Discussion on the transcendental transfer mechanism
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Sheet-fed offset presses use a variety of transfer methods, but in summary, they can be divided into three basic forms: direct transfer, indirect transfer, and transcendental transfer.
In recent years, the speed of printing presses has been increasing, and the speed of sheetfed offset presses has reached 16,000 sheets per hour or higher. Although the indirect transfer paper which is widely used has the characteristics of smooth paper transfer, it can meet the requirements of high-speed printing. However, this transfer method is also a mechanical reciprocating motion, and the paper that has been positioned is still transferred to the impression cylinder twice.叼 paper tooth row, the faster the printing speed, the greater the inertia, the greater the impact force, the more serious the damage to the machine, the inevitable error, affecting the overprinting accuracy, thus hindering the further improvement of the printing speed, and the printing speed is increased to a certain degree. After the degree, due to the limitation of manufacturing precision and the objective reasons such as assembly error and vibration, there will always be errors and affect the accuracy of overprinting.
To this end, on some high-speed presses, the final positioning of the paper is placed on the impression cylinder, ie the final registration rule is placed on the impression cylinder rather than on the paperboard. After the paper passes the pre-pre-regulation and side-prescribed positions on the paperboard, the speed gradually increases from zero to about 25% of the linear speed of the roller surface, and the paper is pushed tightly to the front gauge on the drum for positioning. This type of transfer is called transcendental transfer.
By adopting transcendental transfer paper, the positioning time of the paper at the rule can be shortened as much as possible, and the paper acceleration mechanism of the reciprocating motion accelerates the paper beyond the surface speed of the roller surface, finally locates on the drum, and then the cylinder is smashed. print. This eliminates the problem of destroying the paper positioning accuracy during the transfer process. This method of transferring paper overcomes the shortcomings of direct and indirect transfer, and is advantageous for increasing the speed of printing. It has the following advantages:
1 Compared with the indirect transfer method, the paper has a reduced transfer mechanism, which is compact in structure and has a reduced probability of paper delivery errors.
2 The paper is positioned twice, and the final positioning is set on the impression cylinder, which reduces the number of intermediate handovers and is accurate in positioning, especially for high-speed printing.
3 The paper speed is greater than the linear speed of the impression cylinder (with a certain amount of overshoot), which is conducive to the final positioning.
4 The final positioning plate can be easily adjusted, which can partially solve the problem of overprinting caused by paper deformation.
5 The impact, vibration and noise of the mechanism are small, and the paper transfer stability is good.
6 easy and fast operation, reducing labor intensity.
However, it must be pointed out that transcendental transfer can only be applied to smaller format papers, ie only for smaller presses; for larger format papers or for large presses, the accuracy will be much lower.
Beyond transfer is a new and advanced way of transferring paper. In recent years, such devices have developed very rapidly, and there are many types, such as friction roller type, vacuum belt type, suction wheel type, etc., but their basic principles are similar, and there is no qualitative difference. The following is a description of the composition and working principle of the transcendental transfer mechanism with the more common and advanced suction wheel type overfeed mode.
The suction wheel type overfeeding mechanism is mainly composed of two parts: one part is a power transmission and a variable speed movement mechanism; the other part is a vacuum opening time control and a negative pressure adjustment mechanism.
(1) Variable speed (intermittent) motion analysis of the suction wheel
The suction wheel is the key actuator in the transcendental transfer mechanism, and it is important to analyze its working principle and motion characteristics. The shifting intermittent movement of the suction wheel is usually realized by using a multi-head conjugate disc-shaped indexing cam mechanism as the transmission mechanism. Therefore, it is particularly important to first analyze the motion characteristics of the conjugated disc-shaped indexing cam mechanism.
1 conjugate disc indexing cam mechanism
The transcendental transfer mechanism has two cams with the same profile on the power input shaft, but is reversely mounted according to a certain phase angle to form a pair of parallel common disc-shaped indexing cams. When the pair of disc-shaped indexing cams are continuously rotated, the follower turntable and the suction wheel of the drive mechanism transmit the paper to the paper feed roller according to a certain transmission ratio. The common disc-shaped indexing cam is always in contact with the follower to form a geometric closure
The conjugate disc-shaped indexing cam mechanism has the advantages of simple design analysis process, high indexing precision, small impact vibration, and is suitable for high-speed operation, and the transmission mechanism using the same as the transmissive paper-feeding mechanism is quite ideal.
2 yoke disc indexing cam mechanism motion characteristics and corridor curve
Related studies have shown that the motion characteristics of the common disc-shaped indexing cam mechanism is a general improved trapezoidal acceleration.
3 suction wheel shifting movement
According to the motion characteristics and contour curve of the conjugate disc-shaped indexing cam mechanism of the drive mechanism, the motion form of the follower suction wheel can be divided into the following four stages:
a. Start-up phase (accelerated movement):
At this stage, the suction wheel sucks the stationary paper on the paperboard, and the suction wheel starts to accelerate until the speed exceeds the surface speed of the paper feed roller, so that the bite edge of the paper reaches the front paperboard in time for the second positioning. .
b. Handover phase (constant motion):
At this stage, the paper feed roller jaws bite the paper and hand the paper to the impression cylinder, causing the paper to leave the pre-premature gauge completely quickly.
c. Inert phase (deceleration motion):
At this point, the paper completely leaves the suction wheel, and the portion of the suction wheel that has no suction hole on the surface is finished, and starts to decelerate at a high speed operation, and transitions to a stationary state, so that the initial portion of the suction hole on the surface of the suction wheel is at the surface of the suction plate. .
d. Static phase: At this stage, the suction wheel is about to complete the entire motion cycle, waiting for the beginning of the next delivery cycle.
(2) Working principle of the suction control mechanism
The negative pressure provided by the air pump of the mechanism controls the vacuum volume through the regulating valve, and the set negative pressure enters the main air duct of the mechanism. A ventilation valve is arranged on each of the suction air ducts, and when the valves are all opened, the negative pressure is directed to the air ducts of the respective suction wheels. When the piston rod is in the open position, the negative pressure will enter the air chamber, and the suction wheel can absorb the paper by the action of the air pressure, and realize the transcendental transfer under the action of the shifting mechanism; when the piston rod is in the closed position, the negative The pressure cannot enter the vacuum chamber, the air chamber is open to the atmosphere, and the suction wheel loses the function of delivering the paper; and when the paper detecting device finds that there is some problem with the paper being conveyed, such as double sheet, empty sheet, skew, etc., the mechanical automatic The control mechanism immediately produces a series of chain reactions, one of which is that the supply of air in the air chamber is stopped, so that the suction wheel cannot act on the paper stopped on the paperboard.
Adjustment of the suction wheel beyond the transfer mechanism
(1) The important feature of the transcendental transfer mechanism - paper arch
Since the paper is at a standstill when the front gauge is in the pre-position, and the drum is in operation, the speed of the paper must be slightly larger than the surface speed of the impression cylinder (usually around 25%), so that the paper can be pushed tightly and accurately. The final positioning plate of the impression cylinder. Therefore, the front half of the paper will necessarily produce a convexity, often referred to as a arch.
It is generally believed that the arch of the paper reflects the degree of transcendence of the transfer, and also apparently shows that the running speed of the paper is greater than the linear speed of the paper feed roller, so it is one of the important features of the transponder. Without this arch, the transfer speed is only equal to or less than the surface speed of the paper feed roller, which means that the paper has just been reluctantly sent to the registration plate of the paper feed roller or not sent to the front gauge at all. If the above phenomenon occurs, it will lead to the accurate positioning of the paper and the accuracy of the overprinting, and the printing quality will also be affected.
The amount of paper is different, and the amount of arching is different, but it is generally in the range of 1 - 4mm. The usual rule is that the greater the stiffness of the paper, the smaller the amount of arching.
(2) Adjustment method:
1 paper arch adjustment
Paper arch adjustment is very convenient. The adjusting device is generally disposed outside the protective cover of the machine transmission surface (ie, the side of the II), and is composed of two parts: an adjusting handle and a lock tension arch handle. First, the paper is delivered by the suction roller to the paper feed roller, and the machine is stopped at 128.5° (the front edge of the paper is close to the front gauge of the transfer roller, the teeth are not closed), and then the paper arching amount is checked. If it is too large or too small, you can loosen the locking handle and adjust the value dial on the end of the handwheel. After adjusting, tighten the locking handle, then let the paper run several times and check the crown at 128.5° of the machine. .
2 adjustment of the air volume of the suction wheel
a. First turn the machine to the 100 o to 110 o position (the air valve of the suction wheel is still open and the suction hole is holding the paper).
b. Place a common amount of paper and type on each suction wheel to cover the suction wheel.
c. Turn on the air pump to turn on the negative pressure, and turn the adjustment handle so that the vacuum level displayed by the vacuum gauge reaches 0.5pa. The outline of the suction hole can be seen faintly on the paper strip, and there is a resistance when the paper strip is twitched. .
d. There is a damper on the ventilation duct of each suction wheel. As long as the valve is rotated 900, the air source of the suction wheel can be closed, and the suction wheel can be discarded.
In short, when the printing conditions change, especially when the properties of the paper change, the adjustment of the suction wheel type overfeeding mechanism is relatively simple, and no mechanical adjustment is needed, as long as the paper arch and the suction air volume are performed. The corresponding simple adjustment is fine.
Friction-rolling type, vacuum belt type overfeeding mechanism
There is no essential difference between the working principle of the friction roller type, the vacuum belt type overfeeding mechanism and the suction wheel type overfeeding mechanism, and the mechanical mechanism. The main difference is that the shifting mechanism used is different. The two types of transcendental transfer mechanisms will be briefly described below to compare and supplement with the suction wheel type overfeed mechanism.
1. Friction roller type overfeeding mechanism
The friction roller type overfeeding mechanism is mainly composed of an upper transfer roller, a lower transfer roller, a paper stopper and the like. The swing of the upper transfer paper roller and the paper stopper lever and the rotation of the lower paper feed roller are respectively driven by the two cams on the cylinder of the impression cylinder through corresponding mechanisms.
The basic working process is: after the paper reaches the front gauge and is pre-positioned, the side gauge falls to complete the initial positioning, and then the upper paper roller falls to press the paper. After that, the lower transfer roller starts to rotate, and the front gauge starts to let the paper advance. Under the guidance of the paper stopper, the paper advances in a certain gap, and the paper is quickly conveyed by the friction of the upper and lower rollers, and the paper is accelerated by the rapid acceleration. The speed of the surface of the drum reaches the positioning plate of the drum to complete the second positioning. The paper is prepared by the squeegee of the impression cylinder, and the paper is lifted on the upper transfer roller and the stopper.
There are many adjustment items of the friction roller type over-feeding mechanism, such as the pressure of the transfer roller, the rotation arc length of the lower transfer roller, the gap between the paper stopper and the paperboard, etc., and the operation is not convenient. In addition, it has a very obvious defect: due to the upper and lower friction drive, the second print is easy to smear the partial print of the first print. Therefore, this type of paper delivery mechanism must be greatly improved.
2. Vacuum belt type beyond paper transfer mechanism
The vacuum belt type overfeeding mechanism is composed of a front plate, a nozzle, a porous tape and the like.
The basic working process is:
(1) After the paper arrives at the pre-positioning position, the side gauge falls and is positioned, and the inhalation stops at this time;
(2) The nozzle starts to inhale, and the paper is adsorbed on the tape. At the same time, the paper is rotated by the yaw of the front plate, and the piston in the gas distribution wheel is opened, and the gas path forms a vacuum under the action of the vacuum pump. The vacuum belt and the gas distribution port communicate with the paper, and the paper is accelerated by the friction between the paper and the tape. At this time, the front gauge drops the paper.
(3) The paper reaches the positioning plate of the drum at a speed greater than the surface of the drum to complete the second positioning.
(4) The impression cylinder is smashed by the paper to convey the paper. At this time, the suction is converted into a blower, and an air cushion is formed between the paper feed table and the paper to facilitate the rapid transfer of the paper. The tape starts to reverse, and the front positioning plate enters the positioning position again, and the next paper is prepared for positioning.
The adjustment of the vacuum belt type overfeeding mechanism is also relatively simple: there is no need to adjust the mechanical aspect, generally only the vacuum degree and the blowing pressure can be adjusted. Therefore, it is more convenient to use than the friction roller type overfeeding mechanism. Moreover, since the device is disposed under the paperboard and is fed by paper suction, the problem of smudging is also solved.