9.525 mm and are used, in particular, in motorcycles and cars (drive to the camshaft). The chains show sufficient performance.
Roller chains with bent plates PRI are recruited from identical links, similar to a transition link (see Fig. 12.2, e). Due to the fact that the plates work in bending and therefore have increased flexibility, these chains are used for dynamic loads (impacts, frequent reversals, etc.).
In the designation of a roller or bush chain indicate: type, pitch, breaking load and GOST number (for example, Chain PR-25.4-5670 GOST 13568 -75 *). For multi-row chains, the number of rows is indicated at the beginning of the designation.
Toothed chains (Table 2) are link chains from plate sets. Each plate has two teeth with a cavity between them to accommodate a sprocket tooth. The working (outer) surfaces of the teeth of these plates (the contact surfaces with the sprockets are limited by planes and are inclined to one another at a wedging angle a equal to 60 °). With these surfaces, each link sits on two sprocket teeth. The sprocket teeth are trapezoidal.
The plates in the links are spaced apart by the thickness of one or two plates of the mating links.
At present, the main production is chains with rolling joints, which are standardized (GOST 13552-81 *).
To form hinges, prisms with cylindrical working surfaces are inserted into the holes of the links. The prisms rest on the flats. With a special profiling of the bore of the plates and the corresponding surfaces of the prisms, an almost pure rolling can be obtained in the hinge. There is experimental and operational data that the resource of gear chains with rolling joints is many times higher than that of chains with sliding joints.
To avoid lateral slipping of the chain from the sprockets, guide plates are provided, which are ordinary plates, but without recesses for the teeth of the sprockets. Internal or side guide plates are used. Internal guide plates require a matching groove on the sprockets. They provide best direction at high speeds and are of primary use.
The advantages of toothed chains in comparison with roller chains are less noise, increased kinematic accuracy and allowable speed, as well as increased reliability due to the multi-plate design. However, they are heavier, more difficult to manufacture, and more expensive. Therefore, they are of limited use and are supplanted by roller chains.
Traction chains are subdivided into three main types: lamellar but GOST 588-81 *; collapsible in accordance with GOST 589 85; round-grain (normal and high strength), respectively, according to GOST 2319-81.
Plate chains serve to move goods at any angle to the horizontal plane in transporting machines (conveyors, elevators, escalators, etc.). They usually consist of plain plates and pins with or without bushings; they are characterized by
large pitches as side plates are often used to secure the conveyor belt. The speed of movement of chains of this type usually does not exceed 2 ... 3 M / S.
Round Link Iepi used mainly for suspension and lifting of loads.
There are special chains that transfer movement between sprockets with mutually perpendicular axes. The rollers (axes) of two adjacent links of such a chain are mutually perpendicular.
§ 3. BASIC PARAMETERS OF DRIVE CHAIN GEARS
The capacities for the transmission of which chain drives are used vary in the range from fractions to hundreds of kilowatts, in general mechanical engineering, usually up to 100 kW. The center-to-center distances of the chain drives are up to 8 m.
The sprocket speeds and speed are limited by the impact force between the sprocket tooth and the chain pivot, wear and gear noise. The highest recommended and maximum sprocket speeds are given in table. 3. The speed of movement of the chains usually does not exceed 15 m / s, however, in gears with chains and sprockets High Quality at effective ways lubrication reaches 35 m / s.
Average chain speed, m / s,
V = znP / (60 * 1000)
where z is the number of teeth of the sprocket; NS speed of its rotation, min-1; R-
Chain transmission refers to gearing transmissions with flexible coupling. It consists of a drive and driven sprocket, bent by a chain The advantages of chain drives... 1. Compared to gear drives, chain drives can transmit motion between shafts at significant center distances (up to 5 m).
2. Compared to belt drives: they are more compact, can transmit high power, require significantly less pre-tensioning force, provide a constant gear ratio (there is no slipping and slipping).
3. They can transmit the movement of one chain to several sprockets. Disadvantages. 1. Significant noise during operation due to the impact of the chain link on the sprocket tooth when engaging, especially with small numbers of teeth and a large pitch (this drawback limits the use of chain drives at high speeds).
2. Relatively fast wear of chain joints; the need to use a lubrication system.
3. The lengthening of the chain due to the wear of the joints and its coming off the sprockets, which requires the use of tensioners.
Application. Chain drives are used in machine tools, industrial robots, transport, agricultural and other machines to transfer motion between parallel shafts over long distances, when the use of gear drives is impractical and belt drives are impossible. The most widely used are chain drives with a power of up to 120 kW at a peripheral speed of up to 15 m / s.
32. Classification of circuits
Drive chain - main element chain transmission - consists of individual links connected by hinges. In addition to drive chains, there are traction and load chains that are not covered here. The main types of standardized drive chains are roller, sleeve and toothed. Roller drive chains. Consist of two rows of outer and inner plates (Fig. 14. 2). Axles are pressed into the outer plates, passed through bushings, which, in turn, are pressed into the inner plates. Hardened rollers are loosely fitted on the bushings. With relative rotation of the links, the axle rotates in the bushing, forming a sliding hinge. The engagement of the chain with the sprocket occurs through a roller, which, turning on the bushing, rolls over the sprocket tooth. This design evens out the pressure of the tooth on the sleeve and reduces wear on both the sleeve and the tooth. Roller chains are widespread. They are used at speeds v ≤ 15 m / s. Bush drive chains are similar in design to roller chains, but do not have rollers, which makes the chain cheaper, reduces its weight, but significantly increases the wear of the chain bushings and sprocket teeth. Bush chains are used in non-critical gears with v ≤ 1 m / s. Bush and roller chains are made single-row (Fig. 14.2) and multi-row (Fig. 14.3) with the number of rows 2, 3 and 4. A multi-row chain with a smaller pitch allows you to replace a single-row chain with a large pitch and thereby reduce the diameters of the sprockets, reduce dynamic loads in the transmission ... The ends of the axles are riveted, so the chain links are one-piece. Toothed drive chains consist of links made up of a set of plates and pivotally connected to each other (Fig. 14.4). Each plate has two teeth and a cavity between them to accommodate a sprocket tooth. The number of plates determines the width of the chain B, which depends on the transmitted power. The working faces are the planes of the plates located at an angle of 60 °. With these faces, each chain link is wedged between two sprocket teeth that have a trapezoidal profile (see Fig. 14.7). This allows the gear chains to run smoother, quieter, better absorb shock and allow higher speeds. However, compared to others, timing chains are heavier and more expensive. They are used at speeds v ≤ 35 m / s. To eliminate the lateral fall of the chain from the sprockets, guide plates 1 are used (see Fig. 14. 4) located in the middle or on the sides of the chain.
where T is the torque on the sprocket; d - pitch diameter of the drive sprocket (see Fig. 12 and 13).
Pulling forces:
The leading branch of the chain of a working transmission (Fig. 16)
F 1 = F t + F 0 + F v;(11)
Driven branches of the chain
F 2 = F 0 + F v;(12)
From sagging chain
F 0 = K f ∙ q ∙ a ∙ g, (13)
where K f is the coefficient of sagging, depending on the location of the drive and the size of the chain sagging boom f
With f = (0.01 ÷ 0.002) afor horizontal gearsK f = 6; for inclined (≈ 40 °) - K f = 3; for verticalK f =1
q- weight of 1 m of chain, kg (see table 1);
a- center distance, m;g = 9.81 m / s 2 ;
From centrifugal forces;
F u = q v 2 ,(14)
where v– average speed chains in m / c.
Rice. 16. Tension forces in the chain drive
The shaft and support absorb the tension forces from the chain slack and from the circumferential force. Approximately
F s = F t ∙ K in +2 F 0, (15)
where
TO B
- shaft load factor (Table 3).Load on shafts and supports in chain drive significantly less than in a belt drive.
Table 3. Shaft load factor value TO v
|
Inclination of the line of centers of stars to the horizon, degrees |
The nature of the load |
To in |
|
0 ÷ 40 |
Calm Shock |
1,15 1,30 |
|
40 ÷ 90 |
Calm Shock |
1,05 1,15 |
Methodology for the selection and testing of chains taking into account their durability
The main criterion for the performance of drive chains is the wear resistance of their hinges. As shown by theoretical and experimental studies, the load capacity of the chain is directly proportional to the pressure in the joints, and the durability is inversely proportional.
Calculation of the chain for the durability of the hinges. Medium pressure R in the hinge should not exceed the permissible value (specified in Table 1), i.e.
where F t = 2 t / d - circumferential force transmitted by the chain; T is the torque; d is the pitch circle diameter of the sprocket (if the transmission power P is given, then F t = p / v, where v- chain speed); A -projection area of the bearing surface of the joint, for roller and bush chains A = dB; for toothed chains A = 0.76 dB; m- the number of rows of the chain; TO - operating factor;
K = K 1 ∙ K 2 ∙ K 3 ∙ K 4 ∙ K 5 ∙ K 6 (17)
(the values of the coefficients
K 1 ÷ K 6 - see Table 4).The joint pressure must be between 0.6 [p] ≤p ≤1.05.
If the obtained value of the pressure in the hinge exceeds or is significantly less than the allowable one, then, by changing d, T, the chain row m or parameters affecting K, the specified condition is satisfied.
Table 4. Meaning different ratios when calculating the chain for the wear resistance of the hinges
|
Coefficient |
Working conditions |
Meaning |
|
TO 1 - dynamism |
With a quiet load Under jolt or variable load |
1,25-1,5 |
|
K 2 - center distance |
a<25t a = (30 ÷ 50) t a = (60 ÷ 8 0) t |
1,25 |
|
K 3- lubrication method |
Lubrication: continuous drip periodic |
|
|
TO 4 - the slope of the line of centers in the horizon |
When the line of centers is inclined to the horizon, deg .: up to 60 over 60 |
|
|
TO 5 - operating mode |
When working: single shift two-shift continuous |
1,25 |
|
TO 6 - chain tension control method |
With movable supports With pull sprockets With a squeeze roller |
1,25 |
We transform the formula (16):
a) we express the circumferential force in terms of the torque on the driving sprocket T 1, chain pitch tand the number of teeth of this sprocketz 1;
b) we represent the area of the bearing surface of the hinge as a function of the stept... Then we get an expression for determining the step of the chain:
for roller and bush chains
for toothed chain with sliding joint
where T - number of rows in a roller or bushing chain;
𝜓 p = B / t = 2 ÷ 8 - the ratio of the width of the toothed chain.
Calculation of the chain for breaking load (by safety factor). In critical cases, the selected chain is checked by the safety factor
where F -
Σ F 1 = F t ∙ K B + F v+ F
[s ] - the required (admissible) safety factor (selected according to Table 1).
Durability in terms of number of engagement with both sprockets (number of strokes) is checked according to the formula
where z c - the total number of chain links;zn- the number of teeth and the frequency of rotation of the sprocket (leading or driven);U- the actual number of chain links in engagement for 1 s;v -peripheral speed, m / s;L- chain length, m; [ U]- the permissible number of chain entrances in engagement for 1 s (see Table 1).
The sequence of the design calculation of chain drives.
1. Select the type of chain according to its expected speed and from the operating conditions of the transmission (roller, sleeve, gear).
2. By gear ratio and choose from table 1 the number of teeth of a small sprocket z 1, by the formula (9) determine the number of teeth of the larger sprocket z 2.
Check condition z 2 3. Determine the torque T x
on a small sprocket, according to Table 1, select the allowable pressure in the hinges [ R ], set calculated coefficients K 1, K 2, K 3, K 4, K 5, K 6
and by formula (17) determine the operating factor K .
Then from the condition of the wear resistance of the hinges [see. formulas (18), (19)] determine the step of the chain. The resulting step valuet round up to standard (see table 1). 4. The accepted step is checked by the permissible angular velocity of the small sprocket (see Table 1). If the condition is not metω = ω max Increase the number of roller (sleeve) chain rows or the width of the toothed chain. 5. By the formula (8) determine the average speed of the chainv and strength F t,
then use the formula (16) to check the wear resistance of the chain. If the condition is not met R
<[р]
increase the chain step and repeat the calculation. 6. Determine the geometric dimensions of the gear. 7. For especially critical chain drives according to the formula (20), check the selected chain according to the safety factor. 8. Using the formula (21), check the transmission by the number of strokes per 1 s. The chain pitch is selected depending on the maximum permissible speed n 1max smaller asterisk. Number of teeth z 1 smaller sprockets are taken according to the formula, while taking into account that with an increase in the number of teeth z 1 pivot pressure, pitch and chain width decrease, and chain life is correspondingly increased. Sprocket circle diameters: Dividing Outdoor
Number of star teeth:z 1
= 37-2and(but not less than 17),z 2
=
z 1
(but not more than 140): here u = n 1 / n 2 = z 2 / z 1. Chain wedging angle α = 60 °
(see page 13.2). Double angle of the tooth cavity: 2β = α -φ. Sharpening angle of the tooth: γ = 30 ° -φ, where φ = 360 ° / Z. Sprocket ring gear width: B = b + 2S, where S- thickness of the chain plate. Chain Drive Parameters - Center Distance a, chain lengthL -determined by the formulas for roller chains. The forces acting in the transmission are determined in the same way as in the case of transmission by roller chains. The main parameter of the toothed chain is its width in mm, determined by the formula Here P is the transmitted power, kW; coefficient TO
has the same meaning as in roller chain transmission [cf. formula (17)]; [P 10
] - power, kW, allowed for transmission by a toothed chain 10 mm wide (see Table 5). Since the values R 10 are shown in the table depending on the stept and speed v, and at the beginning of the calculation these values are unknown, then it is necessary to perform the calculation by the method of successive approximations: taking a preliminary approximate value of the steptfind the speed of the chain
According to these values, the value [ R 10 ]
and calculated by the formula (24) the chain widthb.The result obtained is rounded to the nearest higher value according to the table. 2. Optimal results can be obtained by calculating a number of options on a computer with various combinations of valuest,z 1,
b;
while the initial data ( R,
n 1, n 2 , installation and operating conditions) should generally not be changed. Table 5. Values[
R 10
]
, kW, for drive toothed chains type 1 (one-sided gearing) with a nominal width of 10 mm t, mm Chain speed v, m / with 12,7
15,875
19,05
25,4
31,75
2,35
The calculation ends with the determination of the geometric parameters of the transmission, the loads acting in it, checking the strength factor of the chain - in the same way as described above in the calculation of the transmission by drive roller chains, with the difference, however, that the calculated strength factor must not be less than the standard [ s] specified in table. 6. Table 6. Standard safety factor
[
s
]
drive gear chains type 1 (single-sided gearing)
t, mm Smaller sprocket speedn 1 obmin 12,7
15,875
19,05
25,4
31,75
Experimental observations show that the main reasons for the failure of chain drives are: 1. Wear of hinges (due to impacts when the chain engages with the teeth of the sprocket and due to their wear from friction) leading to lengthening of the chain and disruption of its engagement with the sprockets (the main criterion for performance for most gears). The limiting elongation of the chain due to wear of the hinges should not exceed 3%, since the correct engagement of the chain hinges and teeth is disturbed. 2. Fatigue failure of plates along the lugs is the main criterion for high-speed heavily loaded roller chains running in closed crankcases with good lubrication. 3. Rotation of rollers and bushings in the plates at the press-in points is a common cause of chain failure due to insufficiently high quality workmanship. 4. Chipping and destruction of rollers. 5. Achieving the maximum sagging of the idle branch is one of the criteria for gears with an unregulated center distance, operating in the absence of tensioning devices and tight dimensions. 6. Wear of sprocket teeth. In accordance with the above reasons for the failure of chain drives, it can be concluded that the service life of the transmission is most often limited by the life of the chain. The durability of the chain primarily depends on wear resistance of the hinges. According to this criterion, the design calculation of the chain drive is carried out using medium pressure in the hinge.p u... Protection against excessive stretching of the chain during operation or from overloads and destruction during start-up is ensured by checking the strength of the chain. The material and heat treatment of the chains is critical to their durability. Plates are made of medium-carbon or alloy hardened steels: 45, 50, 40X, 40XH, ZOKHNZA with hardness mainly 40 ... 50 HRCe; toothed chain plates are predominantly made of 50 steel. Curved plates are generally made of alloy steels. Plates, depending on the purpose of the chain, are hardened to a hardness of 40 .-. 50 HRC NS... Details of the hinges, rollers, bushings and prisms - are made mainly of case-hardened steels 15, 20, 15X, 20X, 12XNZ, 20XIZA, 20X2H4A, ZOKHNZA and are hardened to 55-65 HRC NS... Due to the high requirements for modern chain drives, it is advisable to use alloy steels. The use of gas cyanidation of the working surfaces of the hinges is effective. A multiple increase in the chain life can be achieved by diffusion chromium plating of the hinges. The fatigue strength of the roller chain plates is significantly increased by crimping the edges of the holes. Shot blasting is also effective. Plastics are used in the joints of roller chains for operation without lubricant or when it is poorly supplied. The resource of chain drives in stationary machines should be 10 ... 15 thousand hours of work. Friction losses in chain drives consist of losses: a) friction in hinges; b) friction between the plates; c) friction between the sprocket and the chain links, and in roller chains also between the roller and the bushing, when the links enter and disengage; d) for friction in supports; e) losses due to oil splashing. The main ones are the friction losses in the joints and bearings. Losses due to oil splashing are significant only when the chain is lubricated by dipping at the maximum speed for this type of lubrication v = 10 ... 15 m / s. The chain drives are positioned so that the chain moves in a vertical plane, and the relative position in height of the driving and driven sprockets can be arbitrary. The optimum chain drive locations are horizontal and inclined at an angle of up to 45 ° to the horizontal. Vertically located gears require more careful adjustment of the chain tension, since its sagging does not provide self-tension; therefore, at least a slight mutual displacement of the sprockets in the horizontal direction is advisable. Chain drives can be driven by either the upper or lower branches. The leading branch must be top in the following cases: a) in gears with a small center distance (a<30P при
and> 2) and in gears close to vertical, in order to avoid the capture of additional teeth by the sagging upper driven branch; b) in horizontal gears with a large center distance (a> 60P) and a small number of teeth of the sprockets in order to avoid contact of the branches. As the hinges wear out and contact crimps, the chain stretches, the arrow sags f the driven branch increases, causing the chain to overwhelm the sprocket. For gears with inclination angle θ<45° наклона к
горизонту [f]<0,02a; at θ
> 45 ° [ f]
< 0,015a, where a- center distance. Therefore, chain drives, as a rule, must be able to adjust its tension. Pre-tension is essential in vertical gears. In horizontal and inclined gears, the chain engagement with sprockets is ensured by tension from the chain's own gravity, but the chain sagging boom should be optimal within the above limits. The chain tension is controlled by devices similar to those used for belt tension, i.e. by moving the shaft of one of the sprockets, pressure rollers or pull-off sprockets. Tensioners should compensate for chain elongation within two links, with a larger stretch, two chain links are removed. The increase in the chain pitch due to wear in the joints is not compensated by its tension. As the chain wears down, the joints move closer to the tops of the teeth and there is a risk of the chain slipping off the sprockets. The adjusting sprockets and rollers should, if possible, be installed on the driven branch of the chain in the places of its greatest sagging. If it is impossible to install them on the driven branch, they are placed on the leading one, but to reduce vibrations - from the inside, where they work as pull-backs. In gears with a PZ-1 toothed chain, the control sprockets can only work as pull-back wheels, and the rollers as tension wheels. The number of teeth of the adjusting sprockets is chosen equal to the number of the small working sprocket or more. In this case, there must be at least three chain links in engagement with the adjusting sprocket. The movement of the adjusting sprockets and rollers in chain drives is similar to that in belt drives and is carried out by a load, a spring or a screw. The most widespread is the design of an asterisk with an eccentric axis, compressed by a spiral spring. It is known the successful use of chain drives with roller chains of improved quality in closed crankcases with good lubrication with fixed sprocket axles without special tensioning devices. To create conditions for abundant chain lubrication, protection from contamination, noiselessness and safety of work, chain drives are enclosed in crankcases. The internal dimensions of the crankcase must ensure the possibility of the chain sagging and its easy maintenance. The radial clearance between the inner wall of the crankcase and the outer surface of the sprockets is taken equal to l = (t + 30)
mm. The gap, taking into account the chain slack, is assigned within 0.1 a, and the width of the crankcase will be 60
mm larger than the chain width. Carters are supplied with a window and an oil level indicator. a) by dipping the chain in oil to a depth equal to the width of the plate. Apply when V≤ 10 m / s. b) by spraying with the help of special rings, baffle plates, along which the oil flows onto the chain. Apply when V= 6 ... 12 m / s in cases when the oil level cannot be raised to the chain horizon; c) circulating jet lubrication from the pump is the most advanced method. It is used for high-speed powerful gears; d) circulating lubrication with oil droplets spraying in a stream of compressed air. Apply when V> 12 m / s. In medium-speed gears that do not have sealed crankcases, you can use a consistent intra-hinge or drip lubricant. Grease is carried out periodically after 120 ... 180 hours by immersing the chain in a heated grease. This lubricant is applicable whenV≤ 4 m / with . Lubrication of the chain has a decisive influence on its durability. The lubricant increases the wear resistance and endurance of the chain, as well as softens the impact of the links on the sprocket teeth and reduces the heating temperature of the chain. The most commonly used lubricating oils are liquid lubricating oils. For critical power transmissions, continuous crankcase lubrication of the following types should be used, if possible: a) by dipping the chain in an oil bath, and the immersion of the chain in oil at the deepest point should not exceed the width of the plate; apply up to a chain speed of 10 m / s in order to avoid inadmissible agitation of the oil; b) spraying with the help of special spray projections or rings and reflective shields, along which the oil flows onto the chain, is used at a speed of 6 ... 12 m / s in cases where the oil level in the bath cannot be raised to the location of the chain; c) circulating jet lubrication from a pump, the most advanced method, is used for powerful high-speed gears; d) circulating centrifugal with oil supply through channels in shafts and sprockets directly to the chain; used with limited transmission dimensions, for example, in transport vehicles; e) circulating lubrication by spraying oil droplets in a stream of air under pressure; used at a speed of more than 12 m / s. In medium-speed gears that do not have sealed crankcases, plastic intra-hinge or drip lubrication. Plastic intra-hinge lubrication is carried out periodically, after 120 ... 180 hours, by immersing the chain in oil heated to a temperature that ensures its liquefaction. Grease is suitable for chain speeds up to 4 m / s and drip lubrication up to 6 m / s. In gears with coarse pitch chains, the limiting speeds for each lubrication method are slightly lower. With periodic operation and low speeds of the chain movement, periodic lubrication with a manual oiler (every 6 ... 8 hours) is permissible. Oil is supplied to the lower branch at the entrance to the sprocket engagement. With manual drip and spray lubrication from the pump, it is necessary to ensure that the lubricant is distributed over the entire width of the chain and gets between the plates to lubricate the joints. It is preferable to supply lubricant to the inner surface of the chain, from where, under the action of centrifugal force, it is better supplied to the joints. The choice of the type of lubricant (table 7) and the type of lubricant according to GOST 17479.4-87 (table 8) depends on the chain speed v and pressure in the chain pivot p. Table 7 Chain drive lubrication at peripheral speed v, m / s ≤
4 ≤
7 ≥
12 Drip 4 ... 10 cpm In oil Circulating under pressure Spraying Table 8 Joint pressure p, MPa Chain speed v, m / s Joint pressure p, MPa Chain speed v, m / s Drip In an oil bath ≤
10 ≤
1 ≥
5 ≤
10 ≤
5 ≥
10 ≤
1 ≥
5 ≤
5 ≥
10 ≤
1 ≥
5 ≤
5 ≥
10 ≥
30 ≤
1 ≥
5 ≥
30 ≤
5 ≥
10 Abroad, they began to produce chains for light operating conditions that do not require lubrication, the rubbing surfaces of which are covered with self-lubricating antifriction materials. 1. In drives with high-speed motors, the chain drive is usually installed after the gearbox. 3. To ensure sufficient self-tensioning of the chain, the angle of inclination of the line of centers of the sprockets to the horizon should not be more than 60 °. At θ> 60 0, a pull-back sprocket is installed on the driven branch in the places of the greatest sagging of the chain. 4. The diameter of the pull-back sprocket is larger than the diameter of the change of the transmission sprocket, it must mesh with at least three chain links. 5. Since the cross section of the chain is not flexible, the shafts of the chain drive must be parallel and the sprockets are installed in the same plane. 6. The use of three- and four-row chains is undesirable, as they are expensive and require increased precision in the manufacture of sprockets and transmission mounting. 7. To increase the durability of the chain drive, it is necessary to take as many teeth as possible of the smaller (driving) sprocket, since with a small number of teeth there are a small number of links in engagement, which reduces the smoothness of the transmission and increases chain wear due to the large angle of rotation of the hinge. The design of the sprocket ring for roller chains is shown in Fig. 17. Rice. 17. Design of roller chain sprockets The main dependencies for the construction of sprockets of this type are shown in Table 9. Table 9. Main dependencies for the design of sprockets Parameter Calculation formulas pitch diameter diameter of projections D e = P c ∙ cavity diameter D i = d d -2r groove diameter D c = P c ∙ ctg(180 ° / z) -1.3 ∙ h tooth width b = 0.9 ∙ B VN -0.15 crown width B = (n-1) ∙ A + b radius of rounding of a tooth R = 1.7 ∙ d 1 cavity radius r = 0.5025 ∙ d 1 -0.05 fillet radius r 1 = 1.3025 ∙ d 1 +0.05 tooth head radius r 2 = d 1 ∙ (1.24cos φ + 0.08cos β -1.3025) -0.05 half of the angle of the tooth φ = 17 ° -64 ° / z mating angle β = 18 ° -60 ° / z half the angle of the valley α = 55 ° -60 ° / z f = 0.2b tooth bevel angle γ≈ 20 ° bias e = 0.03 ∙ P c rim thickness δ = 1.5 ∙ (D e -d d) disc thickness С = (1.2 ... 1.3) ∙ δ Numerical values B VN, A, d 1 and h are taken depending on the step of the chain P c according to table 10. Table 10 P c, mm Distance between internal plates B VN, mm Distance between axes of symmetry multi-row chains A, mm d 1, mm internal plates h, mm In the manufacture of sprockets, they usually take the 2nd accuracy class according to GOST 591-69. An example of a drawing of a sprocket for a roller chain is shown in Fig. 18. The table of parameters of the gear rim is placed in the upper right corner of the drawing. It consists of two parts, separated by a solid main line. In the first part of the table, the designation of the mating circuit is given. In the second part, the parameters of the sprocket are indicated: the number of teeth - z; tooth profile with reference to the standard (GOST 591-69) and an indication of the offset; accuracy class - 2nd; cavity radius - r; radius of conjugation - r 1; the radius of the tooth head - r 2; half the angle of the valley - α
;
conjugation angle - β.Toothed chain transmission calculation
Performance criteria and damage types for chain drives
Chain materials
Friction loss. Gear design
Tensioning the chains
Carters
Lubrication
Sprocket design
