The function of the paper conveying mechanism is to convey the paper separated from the feeder part to the front and side gauges for positioning smoothly, accurately and without scribing. The higher the printing speed, the more the paper specifications or thickness changes, and the higher the work requirements of the paper transport mechanism. Comprehensive analysis of the current sheet-fed offset printing presses of various brands, the commonly used paper conveying mechanisms are mainly two categories: one is the conveyor belt type uniform speed paper conveying mechanism; the other is the vacuum suction belt type deceleration paper conveying mechanism. Its structural characteristics and working performance are different. This article will compare and analyze these two institutions.

1. Working principle

1. Conveyor belt type paper conveying uniform speed mechanism

As shown in Figure 1. On a rectangular paper conveying plate 1 inclined to the positioning member, 4 to 8 conveyor belts 2 are evenly surrounded, and the knurled active thread belt shaft 3 drives the belt around the tension wheel 4 and driven by friction The pulley shaft 5 turns over the paper conveying plate. Paper feeding frame 6 is evenly equipped with paper pressing wheel 7, brush wheel 8, paper blocking brush 9, paper ball 10, etc. They are all in contact with the belt with a certain pressure, between the belt and the belt Under the action of friction, it rotates with the movement of the belt, and the paper is clamped and conveyed to the front gauge 12 at a steady and uniform speed until it reaches the static position of the front gauge. The paperboard 11 has a gap of three sheets of paper away from the paperboard to prevent the paper edges and paper corners from warping.

Figure 1 Analysis of the paper feeding mechanism of sheet-fed offset printing machine

The conveyor-type uniform speed paper conveying device can stabilize the paper on the entire web, so that the paper reaches the front gauge smoothly, and the structure is very simple. However, due to the friction of the paper pressure wheel, brush wheel, brush and other mechanical parts on the surface of the paper, it will affect the quality of the paper surface and the existing printing changes to a certain extent; Rebound, and the faster the machine speed, the stronger the impact and rebound.

2. Vacuum suction belt type deceleration paper conveying mechanism

Figure 2 shows the vacuum suction belt type paper conveying method. Two suction belts 2 are symmetrically arranged on the same rectangular paper conveying plate 1, and the transmission is also driven by the driving belt shaft 3 through the friction force to drive the suction belt around the tension wheel 4 and the driven belt shaft 5 through the paper Turn over the tablet. When the paper reaches the paper conveying plate, the suction belt is sucked by the vacuum suction chamber 6, and the paper is slowly fed to the front gauge until it reaches the position of the front gauge for static positioning.

Figure 2 Vacuum suction belt conveying method

Figure 3 shows the working principle of the deceleration mechanism.

Figure 3 Working principle of deceleration airport

The drive shaft 1 is provided with an eccentric gear 2, the center of the shaft 1 is B, the axis of the eccentric gear 2 is fixed on the frame, and the distance between AB is the eccentric distance "e" of the eccentric gear 2. The eccentric gear 2 meshes with the movable gear 3, and the gear 3 meshes with the gear 4 fixed on the frame. At the same time, the axis C of the gear 3 and the shaft 1 are connected by the rod 5, and the axis D of the gear 4 is connected by the rod 6, the sprocket 7 is coaxial with the gear 4, the sprocket 9 is coaxial with the driving belt shaft 10, and the wire belt The shaft 10 is surrounded by a tensioned vacuum suction belt 11.

When the paper conveying mechanism is working, the power of the deceleration mechanism is input by the driving shaft 1, driving the eccentric gear 2 to rotate around the axis A, the movable gear 3 and the eccentric gear 2 are always meshed and rotated through the connecting rod 5, and the movable gear 3 is connected through the connecting rod 6 The fixed gear 4 is always meshed and rotated, thereby sequentially transmitting power to the vacuum suction belt 11 for paper conveyance. Due to the eccentricity of the gear 2, the distance AE between the meshing point E of the gear 3 and the axis A changes periodically. due to:

Linear velocity at point E V = speed of gear 2 n × length of AE

Therefore, the linear velocity V at point E changes periodically. Then, the working speeds of gear 3, gear 4, sprocket 7, chain 8, sprocket 9, driving belt shaft 10 and vacuum suction belt 11 are periodically changed, so that the paper on the paper table is also in a cycle The speed of sexual change is conveyed to the front gauge. Through the design, the paper is conveyed in the direction of the front gauge at a high speed on the paper conveying table; when the front gauge is about to be reached, the suction belt gradually decelerates and the impact with the front gauge is minimized.

Vacuum suction belt type decelerating paper feeding device can adapt to small errors in paper thickness and cutting; the contact between the paper and the suction belt is stable without slipping; there is no contact rubbing on the surface of the paper; the paper is gentle and impact when it reaches the front rule small. However, due to the addition of an air pump, air circuit system and gear-link reduction mechanism, the structure is complex, the noise is high, and the maintenance is difficult.

2. Work requirements

1. Conveyor belt type paper conveying mechanism

(1) The thickness of the six wires must be uniform

Paper conveying speed on the paper table V1 = line moving speed V2 = active line belt shaft speed n × (shaft radius R + line belt thickness H)

If the thickness H of the tape is not equal, the feeding speed V is also not equal, and the paper is irregularly fast and slow, causing skew. Therefore, the thickness of the six wires should be even, especially the slits of the wires should be stitched after sectioning to ensure smoothness.

(2) The tension of the six thread belts must be consistent and moderate

The tension between the six tapes and the active tape spool determines the amount of frictional force given to the tape by the tape spool, that is, the speed of the tape movement, and thus the speed of paper transport on the tape. If the six thread belts are tensioned differently, the paper is bound to be skewed; the thread belt is stretched too tightly, the tension of the belt is too large, and the belt is easy to stretch and aging. Change the up and down position of the tension wheel by pushing the tension arm to tighten the string. Generally, it is better to be able to lift the belt about 20mm away from the paper table. In the long-term use of the belt, it will inevitably appear to be elongated. Therefore, it is necessary to check the tightness of the belt frequently and tighten it in time.

(3) The position of the pressure roller on the line with the ball hitting the person and the pressure with the line should be appropriate

The first row of pinch rollers is evenly arranged on the driving belt spool. The paper pressing point of the bottom row of paper pressing wheels is about 3 ~ 5mm away from the trailing edge of the pre-positioned paper. Do not press the trailing edge of the positioning paper, so as to avoid the trailing edge being pressed and the paper skew Or not in place.

In order to prevent the paper on the paper feeding table from arching toward the middle, the left and right paper pressing wheels can be slightly spread outward, so that the paper feeding force is slightly skewed outward, so as to ensure that the paper is smooth and scraped. Of course, if the skew is too large, the paper will not be lost, and the paper may even be scratched.

The pressure of each paper pressing wheel and the linear belt should be uniform and consistent, in order to smoothly transport the paper forward. The pressure of the left and right platen rollers is uneven. After too much force, the paper is fed fast on the side, and the paper fed on the light side is slow, causing the paper to skew. The pressure of the pressure roller is too high, leaving indentation on the paper or scratching the original marks on the paper.

(4) The position of the brush, brush wheel, and paper ball must be correct

The brush, brush wheel, and paper pressing ball are located at the front of the paperboard. Their function is to prevent the paper from rebounding and floating when it reaches the front gauge, so that the paper is accurately positioned. The position of the brush is suitable to press the trailing edge of the paper being positioned 5 ~ 10mm, so that the rebound paper is gently blocked by the bristles. The outer circle of the brush wheel is tangent to the rear edge of the positioning paper, which not only prevents the paper from rebounding, but also allows the bristles to gently push the paper back along the front gauge through the rotation of the wheel, so that the rebound paper continues to reach the front gauge. The weight of the paper ball is large. When printing thick paper, it is placed in the front half of the paper, and its role is to rotate with the brush. When printing thin paper, it is lifted and the brush wheel is moved forward to 25 ~ 30mm from the trailing edge of the paper.

The pressure of the brush, brush wheel, paper ball and thread belt should be properly controlled. The pressure is too small to prevent the paper from rebounding and pushing it forward; the pressure is too large, the side gauge cannot pull the paper, the paper is skewed, or Can not be pulled in place; moreover, it will leave scratches on the surface of the paper, and even make the paper bite wrinkled at the front rule.

(5) When printing paper of different thicknesses, the pressure of the pressure roller etc. needs to be corrected

If the paper thickness of the two batches of prints before and after is different, then the pressure of each pressure roller, brush wheel, brush, pressure ball and thread belt should be corrected one by one.

(6) Printing paper of different specifications needs to move the position of the paper pressure wheel, etc.

If the paper specifications of the two or two batches of prints are different, the position of the last row of paper pressure rollers, brush wheels, brushes, and paper pressure balls should be moved to prevent the printed paper from rebounding and floating when it enters the front gauge.

2. Vacuum suction belt type paper conveying mechanism

(1) The tension of the two suction belts needs to be even and moderate. The friction between the suction belt and the active belt shaft must be able to transfer the suction belt smoothly without slippage.

(2) The vacuum pressure of the suction chamber should be appropriate. It can be adjusted by the inspiratory air pressure regulating valve on the gas gate. If the vacuum pressure in the suction chamber is too small, the suction belt will suck the paper into the front gauge; if the vacuum pressure in the suction chamber is too large, the marks of the suction belt will easily appear on the surface of the paper, making the back of the picture and text dirty or even The tissue is damaged.

(3) As shown in Figure 4, in order to avoid the lack of flatness in the transport of large-format thin paper, the paper is wrinkled outside the suction belt, and the paper corner is warped and jittered. At present, most vacuum suction belts Type decelerating paper conveying mechanism adds 2 ~ 4 conveying line belts on the paper conveying table, and evenly arranges several brush wheels at the trailing edge of the positioning paper, and arranges several pressing papers at 5mm along the trailing edge The main function of the wheel is to control the flatness of the paper on the entire web and prevent the paper from rebounding and fluttering before reaching the specified position. Therefore, the pressure between the paper pressing wheel, the brush wheel and the thread belt is relatively small, and it is only necessary to give the paper a tendency to feed forward slightly, without damaging the imprint of the paper and its surface. If the pressure is too high, it will not only operate the paper, but also affect the smooth feeding of the vacuum suction belt.

Figure 4 The conveyor belt is added to the paper table

(4) The air pump and the suction air circuit should be cleaned frequently. Regular cleaning can prevent paper powder and paper wool from clogging the air pump and air path, affecting the effect of suction and paper feeding.

(5) When printing paper of different thicknesses, it is necessary to calibrate the suction air pressure and the paper pressure roller.
If the paper thickness of the two batches of prints is different, the suction air pressure valve on the air pump must be adjusted to change the vacuum pressure of the suction chamber so that the suction belt can properly absorb the paper. At the same time, the pressure of each paper pressing wheel, brush wheel and thread belt should be corrected one by one.

(6) When printing paper of different specifications, it is necessary to adjust the position of the pressure roller and so on. If the paper specifications of the two batches of prints are different, the position of the pressure roller and the brush wheel should also move accordingly.

3. Comparison of paper feeding speed and paper feeding effect

1. From paper feed nozzle to active spool

Suppose the design speed of the printing press is 15,000 sheets / hour and the fixed speed is 13,000 sheets / hour. The speed of the paper on the stack is 0. When the paper suction nozzle picks up the paper and feeds it forward, the paper accelerates. When it passes the paper feed roller to the active thread belt spool, the vacuum suction belt type deceleration paper transport mechanism only requires the paper transport speed. It matches the printing speed of about 8000 sheets; the conveyor-type uniform-speed paper conveying mechanism requires that the paper conveying speed matches the printing speed of about 13,000 sheets; as shown in Figure 5. The paper conveying speed is abruptly large, the acceleration is large, and the separation and feeding stability requirements are high.

Figure 5 Paper feeding speeds of two different paper feeding mechanisms

2. From the active spool to the front position

Driven by the line belt, the paper is transported on the paper conveying table. Conveyor belt uniform speed paper conveying mechanism always requires that the paper conveying speed matches the printing speed of about 13,000 sheets, stable and without fluctuations; the paper conveying speed of the vacuum suction belt decelerating paper conveying mechanism is gradually accelerated from the corresponding 8000 sheets per hour to the highest 18000 sheets / hour at the time, then gradually decelerate to 8000 sheets / hour, then, accelerate to 18000 sheets / hour again, and then slow down to 8000 sheets / hour. Among them, the paper has undergone many large speed changes, and the conveying stability has been challenged.

3. When the rule is reached

When the paper reaches the ex-husband, the speed is 0, the position is still, and the side gauge pulls the paper smoothly. For the conveyor belt type constant speed paper conveying mechanism, the paper speed instantly decreases from the corresponding 13,000 sheets / hour to zero speed. Due to the strong inertia, on the one hand, the paper bite is wrinkled, especially the thin paper, and the damage is more serious, as shown in Figure 6; on the other hand, the paper rebounds, the trailing tip hits the brush, brush wheel, etc. Cause scratches, as shown in Figure 7. Although the rebounded paper continues to be delivered to the front gauge under the impetus of the brush wheel and the paper pressure ball, it is easy to get out of position, resulting in poor positioning accuracy. For the vacuum suction belt type decelerating paper feed mechanism, the paper speed is reduced from the corresponding 8000 sheets / hour to zero speed, and the inertia is much smaller. It has obvious effects in preventing wrinkling of the bite, scratching of the trailing edge, and improving positioning accuracy. As shown in Figure 8.

Figure 6 Deformation

Figure 7 rebound

Figure 8 Normal

4. Auxiliary institutions

1. The blower at the front gauge replaces the cardboard

A paper pressing board is installed at the port of the paper feed iron plate, as shown in Figure 9. The upper swing-type front gauge installs the pressboard and the front gauge as a whole, and the lower swing-type front gauge installs the pressboard separately above the paper conveying table to prevent the bite portion from warping or arching when the paper reaches the front gauge. The height of the paperboard and the paper table is about three sheets thick. If it is too high, it will not press the paper, which will cause the paper to pass through the position; if it is too low, it will affect the smooth entry of the paper into the front gauge, which will cause the paper to go out of position and may scratch the paper surface or existing marks. Therefore, the thickness of the printed paper changes, and the height of the paperboard should be adjusted accordingly.

Fig. 9 The iron plate is installed on the port of the iron plate

The blower at the front gauge is shown in Figure 10. When the paper is about to reach the front gauge, the air blower blows under the paper bite to form a layer of air cushion, which holds the paper into the front gauge, alleviates the impact of the front gauge on the paper, and prevents the paper from passing through the gas flow Do not manipulate the surface and marks of the paper.

Figure 10 Blowing device installed at the front

This device is currently installed on various offset presses such as Mitsubishi, Komori, and Heidelberg.

2. Paper specification preset system

At present, many brands of offset presses have been equipped with a paper size presetting system. Enter the paper size to be printed on the central console, such as paper type, thickness, size and other data. Each part of the printing machine will be automatically adjusted, such as: Feida position, suction volume, blowing volume, pressure and position of paper pressure roller, brush wheel, paper pressure plate, etc., reduce the time and uncertainty of manual adjustment, and greatly shorten the preparation time of printing.

3. Paper trend monitor

The deviation of the paper entering the previous position not only leads to inaccurate paper positioning, but also greatly reduces the production efficiency due to multiple shutdowns. Usually, the operator adjusts each control unit in the paper conveying device to ensure that the paper reaches the predetermined position accurately. Many brand offset presses now use paper trend monitors. The operator checks the alignment of the paper at the front gauge through the screen, and easily corrects the paper trend position through the button, eliminating the time for manual adjustment, shortening the preparation time of the job, and improving Production efficiency.

4. Suction brush roller device

Offset Heidelberg offset presses are equipped with a suction brush roller above the paper table. As the paper is transported, the suction brush roller sucks away the paper dust on the surface of the paper, reducing machine downtime and improving printing quality.

5. Static elimination device

During paper conveyance, static interference will be caused by friction, and the adhesion of paper will affect the smooth separation and conveyance of paper. Many sheet-fed offset printing presses have increased static elimination capability. For example, the paper table is not flat and smooth, but is processed into fish scales to reduce the static electricity of the paper; or the negatively charged ion wind is blown out through the presser foot nozzle or side blowing device to neutralize the positive Charge; there is also a static elimination bar as shown in FIG. 11.

Figure 11 Static elimination bar on the paper table

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