1. Failure form of chain drive
1) Fatigue failure of the chain
When the chain is working, the chains on both sides of the sprocket are tensioned and relaxed.The chain moves continuously from the loose edge to the tight edge, so all its components work under the action of variable stress. After a certain number of cycles, the chain plate will experience fatigue fracture, or the surface of the sleeve and roller will appear fatigued Pitting (impact fatigue due to polygon effect).Therefore, the fatigue strength of the chain becomes the main factor that determines the carrying capacity of the chain drive.Tests have shown that a chain operating at a well-lubricated medium speed will first appear fatigue fracture on the chain plate.The shorter the chain, the higher the speed, and the more severe the fatigue damage when the cycle is fast.
2) Wear of chain hinges
When the chain is working, the hinge and the sleeve are under relatively large pressure, and they rotate relative to each other during transmission, which causes the hinge to wear and the hinge pitch is elongated, while the tooth pitch is hardly affected by the wear, resulting in a meshing point. If it moves out, in severe cases, chain jumping and off-chaining will occur.
Figure 9-15, after the hinge is worn out, the pitch increases from p to p+Δp, and the meshing point increases from d to d+Δd. The chain pitch increase Δp has the following relationship with the meshing circle’s outward displacement Δd: When the pitch is constant, the tooth height is constant, that is, the allowable external displacement of the meshing circle is constant.The greater the number of teeth z, the greater the outward displacement Δd of the meshing circle, and the greater the possibility of the chain falling off the sprocket. To ensure the life of the chain, the number of teeth should be less. .
3) Gluing of pin and sleeve
ThisSprocketWhen the rotation speed is too high, the impact energy received by the chain link increases, the accumulated heat is large, and the lubricating oil film between the pin and the sleeve is destroyed, so that the working surfaces of the two are directly exposed to high temperature and pressure. contact, resulting in gluing.The gluing determines the limit speed of the chain drive to a certain extent. "Mechanical Engineering Literature", the engineer's gas station.
4) The chain is broken by static force
low speed (v
2. Rated power of roller chain drive
Various failure forms of chain drive limit its carrying capacity under certain conditions.Therefore, when selecting the chain type, the causes and conditions of various failure modes must be fully considered, so as to determine the rated power P0 that it can transmit.
The figure on the right is the rated power curve of the single-row chain made by the experiment.It can be seen from the figure: 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 rotation speed increases, the polygon effect of the chain drive increases, and the transmission capacity mainly depends on the sleeve. And the impact fatigue strength of the roller, the higher the speed, the lower the transmission capacity, and the phenomenon of hinge gluing will occur, which will cause the chain to fail quickly.
In the formula: KA - working condition coefficient, Table 9-9.
Kz—the coefficient of the number of teeth of the small sprocket.Tables 9-10.
KL - chain length factor, Table 9-10.
Kp—multi-strand chain coefficient, Table 9-11.
When the lubrication method recommended in the diagram cannot be guaranteed, the P0 value in the line diagram should be reduced to the following values:
When the lubrication is poor, it will drop to (0.15~0.3) P0;
When the lubrication is poor, the transmission is unreliable and should not be used.
When the actual working life required is less than 15000 hours, it is designed according to the limited life.At this time, the power allowed to be transmitted can be higher.
3. Design calculation of roller chain drive
Known; transmission purpose, working conditions, type of prime mover, transmitted power P, sprocket speed n1, n2 (or i), structural size requirements, etc.
Design content: chain pitch p, number of rows, number of chain links Lp, transmission center distance a; large and small sprocket teeth number z1, z2; axial pressure Q; lubrication method.
1)SprocketNumber of teeth z1, z2 and gear ratio i
The number of teeth of the small sprocket z1 has a great influence on the stability and service life of the chain drive.The number of teeth is small and the outline size is small, but the number of teeth is too small, the motion unevenness is increased, the dynamic load and impact are increased; when the chain enters and exits meshing, the relative angle between the chain links increases, and the wear of the hinge increases; The increased circumferential force accelerates the damage of the chain and sprocket.
Too many teeth will increase the size and quality of the transmission, and the elongation of the pitch after the chain wears is prone to tooth skipping and de-chaining, which will also shorten the service life of the chain.
(rounded up), and .From Table 9-8, try to select v—select z1, and try to use an odd number for z1.
, recommended = 2~3.5.when v
2) Determine the calculated power Pca
The calculated power Pca is determined according to the transmitted power P, taking into account the nature of the load and the type of prime mover, namely
3) The distance between the center of the primary election is 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 participates in too many meshing times per unit time, which aggravates the wear and fatigue of the chain. If a is too large, the load is good, but the chain is long and the lateral vibration is large.generally
(tension or pallet), when the center distance is not adjustable.
4) Number of chain links Lp
Round, preferably even.
5) Determination of pitch and number of rows
Under certain conditions, the larger the pitch, the stronger the carrying capacity of the chain drive, but the larger the pitch, the more serious the polygon effect of the chain drive, the more serious the dynamic load, impact and vibration.Therefore, in order to make the chain drive structure compact and have a long service life, try to use a single-row chain with a small pitch.
If the transmission speed is high, the power transmitted is large; or if the transmission center distance is small, the transmission ratio is large, and a multi-row chain with a small pitch is used.
If the transmission center distance is large and the transmission ratio is small, take a single-row chain with a large pitch.
6) Check the chain speed to determine whether it is consistent with the hypothesis.
7) Determine the actual center distance
To ensure proper sag for loose edges
actual center distance
If the center distance of the transmission 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
After the chain pitch and the number of teeth of the small sprocket are determined, the structure of the sprocket and the dimensions of each part can be determined (Table 9-3), and the maximum diameter dkmax of the hub hole can also be determined, but the dkmax is not less than the installation sprocket If it cannot meet the requirements, special structure sprockets (such as sprocket shafts) can be used or chain transmission parameters can be reselected (increase z1 or p).
9) Calculate the final force Q
In the formula: Fe—the effective circular force transmitted by the chain, N;
KQ—the pressure coefficient of the pressure axis, for horizontal transmission, KQ=1.15; for vertical transmission, KQ=1.05.
10) Structural design, material and size of sprocket.
11) Lubrication and protection of chain drive.
4. Static strength calculation of low-speed chain drive
For the low-speed chain transmission of chain speed, the probability of failure due to insufficient static tensile strength is very high, so the calculation of static tensile strength is often carried out according to the following formula.
In the formula: Sca - the calculated safety factor of the static tensile strength of the chain;
Q—Ultimate tensile load of single-row chain, kN, see Table 9-1;
n—number of rows;
KA—work condition coefficient, look up table 9-9;
F1 - the working tension of the tight edge of the chain, kN.
For low-speed chain transmission, the number of teeth of the small sprocket can be less than 17, but not less than 9.