Chain transmission refers to gears with a flexible link. It consists of the leading 2 and the slave 1 , rounded by a chain (Figure 4.64).
Advantages: Compared with gear drives, chain drives can transfer traffic between shafts at significant interaxial distances (up to 8 m); in comparison with belts are more compact, can transfer large capacities (up to 3000 kW); The forces acting on the shafts are much smaller; since the preliminary tension of the chain is small, can transfer movement by one chain to several asterisks.
Disadvantages: Significant noise due to the impact of the chain link when engaging, especially with small numbers of sprocket teeth and a large pitch (this drawback limits the possibility of using chain drives at high speeds); relatively rapid wear of the chain hinges, which leads to an elongation of the chain and requires tensioning devices; uneven movement of the chain, which causes additional dynamic loads.
Chain drives are used in machines, transport (bicycles) and other machines to transfer traffic between parallel shafts located at a considerable distance, when the gears are unsuitable, and the belt is unreliable. The greatest applications were chain drives with a power of up to 120 kW at circumferential speeds of up to 15 m/s. In drives with high-speed engines, chain transmission is usually installed after the gearbox; Efficiency transfer .
The design of the drive chains. An important element of chain transmission is the drive chain, which consists of jointed joints. The main types of drive chains are roller, bush and gear, which are standardized and manufactured by specialized plants with dimensions set in GOST.
Roller chains. They consist of the outer and inner plates 2 and 1 (Figure 4.65). In the outer plates, the rollers 5 are pressed, passed through the bushings 4, to which
Pressed inner plates. Rollers and bushings form hinges. The rollers 3 are loosely placed on the bushings The chain with the sprocket engages via a roller that rolls over the tooth and reduces its wear. Roller chains are widely used, are recommended at speeds m/s.
Sleeve chains. These chains are similar to roller chains, but do not have rollers. They have a smaller mass, but less wear-resistant. They are used in transmissions whose speed is up to 10 m/s.
Toothed chains. These chains consist of a set of plates with denticles, pivotally connected to each other (Figure 4.66). They have a smaller pitch and therefore allow higher speeds. To eliminate the lateral fall of the chain from the sprocket, guide plates 1, located along the middle of the chain or along its sides are used. Toothed chains, in comparison with others, work more smoothly, with less noise, they better perceive the shock load, but are heavier and more expensive. Recommended at speeds m/s.
Pitch chain. The chain P is the main parameter of the chain transfer and is accepted by GOST. The larger the pitch, the higher the load capacity of the chain, but the stronger the impact of the link on the tooth during the period of the sprouting of the sprocket, the less smoothness, noiselessness and longevity of the transmission. At high speeds, chains with a small step are adopted. In high-speed transmissions at high powers, small-pitch chains are also recommended: large-toothed gears or multi-row roller gears.
Material of chains. Chains should be wear-resistant and durable. The chain plates are made of steel 50, 40ХН and others with hardening to hardness HRGе 40 ÷ 56; axles, bushings, rollers, liners and prisms - from cemented steels, for example 15, 20, 12HNZA and others with quenching to HRCe hardness of 52-55.
Pulling and lubricating the chain. The chain tension as the hinges wears out weakens. The chain is stretched, the arrow of the slack of the driven branch increases, which causes the sprocket to become chained with a chain. The tension of the chain is regulated by devices similar to those used for belt tension, i.e. moving the shaft of one of the sprockets, pressure rollers or pulling stars. Tensioning devices must compensate for the elongation of the chain within the two links, with a larger chain stretching, two of its links are removed.
To lubricate the chain, apply periodic manual lubrication, continuous immersion in the oil bath of the enclosed housing or circulating fluid lubrication from the pump.
The asterisks differ in design from the gears only in the profile of the teeth, the size and shape of which depend on the type of chain - roller or bushing (Figure 4.67, a) and gear Figure 4.67, b).
The pitch P of the asterisks is measured along the chord of the dividing circle. To increase the durability of the chain transmission
take as large a number of teeth as possible with a smaller sprocket, since there will be a larger number of links in the mesh, which will improve the smoothness of the transmission and reduce the wear of the chain. However, when stretching, the chain tends to rise along the profile of the teeth, the more the greater the number of teeth of the sprocket. With a very large number of teeth, the chain jumps off the asterisk. Therefore, the maximum number of teeth of a large sprocket is limited: for a spigot chain , for a roller , for a gear i. The number of teeth of a small sprocket for bushing and roller chains is taken according to empirical dependence
where and is the gear ratio.
For gear chains, the value, min, is increased by 20-30%. It is preferable to take an odd number of sprocket teeth, which in combination with an even number of chain links contributes to a more even wear.
The material of the sprockets must be wear-resistant and well resist shock loads. The asterisks are made of steels 45, 40X, 12HNZA and others with quenching or from cemented steels 15, 20X, etc. The production of sprockets of sprockets from nonmetals is promising, which reduces the noise during transmission operation and the wear of the chain.
Kinematics of chain transmission. The chain for one revolution of the sprocket passes the path Pz, therefore, the chain speed
where P - the chain pitch; - the number of teeth of the leading and driven sprockets; - frequency rotation of the master and driven sprockets.
From the equality of chain speeds on asterisks, the gear ratio
The gear ratio of the chain drive is variable within the rotation of the sprocket by one tooth, which is practically noticeable only for a small number Z. Inconsistency and (usually within 1-2%) causes uneven transmission and oscillation of the circuit itself. The average gear ratio per revolution is constant. For chain transmissions, it is recommended Selecting the number of teeth of the larger star with (4.89)
Transmission geometry. The dividing circle of the asterisks passes through the centers of the chain hinges. From the triangle OLS (see Figure 4.67, c) we find the dividing diameter
where P - step; r is the number of teeth of the star.
It is recommended to determine the interaxial distance using the formula (greater value for large and).
The length of the chain is
Forces in the transfer. Circular force transmitted by the circuit,
where d - is the divisor diameter of the asterisk.
Preloading the chain from the slack of the driven branch
where is the sag coefficient ( for horizontal transmissions, for transmissions , inclined to the horizon up to 40 °, for vertical transmissions); q - mass of 1 m chain, kg/m; a - mszosevoe distance, m; m/s2.
Chain tension from centrifugal forces . Power
loads the links of the chain along its entire contour, but is not perceived as asterisks. The tension of the leading branch of the chain of running transmission
The tension of the driven branch of the chain is equal to the greater of the tensions and . Due to the fact that the hinge of the running link of the chain rests against the tooth, the force is not transmitted to the links located on the star.
Load on the sprocket shafts. The chain acts on the sprocket shafts with a force
where - shaft load factor ( - for vertical transfer, - for horizontal).
When the shock load increase by 10-15%. The direction of the force is taken along the line of the centers of the shafts.
Calculation of chain transfer for wear resistance. The performance of chain drives depends on the longevity of the chain, determined by the wear of the hinges. In accordance with this, the calculation of chain gears ensuring the wear resistance of the hinges is adopted as the main one. Chains selected from the durability condition have sufficient strength. Durability of drive chains for wear is 8-10 thousand hours of operation.
Calculation of transmission with a roller (bush) chain. The load capacity of a chain is determined from the condition that the average pressure in the link hinge does not exceed the permissible (MPa; with increasing pitch and speed reduce):
where - the calculated circumferential force transmitted by the chain; - factor operation (); - the coefficient of the lubrication method ( with continuous lubrication, for periodic); < img src="images/image2477.jpg"> - the slope of the line of centers of stars to the horizon (at , with ); < img src="images/image2480.jpg"> - the coefficient of the method for regulating the tension of the chain (when controlled by pressing bearings , when controlled by pressure asterisks , for unregulated transmission
= 1.25); A - the area of the projection of the bearing surface of the hinge (for roller and bushing chains , where axis diameter; img src="images/image2486.jpg"> - the length of the sleeve); - the coefficient of chain length (for one row ).
Expressing in the formula (4.92) the circumferential force after the moment on the small sprocket, and the area of the projection of the bearing surface of the hinges through the step < img src="images/image2491.jpg">, we get the formula for choosing the pitch of the roller (bush) chain:
where - the calculated torque; allowable pressure in the hinges of the roller chain, MPa.
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