1、The failure form of chain drive
1) Fatigue damage of the chain
When the chain is working, the chain on both sides of the sprocket is tensioned and relaxed on one side. After a certain number of cycles, the chain plate will be fatigue fractured, or fatigue pitting (impact fatigue caused by polygon effect) will appear on the surface of the sleeve and roller. Therefore, the fatigue strength of chain becomes the main factor to determine the bearing capacity of chain transmission. The test shows that the fatigue fracture occurs first on the chain plate of the chain working under the medium speed with good lubrication. The shorter the chain is, the higher the speed is, the more serious the fatigue damage is when the cycle is fast.
2) Wear and tear of chain hinges
When the chain is working, the hinge and the sleeve are under large pressure, and they rotate relatively to each other during the transmission, which leads to the wear of the hinge and the elongation of the pitch of the hinge, while the pitch of the wheel teeth is almost not affected by the wear, which will lead to the outward movement of the engagement point and, in serious cases, the phenomenon of skipping and chain release.
Figure 9-15, after the hinge wears, the pitch increases from p to p+Δp, and the engagement point increases from d to d+Δd. The growth of the chain pitch Δp and the outward movement of the engagement circle Δd have the following relationship: When the pitch is certain, the tooth height is certain, i.e. the allowed outward movement of the engagement circle is certain. The more the number of teeth z, the bigger the outward shift of the engagement circle Δd, the bigger the possibility of the chain falling off from the sprocket, so the number of teeth should be less to ensure the chain life.
3）Gluing of pin and sleeve
WhenSprocketsWhen the rotational speed is too high, the impact energy of chain link engagement increases, the accumulated heat is larger, and the lubricant film between the pin and sleeve is destroyed, so that the working surfaces of both are in direct contact under very high temperature and pressure, resulting in gluing. Gluing determines the limit speed of chain drive to a certain extent. Mechanical Engineering Digest, a refueling station for engineers.
4）Static chain pull off
Low speed (v
2、The rated power of roller chain drive
Various failure forms of chain drive limit its load-bearing capacity under certain conditions. Therefore, when selecting the chain model, the causes and conditions of various failure forms must be considered comprehensively, so as to determine the rated power P0 that it can transmit.
The figure on the right shows the rated power curve of single-row chain made by experiment. From the graph, it can be seen that in the chain drive with good lubrication and medium speed, the bearing capacity of the chain drive mainly depends on the fatigue strength of the chain plate; as the speed increases, the polygon effect of the chain drive increases, and the drive capacity mainly depends on the impact fatigue strength of the sleeve and the roller, and the higher the speed, the lower the drive capacity is, and the hinge gluing phenomenon will occur, which makes the chain fail rapidly.
The figure shows the rated power curve of A series roller chain, which is obtained under the standard experimental conditions: 1) two sprockets are mounted on the horizontal axis and the two sprockets are co-planar; 2) z1=19; 3) Lp=100 links; 4) the load is smooth; 5) it is lubricated according to the recommended way; 6) it can operate continuously for 15000 hours at full load; 7) the relative elongation of the chain due to wear does not exceed 3%. According to the small If the designed chain drive does not match with the above experimental conditions, the P0 value obtained from the figure should be multiplied by a series of correction factors.
Where: KA - working condition coefficient, Table 9-9.
Kz - coefficient of the number of teeth of the small sprocket. Table 9-10.
KL - chain length factor, Table 9-10.
Kp - Multi-row chain coefficient, Table 9-11.
When the lubrication recommended in the diagram cannot be guaranteed, the P0 value in the line diagram should be reduced to the following values:
When, poor lubrication, down to (0.3~0.6) P0; without lubrication, down to 0.15 P0 (life cannot be guaranteed 15000 hours).
When, in case of poor lubrication, down to (0.15 to 0.3) P0;
When, the lubrication is poor, the transmission is not reliable and should not be used.
When the required actual operating life is less than 15,000 hours, the design is based on limited life. The allowed power transfer can be higher at this time.
3、The design calculation of roller chain drive
Known; transmission application, working condition, type of prime mover, transmitted power P, sprocket speed n1, n2 (or i), structural dimensional requirements, etc.
Design content: chain pitch p, number of columns, number of chain links Lp, transmission center distance a; number of teeth of large and small sprockets z1 and z2; shaft pressure Q; lubrication method.
1)SprocketsNumber of teeth z1 , z2 and transmission ratio i
The number of teeth of small chain wheel z1 has a greater impact on the smoothness and service life of chain transmission. The number of teeth is small, the outer dimension is small, but the number of teeth is too small, the unevenness of movement is increased, the dynamic load and impact are increased; when the chain enters and exits the engagement, the relative angle of rotation between the links increases, the wear of the hinge is increased; the circumferential force transmitted by the chain increases, which accelerates the damage of the chain and sprocket.
Too many teeth will increase the transmission size and quality, and the elongation of the pitch after the chain wears out will easily lead to jumping teeth and chain release, which will also shorten the service life of the chain.
The principle of selecting the number of teeth: (1) When the speed of chain drive is high, the number of teeth is more; (2) In order to consider uniform wear, the number of teeth of sprocket wheel should be taken as odd number with the number of chain links as prime numbers, and the following series are preferred: 17, 19, 21, 23, 25, 38, 57, 76, 95, 114.
(rounded), and. From Table 9-8, try to select v- select z1, z1 try to use odd numbers.
, recommended = 2~3.5. When v
2) Determine the calculated power Pca
The calculated power Pca is based on the transmitted power P, taking into account the nature of the load and the type of prime mover, i.e.
(3) Primary center distance a0
If a is small, the transmission structure is compact, but if a is too small, the total length of the chain is too short, and each chain link is engaged too many times per unit time, which aggravates the wear and fatigue of the chain. a is too large, the load bearing is good, but the chain is long and the lateral vibration is big. General
(Tension or pallet), when the center distance is not adjustable.
4) Number of chain links Lp
Rounding, preferably to an even number.
5) Determination of pitch and number of rows
Under certain conditions, the larger the pitch, the stronger the chain transmission load-bearing capacity, but the larger the pitch, the more serious the polygon effect of chain transmission, the more serious the dynamic load, shock and vibration. Therefore, in order to make the chain transmission compact and long life, try to take the single row chain with small pitch.
If the transmission speed is high and the power transferred is large; or the transmission center distance is small and the transmission ratio is large, take the multi-row chain with small pitch.
If the transmission center distance is large and the transmission ratio is small, take the single row chain with large pitch.
When designing, the number of columns of the drive is determined first - Check Table 9-11 to get Kp - Calculate P0 from the above equation - Check the chain number from Figure 9-13 - Check Table 9-1 to get the pitch p.
(6) Verify the chain speed Determine whether it is consistent with the assumptions.
7) Determine the actual center distance
To ensure the right drape of the loose edge
Actual center distance
If the transmission center distance is adjustable, △a takes a large value; if the center distance is not adjustable, △a takes a small value.
8) Maximum diameter of small sprocket hub hole
When the chain pitch and the number of small sprocket teeth are determined, the structure and dimensions of the sprocket parts can be determined (Table 9-3), and the maximum diameter of the hub hole dkmax can be determined, but dkmax is not less than the shaft diameter where the sprocket is installed; if the requirements cannot be met, the sprocket of special structure (such as sprocket shaft) can be used or the chain drive parameters (increase z1 or p) can be selected again.
(9) Calculate the pressure axis force Q
where: Fe-effective circumferential force transmitted by the chain, N;
KQ-Pressure force coefficient, for horizontal drive, KQ=1.15; for vertical drive KQ=1.05.
10) Structural design, materials and dimensions of sprockets.
11) Lubrication and protection of chain drive.
4、Static strength calculation of low-speed chain drive
For the low-speed chain drive of chain speed, the chance of damage due to insufficient tensile static strength is large, so the tensile static strength is often calculated as follows
Where: Sca - calculated safety factor for the tensile static strength of the chain;
Q - ultimate tensile load of single-row chain, kN, check Table 9-1;
n - number of rows;
KA - work condition factor, check Table 9-9;
F1 - tight side working tension of the chain, kN.
For low-speed chain drive, the number of teeth of small sprocket can be less than 17, but not less than 9.