In the process of production, installation and operation of optical cables, it is required to have a certain stretching window, which requires the optical cables to form enough excess length when bundled into cables. The excess length of bundle tube is an important factor to determine the performance of optical cable. First, the formation of cable excess length mainly comes from the secondary coating and cabling process, which together determine the size of cable excess length. Secondary coating process is the most important process to adjust the excess length of optical cable, and the excess length can be adjusted by adjusting other process parameters. Fig. 1ROSENDAHL secondary coating machine is used to discuss the formation process of cable excess length in secondary coating. The flow chart of production equipment is shown in Figure 1: the equipment consists of 1 pay-off unit, 2 ointment filling system and plastic extrusion system, 3 hot water cooling, 4 wheel traction, 5 cold water cooling, 6 crawler traction, 7 wire storage system and wire take-up system. The optical fiber is paid off from the pay-off stand under a certain pay-off tension, enters the main engine extrusion system through the oil gun, and then enters the wheel traction through the hot water tank. In this process, the optical fiber moves linearly. Due to the thixotropy of optical fiber slurry, the chemical bond is broken under the action of shear force, and the viscosity of optical fiber slurry is reduced, which has good fluidity. The optical fiber is straightened in the hot water tank section, and no excess length or negative excess length is formed. Because the optical fiber has a certain tension when stressed (generally < 1%), on the other hand, when the optical fiber is pulled by the wheel, the optical fiber is close to the inside of the beam tube, which is negative relative to the length of the beam tube above the optical fiber. In the cold water tank section, it is the main stage to form the excess length. The excess length is formed due to the large contraction of the bundle tube during cooling, which offsets the previous negative excess length and forms the required excess length. Stranded optical cable also forms a certain excess length, and the bundle tube is longer than the optical cable. Give the fiber enough room to stretch. The bundle tube relative to the length of the optical cable can be calculated by the following formula: L= 1000/cosα (1), where l is the length m of the bundle tube per kilometer of the optical cable and α is the twist angle of the optical cable. Tg α = π (φ1+φ2)/w (2) φ1is the diameter of reinforcement, φ 2 is the diameter of bundle tube, and w is the cable distance. As can be seen from the above two formulas, the actual bundle length per kilometer of optical cable is longer than that of optical cable, and the long part can be used to provide part of the excess length. Together with the excess length formed by the two groups, these two groups form the total excess length of the optical cable, which provides enough stretching window for the optical cable. For the central bundle optical cable, because there is no excess length in the cabled part, the excess length in the secondary sheath is large. Provide enough stretching window for optical cable. Therefore, the corresponding residual length is set for optical cables with different uses. Second, there are many factors that affect the remaining length, which are independent and interrelated. In the second set of production, each link affects the remaining length to varying degrees. To sum up, there are the following aspects that affect the surplus. The influence of pay-off tension on the excess length is that the greater the tension, the greater the stretching degree of the optical fiber, the greater the negative excess length of the bundle tube in the hot water tank and the smaller the final excess length. Therefore, in production, because the pay-off stand is unstable or the pay-off force is too large, the excess length of the bundle tube is unstable, and the length of the optical fiber in the bundle tube is quite different. Some devices are active pay-off and some are passive pay-off, but unstable tension has an impact on the excess length of optical fiber, and passive pay-off has a greater impact. Excess tension is one of the most common process parameters for adjusting excess length in our daily production, and its adjustment is sensitive to the change of excess length. When the excess tension increases, the excess length of the beam tube becomes smaller, on the contrary, when the tension is adjusted, the excess length becomes larger. Adjusting the excess tension is an easy-to-control adjustment method, and the measurement is stable and easy to adjust, but its adjustment range is not very large, and the excess tension can only be adjusted in a small range. The temperature of hot water tank is also the main process parameter to adjust the excess length of beam tube. When other parameters are stable, the excess length will increase with the increase of temperature, and vice versa. The temperature of hot water is generally higher than 45℃, because the crystallization temperature of PBT is generally higher than 45~50℃. If the hot water temperature is too low, the poor crystallization of PBT will affect the properties of its bundle, and the bundle will shrink greatly in the later stage. The temperature difference between hot water and cold water finally determines the remaining length of the tube bundle. Generally speaking, the greater the temperature difference, the greater the shrinkage of the tube bundle and the greater the excess length, and vice versa. The performance of ointment is also an important factor affecting the stability of excess length. In our daily production, we often look at the stability of the ointment level between the extruder head and the hot water tank to judge the stability of the excess length of the bundle tube. The viscosity of ointment is an important factor to determine the remaining length. The viscosity of fiber paste is inversely proportional to its heating temperature. When the temperature rises, the viscosity of fiber pulp decreases, and the viscosity of fiber pulp has a great influence on the residual length of beam. When the viscosity of the optical fiber slurry reaches a certain level, the excess length of the bundle tube is uncontrollable, and the excess length of each optical fiber in the bundle tube may change greatly. In the production process, the chemical bond of fiber paste is destroyed when subjected to shear force, and the viscosity is greatly reduced. The fiber paste has good fluidity and meets the production requirements. When the shear force is removed, after a certain period of time, its chemical bond will recover, reaching the viscosity that the fiber paste will not flow out of the bundle tube. This property of fiber paste is called thixotropy. This can meet the requirements of low viscosity of optical fiber paste and free movement of optical fiber in beam tube production, and make the excess length of optical fiber easy to control. Excessive viscosity makes it difficult to move the optical fiber and control the excess length of the optical fiber. The common fiber paste on the market is unigel. , DAE and Korean paste all have different viscosities and different product models to meet the requirements of different equipment and different types of products. The production speed of some equipment reaches 400 meters per minute, so there are special requirements for the viscosity of fiber paste. The die oil needle and fiber guide for fiber paste extrusion also have some influence on the excess length of the bundle tube. The size of oil needle or fiber guide needle directly affects the extrusion stability of fiber paste. The stability of fiber paste extrusion determines the motion trajectory of optical fibers, so the general fiber paste extrusion is unstable, which is manifested in the great difference in the remaining length of each optical fiber. Appropriate mold configuration is also the main aspect that determines the excess length of the beam tube. Unreasonable mold configuration will wrap a lot of air in the bundle tube when using two sets, which will cause many vacuum bubbles on the surface of the bundle tube, and the bubbles in the bundle tube will not disappear after being placed for a period of time, indicating that air is involved because of the unstable liquid level of the ointment. In the process of cable formation, the formation of excess length mainly comes from the relative length of bundle tube and cable. From the formula 1, it can be seen that after the cable structure is fixed, the remaining length is determined by the torsion angle between the bundle tube and the core wire when the cable is completed. Generally speaking, the greater the twist angle, the greater the remaining length. As can be seen from Equation 2, the factor that determines the twist angle is the pitch of the cable. The smaller the pitch, the greater the twist angle and the greater the excess length. Stranding is also an important source of overlong. Some companies deliberately form a surplus length of zero when producing the second set, so it is enough to form a surplus length through cabling. Thirdly, the significance of redundancy in practical application. In the process of production and use, optical cables need to have a certain stretching window, which is 0.5% for general optical cables and 0.8- 1.0% for self-supporting optical cables. Stretching window has the following relationship with excess length and cable spacing, as shown in Formula (3). = 0 (1+4π rn2/sn2)+2π 2 (rn2-rmin2)/sn2 (3), where is the stretching window of the optical cable, 0 is the residual length of the beam, the stranded pitch of Sn, rn is the stranded radius (Rn= Rc+ Rt)), Rt is the beam radius, and Rc,. The optical cable is in production. When the optical cable is stretched, the optical fiber cannot be stressed, which requires the optical cable to have a certain stretching window. The size of the tensile window directly determines the tensile test of the optical cable, so the tensile test is one of the most important tests in the optical cable test. In some areas, the temperature varies greatly throughout the year. When the working temperature of the optical cable changes, the thermal expansion coefficient between the optical fiber and other components of the optical cable is different, and the optical fiber cannot be pulled by the outside world, so the optical cable must have enough stretching windows. The type test of the optical cable we produce every year is high and low temperature, in order to prevent the optical cable from being damaged when the working temperature changes. Other tests of optical cable, such as flattening, bending, impact resistance, etc., require sufficient excess length of optical fiber in optical cable. When the optical cable is subjected to external force, the optical fiber can get enough strain space, so that the optical fiber will not be damaged by external force. The excess length of optical fiber is the most important control parameter in the production of optical cable, which directly determines the quality and performance of optical cable and is of great significance. There are many factors that affect the surplus length, and they interact and are interrelated. Therefore, in our production process, we must understand how various influencing factors affect the excess length of optical cable, so as to control the production well. Although the production of optical cable is very mature, there are still problems in our actual production, which has brought great losses to the production of optical cable. It is necessary to strictly control the technological parameters of optical cable production in order to produce first-class products.