The mechanism of sewing needle through sewing material with guide thread is called needle bar mechanism. The task of the needle bar mechanism is to drive the needle, guide the upper thread through the sewing material, and form the upper thread loop to prepare for the interweaving of the stitches.
The function of the needle bar mechanism is finally realized by the machine needle. In order to sew the sewing materials together when the sewing machine is working, the needle should do piercing movement. There are vertical and horizontal, straight line and curved lines in the movement mode of machine needle piercing sewing material. Because of different uses, the needle bar mechanism of sewing machine is also different. Most sewing machines do vertical reciprocating linear motion, while some special machines, such as sewing machines, sewing machines and embroidery machines, not only do vertical reciprocating linear motion, but also do lateral swing, but this lateral swing time must start after the needle leaves the sewing material and end before entering the sewing material.
The height positioning of needle bar is an important parameter in the use and maintenance of sewing equipment. The height of double straight needles or multiple straight needles is different, and the height positioning is generally based on the long needle.
The distance between the top dead center and the bottom dead center of the needle bar movement is called the needle bar stroke. Needle bar stroke is an important parameter of sewing equipment, and needle bar stroke and effective working stroke of machine needle are two different things. The effective stroke of the needle refers to the distance from the moment when the needle punches the cloth to the lowest point of the needle.
The selection of straight needle should be as uniform as possible with the thickness of sewing material, the thickness of sewing thread and the technological quality of sewing products, so that the straight needle can ensure that the sewing material and sewing thread will not be damaged when piercing the sewing material, which is conducive to the formation of loops.
1. 1 needle insertion
The process of sewing cloth with machine needles is called inserting needles. Machine needle is one of the key parts of sewing machine, and the action of needle bar mechanism and its mechanism is realized through it. When the machine needle enters the needle, it must overcome the positive resistance from the sewing material and the friction force on the side of the sewing material. Due to the change of the interaction force between the suture, the machine needle and the sewing material, the tension reaches the highest point.
In order to reduce the friction between needle and sewing material, we can adopt the methods of precision manufacturing of needle (especially needle tip), softening of needle coating and sewing material, or adding silicone oil to needle and suture.
In addition to the friction between the needle and the sewing material, there are also the presser foot pressure and the size of the needle plate hole. If the needle is not selected properly, or made badly, or the hole in the needle plate is too large, holes will be formed in the sewing material, which will cause damage to the sewing material, not only affecting the appearance, but also reducing the firmness. Therefore, under the condition of meeting the requirements of sewing strength, we should try to use needles with smaller diameter and choose special needles for special sewing materials to reduce friction and achieve good sewing effect.
1.2 incoming line
The process of sewing needle bringing sewing thread to the reverse side of sewing material is called threading. The thread feeding stage begins when the needle descends until the eye of the needle contacts the sewing material until the required stitch length is formed (the needle bar is at the bottom dead center position).
The length of the thread entrance is not equal to the length of the suture required to form the stitch. The formation of the loop is an important parameter, which can be accurately entered by the shuttle tip and hook of the sewing machine to prevent thread breakage and needle jump. The amount of thread introduced into the sewing machine is about 6 times the actual amount of thread used. Therefore, each thread has to pierce the sewing material many times, and the increase of friction times will affect the strength of the suture and easily lead to thread breakage.
In the process of thread insertion, the thread insertion angle is an important factor affecting the thread stress change, so the thread insertion angle should be reduced as much as possible. The change of the interaction force between suture, sewing material and machine needle mainly depends on the change of the characteristics and thickness of suture and sewing material, so the quality of suture and the correct selection of machine needle are very important.
1.3 loop
The needle carries the needle thread through the sewing material, reaches the lowest point and then rises, forming a loop due to the friction between the sewing material and the needle. The purpose of forming the loop is to make the sewing machine (hook) enter the loop smoothly and realize the interweaving of the bottom thread. The top threaded ring is formed by the following two factors.
(1) When the needle reaches the bottom dead center with the needle thread, the tension of the needle thread is maximum due to the downward tension of the needle eye and the extrusion of the needle edge and the sewing material at the introduction slot. When the needle rises, the tension at the top of the needle eye disappears, and a part of the line segment is in a free state (at this time, as long as a small force is applied to the line segment, the line segment will be deformed). With the rising of the machine needle, the suture itself has a certain resilience, and the supporting force generated by the bottom of the needle eye is one of the conditions for forming the loop.
(2) According to the structure of the needle, the needle slot of the needle can be divided into long slot and short slot (or short slot), with long slot on one side and short slot (or short slot) on the other. The lead-in groove is a long groove with a diameter larger than the top line, and the lead-out groove is a short groove (or missing teeth) structure with a diameter smaller than the top line. When the machine needle withdraws from the sewing material, the sewing thread at one side of the lead-in groove does not rub against the sewing material, but the sewing thread at one side of the lead-out groove squeezes and rubs against the sewing material, so the random needle will not rise together and stay under the sewing material. This friction force is obviously that the short groove edge (vacancy) is greater than the long groove edge, which is another condition for forming silk loops.
When the three-thread sewing machine is sewing empty, the formation of stitch loop mainly depends on bending the needle thread to fasten the straight needle, so that the suture is embedded in the needle groove, which plays a similar role in sewing material, so it can still form stitch under the condition of seamless material, which is different from other machines.
The size and stability of the loop is the key to whether the sewing machine can enter reliably, realize the interweaving of the bottom thread and form the stitch. In order to ensure that the sewing ring can be smoothly entered by the sewing machine head, two conditions need to be met: one is that the sewing ring has a certain width, and the other is that the sewing ring must be perpendicular to the trajectory plane of the sewing machine head when entering the coil.
The formation of the coil is different from the rising size of the needle. The rising amount of the needle should not be too large or too small. If the lifting amount is too small, the coil width is not enough, which is not conducive to the entry of sewing machines. Although the amount of needle return is too large, which is beneficial to the sewing machine, it is difficult for the sewing machine to enter because of the long time of coil formation and the deflection of the coil. The test shows that the ideal looping stage should be when the needle rises about 2 ~ 3 mm for sewing materials with normal thickness and density.
Too large or too small the diameter of the needle plate hole will also have an adverse effect on needle insertion. If the hole of the needle plate is too large, the impact force when the needle enters the sewing material will make the sewing material sag, which may form a pinhole. The sagging sewing material will move with the needle when the needle is lifted, which will affect the normal formation of the coil. Generally speaking, the diameter of the needle plate hole is about 1.5 ~ 2 times that of the machine needle, and the machine needle should be kept in the center of the needle plate hole.
The formation and stability of the coil are related to the specifications and quality of the needle, the nature, texture, density and thickness of the sewing material, the specifications and types of raw materials, the twist and twisting direction of the suture, the needle and needle plate, the presser foot pressure and the cloth pressing state.
2 Hook mechanism
The mechanism of hook coil is called hook machine, which plays the role of hook, split, span, break and pay off the bottom line.
Common sewing machines include rotary shuttle, pendulum shuttle, thread hook, rotary shuttle, fork hook and so on. Swing shuttle is mostly used in double-thread lock stitch sewing machine, needle bending sewing machine and buttonhole sewing machine. Bending needles are mostly used in chain stitch sewing machines, flat sewing machines and double-thread, three-thread, four-thread, five-thread and six-thread sewing machines. The main application objects of fork hook are single-line chain stitch sewing machine, beading machine and so on. Shuttle is mostly used in single-line chain stitch sewing machine and button sewing machine. The difference when turning the hook is that the last thread loop enters the previous thread loop (self-connecting thread loop), and the thread hook is the difference between the thread hook and the straight needle, and the thread hook and the thread hook continuously enter the interconnected thread loop.
2. 1 loop
The process that the needle tip of sewing machine penetrates into the needle coil on the needle side is called inserting ring. Forming a good circulation is to prepare for the reliable entry of the hook tip of the sewing machine. The larger or more stable the needle coil is, the greater the possibility that the stapler will penetrate into the coil, even if the stapler is slightly different from the installation position, it can smoothly enter the coil.
In order to make the hook tip of the sewing machine enter the straight needle circle in a proper time when the needle bar rises, most industrial sewing machines stipulate that the needle bar rises 2.2 ~ 2.5 mm when it reaches the lowest level, and the hook tip of the shuttle or thread reaches the center line of the straight needle, and meets at 1.5 ~ 2.5 mm above the eye of the needle, keeping 0.05 ~ 0./kloc-0 with the back plane of the straight needle. If it is a hook with double straight needles and a single curved needle (such as a flat sewing machine), it is debugged according to the matching size of the hook and the short straight needle and taking into account the long needle.
Different machines have different debugging principles and matching dimensions. For different types of equipment, besides adjusting the parameters specified in the manual, we should also consider the sewing machine form and sewing conditions. According to the special needs of sewing, sewing and mechanical wear, we should appropriately advance or postpone the time when the hook tip coincides with the straight needle, that is, the change of the intersection position above the needle eye, in order to make up for some shortcomings, so as to overcome the failures such as needle jumping and poor thread picking caused by the inability of the thread hook to accurately enter the coil.
The other surface of the hook tip of the thread in the sewing machine protrudes or inclines outward, so as to enlarge the sewing circle after entering the circle, which is beneficial to the interweaving of the bottom thread, and the other protruding surface of the thread hook also has the function of forming a triangular loop of the bottom thread.
2.2 Needle extraction
When the coil is hooked by the sewing machine, the process of withdrawing the needle from the sewing material is called needle withdrawal. When the needle is withdrawn, a part of the suture is withdrawn from the sewing material through the long needle slot, which tightens the suture through the thread take-up mechanism. When the needle has just withdrawn from sewing, the needle coil has been hooked by the sewing machine and is in the process of unfolding or controlling. Before the long needle slot moves relative to the machine needle, the suture rubs against the upper and lower edges of the needle eye, which has a great influence on the strength of the suture. In order to reduce friction, the lower part of the exit side of the needle eye is designed with a cut, which is to reduce the wear of the suture when withdrawing the needle and improve the sewing quality. The author suggests using good quality machine needles to prevent suture damage.
Thread take-up mechanism
The mechanism for conveying, recovering and tightening needle and thread is called thread take-up mechanism. The thread take-up lever is responsible for winding and unwinding in each working cycle, and cooperates with the cloth feeding mechanism to complete the formation of sewing stitches.
Due to the movement of sewing machine, the adjustment of winder and the movement of sewing material, when forming stitches, the stitches are inserted into each other and locked, and the process of meeting the structural requirements of stitches is called tight loop. The thread take-up forms include needle bar thread take-up, connecting rod thread take-up, cam thread take-up, slide bar thread take-up, turntable thread take-up and so on. In industrial sewing machines, connecting rod thread take-up and needle bar thread take-up are the most common. There are two ways to take up the thread: one is to tighten the front coil before inserting the needle; Second, when pulling the needle, use the tension device of the winding machine to tighten it. 1 tightening method is adopted on the coating machine or flat sewing machine (including double-thread chain sewing machine), and the secondary tightening method is adopted on the flat sewing machine. The sewing thread required for the second insertion of machine needles comes from two aspects: one is obtained when tightening the previous needle; The second is to extract from the coil after overcoming the pressure of the tension regulator.
The working feature of the thread take-up mechanism is the up-and-down movement of the thread take-up rod hole. The motion curve of GC industrial sewing machine is like branches and leaves. When the thread take-up lever moves downward, firstly, it supplies thread for the lower needle; Second, when the shuttle hooks the needle thread, it supplies the needle thread to the shuttle until the needle thread bypasses the shuttle frame. When the thread take-up lever moves upward, it is responsible for pulling the upper thread out of the shuttle frame, tightening the stitch already formed in the sewing material to form a firm stitch, and pulling the upper thread out of the ball to prepare for the next stitch. Generally speaking, the stroke and trajectory of the thread take-up rod are relatively fixed. Because it is in a subordinate position, it is impossible to adjust the relevant dimensions of the mechanism, so it should be installed in strict accordance with the positioning requirements.
For some sewing machines, such as the shuttle, you can choose them according to the types of sewing materials. There are n types of rotary shuttles, and their main difference lies in the length of the tail hook. The tail hook is used to control the top thread in the winding process, aiming at soft and thin materials, ordinary materials, thick and hard materials or special sewing materials respectively.
Take-up spring, clamp and slow hook are all subordinate to take-up mechanism. Correct adjustment of tension and take-up accessories are important conditions for maintaining good stitch. In addition, the feeding time of the cloth feeding mechanism is wrong, and when there is a big error in the crochet and needle withdrawal time of the sewing machine in the crochet mechanism, the tightening effect of the thread loop will also be affected.
Supply device
The mechanism for conveying sewing materials is called feeding mechanism.
The process of moving the sewing material in the original position for a certain distance so as to form the next needle in the new position is called feeding. Common feeding methods include: lower feeding mechanism, upper feeding mechanism, needle feeding mechanism (needle and lower composite feeding mechanism), upper and lower composite feeding mechanism, upper and lower composite feeding mechanism, differential feeding mechanism, roller feeding mechanism and belt feeding mechanism. In most cases, the feed motion is a compound motion, that is, the combination of horizontal motion and vertical motion. It is a closed curve movement, that is, the cloth feeding teeth should move alternately up and down, back and forth in each stitch distance. When the cloth feeder completes the feeding movement, it descends to the lower part of the needle plate, separates from the sewing material, and returns to the original position to prepare for the next feeding movement.
The feeding time should start after the straight needle leaves sewing, and end the feeding movement before the next sewing (except needle, lower compound or needle and lower compound feeding mode). Take the flat sewing machine as an example, its positioning standard is that when the needle bar height and the feed tooth height are normal, the feed tooth is flush with the needle plate when the straight needle tip (needle eye) descends to the plane of the needle plate, which is what we usually call triple synchronization. Otherwise, there will be failures such as poor cloth feeding, wrinkling of sewing materials, broken needles, and tightening of stitches.
In the feeding process of sewing machine, most of them rely on the pressure exerted by the presser foot to cooperate with the feeding movement. Due to the function of presser foot, friction is generated between sewing materials and between sewing materials and cloth feeding teeth, which is beneficial to cloth feeding and reduces the slip between sewing materials. In order to reduce the friction between presser foot and sewing material, we can keep the presser foot bottom plate flat and smooth, and the presser foot bottom plate should be parallel to the feed teeth of the needle plate. Under the condition of not affecting the feeding effect, the presser foot pressure can be appropriately reduced and the engineering plastic presser foot with low friction coefficient can be selected.
The pressure of presser foot depends on the nature of sewing material. The pressure is higher when the seam material is thick, and lower when the seam material is soft or thin. The height and pitch of feed teeth should also be different with the thickness of sewing material. Tooth pitch selection method: coarse teeth are medium-thick materials, and fine teeth are thin materials. Also pay attention to the fact that the cloth feeder keeps the corresponding parallelism, levelness and front-back distance in the slot of the needle plate, and the movement direction and speed of sewing material should be consistent with the cloth feeder when feeding, so as to minimize slippage. The needle pitch of industrial sewing machines is usually in millimeters. The denser the needle spacing, the greater the thread consumption. Different sewing materials and the same sewing material have different thicknesses or positions, so it is necessary to choose the appropriate needle spacing.
On the sewing machine or double-needle chain stitch sewing machine, when the thread hook moves to the position orthogonal to the needle or another thread hook, a triangular circle is formed so that the tip of the straight needle or another thread hook can pass through. There are two situations: first, on the three-thread sewing machine, the big bending needle passes through the triangular coil of the small bending needle, while the straight needle passes through the triangular coil of the big bending needle before piercing the sewing material; Secondly, on a double-chain sewing machine or a flat sewing machine, a triangular needle formed by a thread hook is pierced after a straight needle is punched into the sewing material.
The reason why the bottom thread loop is formed is that the hook does not need to pierce the sewing material, and the head should be made thicker and the side has a convex shape, so that the original suture on the hook naturally forms a triangular gap. In addition, the triangle coil should be stable in shape and have enough coil space, which needs to be realized by the retraction of the bottom thread take-up device and the hook itself.
Sewing equipment belongs to a kind of precision machinery. In the process of sewing, we mainly rely on the cooperation of four major institutions (including other auxiliary institutions). No matter where there is a slight deviation, it will affect the sewing quality. As the management and maintenance personnel of sewing equipment, it is necessary to comprehensively analyze and study the matching state, time requirements, dimensional parameters, geometric shape and smoothness of the main parts in each stage, and try to achieve accurate and reliable debugging and maintenance, supplemented by normal lubrication and maintenance and correct operation, so as to achieve good sewing effect.